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This work presents the first experimental results on the electron properties of a nanosecond diffuse fast ionisation wave generated in synthetic dry air at atmospheric pressure under very strong overvoltage. Both density and mean temperature of electrons are investigated by incoherent Thomson scattering. The electron density is also derived from the Stark broadening of oxygen lines resolved by optical emission spectroscopy. The extreme voltages applied question some common hypothesis of the diagnostics implemented. The solutions adopted and the remaining limitations are discussed in the paper. Each diagnostic covers a specific region of interest within the discharge and they show good agreement in conditions where they overlap. It is shown that most of the volume of the pin-to-plane discharge is quite representative of a quasi-steady state glow discharge dominated by the emission of the first and second positive systems of nitrogen. Once its propagation completed within the first two nanoseconds and until the end of the 10 ns pulse, it is characterized by rather homogeneous properties close to the axis. The electron density is of the order of 1015 cm−3 and the mean temperature is about 3 eV within the whole air gap. About 6 ns after the start of the discharge from the pin, a sub-millimetric region of strong ionization develops at the pin, which is consistent with the observation of a continuum of emission spreading from the UV to the near-IR spectral range. Within this part of the discharge, the electron density reaches values greater than 1017 cm−3 with an ionization degree higher than 1%. The radiative recombination of nitrogen ions N2+ and the three-body recombination of N+ with a large number of electrons could help to explain the continuum.

The insulator–metal transition in liquid hydrogen is an important phenomenon to understand the interiors of gas giants, such as Jupiter and Saturn, as well as the physical and chemical behavior of materials at high pressures and temperatures. Here, the path toward an experimental approach is detailed based on spectrally resolved x-ray scattering, tailored to observe and characterize hydrogen metallization in dynamically compressed hydrocarbons in the regime of carbon–hydrogen phase separation. With the help of time-dependent density functional theory calculations and scattering spectra from undriven carbon samples collected at the European x-ray Free-Electron Laser Facility (EuXFEL), we demonstrate sufficient data quality for observing C–H demixing and investigating the presence of liquid metallic hydrogen in future experiments using the reprated drive laser systems at EuXFEL.

As a signal processing method for fast digitizers of the switched-capacitor-type in Thomson scattering diagnostics, a “model fitting” method is proposed. An ideal shape of the signal is estimated by this method by averaging many Thomson scattering signals. After applying this method to a relatively low density LHD plasma, the scattering of electron temperature profiles becomes small. The magnitude of error is also reduced by about 60% at some spatial channels in the core plasma. Simulations of signals with some noises based on the JT-60SA Thomson scattering system enables a showing of the expected error in electron temperature. The error can be suppressed by the “model fitting” method., source:https://doi.org/10.1585/pfr.17.2402032, identifier:0000-0003-2217-6018

A multi-pass Thomson scattering (TS) system based on confining laser pulses in an optical cavity was constructed for measuring very low-density plasma in the TST-2 spherical tokamak device. This paper describes the setup of the optical system, injection of the laser pulse into the cavity, and properties of the confined laser pulse. A combination of Pockels cell plus polarizer, which serves as an optical shutter, allows us to inject and then confine intense laser pulses in the cavity. A photodiode signal monitoring the very weak light leaking from the cavity mirrors demonstrated that the laser pulse makes many round trips, with a round-trip efficiency of approximately 0.73. The effective number of round trips (i.e., the signal enhancement factor) is approximately 3.7. For an injection efficiency of approximately 0.69, a cavity-confined laser pulse, applied to Thomson scattering, will yield a scattered signal that is five times larger than that from a single-pass laser pulse., source:https://doi.org/10.1585/pfr.9.1202005

This work discusses the possible mechanisms that have led to the degradation of the pedestal pressure height and pedestal stability of low triangularity peeling-ballooning (PB) limited pla-smas from JET with the carbon wall (JET-C) to JET with the ITER-like wall (JET-ILW). The work shows that the lower pedestal stability of JET-ILW is not directly caused by the presence of a metal wall, but rather due to the operational constraints that require operation with high gas fuelling. Recent results have suggested the possible role of the distance between the density and temperature pedestal positions (n(e)(pos) - T-e(pos), also called the 'relative shift') with increasing gas fuelling rate on the PB stability of JET-ILW baseline plasmas (Stefanikova et al 2018 Nucl. Fusion 58 056010). The work further extends the analysis of the role of the relative shift, showing that it plays an important role in the difference between the pedestal performance of JET-C and JET-ILW. Moreover, the work also shows that the pedestal density n(e)(ped), Z(eff), and pedestal pressure width w(pe) play an important role in this difference. The pedestal structure and stability have been studied both experimentally and by modelling. The modelling shows that the changes in n(e)(pos) - T-e(pos), n(e)(ped), Z(eff), and w(pe) are sufficient to explain the differences in the pedestal performance between JET-C and JET-ILW PB limited discharges. A hypothesis describing the possible mechanisms leading to the degradation of the pedestal pressure and stability from JET-C to JET-ILW in PB limited plasmas is put forward., QC 20210205

Controlled thermonuclear fusion offers a promising concept for safe and sustainable production of electrical energy. However, there are still many issues to be investigated on the way to a commercial fusion reactor. An important point for detailed studies is connected to wall materials surrounding hot thermonuclear plasma. The JET tokamak (the largest fusion experiment in the world) in the United Kingdom has completed a major upgrade in 2011 in which the materials of the vessel surrounding the fusion fuel have been changed from a carbon-fibre-composite (or JET-C wall) to Beryllium and Tungsten. These new materials are the same as those that will be used in a next step fusion device International Thermonuclear Experimental Reactor ITER (hence the name ITER-like wall or JET-ILW), designed to demonstrate the feasibility of fusion reactor based on the tokamak concept. One of the goals of JET with the ILW is to act as a test bed for ITER technologies and for ITER operating scenarios. The overall purpose of the thesis work is to characterise the effect of the ILW on the structure and stability of edge plasma phenomenon called the pedestal, a steep pressure gradient associated with the H-mode, an operational regime with improved confinement. The aim is to contribute to the understanding of the difference in the pedestal performance between JET-C and JET-ILW. The work is focused on experimental characterisation of the pedestal structure in deuterium discharges by analysing the experimental data (radial profiles of electron temperature and density measured in H-mode plasmas) from Thomson scattering diagnostics at JET and on investigating the differences in pedestal stability between JET-ILW and JET-C plasmas in terms of the pedestal modelling. The pedestal structure is determined using a modified hyperbolic tangent fit to the experimental Thomson scattering profiles. The modelling is performed with the pedestal predictive code Europed, based on the EPED model commonly used t, Kontrollerad termonukleär fusion erbjuder ett lovande koncept för säker och hållbar energiproduktion. Det finns dock fortfarande många frågor som ska undersökas på väg till en kommersiell fusionsreaktor. En viktig aspekt är väggmaterialet som innesluter det varma termonukleära plasmat. JET tokamak, världens största fusionsexperiment beläget i Storbritannien, har genomgått en stor översyn 2011 där kolfiberkompositvägg (eller JET-C vägg) som innesluter fusionsbränslet ersattes med beryllium och volfram. De nya materialen är identiska med de i nästa stegs fusionsexperiment, International Thermonuclear Experimental Reactor ITER (därav namnet “ITER-liknande vägg” eller JET-ILW), som förväntas demonstrera genomförbarheten av en fusionsreaktor baserad på Tokamak-konceptet. Ett av de experimentella målen med JET-ILW är att vara ett test för ITER-teknik och för ITER-driftsscenarier. Det övergripande målet för detta avhandlingsarbete är att beskriva hur strukturen och stabiliteten hos ett kantplasmafenomen som kallas piedestal påverkas av ILW. Piedestal är en brant tryckgradient associerad med H-mod, vilket är ett driftscenario med förbättrad inneslutning. Syftet är att bidra till förståelsen av skillnader i piedestalstrukturen mellan JET-C och JET-ILW. Avhandlingens fokus är att beskriva piedestalstrukturen i deuteriumurladdningar genom att analysera experimentella data (radiella profiler av elektrontemperatur och densitet mätt i H-mod plasma) från Thomsonspridningsdiagnostik vid JET, och att belysa skillnader i piedestalens stabilitet mellan JET-ILW och JET-C-plasma via modellering. Piedestalstrukturen kännetecknas av en modifierad hyperbolisk anpassning till experimentella Thomsonspridningsprofiler. Modelleringen utförs med prediktionskoden Europed, baserad på EPED-modellen som allmänt används för att förutsäga piedestalhöjden i JET. Analysen av experimentella data från JET-ILW och JET-C visar ett antal skillnader i piedestalstrukturen hos plasmaurladdningar under jämförba, QC 20200518

Controlled thermonuclear fusion offers a promising concept for safe and sustainable production of electrical energy. However, there are still many issues to be investigated on the way to a commercial fusion reactor. An important point for detailed studies is connected to wall materials surrounding hot thermonuclear plasma. The JET tokamak (the largest fusion experiment in the world) in the United Kingdom has completed a major upgrade in 2011 in which the materials of the vessel surrounding the fusion fuel have been changed from a carbon-fibre-composite (or JET-C wall) to Beryllium and Tungsten. These new materials are the same as those that will be used in a next step fusion device International Thermonuclear Experimental Reactor ITER (hence the name ITER-like wall or JET-ILW), designed to demonstrate the feasibility of fusion reactor based on the tokamak concept. One of the goals of JET with the ILW is to act as a test bed for ITER technologies and for ITER operating scenarios. The overall purpose of the thesis work is to characterise the effect of the ILW on the structure and stability of edge plasma phenomenon called the pedestal, a steep pressure gradient associated with the H-mode, an operational regime with improved confinement. The aim is to contribute to the understanding of the difference in the pedestal performance between JET-C and JET-ILW. The work is focused on experimental characterisation of the pedestal structure in deuterium discharges by analysing the experimental data (radial profiles of electron temperature and density measured in H-mode plasmas) from Thomson scattering diagnostics at JET and on investigating the differences in pedestal stability between JET-ILW and JET-C plasmas in terms of the pedestal modelling. The pedestal structure is determined using a modified hyperbolic tangent fit to the experimental Thomson scattering profiles. The modelling is performed with the pedestal predictive code Europed, based on the EPED model commonly used t, Kontrollerad termonukleär fusion erbjuder ett lovande koncept för säker och hållbar energiproduktion. Det finns dock fortfarande många frågor som ska undersökas på väg till en kommersiell fusionsreaktor. En viktig aspekt är väggmaterialet som innesluter det varma termonukleära plasmat. JET tokamak, världens största fusionsexperiment beläget i Storbritannien, har genomgått en stor översyn 2011 där kolfiberkompositvägg (eller JET-C vägg) som innesluter fusionsbränslet ersattes med beryllium och volfram. De nya materialen är identiska med de i nästa stegs fusionsexperiment, International Thermonuclear Experimental Reactor ITER (därav namnet “ITER-liknande vägg” eller JET-ILW), som förväntas demonstrera genomförbarheten av en fusionsreaktor baserad på Tokamak-konceptet. Ett av de experimentella målen med JET-ILW är att vara ett test för ITER-teknik och för ITER-driftsscenarier. Det övergripande målet för detta avhandlingsarbete är att beskriva hur strukturen och stabiliteten hos ett kantplasmafenomen som kallas piedestal påverkas av ILW. Piedestal är en brant tryckgradient associerad med H-mod, vilket är ett driftscenario med förbättrad inneslutning. Syftet är att bidra till förståelsen av skillnader i piedestalstrukturen mellan JET-C och JET-ILW. Avhandlingens fokus är att beskriva piedestalstrukturen i deuteriumurladdningar genom att analysera experimentella data (radiella profiler av elektrontemperatur och densitet mätt i H-mod plasma) från Thomsonspridningsdiagnostik vid JET, och att belysa skillnader i piedestalens stabilitet mellan JET-ILW och JET-C-plasma via modellering. Piedestalstrukturen kännetecknas av en modifierad hyperbolisk anpassning till experimentella Thomsonspridningsprofiler. Modelleringen utförs med prediktionskoden Europed, baserad på EPED-modellen som allmänt används för att förutsäga piedestalhöjden i JET. Analysen av experimentella data från JET-ILW och JET-C visar ett antal skillnader i piedestalstrukturen hos plasmaurladdningar under jämförba, QC 20200518

In this lecture an overview of applications of incoherent Thomson scattering (TS) as well as collective Thomson scattering (CTS) will be given. These are the most accurate methods for measuring the electron and ion properties, because the method is direct and non-intrusive. A CTS system based on the fundamental wavelength of a seeded Nd:YAG laser, being developed for the high density, low-temperature plasma of the linear plasma generator Magnum-PSI will be described also. The small Debye length of dense low temperature plasma enables application of CTS at relatively short laser wavelength. The combination of this CTS system and existing incoherent TS system enables determination of electron density and temperature as well as ion temperature and plasma velocity of the near surface plasma. In this lecture, the theoretical background and experimental challenges of the work will be given along with some examples that demonstrate the capabilities of such systems.

This paper quantitatively characterizes a kHz atmospheric pressure He plasma jet without target powered by a pulse of positive applied voltage. It focuses on a quantitative comparison between experimental measurements and numerical results of a two-dimensional fluid model using the same configuration, for different values of magnitude and width of pulsed applied voltage. Excellent agreement is obtained between experiments and simulations on the gas mixture distribution, the length and velocity of discharge propagation and the electric field in the discharge front. For the first time in the same jet, the experimentally measured increase of the electric field in the plume is confirmed by the simulations. The electron density and temperature, measured behind the high field front, are found to agree qualitatively. Moreover, the comparison with simulations shows that discharge propagation stops when the potential in the discharge head is lower than a critical value. Hence, pulse width and magnitude allow to control propagation length. For long pulses, the potential in the discharge front reaches this critical value during the pulse. For shorter pulses, propagation is determined by the pulse shape, as the critical value is reached around 90-130 ns after the fall of the pulse. The results suggest that the magnitude of this critical value is defined by the gas mixture at the position of the front.

In this lecture an overview of applications of incoherent Thomson scattering (TS) as well as collective Thomson scattering (CTS) will be given. These are the most accurate methods for measuring the electron and ion properties, because the method is direct and non-intrusive. A CTS system based on the fundamental wavelength of a seeded Nd:YAG laser, being developed for the high density, low-temperature plasma of the linear plasma generator Magnum-PSI will be described also. The small Debye length of dense low temperature plasma enables application of CTS at relatively short laser wavelength. The combination of this CTS system and existing incoherent TS system enables determination of electron density and temperature as well as ion temperature and plasma velocity of the near surface plasma. In this lecture, the theoretical background and experimental challenges of the work will be given along with some examples that demonstrate the capabilities of such systems.

This paper quantitatively characterizes a kHz atmospheric pressure He plasma jet without target powered by a pulse of positive applied voltage. It focuses on a quantitative comparison between experimental measurements and numerical results of a two-dimensional fluid model using the same configuration, for different values of magnitude and width of pulsed applied voltage. Excellent agreement is obtained between experiments and simulations on the gas mixture distribution, the length and velocity of discharge propagation and the electric field in the discharge front. For the first time in the same jet, the experimentally measured increase of the electric field in the plume is confirmed by the simulations. The electron density and temperature, measured behind the high field front, are found to agree qualitatively. Moreover, the comparison with simulations shows that discharge propagation stops when the potential in the discharge head is lower than a critical value. Hence, pulse width and magnitude allow to control propagation length. For long pulses, the potential in the discharge front reaches this critical value during the pulse. For shorter pulses, propagation is determined by the pulse shape, as the critical value is reached around 90-130 ns after the fall of the pulse. The results suggest that the magnitude of this critical value is defined by the gas mixture at the position of the front.

Controlled thermonuclear fusion offers a promising concept for safe and sustainable production of electrical energy. However, there are still many issues to be investigated on the way to a commercial fusion reactor. An important point for detailed studies is connected to wall materials surrounding hot thermonuclear plasma. The JET tokamak (the largest fusion experiment in the world) in the United Kingdom has completed a major upgrade in 2011 in which the materials of the vessel surrounding the fusion fuel have been changed from a carbon-fibre-composite (or JET-C wall) to Beryllium and Tungsten. These new materials are the same as those that will be used in a next step fusion device International Thermonuclear Experimental Reactor ITER (hence the name ITER-like wall or JET-ILW), designed to demonstrate the feasibility of fusion reactor based on the tokamak concept. One of the goals of JET with the ILW is to act as a test bed for ITER technologies and for ITER operating scenarios. The overall purpose of the thesis work is to characterise the effect of the ILW on the structure and stability of edge plasma phenomenon called the pedestal, a steep pressure gradient associated with the H-mode, an operational regime with improved confinement. The aim is to contribute to the understanding of the difference in the pedestal performance between JET-C and JET-ILW. The work is focused on experimental characterisation of the pedestal structure in deuterium discharges by analysing the experimental data (radial profiles of electron temperature and density measured in H-mode plasmas) from Thomson scattering diagnostics at JET and on investigating the differences in pedestal stability between JET-ILW and JET-C plasmas in terms of the pedestal modelling. The pedestal structure is determined using a modified hyperbolic tangent fit to the experimental Thomson scattering profiles. The modelling is performed with the pedestal predictive code Europed, based on the EPED model commonly used t, Kontrollerad termonukleär fusion erbjuder ett lovande koncept för säker och hållbar energiproduktion. Det finns dock fortfarande många frågor som ska undersökas på väg till en kommersiell fusionsreaktor. En viktig aspekt är väggmaterialet som innesluter det varma termonukleära plasmat. JET tokamak, världens största fusionsexperiment beläget i Storbritannien, har genomgått en stor översyn 2011 där kolfiberkompositvägg (eller JET-C vägg) som innesluter fusionsbränslet ersattes med beryllium och volfram. De nya materialen är identiska med de i nästa stegs fusionsexperiment, International Thermonuclear Experimental Reactor ITER (därav namnet “ITER-liknande vägg” eller JET-ILW), som förväntas demonstrera genomförbarheten av en fusionsreaktor baserad på Tokamak-konceptet. Ett av de experimentella målen med JET-ILW är att vara ett test för ITER-teknik och för ITER-driftsscenarier. Det övergripande målet för detta avhandlingsarbete är att beskriva hur strukturen och stabiliteten hos ett kantplasmafenomen som kallas piedestal påverkas av ILW. Piedestal är en brant tryckgradient associerad med H-mod, vilket är ett driftscenario med förbättrad inneslutning. Syftet är att bidra till förståelsen av skillnader i piedestalstrukturen mellan JET-C och JET-ILW. Avhandlingens fokus är att beskriva piedestalstrukturen i deuteriumurladdningar genom att analysera experimentella data (radiella profiler av elektrontemperatur och densitet mätt i H-mod plasma) från Thomsonspridningsdiagnostik vid JET, och att belysa skillnader i piedestalens stabilitet mellan JET-ILW och JET-C-plasma via modellering. Piedestalstrukturen kännetecknas av en modifierad hyperbolisk anpassning till experimentella Thomsonspridningsprofiler. Modelleringen utförs med prediktionskoden Europed, baserad på EPED-modellen som allmänt används för att förutsäga piedestalhöjden i JET. Analysen av experimentella data från JET-ILW och JET-C visar ett antal skillnader i piedestalstrukturen hos plasmaurladdningar under jämförba, QC 20200518

Controlled thermonuclear fusion offers a promising concept for safe and sustainable production of electrical energy. However, there are still many issues to be investigated on the way to a commercial fusion reactor. An important point for detailed studies is connected to wall materials surrounding hot thermonuclear plasma. The JET tokamak (the largest fusion experiment in the world) in the United Kingdom has completed a major upgrade in 2011 in which the materials of the vessel surrounding the fusion fuel have been changed from a carbon-fibre-composite (or JET-C wall) to Beryllium and Tungsten. These new materials are the same as those that will be used in a next step fusion device International Thermonuclear Experimental Reactor ITER (hence the name ITER-like wall or JET-ILW), designed to demonstrate the feasibility of fusion reactor based on the tokamak concept. One of the goals of JET with the ILW is to act as a test bed for ITER technologies and for ITER operating scenarios. The overall purpose of the thesis work is to characterise the effect of the ILW on the structure and stability of edge plasma phenomenon called the pedestal, a steep pressure gradient associated with the H-mode, an operational regime with improved confinement. The aim is to contribute to the understanding of the difference in the pedestal performance between JET-C and JET-ILW. The work is focused on experimental characterisation of the pedestal structure in deuterium discharges by analysing the experimental data (radial profiles of electron temperature and density measured in H-mode plasmas) from Thomson scattering diagnostics at JET and on investigating the differences in pedestal stability between JET-ILW and JET-C plasmas in terms of the pedestal modelling. The pedestal structure is determined using a modified hyperbolic tangent fit to the experimental Thomson scattering profiles. The modelling is performed with the pedestal predictive code Europed, based on the EPED model commonly used t, Kontrollerad termonukleär fusion erbjuder ett lovande koncept för säker och hållbar energiproduktion. Det finns dock fortfarande många frågor som ska undersökas på väg till en kommersiell fusionsreaktor. En viktig aspekt är väggmaterialet som innesluter det varma termonukleära plasmat. JET tokamak, världens största fusionsexperiment beläget i Storbritannien, har genomgått en stor översyn 2011 där kolfiberkompositvägg (eller JET-C vägg) som innesluter fusionsbränslet ersattes med beryllium och volfram. De nya materialen är identiska med de i nästa stegs fusionsexperiment, International Thermonuclear Experimental Reactor ITER (därav namnet “ITER-liknande vägg” eller JET-ILW), som förväntas demonstrera genomförbarheten av en fusionsreaktor baserad på Tokamak-konceptet. Ett av de experimentella målen med JET-ILW är att vara ett test för ITER-teknik och för ITER-driftsscenarier. Det övergripande målet för detta avhandlingsarbete är att beskriva hur strukturen och stabiliteten hos ett kantplasmafenomen som kallas piedestal påverkas av ILW. Piedestal är en brant tryckgradient associerad med H-mod, vilket är ett driftscenario med förbättrad inneslutning. Syftet är att bidra till förståelsen av skillnader i piedestalstrukturen mellan JET-C och JET-ILW. Avhandlingens fokus är att beskriva piedestalstrukturen i deuteriumurladdningar genom att analysera experimentella data (radiella profiler av elektrontemperatur och densitet mätt i H-mod plasma) från Thomsonspridningsdiagnostik vid JET, och att belysa skillnader i piedestalens stabilitet mellan JET-ILW och JET-C-plasma via modellering. Piedestalstrukturen kännetecknas av en modifierad hyperbolisk anpassning till experimentella Thomsonspridningsprofiler. Modelleringen utförs med prediktionskoden Europed, baserad på EPED-modellen som allmänt används för att förutsäga piedestalhöjden i JET. Analysen av experimentella data från JET-ILW och JET-C visar ett antal skillnader i piedestalstrukturen hos plasmaurladdningar under jämförba, QC 20200518

Controlled thermonuclear fusion offers a promising concept for safe and sustainable production of electrical energy. However, there are still many issues to be investigated on the way to a commercial fusion reactor. An important point for detailed studies is connected to wall materials surrounding hot thermonuclear plasma. The JET tokamak (the largest fusion experiment in the world) in the United Kingdom has completed a major upgrade in 2011 in which the materials of the vessel surrounding the fusion fuel have been changed from a carbon-fibre-composite (or JET-C wall) to Beryllium and Tungsten. These new materials are the same as those that will be used in a next step fusion device International Thermonuclear Experimental Reactor ITER (hence the name ITER-like wall or JET-ILW), designed to demonstrate the feasibility of fusion reactor based on the tokamak concept. One of the goals of JET with the ILW is to act as a test bed for ITER technologies and for ITER operating scenarios. The overall purpose of the thesis work is to characterise the effect of the ILW on the structure and stability of edge plasma phenomenon called the pedestal, a steep pressure gradient associated with the H-mode, an operational regime with improved confinement. The aim is to contribute to the understanding of the difference in the pedestal performance between JET-C and JET-ILW. The work is focused on experimental characterisation of the pedestal structure in deuterium discharges by analysing the experimental data (radial profiles of electron temperature and density measured in H-mode plasmas) from Thomson scattering diagnostics at JET and on investigating the differences in pedestal stability between JET-ILW and JET-C plasmas in terms of the pedestal modelling. The pedestal structure is determined using a modified hyperbolic tangent fit to the experimental Thomson scattering profiles. The modelling is performed with the pedestal predictive code Europed, based on the EPED model commonly used t, Kontrollerad termonukleär fusion erbjuder ett lovande koncept för säker och hållbar energiproduktion. Det finns dock fortfarande många frågor som ska undersökas på väg till en kommersiell fusionsreaktor. En viktig aspekt är väggmaterialet som innesluter det varma termonukleära plasmat. JET tokamak, världens största fusionsexperiment beläget i Storbritannien, har genomgått en stor översyn 2011 där kolfiberkompositvägg (eller JET-C vägg) som innesluter fusionsbränslet ersattes med beryllium och volfram. De nya materialen är identiska med de i nästa stegs fusionsexperiment, International Thermonuclear Experimental Reactor ITER (därav namnet “ITER-liknande vägg” eller JET-ILW), som förväntas demonstrera genomförbarheten av en fusionsreaktor baserad på Tokamak-konceptet. Ett av de experimentella målen med JET-ILW är att vara ett test för ITER-teknik och för ITER-driftsscenarier. Det övergripande målet för detta avhandlingsarbete är att beskriva hur strukturen och stabiliteten hos ett kantplasmafenomen som kallas piedestal påverkas av ILW. Piedestal är en brant tryckgradient associerad med H-mod, vilket är ett driftscenario med förbättrad inneslutning. Syftet är att bidra till förståelsen av skillnader i piedestalstrukturen mellan JET-C och JET-ILW. Avhandlingens fokus är att beskriva piedestalstrukturen i deuteriumurladdningar genom att analysera experimentella data (radiella profiler av elektrontemperatur och densitet mätt i H-mod plasma) från Thomsonspridningsdiagnostik vid JET, och att belysa skillnader i piedestalens stabilitet mellan JET-ILW och JET-C-plasma via modellering. Piedestalstrukturen kännetecknas av en modifierad hyperbolisk anpassning till experimentella Thomsonspridningsprofiler. Modelleringen utförs med prediktionskoden Europed, baserad på EPED-modellen som allmänt används för att förutsäga piedestalhöjden i JET. Analysen av experimentella data från JET-ILW och JET-C visar ett antal skillnader i piedestalstrukturen hos plasmaurladdningar under jämförba, QC 20200518

This thesis work concerns methods for improved spectroscopy including stray-light suppression, dynamic range improvement and signal enhancement. These methods were applied in Raman and Thomson- scattering spectroscopy in combustion and plasma. First, the stray-light suppression method Periodic Shadowing (PS) is implemented in an experimental setup with a high-power high repetition rate laser system used for Raman measurements in premixed, laminar, flat flames. It is shown that the background is reduced by a factor of 10 or more. Weak spectral lines can be distinguished and therefore better molecular species identification, concentration and temperature evaluation were performed. Second, initial test for Thomson scattering measurements in a gas discharge plasma was carried out with the experimental setup. Third, the method of using a Digital Micro-mirror Device (DMD) is combined with the PS method to boost the dynamic range in spectroscopic imaging. A novel DMD-PS spectrometer was designed and built and shows significantly enhanced performance, improving the dynamic range by a factor of at least 50. Such a high-dynamic-range and stray-light-reduced spectrometer has potential to be implemented in future imaging spectroscopy., Combustion is a commonly used physical phenomenon in our daily life, from cooking to driving, heating and manufacturing. Typically, combustion starts to happen when fuel is mixed with an oxidant (usually oxygen in the air) and reaches a critical temperature. The process usually contains a complicated sequence of chemical reactions and also gives out intensive heat and light. As an important scientific task, significant research effort has been focused on understanding the physical and chemical processes in combustion. Plasma is one of the fundamental states of matter in which an ionized substance becomes highly electrically conductive. Lightning is an example in nature of a partially ionized plasma. Plasma can be artificially generated as well. When electric current flows through a gaseous medium, gas discharge occurs and plasma is generated. Measuring the parameters of such laboratory plasma, termed plasma diagnostics, helps researchers to understand the physical phenomena. Optical spectroscopy has played a significant role in many research fields, including combustion and plasma diagnostics. The interaction between matter and light can provide considerable amount of useful information. Therefore, specific light sources, such as lasers, can be employed as a probe to study the object of interest. With the advent of laser techniques, it is possible to achieve in-situ and non-invasive investigations with high spatial and temporal resolution. Raman spectroscopy is a diagnostic method based on Raman scattering, which is an inelastic light matter interaction. When light interacts with molecules, an energy exchange will take place between a photon and a molecule and the photon will scatter off with a different energy. The energy change of the photon coming in and out is determined in accordance with the species, providing a specific fingerprint on the spectrum. Similar to Rayleigh scattering, Thomson scattering is an elastic scattering process when light is scattered by c

We show how optical free-electron lasers and enhanced incoherent Thomson scattering radiation sources can be realized with Traveling-Wave Thomson-Scattering (TWTS) today. Emphasis is put on the realization of optical free-electron lasers (OFELs) with existing state-of-the-art technology for laser systems and electron accelerators. The conceptual design of optical setups for the preparation of laser pulses suitable for TWTS OFELs and enhanced Thomson sources is presented. We further provide expressions to estimate the acceptable alignment tolerances of optical components for TWTS OFEL operation. Examples of TWTS OFELs radiating at 100 nm, 13.5 nm and 1.5 Å as well as an incoherent source at 40 pm highlight the feasibility of the concept and detail the procedure to determine the optical components parameters of a TWTS setup.

This thesis work concerns methods for improved spectroscopy including stray-light suppression, dynamic range improvement and signal enhancement. These methods were applied in Raman and Thomson- scattering spectroscopy in combustion and plasma. First, the stray-light suppression method Periodic Shadowing (PS) is implemented in an experimental setup with a high-power high repetition rate laser system used for Raman measurements in premixed, laminar, flat flames. It is shown that the background is reduced by a factor of 10 or more. Weak spectral lines can be distinguished and therefore better molecular species identification, concentration and temperature evaluation were performed. Second, initial test for Thomson scattering measurements in a gas discharge plasma was carried out with the experimental setup. Third, the method of using a Digital Micro-mirror Device (DMD) is combined with the PS method to boost the dynamic range in spectroscopic imaging. A novel DMD-PS spectrometer was designed and built and shows significantly enhanced performance, improving the dynamic range by a factor of at least 50. Such a high-dynamic-range and stray-light-reduced spectrometer has potential to be implemented in future imaging spectroscopy., Combustion is a commonly used physical phenomenon in our daily life, from cooking to driving, heating and manufacturing. Typically, combustion starts to happen when fuel is mixed with an oxidant (usually oxygen in the air) and reaches a critical temperature. The process usually contains a complicated sequence of chemical reactions and also gives out intensive heat and light. As an important scientific task, significant research effort has been focused on understanding the physical and chemical processes in combustion. Plasma is one of the fundamental states of matter in which an ionized substance becomes highly electrically conductive. Lightning is an example in nature of a partially ionized plasma. Plasma can be artificially generated as well. When electric current flows through a gaseous medium, gas discharge occurs and plasma is generated. Measuring the parameters of such laboratory plasma, termed plasma diagnostics, helps researchers to understand the physical phenomena. Optical spectroscopy has played a significant role in many research fields, including combustion and plasma diagnostics. The interaction between matter and light can provide considerable amount of useful information. Therefore, specific light sources, such as lasers, can be employed as a probe to study the object of interest. With the advent of laser techniques, it is possible to achieve in-situ and non-invasive investigations with high spatial and temporal resolution. Raman spectroscopy is a diagnostic method based on Raman scattering, which is an inelastic light matter interaction. When light interacts with molecules, an energy exchange will take place between a photon and a molecule and the photon will scatter off with a different energy. The energy change of the photon coming in and out is determined in accordance with the species, providing a specific fingerprint on the spectrum. Similar to Rayleigh scattering, Thomson scattering is an elastic scattering process when light is scattered by c

Thomson scattering using a Bragg grating notch filter is used to determine the electron properties of a pulsed, kHz-driven, non-thermal atmospheric pressure plasma jet in helium expanding in air. The plasma jet is allowed to freely expand or interact with targets with different electrical properties, i.e. glass, copper and water. With the same setup, Raman scattering is used to determine spatially- and time-resolved the densities and rotational temperatures of oxygen and nitrogen molecules entrained into the jet. Fast imaging is used to determine the development of the discharge in the plasma jet as well as its behavior in the plasma-target interaction zone. As the discharge approaches the target, the rise of electron density was followed by the fall of electron temperature. The discharge is influenced only over a few millimeters before it hits the target. The electron density and temperature during the spreading of the discharge on the low-permittivity target are measured to be resp. 2 × 1019 m−3 and ≈1 eV. During the return stroke on the high-permittivity and the metallic target the densities rise with a factor 1.5 resp. 2.2, and the temperature with a factor 2.5 for both cases. The discharges on the high- and low-permittivity targets extinguished soon after the initial impact of the ionization front, while the diffuse discharge on the metallic target extinguished only after the end of the voltage pulse (with a duration of 1 μs). In the diffuse discharge the electron temperature reaches 3.4 eV, the gas temperature increases by approximately 100 K and the electron density increases by approximately a factor three with respect to before its formation.

The electron temperature and density pedestals tend to vary in their relative radial positions, as observed in DIII-D (Beurskens et al 2011 Phys. Plasmas 18 056120) and ASDEX Upgrade (Dunne et al 2017 Plasma Phys. Control. Fusion 59 14017). This so-called relative shift has an impact on the pedestal magnetohydrodynamic (MHD) stability and hence on the pedestal height (Osborne et al 2015 Nucl. Fusion 55 063018). The present work studies the effect of the relative shift on pedestal stability of JET ITER-like wall (JET-ILW) baseline low triangularity (δ) unseeded plasmas, and similar JET-C discharges. As shown in this paper, the increase of the pedestal relative shift is correlated with the reduction of the normalized pressure gradient, therefore playing a strong role in pedestal stability. Furthermore, JET-ILW tends to have a larger relative shift compared to JET carbon wall (JET-C), suggesting a possible role of the plasma facing materials in affecting the density profile location. Experimental results are then compared with stability analysis performed in terms of the peeling-ballooning model and with pedestal predictive model EUROPED (Saarelma et al 2017 Plasma Phys. Control. Fusion). Stability analysis is consistent with the experimental findings, showing an improvement of the pedestal stability, when the relative shift is reduced. This has been ascribed mainly to the increase of the edge bootstrap current, and to minor effects related to the increase of the pedestal pressure gradient and narrowing of the pedestal pressure width. Pedestal predictive model EUROPED shows a qualitative agreement with experiment, especially for low values of the relative shift.

The electron temperature and density pedestals tend to vary in their relative radial positions, as observed in DIII-D (Beurskens et al 2011 Phys. Plasmas 18 056120) and ASDEX Upgrade (Dunne et al 2017 Plasma Phys. Control. Fusion 59 14017). This so-called relative shift has an impact on the pedestal magnetohydrodynamic (MHD) stability and hence on the pedestal height (Osborne et al 2015 Nucl. Fusion 55 063018). The present work studies the effect of the relative shift on pedestal stability of JET ITER-like wall (JET-ILW) baseline low triangularity (d) unseeded plasmas, and similar JET-C discharges. As shown in this paper, the increase of the pedestal relative shift is correlated with the reduction of the normalized pressure gradient, therefore playing a strong role in pedestal stability. Furthermore, JET-ILW tends to have a larger relative shift compared to JET carbon wall (JET-C), suggesting a possible role of the plasma facing materials in affecting the density profile location. Experimental results are then compared with stability analysis performed in terms of the peeling-ballooning model and with pedestal predictive model EUROPED (Saarelma et al 2017 Plasma Phys. Control. Fusion). Stability analysis is consistent with the experimental findings, showing an improvement of the pedestal stability, when the relative shift is reduced. This has been ascribed mainly to the increase of the edge bootstrap current, and to minor effects related to the increase of the pedestal pressure gradient and narrowing of the pedestal pressure width. Pedestal predictive model EUROPED shows a qualitative agreement with experiment, especially for low values of the relative shift., For complete list of authors see http://dx.doi.org/10.1088/1741-4326/aab216

We report the first experimental observation of the depolarization of the Thomson scattering (TS) radiation, a relativistic effect expected to occur in very high T-e plasmas and never observed so far in a fusion machine. A set of unused optical fibers in the collection optics of the high resolution Thomson scattering system of JET has been used to detect the depolarized TS radiation during a JET campaign with T-e <= 8 keV. A linear polarizer with the axis perpendicular to the direction of the incident E-field was placed in front of a fiber optic pair observing a region close to the plasma core, while another fiber pair with no polariser simultaneously observed an adjacent plasma region. The measured intensity ratio was found to be consistent with the theory, taking into account sensitivity coefficients of the two measurement channels determined with post-experiment calibrations and Raman scattering. This depolarization effect is at the basis of polarimetric TS, a different and complementary method for the analysis of TS spectra that can provide significant advantages for T-e measurements in very hot plasmas such as in ITER (T-e <= 40 keV)., For complete list of authors see http://dx.doi.org/10.1088/1741-4326/aab3fd

The electron temperature and density pedestals tend to vary in their relative radial positions, as observed in DIII-D (Beurskens et al 2011 Phys. Plasmas 18 056120) and ASDEX Upgrade (Dunne et al 2017 Plasma Phys. Control. Fusion 59 14017). This so-called relative shift has an impact on the pedestal magnetohydrodynamic (MHD) stability and hence on the pedestal height (Osborne et al 2015 Nucl. Fusion 55 063018). The present work studies the effect of the relative shift on pedestal stability of JET ITER-like wall (JET-ILW) baseline low triangularity (d) unseeded plasmas, and similar JET-C discharges. As shown in this paper, the increase of the pedestal relative shift is correlated with the reduction of the normalized pressure gradient, therefore playing a strong role in pedestal stability. Furthermore, JET-ILW tends to have a larger relative shift compared to JET carbon wall (JET-C), suggesting a possible role of the plasma facing materials in affecting the density profile location. Experimental results are then compared with stability analysis performed in terms of the peeling-ballooning model and with pedestal predictive model EUROPED (Saarelma et al 2017 Plasma Phys. Control. Fusion). Stability analysis is consistent with the experimental findings, showing an improvement of the pedestal stability, when the relative shift is reduced. This has been ascribed mainly to the increase of the edge bootstrap current, and to minor effects related to the increase of the pedestal pressure gradient and narrowing of the pedestal pressure width. Pedestal predictive model EUROPED shows a qualitative agreement with experiment, especially for low values of the relative shift., QC 20180411

We report the first experimental observation of the depolarization of the Thomson scattering (TS) radiation, a relativistic effect expected to occur in very high T-e plasmas and never observed so far in a fusion machine. A set of unused optical fibers in the collection optics of the high resolution Thomson scattering system of JET has been used to detect the depolarized TS radiation during a JET campaign with T-e <= 8 keV. A linear polarizer with the axis perpendicular to the direction of the incident E-field was placed in front of a fiber optic pair observing a region close to the plasma core, while another fiber pair with no polariser simultaneously observed an adjacent plasma region. The measured intensity ratio was found to be consistent with the theory, taking into account sensitivity coefficients of the two measurement channels determined with post-experiment calibrations and Raman scattering. This depolarization effect is at the basis of polarimetric TS, a different and complementary method for the analysis of TS spectra that can provide significant advantages for T-e measurements in very hot plasmas such as in ITER (T-e <= 40 keV)., QC 20200416

We report the first experimental observation of the depolarization of the Thomson scattering (TS) radiation, a relativistic effect expected to occur in very high Te plasmas and never observed so far in a fusion machine. A set of unused optical fibers in the collection optics of the high resolution Thomson scattering system of JET has been used to detect the depolarized TS radiation during a JET campaign with Te ≤ 8 keV. A linear polarizer with the axis perpendicular to the direction of the incident E-field was placed in front of a fiber optic pair observing a region close to the plasma core, while another fiber pair with no polarizer simultaneously observed an adjacent plasma region. The measured intensity ratio was found to be consistent with the theory, taking into account sensitivity coefficients of the two measurement channels determined with post-experiment calibrations and Raman scattering. This depolarization effect is at the basis of polarimetric TS, a different and complementary method for the analysis of TS spectra that can provide significant advantages for Te measurements in very hot plasmas such as in ITER (Te ≤ 40 keV).

Generating and controlling ultrashort, pulse-front tilted laser pulses is essential for Traveling-Wave Electron Acceleration (TWEAC), Traveling-Wave Thomson Scattering (TWTS) and Traveling-Wave Optical FELs (TWTS-OFELs). All these applications require controlling angular and group-delay dispersion, while keeping experimental setups as compact as possible. However, the varying requirements with respect to laser power, extent of focal region, incident angles and laser mode quality lead to differing strategies in designing experimental setups. In this overview poster we provide answers to the question: What experimental efforts in terms of laser system and optics are necessary in current labs for first proof-of-principle realizations of the different applications of "Traveling-Wave" laser pulses -- ranging from low-bandwidth and yield-enhanced Thomson sources (TWTS), laser-based electron accelerators beyond the LWFA depletion and dephasing limits (TWEAC) and ultimately an optical free-electron laser (TWTS-OFEL)?

Generating and controlling ultrashort, pulse-front tilted laser pulses is essential for Traveling-Wave Electron Acceleration (TWEAC), Traveling-Wave Thomson Scattering (TWTS) and Traveling-Wave Optical FELs (TWTS-OFELs). All these applications require controlling angular and group-delay dispersion, while keeping experimental setups as compact as possible. However, the varying requirements with respect to laser power, extent of focal region, incident angles and laser mode quality lead to differing strategies in designing experimental setups. In this overview poster we provide answers to the question: What experimental efforts in terms of laser system and optics are necessary in current labs for first proof-of-principle realizations of the different applications of "Traveling-Wave" laser pulses -- ranging from low-bandwidth and yield-enhanced Thomson sources (TWTS), laser-based electron accelerators beyond the LWFA depletion and dephasing limits (TWEAC) and ultimately an optical free-electron laser (TWTS-OFEL)?

All across physics research, incoherent and coherent light sources are extensively utilized. Especially highly brilliant X-ray sources such as third generation synchrotrons or free-electron lasers have become an invaluable tool enabling experimental techniques that are unique to these kinds of light sources. But these sources have developed to large scale facilities and a demand in compact laboratory scale sources providing radiation of similar quality arises nowadays. This thesis focuses on Traveling-Wave Thomson-Scattering (TWTS) which allows for the realization of ultra-compact, inherently synchronized and highly brilliant light sources. The TWTS geometry provides optical undulators, through which electrons pass and thereby emit radiation, with hundreds to thousands of undulator periods by utilizing pulse-front tilted lasers pulses from high peak-power laser systems. TWTS can realize incoherent radiation sources with orders of magnitude higher photon yield than established head-on Thomson sources. Moreover, optical free-electron lasers (OFELs) can be realized with TWTS if state-of-the-art technology in electron accelerators and laser systems is utilized. Tilting the laser pulse front with respect to the wavefront by half of this interaction angle optimizes electron and laser pulse overlap by compensating the spatial offset between electrons and the laser pulse-front at the beginning of the interaction when the electrons are far from the laser pulse axis. The laser pulse-front tilt ensures continuous overlap between electrons and laser pulse while the electrons cross the laser pulse cross-sectional area. Thus the interaction distance can be controlled in TWTS by the laser pulse width rather than laser pulse duration. Utilizing wide, petawatt class laser pulses allows realizing thousands of optical undulator periods. This thesis will show that TWTS OFELs emitting ultraviolet radiation are realizable today with existing technology for electron accelerators and laser

All across physics research, incoherent and coherent light sources are extensively utilized. Especially highly brilliant X-ray sources such as third generation synchrotrons or free-electron lasers have become an invaluable tool enabling experimental techniques that are unique to these kinds of light sources. But these sources have developed to large scale facilities and a demand in compact laboratory scale sources providing radiation of similar quality arises nowadays. This thesis focuses on Traveling-Wave Thomson-Scattering (TWTS) which allows for the realization of ultra-compact, inherently synchronized and highly brilliant light sources. The TWTS geometry provides optical undulators, through which electrons pass and thereby emit radiation, with hundreds to thousands of undulator periods by utilizing pulse-front tilted lasers pulses from high peak-power laser systems. TWTS can realize incoherent radiation sources with orders of magnitude higher photon yield than established head-on Thomson sources. Moreover, optical free-electron lasers (OFELs) can be realized with TWTS if state-of-the-art technology in electron accelerators and laser systems is utilized. Tilting the laser pulse front with respect to the wavefront by half of this interaction angle optimizes electron and laser pulse overlap by compensating the spatial offset between electrons and the laser pulse-front at the beginning of the interaction when the electrons are far from the laser pulse axis. The laser pulse-front tilt ensures continuous overlap between electrons and laser pulse while the electrons cross the laser pulse cross-sectional area. Thus the interaction distance can be controlled in TWTS by the laser pulse width rather than laser pulse duration. Utilizing wide, petawatt class laser pulses allows realizing thousands of optical undulator periods. This thesis will show that TWTS OFELs emitting ultraviolet radiation are realizable today with existing technology for electron accelerators and laser

Plasma detachment in tokamak divertors reduces the particle and power fluxes to the plasma facing components and is essential for successful operation of ITER. The linear plasma generator Pilot-PSI can produce a high density (∼10 21 m −3), low temperature (∼1 eV) plasma which is similar to that expected at the ITER divertor strike-points during the partially detached regime [1]. Given the simple geometry of the device, Pilot-PSI allows to diagnose the plasma beam at multiple axial positions. In this study, the incoherent Thomson scattering (TS) diagnostic [2] is exploited to measure the radial plasma T e and n e profiles at two locations of the hydrogen plasma beam: near the plasma source (upstream) and 2 cm from the target plate. At the target, the TS measurements are supported by an embedded Langmuir probe. These measurements prove the existence of parallel plasma pressure loss as well as particle loss, confirming that physical processes believed to cause detachment in tokamak divertors also hold in Pilot-PSI (ion-neutral friction, volume recombination). It is found that the fractional reduction of the plasma pressure varies between ∼4 and ∼5000, depending strongly on the pressure of the background neutrals. The importance of individual loss channels is discussed.

Traveling-Wave Thomson-Scattering (TWTS) allows for the realization of ultra-compact, inherently synchronized and highly brilliant light sources by providing optical undulators with hundreds to thousands of undulator periods from high-power, pulse-front tilted lasers pulses. With TWTS the realization of optical free-electron lasers (OFELs) as well as incoherent radiation sources with orders of magnitude higher photon yields than classic head-on Thomson sources becomes possible with state-of-the-art technology in electron accelerators and laser systems. The talk will show how pulse-front tilted, petawatt class laser pulses and relativistic electrons work together in a side-scattering geometry, where laser end electron propagation direction of motion enclose an angle, to realize long but compact optical undulators with centimeter to meter-scale interaction distances at sub-millimeter undulator periods. Example setups of TWTS OFELs emitting ultraviolet radiation are presented which are realizable today with existing technology for electron accelerators and laser systems. Especially the ultra-low emittance of laser wakefield accelerated electron bunches can be exploited to compensate for their one percent level energy spreads. Further an experimental setup is presented to generate the tilted TWTS laser pulses. This setup strategy provides dispersion compensation, required due to angular dispersion of the laser pulse, and is especially relevant when building compact, high-yield hard X-ray TWTS sources in large interaction angle setups.

Traveling-Wave Thomson-Scattering (TWTS) can realize ultra-compact, inherently synchronized and highly brilliant light sources from the ultraviolet to the hard X-ray range. In TWTS ultrashort laser pulses and relativistic electron bunches are utilized in a side-scattering geometry where laser pulse and electron bunch direction of motion enclose an interaction angle. Thereby the laser electric field provides the undulator field in which electrons oscillate and emit radiation during interaction. By employing tilted laser pulses TWTS ensures continuous overlap of laser and electrons while these traverse the laser cross-sectional area. Tilting the laser pulse-front compensates the spatial separation of electrons and laser at begin and end of the interaction originating from their different propagation directions. Combining laser pulse-front tilt and side-scattering in TWTS enables interaction over hundreds to thousands of optical undulator periods, enough to allow for optical free-electron laser (OFEL) operation since microbunching of the electron bunch and thus coherent radiation amplification can be achieved. After shortly introducing the TWTS scattering geometry, the design of optical setups to generate the tilted TWTS laser pulses is presented in the talk. This setup strategy provides dispersion compensation, required due to angular dispersion of the laser pulse, and is especially relevant when building compact, high-yield hard X-ray TWTS sources in large interaction angle setups. Determining parameters of the setup is illustrated in an example of an ultraviolet TWTS OFEL and an outlook is given on the design of hard X-ray TWTS sources.

Generating and controlling ultrashort, pulse-front tilted laser pulses is essential for Traveling-Wave Electron Acceleration (TWEAC), Traveling-Wave Thomson Scattering (TWTS) and Traveling-Wave Optical FELs (TWTS-OFELs). All these applications require controlling angular and group-delay dispersion, while keeping experimental setups as compact as possible. However, the varying requirements with respect to laser power, extent of focal region, incident angles and laser mode quality lead to differing strategies in designing experimental setups. This overview poster provides answers to the question: What experimental efforts in terms of laser system and optics are necessary in current labs for first proof-of-principle realizations of the different applications of "Traveling-Wave" laser pulses -- ranging from low-bandwidth and yield-enhanced Thomson sources (TWTS), laser-based electron accelerators beyond the LWFA depletion and dephasing limits (TWEAC) and ultimately an optical free-electron laser (TWTS-OFEL)?

Traveling-Wave Thomson-Scattering (TWTS) allows for the realization of ultra-compact, inherently synchronized and highly brilliant light sources by providing optical undulators with hundreds to thousands of undulator periods from high-power, pulse-front tilted lasers pulses. With TWTS the realization of optical free-electron lasers (OFELs) as well as incoherent radiation sources with orders of magnitude higher photon yields than classic head-on Thomson sources becomes possible with state-of-the-art technology in electron accelerators and laser systems. The talk will show how pulse-front tilted, petawatt class laser pulses and relativistic electrons work together in a side-scattering geometry, where laser end electron propagation direction of motion enclose an angle, to realize long but compact optical undulators with centimeter to meter-scale interaction distances at sub-millimeter undulator periods. Example setups of TWTS OFELs emitting ultraviolet radiation are presented which are realizable today with existing technology for electron accelerators and laser systems. Especially the ultra-low emittance of laser wakefield accelerated electron bunches can be exploited to compensate for their one percent level energy spreads. Further an experimental setup is presented to generate the tilted TWTS laser pulses. This setup strategy provides dispersion compensation, required due to angular dispersion of the laser pulse, and is especially relevant when building compact, high-yield hard X-ray TWTS sources in large interaction angle setups.

Traveling-Wave Thomson-Scattering (TWTS) can realize ultra-compact, inherently synchronized and highly brilliant light sources from the ultraviolet to the hard X-ray range. In TWTS ultrashort laser pulses and relativistic electron bunches are utilized in a side-scattering geometry where laser pulse and electron bunch direction of motion enclose an interaction angle. Thereby the laser electric field provides the undulator field in which electrons oscillate and emit radiation during interaction. By employing tilted laser pulses TWTS ensures continuous overlap of laser and electrons while these traverse the laser cross-sectional area. Tilting the laser pulse-front compensates the spatial separation of electrons and laser at begin and end of the interaction originating from their different propagation directions. Combining laser pulse-front tilt and side-scattering in TWTS enables interaction over hundreds to thousands of optical undulator periods, enough to allow for optical free-electron laser (OFEL) operation since microbunching of the electron bunch and thus coherent radiation amplification can be achieved. After shortly introducing the TWTS scattering geometry, the design of optical setups to generate the tilted TWTS laser pulses is presented in the talk. This setup strategy provides dispersion compensation, required due to angular dispersion of the laser pulse, and is especially relevant when building compact, high-yield hard X-ray TWTS sources in large interaction angle setups. Determining parameters of the setup is illustrated in an example of an ultraviolet TWTS OFEL and an outlook is given on the design of hard X-ray TWTS sources.

Plasma detachment in tokamak divertors reduces the particle and power fluxes to the plasma facing components and is essential for successful operation of ITER. The linear plasma generator Pilot-PSI can produce a high density (∼10 21 m −3), low temperature (∼1 eV) plasma which is similar to that expected at the ITER divertor strike-points during the partially detached regime [1]. Given the simple geometry of the device, Pilot-PSI allows to diagnose the plasma beam at multiple axial positions. In this study, the incoherent Thomson scattering (TS) diagnostic [2] is exploited to measure the radial plasma T e and n e profiles at two locations of the hydrogen plasma beam: near the plasma source (upstream) and 2 cm from the target plate. At the target, the TS measurements are supported by an embedded Langmuir probe. These measurements prove the existence of parallel plasma pressure loss as well as particle loss, confirming that physical processes believed to cause detachment in tokamak divertors also hold in Pilot-PSI (ion-neutral friction, volume recombination). It is found that the fractional reduction of the plasma pressure varies between ∼4 and ∼5000, depending strongly on the pressure of the background neutrals. The importance of individual loss channels is discussed.

We present an experimental setup strategy for the realization of an optical free-electron laser (OFEL) in the Traveling-Wave Thomson-Scattering geometry (TWTS). In TWTS, the electric field of petawatt class, pulse-front tilted laser pulses is used to provide an optical undulator field. This is passed by a relativistic electron bunch so that electron direction of motion and laser propagation direction enclose an interaction angle. The combination of side scattering and pulse-front tilt provides continuous overlap of electrons and laser pulse over meter scale distances which are achieved with centimeter wide laser pulses. An experimental challenge lies in shaping of these wide laser pulses in terms of laser dispersion compensation along the electron trajectory and focusing. The poster shows how diffraction gratings in combination with mirrors are used to introduce and control dispersion of the laser in order to provide a plane wave laser field along the electron trajectory. Furthermore we give limits on alignment tolerances to operate the OFEL. Example setups illustrate functioning and demonstrate feasibility of the design.

We present an experimental setup strategy for the realization of an optical free-electron laser (OFEL) in the Traveling-Wave Thomson-Scattering geometry (TWTS). In TWTS, the electric field of petawatt class, pulse-front tilted laser pulses is used to provide an optical undulator field. This is passed by a relativistic electron bunch so that electron direction of motion and laser propagation direction enclose an interaction angle. The combination of side scattering and pulse-front tilt provides continuous overlap of electrons and laser pulse over meter scale distances which are achieved with centimeter wide laser pulses. An experimental challenge lies in shaping of these wide laser pulses in terms of laser dispersion compensation along the electron trajectory and focusing. The poster shows how diffraction gratings in combination with mirrors are used to introduce and control dispersion of the laser in order to provide a plane wave laser field along the electron trajectory. Furthermore we give limits on alignment tolerances to operate the OFEL. Example setups illustrate functioning and demonstrate feasibility of the design.

We present both the in-situ far field radiation diagnostics in the particle-in-cell code PIConGPU and the offline radiation diagnostic code ClaRa. The first was developed to close the gap between simulated plasma dynamics and radiation observed in laser plasma experiments. The second is used to quantitatively simulate radiation observed in e.g. Thomson scattering experiment. Both methods are based on the far field approximation of the Liénard-Wiechert potential. Their predictive capabilities, both qualitative and quantitative, have been tested against analytical models. We will discuss the advantages of the in-situ approach of PIConGPU over ClaRa that allows predicting both coherent and incoherent radiation spectrally from infrared to x-rays and provides the capability to resolve the radiation polarization and determine the temporal and spatial origin of the radiation. Furthermore, we explain why the direct integration into the highly-scalable GPU framework of PIConGPU allows computing radiation spectra for thousands of frequencies, hundreds of detector positions and billions of particles efficiently. In this talk we will demonstrate these capabilities on resent simulations of laser wakefield acceleration (LWFA), high harmonics generation during target normal sheath acceleration (TNSA) and Thomson scattering during laser electron interactions.

We demonstrate that optical free-electron lasers (OFELs) realized with Traveling-Wave Thomson-Scattering (TWTS) are by orders of magnitude more efficient and brilliant than corresponding OFELs utilizing head-on Thomson scattering geometries. In addition, we emphasize that TWTS OFELs as opposed to standard head-on Thomson scattering geometries scale favorably with regard to space-charge effects degrading emittance, energy losses through photon recoil and transverse coherence of the resulting FEL beam. The presented TWTS OFELs scenarios are assumed to be driven either by conventional, rf-accelerated electrons or by currently available laser wakefield-accelerated electrons featuring energy spreads at the one percent level.

Traveling-Wave Thomson-Scattering (TWTS) allows for the realization of ultra-compact, inherently synchronized and highly brilliant light sources by providing optical undulators with hundreds to thousands of undulator periods from high-power, pulse-front tilted lasers pulses. With TWTS the realization of optical free-electron lasers (OFELs) as well as incoherent radiation sources with orders of magnitude higher photon yields than classic head-on Thomson sources becomes possible with state-of-the-art technology in electron accelerators and laser systems. TWTS employs a side-scattering geometry where laser and electron propagation direction of motion enclose an angle. Tilting the laser pulse front with respect to the wave front by half of this interaction angle optimizes electron and laser pulse overlap by compensating the spatial offset between electrons and the laser pulse-front at the beginning of the interaction when the electrons are far from the laser pulse axis. The laser pulse-front tilt ensures continuous overlap between electrons and laser pulse while the electrons cross the laser pulse cross-sectional area. Thus the interaction distance can be controlled in TWTS by the laser pulse width rather than laser pulse duration. Utilizing wide, petawatt class laser pulses allows to realize thousands of optical undulator periods. The talk will show that TWTS OFELs emitting ultraviolet radiation are realizable today with existing technology for electron accelerators and laser systems. Especially the ultra-low emittance of laser wakefield accelerated electron beams can be exploited to compensate for their one percent level energy spreads. We discuss an experimental setup to generate the tilted TWTS laser pulses. The method presented provides dispersion compensation, required due to angular dispersion, and is especially relevant when building compact, high-yield hard X-ray TWTS sources in large interaction angle setups.

We present both the in-situ far field radiation diagnostics in the particle-in-cell code PIConGPU and the offline radiation diagnostic code ClaRa. The first was developed to close the gap between simulated plasma dynamics and radiation observed in laser plasma experiments. The second is used to quantitatively simulate radiation observed in e.g. Thomson scattering experiment. Both methods are based on the far field approximation of the Liénard-Wiechert potential. Their predictive capabilities, both qualitative and quantitative, have been tested against analytical models. We will discuss the advantages of the in-situ approach of PIConGPU over ClaRa that allows predicting both coherent and incoherent radiation spectrally from infrared to x-rays and provides the capability to resolve the radiation polarization and determine the temporal and spatial origin of the radiation. Furthermore, we explain why the direct integration into the highly-scalable GPU framework of PIConGPU allows computing radiation spectra for thousands of frequencies, hundreds of detector positions and billions of particles efficiently. In this talk we will demonstrate these capabilities on resent simulations of laser wakefield acceleration (LWFA), high harmonics generation during target normal sheath acceleration (TNSA) and Thomson scattering during laser electron interactions.

We present an experimental setup strategy for the realization of an optical free-electron laser (OFEL) in the Traveling-Wave Thomson-Scattering geometry (TWTS). In TWTS, the electric field of petawatt class, pulse-front tilted laser pulses is used to provide an optical undulator field. This is passed by a relativistic electron bunch so that electron direction of motion and laser propagation direction enclose an interaction angle. The combination of side scattering and pulse-front tilt provides continuous overlap of electrons and laser pulse over meter scale distances which are achieved with centimeter wide laser pulses. An experimental challenge lies in shaping of these wide laser pulses in terms of laser dispersion compensation along the electron trajectory and focusing. The poster shows how diffraction gratings in combination with mirrors are used to introduce and control dispersion of the laser in order to provide a plane wave laser field along the electron trajectory. Furthermore we give limits on alignment tolerances to operate the OFEL. Example setups illustrate functioning and demonstrate feasibility of the design.

We demonstrate that optical free-electron lasers (OFELs) realized with Traveling-Wave Thomson-Scattering (TWTS) are by orders of magnitude more efficient and brilliant than corresponding OFELs utilizing head-on Thomson scattering geometries. In addition, we emphasize that TWTS OFELs as opposed to standard head-on Thomson scattering geometries scale favorably with regard to space-charge effects degrading emittance, energy losses through photon recoil and transverse coherence of the resulting FEL beam. The presented TWTS OFELs scenarios are assumed to be driven either by conventional, rf-accelerated electrons or by currently available laser wakefield-accelerated electrons featuring energy spreads at the one percent level.

We present an experimental setup strategy for the realization of an optical free-electron laser (OFEL) in the Traveling-Wave Thomson-Scattering geometry (TWTS). In TWTS, the electric field of petawatt class, pulse-front tilted laser pulses is used to provide an optical undulator field. This is passed by a relativistic electron bunch so that electron direction of motion and laser propagation direction enclose an interaction angle. The combination of side scattering and pulse-front tilt provides continuous overlap of electrons and laser pulse over meter scale distances which are achieved with centimeter wide laser pulses. An experimental challenge lies in shaping of these wide laser pulses in terms of laser dispersion compensation along the electron trajectory and focusing. The poster shows how diffraction gratings in combination with mirrors are used to introduce and control dispersion of the laser in order to provide a plane wave laser field along the electron trajectory. Furthermore we give limits on alignment tolerances to operate the OFEL. Example setups illustrate functioning and demonstrate feasibility of the design.

We adopted laser Thomson scattering for measuring the electron density and the electron temperature of microwave plasmas produced in helium at the pressures higher than the atmospheric pressure. The electron density decreased while we observed the increase in the electron temperature with the pressure. These are reasonable results by considering the decrease in the reduced electric field, the dominant loss of electrons via three-body recombination with helium as the third body, and the production of electrons with medium energy via heavy particle collisions at the high gas pressure. The temporal variation of the electron temperature had the rise and the fall time constants of approximately 10 ns. The rapid heating and cooling of the electron temperature are due to the fast energy transfer from electrons to helium because of the high collision frequency in the high-pressure discharge.

Optical FELs (OFELs) based on Traveling-wave Thomson scattering (TWTS) optimally exploit the high spectral photon density in high-power laser pulses by spatially stretching the laser pulse and overlapping it with the electrons in a side scattering setup. The introduction of a laser pulse-front tilt provides for interaction lengths appropriate for FEL operation. With careful dispersion control, electrons witness an undulator field of almost constant strength and wavelength over hundreds to thousands of undulator periods, thus giving enough time for self-amplified spontaneous emission (SASE) to seed the FEL instability and the realization of large laser gains. We provide an overview on the differences between TWTS OFELs, head-on OFEL designs and magnetic undulator FELs. In this dicussion we emphasize the respective impact on transverse coherence, quantum recoil and space-charge.

In Traveling-Wave Thomson-Scattering (TWTS) an optical, high-power laser pulse is scattered off a relativistic electron pulse to realize optical ree-electron lasers (OFELs) with a wavelength ranging from ultraviolet to Angstrom [*]. Such TWTS-OFELs optimally exploit the high spectral photon density in high-power laser pulses by spatially stretching the laser pulse and overlapping it with the electrons in a side scattering setup. The introduction of a laser pulse-front tilt provides for interaction lengths appropriate for FEL operation, so that beam electrons witness an undulator field of near-constant strength and wavelength over hundreds to thousands of undulator periods, thus giving enough time for self-amplified spontaneous emission (SASE) to seed the FEL instability and the realization of large laser gains. In contrast to head-on OFEL schemes, TWTS-OFEL operates at sub-mm to mm effective undulator wavelength. Thus previous show-stoppers to OFELs due to small transverse coherence, large space charge or significant quantum effects are avoided. One of the key advantages of this approach is its scalability to x-ray wavelengths with existing lasers. We present the complete analytical description of the TWTS field and a self-consistent 1.5D theory of TWTS OFELs. We discuss the main scalings of resulting TWTS OFELs with respect to electron and laser beam properties.

We show how all-optical free-electron lasers (OFELs) from EUV to the X-ray range can be realized using existing high-power lasers and electron accelerators.

Optical free-electron lasers (OFELs) from the ultra violet to x-ray range can be realized with Traveling-Wave Thomson-Scattering (TWTS). This becomes possible in TWTS by increasing the photon scattering efficiency of standard Thomson scattering geometries more than one order of magnitude by changing the interaction geometry. In TWTS a side-scattering geometry is used where the laser and electron propagation directions enclose the interaction angle $\phi$. Together with a tilt of the laser pulse front the interaction distance is increased in TWTS beyond the limits of head-on Thomson scattering. TWTS implements dispersion control of the laser pulse to compensate for variations of the optical undulator period originating from the pulse-front tilt. Altogether, the combination of side-scattering, pulse-front tilt and dispersion control in TWTS allows for meter-scale interaction distances in which the electron beam becomes microbunched and OFEL operation is achieved. These TWTS OFELs provide transverse coherence as well as brilliances an order of magnitude enhanced over standard head-on Thomson scattering geometries. We present the scaling laws of TWTS OFELs derived from a fully analytic theory of the electron laser interaction in TWTS scattering geometries. TWTS OFELs can be realized in an all-optical setup with a meter-scale footprint using laser wakefield accelerated electrons featuring both ultralow transverse emittances and large energy spreads.