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Dynamic gas-puff z-pinch implosions are efficient sources of intense x-ray radiation and are of interest for magneto-inertial fusion studies with applied magnetic fields. However, these implosions are susceptible to magneto-Rayleigh-Taylor instabilities (MRTI) which can lead to axially non-uniform implosions, reducing the effective compression ratio and implosion kinetic energy-to-radiation efficiency. Recent observations of triple nozzle gas-puff implosions on the 1 MA, 220 ns COBRA generator at Cornell University 1 , 2 suggest that these implosions are more stable than predicted by linear MRTI theory based on the time-varying acceleration profiles. Furthermore, the reduced instability growth rates, characterized by effective Atwood numbers much less than one, are observed to depend on gas species and initial fill density. To explore the nature of these differences, detailed Thomson scattering and laser interferometry measurements have been obtained for neon, argon, and krypton gas puffs at various fill densities midway through the implosion. The results reveal two distinct radial profiles for varying initial conditions – a shock-like profile with a sharp discontinuity in density and velocity across the sheath and a snowplow-like profile with a gradual increase in density and velocity across the sheath. Snowplow-like implosions exhibit additional non-thermal broadening of the Thomson scattering spectra in a manner consistent with non-directed hydrodynamic turbulence. These observations suggest that under the appropriate conditions the forward shock may become unstable, resulting in turbulent flow within the imploding plasma sheath. These findings, and implications to the overall implosion stability and performance are presented.

Terahertz (THz) streaking is a novel method to measure the temporal profile of a relativistic electron beam with a few femtosecond (fs) duration. Recently, a THz oscilloscope was proposed for hundred-fs metrology with a significantly increased dynamic range, which used a circular polarized THz pulse instead of a linearly polarized one. In this paper, we propose a THz oscilloscope for temporal characterization of relativistic electron beam with few picosecond (ps) duration, which is crucial for THz free-electron lasers and Thomson scattering X-ray sources, etc.

A new experiment, called the PHAse Space MApping (PHASMA) experiment, features laser induced fluorescence diagnostics for ion measurements, Thomson scattering diagnostics for electron velocity distribution function measurements, and a microwave scattering system for turbulence measurements. PHASMA is designed to enable the direct measurement of ion and electron vdfs in space-relevant plasma phenomena including reconnection, shocks, and turbulence. To create the conditions necessary for different experimental regimes, PHASMA employs a 2 kW, steady-state helicon source capable of generating variable-density background hydrogen, helium, argon, krypton, and xenon plasmas with controllable plasma pressure (relative to the magnetic pressure), collisionality, and azimuthal flow shear. Reconnecting flux ropes arise through the merging of discharges from two pulsed plasma guns.

□A multi-pass Thomson scattering (MPTS) diagnostic system is primarily designed for the HUST Field-Reversed Configuration (HFRC) plasma device. To meet the requirements of high repetition frequency and high TS signal intensity at the same time, one burst of 15 Nd:YAG laser pulses of 50kHz, passing through a polarization-based coaxial multi-pass configuration which enhances scattering light intensity, serves as the laser source of the system. The system covers half plasma cross section, analyzing scattering light by interference-filter avalanche photodiode based polychromators. The expected measurability of this design, namely an electron temperature of 100eV to 1keV and 4 spatial points with 2cm resolution, satisfies the requirements of the HFRC in the near future.

In this paper, RF behavior studies of continuous wave (CW) gyrotron at an operating frequency of 384 GHz for collective thomson scattering application in fusion research are presented. The design is carried out for the output power level of ≈ 41 kW. The mode competition is carefully investigated for the probable modes along with their competitors at first and second harmonic and TE 22,8 is chosen as the operating mode. The cold cavity design and self-consistent computations have been done to determine the geometrical and beam parameters for the desired mode using the in-house code, Gyrotron Design Suite Second Harmonic Version 2019 (GDS2H-V. 2019).

We report on the development of a 946 nm Nd:YAG laser with < 3 ns pulse duration and the output pulse energy exceeding 20 mJ. The laser is designed to be used as master oscillator of 946 nm Nd:YAG calibration laser for plasma diagnostics in the ITER divertor Thomson scattering [1]. This application requires nanosecond pulses with high pulse shape stability.

Streamer discharge plasma, a type of non-thermal plasma, has received global attention as a source of reactive radicals, and is used for many applications such as ozone generation, decomposition of NOx and other gas pollutants, cleaning water, disinfection, deodorization, and medical applications. The tip of streamer discharge, known as the streamer head, in particular contributes to radical production. The peak electric field is located on the streamer head on the axis of symmetry of the discharge, likely resulting in many radical types. Very remarkable results in NO removal efficiency and superior ozone generation yield performed by streamer discharge have reported. Improving gas treatment methods requires understanding of physical characteristics of streamer discharge and streamer head, for example, electron temperature and electron density. This study investigates characteristics of streamer discharge by observing the propagation process of streamer head in a needle to conic electrode with positive voltage using a high speed gated emICCD camera. Then, incoherent laser Thomson scattering (LTS) diagnostic for streamer discharge and streamer head with positive voltage was performed. LTS diagnostic is considered to be the most reliable technique measuring electron temperature and density in plasma simultaneously. In addition, LTS diagnostic has high resolution temporally and spatially, therefore, LTS diagnostic can measure location dependence of electron temperature and density in streamer discharge including streamer head. The measurement point was 1 mm and 2 mm from tip of the high voltage needle electrode, and Thomson scattering signals were measured at the point of initial phase of streamer head propagation. In the results, electron temperature of streamer discharge was 4 to 6 eV, electron density of streamer discharge was 1021 m−3 order. This study has proven that LTS diagnostic can measure electron temperature and density in streamer discharge plasma.

This paper describes the data acquisition system for the Thomson scattering diagnostic on GDT (Gas Dynamic Trap) located in the Budker Institute of Nuclear Physics (Russia, Novosibirsk). The data acquisition system includes 16-channel high-speed recorders and photodetector modules. It allows recording scattered laser radiation pulses of ~10 ns duration with 5 GSPS sampling rate and 10-bit amplitude resolution.

A multi-terawatt laser system generates two synchronized ultra-intense near-infrared light pulses. One pulse Thomson back-scatters from the relativistic electrons that are laser-wakefield-accelerated by the other pulse. Single x-ray photons are created when greater than 500 laser photons are nonlinearly scattered by individual free electrons.

Equipment for sub- THz collective Thomson scattering diagnostics in the Large Helical Device has been developed. The probe beam source is a 303 GHz Gyrotron. It operates in a pulse mode. No parasitic mode is excited during the whole pulse width including the turn-on and turn-off phases. Single mode oscillation at several frequencies has been confirmed. For power transmission, a 3.5 inch corrugated waveguide line installed for electron heating with lower frequency gyrotrons will be used. The test with the 303 GHz gyrotron has shown a sufficiently low transmission loss.

We present experimental results of a method that measures high flux, MeV scale x-rays produced via Thomson backscatter (TBS) of an intense laser pulse (a o ≈ 1) interacting with a counter-propagating Laser wakefield acceleration (LWFA) electron bunch. We predict the spectra of broadband 50–100 MeV TBS gamma-ray pulses in a single shot by combining two diagnostics - a pair-producing magnetic spectrometer and a cerium doped lutetium-yttrium oxyorthosilicate (LYSO(Ce)) scintillator - to capture two independent, cross-confirming signals. By combining these two diagnostics in a single shot, we are able to compare signal originating from pair production in the gamma-conversion spectrometer with the transverse electromagnetic shower profile in the LYSO scintillator, to deconvolve a photon spectrum. We make use of Geant4 [1] simulations and Thomson scattering theory [2] to theoretically reconstruct potential candidate spectra based on the LWFA electron energies and laser parameters, and, using a chi-squared fitting method, fit the signal simulated in Geant4 to signal measured on both detectors obtaining the most probable spectra. Below, we report a preliminary most probable spectrum with a mean energy of 60 MeV.

Non-invasive laser diagnostics such as Thomson scattering and Two-Photon Absorption Laser Induced Fluorescence (TALIF) are powerful tools to measure electron properties and to probe the plasma chemistry with high spatial and temporal resolutions. Using laser Thomson scattering, the electron density and electron temperature of a 140-ns pulsed helium plasma jet were resolved in distributions of bell-shaped profiles with lower values in the center and higher at the outer edge of the 1-mm diameter helium flow column. Laser energy dependence studies were performed to validate the quantitative measurements. Absolute atomic oxygen measurements for the same plasma jet using TALIF, however, revealed a Gaussian-shaped distribution of the O-density. Dependence of the electron properties and atomic oxygen generation on the peak voltage of the applied nanosecond pulses and the oxygen concentration in helium are discussed.

The conditions and dynamics of neon gas puff z-pinch plasmas near pinch time are studied on the COBRA pulsed power generator (current rise time of ~240 ns and ~0.9 MA peak current). A 526.5 nm, 10 J Thomson scattering diagnostic laser enables probing of the plasma conditions of these implosions with both spatial and temporal resolution. Plasma conditions imply that collective scattering spectral profiles will be observed, in which case electron and ion temperatures and plasma fluid element drift velocity can be obtained. The use of two laser pulses, both with a FWHM of 2.2 ns, that can be separated by 10ns allows observation of time-resolved spectra for a total consecutive time of ~6 ns in the present experiments. Spectra from the same scattering volume but collected by optics at differing angles with respect to the laser repeatedly imply ion temperatures that are inconsistent across viewing angles if the width of the ion features in the scattered spectral profile are interpreted as solely due to ion temperature. This indicates the presence of a source of non-thermal peak broadening that changes with scattering angle. Multiple methods of fitting the Thomson scattering spectral profile show that the presence of hydrodynamic drift velocity distributions in the plasma at stagnation can, together with ion temperature, give a better fit to the observed ion feature peak widths than ion temperature alone.

Streaked Thomson scattering measurements have been performed on plasma jets created from a 15 $\mu\mathrm{m}$ thick radial Al, Ti, or Cu foil load on COBRA, a 1 MA pulsed power machine. The streaked system enables collecting scattered light from two separate laser pulses separated in time by between 3 and 14 ns from two different scattering angles. The time separation is created by splitting the initial 2.2 ns full width half max duration, 10 J, 526.5 nm laser beam into two separate pulses, each with 2.5 J, and delaying one beam relative to the other. To collect streaked spectra from two angles simultaneously a split fiber bundle was used to couple light from the plasma to the spectrometer, and the size of the fibers, 100 $\mu \mathrm{m}$ diameter, was used as the effective spectral slit width. The 2.5 J laser pulse is shown to heat the plasma jet by inverse bremsstrahlung radiation, as measured by the streaked Thomson scattering system. Analysis of the streak camera image showed that the electron temperature of the Al jet was increased from 20 eV up to 50 eV within about 2 ns for both laser pulses when they were separated by 12 ns in time. Initial results from the Cu jets showed more heating then the Al jets while Ti jets showed the most heating of these three materials. In addition to this increased heating, the scattering profiles from Ti and Cu jets had ion-acoustic features that were more difficult to interpret then those from Al jets. Results from the two scattering angles showed reasonable agreement of the measured electron temperature, and implied that the electron density was at least $2\times 10^{18}\text{cm}^{-3}$ . Results will also be presented with the laser pulses separated by only 4 ns, creating a more continuous laser pulse. In addition, the effects of the current polarity in the radial foil on the measured electron temperature will be shown.

Planar magnetrons have been applied for decades for low-pressure, plasma-assisted thin film deposition, with applications in sectors such as microelectronics and tribology. Their operation has expanded in recent years to a HiPIMS (High-impulse pulsed magnetron sputtering) regime [1], which can, in some cases, offer superior thin film quality and adhesion than is attainable with DC operation. Modeling of this regime requires an advanced understanding of features such as plasma instabilities [2] and, more fundamentally, particle properties.

Both experimental and numerical approaches have been done on switching arc plasmas for fundamental studies. In the experimental approaches, power-semiconductor switching has been used to control intentional current injections and voltage applications to switching arcs with a high accuracy in time. The systematic experiments provided the interruption probability property for different gas kinds and gas flow rates, as well as the dielectric recovery properties between the electrodes. Time evolutions in electron density were derived by Laser Thomson Scattering (LTS) and Shack-Hartmann (SH) method for different gases. On the other hand, for this nozzle space, numerical models were developed to simulate arc dynamic behaviors with and without local thermodynamic equilibrium (LTE) assumptions for different gases. It was found that the time evolution in electron density derived by the chemically non-equilibrium model is in good agreement with the experimental results by LTS.

A 300 GHz pulse gyrotron has been developed for collective Thomson scattering diagnostics in the Large Helical Device with a careful design consideration. Single-mode oscillation of more than 320 kW was attained. The frequency spectrum was very narrow and stable across the pulse width up to 80 μs. No parasitic mode was observed during the whole pulse width including the turn-on and turn-off phases. No-parasitic-mode oscillation is likely due to a very fast rise time of the beam voltage as well as due to adoption of a whispering gallery mode.

Conventional diagnostics cannot be used to measure plasma parameters of atmospheric pressure plasmas if plasma size is small (

Thomson scattering measurements have been performed on laboratory plasma jets in order to measure the flow velocities and electron temperatures inside and near the edge of the jet. These jets were created using $\mathrm {a}15 \mu \mathrm {m}$ thick radial Al foil load on COBRA, a 1 MA pulsed power machine. The laser used for these measurements had a maximum energy of 10 J at 526.5 nm. Using this 10 J laser, however, significantly heats the $5\mathrm {x} 10 ^{18}$ cm $^{-3}$ jet by inverse bremsstrahlung, creating a density bubble in the jet. This was demonstrated by reducing the probe laser energy to 1 J, which reduced the measured electron temperature in the jet from about 40 to 20 eV and nearly eliminated the bubble. These measurements were made using a time gated spectrometer, with at least a 5 ns gate window, in order to encompass the entire 3 ns laser pulse. Therefore, no time dependent electron temperature measurements could be made from the Thomson scattering spectra. We are now developing an experimental arrangement to send the collected scattered spectra into a streak camera, in order to determine the plasma temperature as a function of time throughout the 3 ns laser pulse. With the streak camera data, we hope to track the changing plasma parameters during the laser heating of the plasma jet. In the future we hope to extend this diagnostic arrangement to other configurations, enabling time resolved measurements of the plasma parameters of various plasmas.

The dynamics of argon and neon gas puff z-pinch plasmas are studied on COBRA, Cornell's pulsed power generator. COBRA can produce current pulses with rise times ranging from ~100 ns, with 0.7 MA – 1.1 MA peak current, to ~240 ns with about 0.9MA peak current. This provides a way to drive gas-puff plasmas that do not have time to pinch during the current rise time, as well as plasmas that are imploded by the current pulse. Radial tailoring of the gas puff mass-density profile using a triple-nozzle coaxial valve (two annular gas puffs and a central jet) enables production of both more stable and less stable (in regards to the magneto-Rayleigh Taylor instability) z-pinch implosions. A 527nm, 10J Thomson scattering diagnostic laser enables probing of the flow dynamics and plasma conditions of these implosions at a single time but at multiple spatial locations and with spatial resolution ranging from ~0.25 – 1mm. Extreme ultraviolet imaging and laser schlieren imaging at multiple times enable monitoring of the implosion morphology as a function of time.

Previous Staged Z-pinch (SZP 1 experiments at the University of California-Irvine demonstrated that gas liners (or gas-puffs) can efficiently couple energy to a target plasma and implode it uniformly. In those experiments, a 1.5 MA, $1 \mu \mathrm {s}$ current driver was used to implode a magnetized, Kr liner onto a D+ target, producing ~1010 neutrons per shot. Time-of-flight data suggested that primary and secondary neutrons were produced. Recent MHD simulations 2 have suggested that liner composition is important for target shock-heating before the main adiabatic implosion and that pre-magnetization is crucial to achieve uniform implosions.A recent series of experiments were carried out to investigate implosion dynamics using Cornell’s 1 MA, 200 ns current driver COBRA with a goal to help us better understand the SZP physics and benchmark MHD codes. We used an optimized gas injector 3 composed of an annular (1 cm radius) high atomic number (e.g., Ar or Kr) gas-puff and an on-axis plasma gun that delivers the ionized hydrogen target. Liner implosion velocity and stability were studied using laser shadowgraphy and interferometry as well as gated visible and XUV imaging. Target temperature and density were measured with Thomson scattering and X-ray spectroscopy. Experimental data is presented and preliminary analysis is discussed.

Thomson scattering (TS) is the most accurate and useful method of measuring plasma electron temperature and density in fusion-oriented experiments. The small power (∼10−8W) and short duration (∼10 ns and less) of scattering signal are the main technical problems of the method. That is why highest requirements are made to photodetection and recording systems. Developed by Budker Institute of Nuclear Physics (Novosibirsk, Russia) fast DAQ system allows recording profiles of short duration (3-5 ns) scattered pulses in 0-200MHz bandwidth with 2GSPS rate and 12-bit resolution. The system consists of photo detector modules (with 200MHz bandwidth), fast (2GSPS) and slow (10MSPS) ADC modules and synchronization subsystem. The 48-channel version of this system was developed as a prototype for ITER (Cadarache, France) divertor Thomson scattering diagnostic. The system was tested and put into operation on GLOBUS-M tokamak (St. Petersburg, Russia).

This paper describes prototype design of the high-speed recorder based on SCA (switch capacitor array) technology for Thomson scattering diagnostic on ITER. The recorder is based on 9-channel DRS4 chip, developed in PSI (Paul Scherrer Institute) and FPGA with System on Crystal (Cyclone V). The recorder is capable of simultaneously data processing from 8 measurement channel with 14 bits dynamic range and with up to 5 GHz sampling rate.

A high power 300 GHz band pulse gyrotron has been developed based on the design concept for stable and single mode oscillation verified with a prototype gyrotron. The oscillation frequency is 303.3 GHz and the power target is 300 kW. Oscillation test has been carried out and a maximum single mode oscillation power higher than 320 kW was attained. No competing mode was observed during the turn on and turn off phases of the oscillation pulse. The frequency spectrum is very narrow and stable during the pulse width. The radiation beam has a Gaussian like pattern. The test results have proved that this gyrotron can be practically used in the collective Thomson scattering (CTS) diagnostics in the Large Helical Device (LHD).

Scattered millimeter waves entering from the collective Thomson scattering diagnostic at ASDEX Upgrade fusion device are generally elliptically polarized. In order to convert the millimeter waves to linearly polarized waves (required for the detector), birefringent window assemblies (sapphire) have been developed to replace grooved metal mirrors. This allows a significantly more compact receiver design which is less susceptible to misalignment. The setup has been tested and implemented at ASDEX Upgrade.

Microwave diagnostics will play an increasingly important role in burning plasma fusion energy experiments like ITER and beyond. The Collective Thomson Scattering (CTS) diagnostic to be installed at ITER is an example of such a diagnostic with great potential in present and future experiments. The ITER CTS diagnostic will inject a 1 MW 60 GHz gyrotron beam into the ITER plasma and observe the scattering off fluctuations in the plasma - to monitor the dynamics of the fast ions generated in the fusion reactions.

We present results of theoretical study of influence of the electron velocity spread and the radial width on the efficiency and mode competition in a 300-GHz gyrotron operating in the TE22,2,1 mode, with which 300-kW level high power single mode oscillation has been demonstrated. Effects of a finite voltage rise time corresponding to the real power supply of this gyrotron is also considered. Simulations tracking eight competing modes show that the electron velocity spread and the finite beam width influence not only the efficiency of the gyrotron operation, but also the mode competition scenario during the startup phase. Combination of the finite rise time with the electron velocity spread or the finite beam width affects the mode competition scenario. The simulation reproduces the experimental observation of high power single mode oscillation of the TE22,2 mode.

A low loss transmission line is one of key elements for realization of sub THz band collective Thomson scattering (CTS) diagnostics in large fusion devices. This paper reports transmission tests of corrugated waveguides with 300 GHz band gyrotron power. Two types of corrugated waveguide are considered. One is 1.25 inch corrugated waveguide. The other is 3.5 inch corrugated waveguide already installed in the Large Helical Device (LHD) for electron heating with lower frequency gyrotrons. At first, transmission test with existing 1.25 inch waveguides was carried out. This waveguide is not optimized for 300 GHz band and substantial attenuation along waveguides was observed. A theoretical analysis explaining the large attenuation is performed.