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An understanding of the behaviour of the D or He fuel used in tokamak discharges is essential in analyses such as modelling edge and divertor transport and the erosion of the vessel walls. However, poor agreement is found between measurements made on the JET tokamak and collisional-radiative models used to predict the hydrogen-like D and He line intensities. The range of temperatures of the plasmas emitting the radiation is also limited, in contrast to that for many impurities for which a wide range is possible. This is particularly so for He II whose line intensities tend to have the same near-constant ratios in most pulses, suggesting that the emission originates in plasma regions with very similar electron temperatures. To gain understanding and to allow quantitative comparisons with theoretical models, extensive observations of the VUV Lyman series have been made, for all discharge scenarios run during three He campaigns. Those for He discharges in both JET ITER-like wall (JET-ILW) and JET C (JET-C) campaigns are presented here. He discharges have the advantage of fewer impurities resulting in less complex spectra than when D is used as the fuel. However, the characteristics of the observed discrepancies are similar in both species, allowing He to be used as a proxy for D in order to gain understanding of the discrepancy. In addition, the study of He avoids the complication of molecular species contributing to the level populations. Opacity effects are also expected to be less severe in He discharges. Nevertheless, so as to ensure that the measurements are not unduly affected by opacity, comparisons have also been made with emission from Balmer and Paschen series members. Measurements of both line intensities and their ratios are presented for all-pulse surveys and for individual pulses. In exceptional cases in which the He emission is intense a dependence on the He II line intensity is demonstrated. The discrepancy between these measurements and the theoretical models is illustrated. [ABSTRACT FROM AUTHOR]

The upgrade of experimental advanced superconducting tokamak lower divertor to use tungsten (W) target is undertaken, however the target erosion and W impurity accumulation in the core pose a great challenge during high power discharge with external impurity seeding. This paper aims to study the influence of argon (Ar) seeding on the target erosion and the W impurity transport on steady-state operation. The scrape-off layer/divertor plasma as well as W impurity are self-consistently simulated by using SOLPS-ITER code package. With the constant input power, the Ar seeding rate is varied to study the changes on divertor plasma, incident Ar flux, the target erosion and W impurity transport. The simulation results indicate that the external Ar impurity influences W target erosion through both reducing the plasma temperature and introducing heavier incident particles, which significantly complicates the erosion process. The eroded W flux has great impact on the W core accumulation, and the divertor plasma condition influences W impurity transport remarkably. A correlation between peak electron temperature (Te) at the outer target and the W concentration (CW) at the core-edge-interface is found. Moreover, the influence of the W impurity on the background boundary plasma is also evaluated. [ABSTRACT FROM AUTHOR]

This study analyzes the dependences of the size of the luminous region of hydrocarbon pellet cloud on the ablation rate, as well as on the ambient plasma electron density and temperature. The data were obtained in a series of experiments where spherical solid polystyrene pellets with a diameter of ≈900 μm at a speed of ≈450 m s−1 were injected into hydrogen plasma of the large helical device. The ambient plasma with an electron temperature of 0.5–1.5 keV and a density of (1–7) × 1019 m−3 was maintained by injection of neutral atom beams with total the heating power of 3–14 MW. A scaling law was derived for determining the transverse size of the cloud's luminous region from the known values of the pellet's current radius as well as the values of the density and temperature of the ambient plasma at the place of image acquisition. The interdependencies between various dimensions of the luminous region obtained in the line spectrum are established. The presence of such dependences makes it possible to describe the dimensions of the luminous region comprehensively by means of the scaling law for its transverse size. [ABSTRACT FROM AUTHOR]

The aim of this paper is to study the influences of plasma heating and plasma density on impurity production and transport during the plasma-termination phase. We have analyzed the ramp-down (RD) phase of a set of representative high-current JET ITER-like wall discharges: #92 437 (disrupted) and #92 442 (soft landing), characterized by a high plasma current of Ip = 3.5MA. Analysis is performed for different time slots within the RD phase, corresponding to different levels of electron line density and auxiliary heating power. Since the deuterium gas fluxes are different, the influence of the separatrix density is also analyzed. The main conclusion from the simulations is the observation that for the same average-electron density, a decrease of the separatrix density leads to an increase of the plasma temperature at the divertor plate, leading to increased W production and consequently to a larger W concentration and radiation in the core. When the central electron temperature approaches the 2 keV level, corresponding to the maximum W and Ni cooling rate, the radiation in the plasma's center is enhanced. Ni radiation is more important in the RD phase. [ABSTRACT FROM AUTHOR]

The Super H-mode, a regime with high pedestal pressure and stored energy, is explored on DIII-D and combined with an ion transport barrier in the plasma core to increase performance. A significant improvement of ion temperatures and confinement is facilitated by favorable conditions such as high rotational shear and high ion pedestal temperatures. As a result of a rise in density and simultaneous decrease in rotation, the ion transport barrier disappears during the discharge evolution, leading to a transition from a very high confinement state at early times, to a reduced but still high confinement phase. Additionally, in many discharges, a global magnetohydrodynamic (MHD) event consistent with the coupling of a destabilized internal mode to an edge localized mode causes a large energy loss and leads to a reorganization of the plasma into a lower temperature, higher density state. Depending on the magnitude of the global MHD event, the plasma edge collisionality can increase significantly and shift the operational boundary from the peeling to the ballooning side, which can be understood as a drop out of the Super H-mode channel into standard H-mode. Hence, in Super H-mode discharges with ion transport barriers, both the improved pedestal height and rotational shear contribute to the high stored energy. At very low levels of rotation, the confinement factor for SH modes is still expected to exceed standard H-mode by 20%–30%. With their overall stationarity and high-performance levels, Super H-mode discharges provide an attractive regime for ITER and may enable a more compact design of future fusion power plants. [ABSTRACT FROM AUTHOR]

Kappa distributions are ubiquitous in space and astrophysical poorly collisional plasmas, such as the solar wind, suggesting that microscopic and macroscopic properties of these non-equilibrium plasmas are highly conditioned by the wave–particle interactions. The present work addresses the evolution of anisotropic bi-Kappa (or bi-κ-)distributions of electrons triggering instabilities of electromagnetic electron-cyclotron (or whistler) modes. The new quasi-linear approach proposed here includes time variations of the κ parameter during the relaxation of temperature anisotropy. Numerical results show that κ may increase for short interval of times at the beginning, but then decreases toward a value lower than the initial one, while the plasma beta and temperature anisotropy indicate a systematic relaxation. Our results suggest that the electromagnetic turbulence plays an important role on the suprathermalization of the plasma, ultimately lowering the parameter κ. Even though the variation of κ is in general negative () this variation seems to depend of the initial conditions of anisotropic electrons, which can vary very much in the inner heliosphere. [ABSTRACT FROM AUTHOR]

Recent studies have shown that helicon waves may be an attractive option for driving an efficient off-axis current drive in order to supplement bootstrap current for steady state operation in fusion reactors. Based on the China fusion engineering test reactor parameters, the helicon wave trajectories, profiles of driven current and current driven efficiency are calculated with the ray-tracing code GENRAY. Both the peak position and the peak value of driven current profiles are dependent on the wave frequency. In the frequency range of 1.6 GHz–2.0 GHz, the peak value increases with the increase of frequency, and the peak position has a slight change (concentrate around ρ= 0.75). High electron temperature and low electron density seem to be favorable for the helicon wave absorption. A driving current of I/P > 70 kA MW−1 at the off-axis position of ρ < 0.7 can be obtained with the electron temperature at the plasma center, T e(0) < 25.4 keV and the electron density at the plasma center, n e(0) ∼ 0.7 × 7.8 × 1019m−3–0.8 × 7.8 × 1019m−3. The current drive efficiency decreases with the increase of the electron density. Peak positon of the driven current profile is insensitive to the parallel refractive index n ∥. It is found that the poloidal launch angle has some effect on the peak positon and its value. [ABSTRACT FROM AUTHOR]

Investigation of helicon wave heating and current drive (CD) in toroidal plasma has been carried out with the ray-tracing code GENRAY. The wave trajectories, profiles of power deposition and driven-currents are presented. It is shown that increasing the poloidal launch angle does not change the absorption rate of the wave, but moves the peak of the wave deposition toward the plasma core. Wave frequencies in the range of our interest (460 MHz ∼ 480 MHz) do not change the absorption of the wave. Under low parameter plasmas, we find that both the peak position and magnitude of the wave power deposition are dependent on the launched parallel refractive index n||. With increasing the refractive index, the driving efficiency first increases and then decreases. The parallel refractive index figure of merit for mid-plane launching in HL-2M configuration is 3.2. With high plasma parameters, both the position and magnitude of the driving current are weakly dependent on n||. The magnitude of the driven current is determined by both the absorption rate and the value of ξe, which is related to n||. It is also demonstrated that driven current increases with increase of the core plasma temperature and decrease of the core plasma density. The plasma ohmic current seems do not significantly affect the helicon CD efficiency, but moves the driven current toward the plasma core region and narrows the width of the driven current profile. In addition, the preliminary calculations including scrape-off-layer (SOL) suggest that the power absorption rate loss in SOL is about 5%–20%. These results provide a reference and theoretical basis for the design and construction of HL-2M helicon wave system. [ABSTRACT FROM AUTHOR]

The onset of helical states has been shown to strongly affect the transport properties and the plasma performance of both the core region (|R − R0|/a ≲ 0.5) and the plasma edge (|R − R0|/a ≳ 0.95) in the RFX-mod reversed field pinch experiment. Both regions have been widely investigated and a strong effort is still dedicated to deepening the comprehension of the phenomena affecting them. Conversely, the region bounded between the plasma core and the plasma edge (0.6 ≲ |R − R0|/a ≲ 0.95) has been poorly analyzed, despite the large plasma volume enclosed in this domain. Improvements in RFX-mod global plasma performance are thus closely linked to the identification of the physical mechanisms occurring in this region, referred to as the outer region. A specifically conceived method of analysis, which gathers together data from a Thomson scattering system and two edge temperature diagnostics, allows the study of the entire temperature profile of RFX-mod. A statistical analysis performed on a large discharge database shows that the temperature profile in the central region of the plasma column is mainly established by the plasma current and the degree of stochasticity of the magnetic field. On the other hand, the temperature profile in the outer plasma region is found to vary with electron density and plasma equilibrium. [ABSTRACT FROM AUTHOR]

The external Ne impurity seeding has been applied to reduce the heat flux to the divertor target as well as plasma temperature to prolong the lifetime of the EAST upper tungsten (W) target. The erosion of the W target during the Ne seeding in different divertor operation regimes has been assessed by using the semi-empirical formula, while the plasma background was provided by SOLPS modeling. The simulation results showed that the Ne impurity played a critical role in the W erosion, which depended strongly on the operation regimes. By increasing the Ne seeding rate, the discharge regime was varied from the attached to detached condition. During the attached regime, the insufficient seeded Ne impurity could reduce the heat flux to the target, but the erosion of the W target was obviously enhanced, which may shorten the target lifetime. By the sufficient impurity seeding, the detached condition could be achieved, and the W target erosion was obviously suppressed. The high-power discharge brings great challenge to the W target lifetime, and the erosion is mainly induced by Ne and D ions. The contribution of each species changes greatly with different Ne seeding rates. In addition, the lifetime of the EAST upper W divertor has also been evaluated in different operation regimes during high input power discharge. [ABSTRACT FROM AUTHOR]

The spectral parameters of x-ray radiation in the wavelength range from 25 to 450 Å emitted by plasma of a vacuum discharge with a storage energy ≤45 J, induced on the Fe cathode by radiation from a neodymium laser with a power density up to 5 · 1012 W cm−2, are studied. On the basis of the collisional-radiative equilibrium model, a method was developed to produce a sufficiently accurate approximation of the experimental spectrum by a linear combination of the calculated spectra of plasma components with a given electron temperature and density. It was shown that the main part (>80%) of the radiation energy in the experimental spectra is provided by relatively cold peripheral plasma with a temperature of no more than 10 eV and a density of 5 · 1017 cm−3. It was established experimentally that a decrease in the laser power density due to the defocusing of the laser beam leads to an increase in the intensity of plasma radiation in the entire spectral range and a change in the spectral structure in the shortwave range. According to simulations, these changes are caused by an increase in the temperature and density of the micropinch formed in the discharge plasma up to 160 eV and 3 · 1021 cm−3, respectively. [ABSTRACT FROM AUTHOR]

The electron cyclotron emission (ECE) in fusion devices is non-trivial to model in detail at frequencies well below the fundamental resonance where the plasma is optically thin. However, doing so is important for evaluating the background for microwave diagnostics operating in this frequency range. Here we present a general framework for estimating the ECE levels of fusion plasmas at such frequencies using ensemble-averaging of rays traced through many randomized wall reflections. This enables us to account for the overall vacuum vessel geometry, self-consistently include cross-polarization, and quantify the statistical uncertainty on the resulting ECE spectra. Applying this to ITER conditions, we find simulated ECE levels that increase strongly with frequency and plasma temperature in the considered range of 55–75 GHz. At frequencies smaller than 70 GHz, we predict an X-mode ECE level below 100 eV in the ITER baseline plasma scenario, but with corresponding intensities reaching keV levels in the hotter hybrid plasma scenario. Benchmarking against the SPECE raytracing code reveals good agreement under relevant conditions, and the predicted strength of X-mode to O-mode conversion induced by wall reflections is consistent with estimates from existing fusion devices. We discuss possible implications of our findings for ITER microwave diagnostics such as ECE, reflectometry, and collective Thomson scattering. [ABSTRACT FROM AUTHOR]

We report on numerical simulation studies of the dynamical behavior of edge localized modes (ELMs) under the influence of repetitive injection of pellets. In our nonlinear 2-fluid model the ELMs are excited by introducing a particle source in the confinement region and a particle sink in the edge region. The injection of pellets is simulated by periodically raising the edge density in a pulsed manner. We find that when the edge density is raised to twice the normal edge density with a duty cycle (on time:off time) of 1:2, the ELMs are generated on an average at a faster rate and with reduced amplitudes. These changes lead to significant improvements in the plasma beta indicative of an improvement in the energy confinement due to pellet injection. Concurrently, the plasma density and temperature profiles also get significantly modified. A comparative study is made of the nature of ELM dynamics for different magnitudes of edge density enhancements. We also discuss the relative impact on ELMs from resonant magnetic perturbations compared to pellet injection in terms of changes in the plasma temperature, density, location of the ELMs and the nonlinear spectral transfer of energies. [ABSTRACT FROM AUTHOR]

The core-crash sawtooth caused by m/n = 2/1 double tearing mode (DTM) in Tokamak is investigated using a three-dimensional toroidal MHD code (CLT). It is found that mode–mode coupling effect plays a crucial role during the core-crash sawtooth. The n = 0 component and n > 1 components generated by mode–mode coupling of the n = 1 modes take up more than 40% of the total kinetic energy and become unneglectable during the core-crash sawtooth. The n = 0 component is crucial for the saturation of DTM. Without the n = 0 component, the DTM will not saturate. The high n components are responsible for the explosive growth of the DTM that finally results in the fast crash of the core plasma temperature. [ABSTRACT FROM AUTHOR]

The Uragan-3M device is equipped with two antennas which are fed by RF power with the frequency below ion cyclotron. The frame antenna was used for pre-ionization and the three-half turn antenna makes plasma heating. In this experimental series, the radial profiles of C III, O V and C V optical line intensity and the second cyclotron harmonic emission are measured using a pulse-by-pulse technique. The results of these measurements and Biot–Savart calculations of the Uragan-3M magnetic configuration could be explained by the existence of a small central area with relatively high electron temperature and good plasma confinement surrounded by a zone where the electron temperature and confinement are worse. The relatively low average electron temperature and high RF power needed to sustain plasma are the consequences. [ABSTRACT FROM AUTHOR]

The traveling relativistic solitons in electron–positron hot plasmas is investigated. The influence of the positron motion on the soliton’s structure is taken into account. A closed set of equations consisting of scalar potential, vector potential and enthalpy in ultra-relativistic regime is presented; symmetric and antisymmetric solitary solutions are obtained numerically. The non-linear dispersion relation of the system is derived in the quasi-neutral limit for the purpose of analyzing the modulational instability. Also, the variations of the modulational instability growth rate are investigated. In contrast to the standing solitons, the drifting velocity and plasma fluid velocity of moving solitons both play an important role in the formation of the solutions. In cases where the fluid velocity of the plasma overtakes the drift velocity of the solitary wave, the magnitudes of the scalar and vector potentials increase with the increase of the plasma temperature, otherwise, they decrease. Also compared with single-humped vector potentials, two-humped vector potentials are excited at a lower temperature if the fluid velocity exceeds the soliton drifting velocity. [ABSTRACT FROM AUTHOR]

An analytical model for heating matter in the semi-infinite space by a high-energy ion flow is presented. The obtained solution obtained describes the temporal evolution of the thermodynamics and hydrodynamics of plasma in the heated region. It counts for the general stopping power dependence on plasma temperature. While the matter is heated, the mean free path of heating ions changes functionally with temperature. Namely, it does not depend on the plasma temperature at the beginning and increases with the temperature after it passes a thermal threshold. At this threshold, the thermal velocity of plasma electrons is equal to the velocity of the ions in the heating beam. The solution shows that the most advantageous regime for the initialization of the quasi-stationary shock wave realizes when the ion energy is equal to the thermal threshold. This regime corresponds to the maximum shock pressure and the slowest subsequent pressure decay with time. Special attention is devoted to the powerful shock wave driven by the laser-accelerated fast ion beam. For example, a Gbar shock pressure may be achieved by heating an aluminum target by a proton beam with intensity of about and particle energy of about 4 MeV. [ABSTRACT FROM AUTHOR]

The nature of turbulent transport in tokamak plasmas results in temperature profiles that are called resilient or stiff, and the stabilization of magnetic islands by a localized heat source is expected to be extremely sensitive to the stiffness strength. Theoretical expectations are verified with nonlinear simulations, showing a good agreement and confirming the enhanced stabilization efficiency due to large profile stiffness, when the power used for the control is small compared with the heating power producing the equilibrium profiles. Heat sources, which are present in the island region before RF heating is applied, contribute to reducing the island size, but at the same time, severely dampen the control capability. [ABSTRACT FROM AUTHOR]

Overdense plasmas have been attained with 2.45 GHz microwave heating in the low-field, low-aspect-ratio CNT stellarator. Densities higher than four times the ordinary (O) mode cutoff density were measured with 8 kW of power injected in the O-mode and, alternatively, with 6.5 kW in the extraordinary (X) mode. The temperature profiles peak at the plasma edge. This was ascribed to collisional damping of the X-mode at the upper hybrid resonant layer. The X-mode reaches that location by tunneling, mode-conversions or after polarization-scrambling reflections off the wall and in-vessel coils, regardless of the initial launch being in O- or X-mode. This interpretation was confirmed by full-wave numerical simulations. Also, as the CNT plasma is not completely ionized at these low microwave power levels, electron density was shown to increase with power. A dependence on magnetic field strength was also observed, for O-mode launch. [ABSTRACT FROM AUTHOR]

Main processes of plasma-wall interaction and impurity transport in fusion devices and their impact on the availability of the devices are presented and modelling tools, in particular the three-dimensional Monte-Carlo code ERO, are introduced. The capability of ERO is demonstrated on the example of tungsten erosion and deposition modelling. The dependence of tungsten deposition on plasma temperature and density is studied by simulations with a simplified geometry assuming (almost) constant plasma parameters. The amount of deposition increases with increasing electron temperature and density. Up to 100% of eroded tungsten can be promptly deposited near to the location of erosion at very high densities (∼1 × 1014 cm−3 expected e.g. in the divertor of ITER). The effect of the sheath characteristics on tungsten prompt deposition is investigated by using particle-in-cell (PIC) simulations to spatially resolve the plasma parameters inside the sheath. Applying PIC data instead of non-resolved sheath leads in general to smaller tungsten deposition, which is mainly due to a density and temperature decrease towards the surface within the sheath. Two-dimensional tungsten erosion/deposition simulations, assuming symmetry in toroidal direction but poloidally spatially varying plasma parameter profiles, have been carried out for the JET divertor. The simulations reveal, similar to experimental findings, that tungsten gross erosion is dominated in H-mode plasmas by the intra-ELM phases. However, due to deposition, the net tungsten erosion can be similar within intra- and inter-ELM phases if the inter-ELM electron temperature is high enough. Also, the simulated deposition fraction of about 84% in between ELMs is in line with spectroscopic observations from which a lower limit of 50% has been estimated. [ABSTRACT FROM AUTHOR]

The measured divertor heat flux profiles are compared to the EMC3-EIRENE simulations for two different times of an LHD discharge, corresponding to higher and lower edge temperatures. The relation between the three-dimensional magnetic field structure and the heat flux distributions on the divertor has been analysed. The modelled heat flux for the lower plasma temperature case has a better agreement with the experimental result obtained by the Langmuir probes, which shows a qualitative reproduction of the experimental profile shape. However, the heat flux distribution for the high plasma temperature case shows a different behaviour between the simulation results and the experimental measurements. The detailed analysis of the heat flux distribution for the higher temperature case which has a larger discrepancy has been performed, both quantitatively and qualitatively. The radiation of the eroded impurity from divertor target plates has a minor effect on the heat flux distribution. Non-uniform cross-field transport coefficients are used in the simulations and its impact on the heat flux distributions is discussed for the case of the high plasma temperature. [ABSTRACT FROM AUTHOR]

The evolution characteristics of type-I ELMy high-confinement mode pedestal are examined in EAST based on the recently developed Thomson scattering system. The influence of the plasma current on pedestal evolvement has been confirmed experimentally. In the higher Ip case (500 kA) the pedestal height shows an increase trend until the onset of next ELM and in the lower Ip cases (300 and 400 kA), however, this buildup saturates at the first ∼30% of the ELM cycle. In contrast, the width increases only during the first ∼70% of the ELM cycle and then keeps almost stable in three Ip cases, but resulting in different widening size of ∼1.5, 1 and 0.5 cm for 300, 400 and 500 kA respectively. Experimental results show that the pedestal pressure width has good correlation with poloidal beta as where the fitting coefficient 0.16 is not changed with different plasma currents but a little larger than that of other machines. For each current level, the pedestal density increases while the pedestal temperature decreases. But with increasing platforms, the pedestal height prior to the ELM onset shows a near quadratic (within error bars) increase. Experimental measurements demonstrate that the decrease of with increasing comes mostly from the reduction of the plasma temperature drop, while the pedestal density height keeps relatively stable. Additional injection of LHW has been proved to modify the pedestal structure which should be responsible for the remaining scatter of the experimental data. [ABSTRACT FROM AUTHOR]

We report the first calculations of the 2ωpe instability in an IFE-scale corona of a direct drive target being driven by a StarDriver-class laser. Using realistic plasma profiles (density, temperature, flow velocity and Z) taken from full hydrodynamic simulations of the capsule implosion, we propagate the StarDriver beamlets to the ¼ critical surface using ray-tracing. We then use that ‘k-space’ of laser modes in a model of the absolute 2ωpe instability in the inhomogeneous plasma at ¼ critical density. Our calculations indicate that 15–20 THz of bandwidth at 351 nm wavelength (about 2% bandwidth) significantly reduces or suppresses the absolute instability at most times in the drive pulse. A bandwidth of 35 THz suppresses it at all times in the drive pulse. We note that 2% bandwidth ~20 THz is achievable using laser gain media available today. [ABSTRACT FROM AUTHOR]

Operation with a Be/W wall at JET (JET-ILW) has an impact on scenario development and energy confinement with respect to the carbon wall (JET-C). The main differences observed were (1) strong accumulation of W in the plasma core and (2) the need to mitigate the divertor target temperature to avoid W sputtering by Be and other low Z impurities and (3) a decrease of plasma energy confinement. A major difference is observed on the pedestal pressure, namely a reduction of the pedestal temperature which, due to profile stiffness the plasma core temperature is also reduced leading to a degradation of the global confinement. This effect is more pronounced in low βN scenarios. At high βN, the impact of the wall on the plasma energy confinement is mitigated by the weaker plasma energy degradation with power relative to the IPB98(y, 2) scaling calculated empirically for a CFC first wall. The smaller tolerable impurity concentration for tungsten (<10−5) compared to that of carbon requires the use of electron heating methods to prevent W accumulation in the plasma core region as well as gas puffing to avoid W entering the plasma core by ELM flushing and reduction of the W source by decreasing the target temperature. W source and the target temperature can also be controlled by impurity seeding. Nitrogen and Neon have been used and with both gases the reduction of the W source and the target temperature is observed. Whilst more experiments with Neon are necessary to assess its impact on energy confinement, a partial increase of plasma energy confinement is observed with Nitrogen, through the increase of edge temperature. The challenge for scenario development at JET is to extend the pulse length curtailed by its transient behavior (W accumulation or MHD), but more importantly by the divertor target temperature limits. Re-optimisation of the scenarios to mitigate the effect of the change of wall materials maintaining high global energy confinement similar to JET-C is underway and JET has successfully achieved H98(y,2)  =  1 for plasma currents up to 2.5 MA at moderate βN. [ABSTRACT FROM AUTHOR]

Polarimetric Thomson scattering (TS) is an alternative method for the analysis of Thomson scattering spectra in which the plasma temperature Te is determined from the depolarization of the TS radiation. This is a relativistic effect and therefore the technique is suitable only for very hot plasmas (Te  >  10 keV) such as those of ITER. The practical implementation of polarimetric TS requires a method to calibrate the polarimetric response of the collection optics carrying the TS light to the detection system, and in particular to measure the additional depolarization of the TS radiation introduced by the plasma-exposed first mirror. Rotational Raman scattering of laser light from diatomic gases such as H2, D2, N2 and O2 can provide a radiation source of predictable intensity and polarization state from a well-defined volume inside the vacuum vessel and is therefore suitable for these calibrations. In this paper we discuss Raman polarimetry as a technique for the calibration of a hypothetical polarimetric TS system operating in the same conditions of the ITER core TS system and suggest two calibration methods for the measurement of the additional depolarization introduced by the plasma-exposed first mirror, and in general for calibrating the polarimetric response of the detection system. [ABSTRACT FROM AUTHOR]

The high plasma temperatures expected at reactor conditions in magnetic confinement fusion toroidal devices suggest that near-marginal operation could be a reality in future devices and reactors. By near-marginal it is meant that the plasma profiles might wander around the local critical thresholds for the onset of instabilities. Self-organized criticality (SOC) was suggested in the mid 1990s as a more proper paradigm to describe the dynamics of tokamak plasma transport in near-marginal conditions. It advocated that, near marginality, the evolution of mean profiles and fluctuations should be considered simultaneously, in contrast to the more common view of a large separation of scales existing between them. Otherwise, intrinsic features of near-marginal transport would be missed, that are of importance to understand the properties of energy confinement. In the intervening 20 years, the relevance of the idea of SOC for near-marginal transport in fusion plasmas has transitioned from an initial excessive hype to the much more realistic standing of today, which we will attempt to examine critically in this review paper. First, the main theoretical ideas behind SOC will be described. Secondly, how they might relate to the dynamics of near-marginal transport in real magnetically confined plasmas will be discussed. Next, we will review what has been learnt about SOC from various numerical studies and what it has meant for the way in which we do numerical simulation of fusion plasmas today. Then, we will discuss the experimental evidence available from the several experiments that have looked for SOC dynamics in fusion plasmas. Finally, we will conclude by identifying the various problems that still remain open to investigation in this area. Special attention will be given to the discussion of frequent misconceptions and ongoing controversies. The review also contains a description of ongoing efforts that seek effective transport models better suited than traditional equations to capture SOC dynamics. Most of these models, based on the use of fractional transport equations and related concepts, could prove useful both in reactor operation and experiment control and design. [ABSTRACT FROM AUTHOR]

The ITER tokamak, the next experimental step toward the development of nuclear fusion reactors, will explore the burning plasma regime in which the plasma temperature is sustained mostly by fusion heating. Regulation of the fusion power through modulation of fueling and external heating sources, referred to as burn control, is one of the fundamental problems in burning plasma research. Active control will be essential for achieving and maintaining desired operating points, responding to changing power demands, and ensuring stable operation. Most existing burn control efforts use either non-model-based control techniques or designs based on linearized models. These approaches must be designed for particular operating points and break down for large perturbations. In this work, we utilize a spatially averaged (zero-dimensional) nonlinear model to synthesize a multi-variable nonlinear burn control strategy that can reject large perturbations and move between operating points. The controller uses all of the available actuation techniques in tandem to ensure good performance, even if one or more of the actuators saturate. Adaptive parameter estimation is used to improve the model parameter estimates used by the feedback controller in real-time and ensure asymptotic tracking of the desired operating point. In addition, we propose the use of a model-based online optimization algorithm to drive the system to a state that minimizes a given cost function, while respecting input and state constraints. A zero-dimensional simulation study is presented to show the performance of the adaptive control scheme and the optimization scheme with a cost function weighting the fusion power and temperature tracking errors. [ABSTRACT FROM AUTHOR]

We use a parameter βr for all plasmas that allows detecting the pollution of the plasma bulk by highly radiative impurities. This parameter is defined as the radiative loss of the mixture of impurities relative to their mean Z2 and was used in previous works to characterize the efficiency of radiative mantles in Neon seeded discharges (Telesca G et al 2000 Nucl. Fusion40 1845, Carraro L et al 2000 Nucl. Fusion40 1983). We show that this parameter, though global, is very sensitive to the presence of highly radiative impurities in the bulk of the discharge. We use it to compare JET plasmas in the carbon environment and in the ITER-like wall (ILW), where it is highly correlated to the level of a bundle of spectroscopic lines of tungsten passing through the center of the discharge. In the carbon environment, the value of βr is around 10−40 MW m6, indicating the absence of highly radiative impurities in the plasma. No change or even a small decrease is observed when going from L-mode to H-mode, this robustness being in agreement with the multi-machine scaling (Matthews G F et al 1997 J. Nucl. Mater.241–243 450). In the ILW machine, the value of βr is found to depend on the type of additional heating and confinement state of the plasma. We observe that neutral-beam injection introduces little W into the plasma, with a βr between 2 × 10−40 and 3 × 10−40 MW m6. Ion-cyclotron radio-frequency waves yield a βr of order 5 in L-mode and 10 × 10−40 MW m6 in H-mode when no edge-localized modes (ELMs) are present. Conversely when ELMs are present, the parameter goes back to 5 × 10−40 MW m6, illustrating the positive effect they can have on the bulk pollution by tungsten. [ABSTRACT FROM AUTHOR]