Showing papers by "Princeton Plasma Physics Laboratory published in 1999"

Plasma confinement in JET H?mode plasmas with H, D, DT and T isotopes

Abstract: The scaling of the energy confinement in H mode plasmas with different hydrogenic isotopes (hydrogen, deuterium, DT and tritium) is investigated in JET. For ELM-free H modes the thermal energy confinement time τth is found to decrease weakly with the isotope mass (τth ~M-0.25±0.22), whilst in ELMy H modes the energy confinement time shows practically no mass dependence (τth ~M0.03±0.1). Detailed local transport analysis of the ELMy H mode plasmas reveals that the confinement in the edge region increases strongly with the isotope mass, whereas the confinement in the core region decreases with mass (τthcore ∝ M-0.16), in approximate agreement with theoretical models of the gyro-Bohm type (τgB ~M-0.2).

Shearing rate of time-dependent E×B flow

Abstract: Theory of E×B shear suppression of turbulence in toroidal geometry [Phys. Plasmas 2, 1648 (1995)] is extended to include fast time variations of the E×B flows often observed in nonlinear simulations of tokamak turbulence. It is shown that the quickly time varying components of the E×B flows, while they typically contribute significantly to the instantaneous E×B shearing rate, are less effective than the slowly time varying components in suppressing turbulence. This is because the shear flow pattern changes before eddies get distorted enough. The effective E×B shearing rate capturing this important physics is analytically derived and estimated from zonal flow statistics of gyrofluid simulation. This provides new insights into understanding recent gyrofluid and gyrokinetic simulations that yield a reduced, but not completely quenched, level of turbulence in the presence of turbulence-driven zonal flows.

Effects of Collisional Zonal Flow Damping on Turbulent Transport

Abstract: Results from 3D global gyrokinetic particle simulations of ion-temperature-gradient driven microturbulence in a toroidal plasma show that the ion thermal transport level in the interior region exhibits significant dependence on the ion-ion collision frequency even in regimes where the instabilities are collisionless. This is identified as arising from the Coulomb collisional damping of turbulence-generated zonal flows.

Physics of energetic ions

Abstract: Physics knowledge (theory and experiment) in energetic particles relevant to design of a reactor scale tokamak is reviewed, and projections for ITER are provided in this Chapter of the ITER Physics Basis. The review includes single particle effects such as classical alpha particle heating and toroidal field ripple loss, as well as collective instabilities that might be generated in ITER plasmas by energetic alpha particles. The overall conclusion is that fusion alpha particles are expected to provide an efficient plasma heating for ignition and sustained burn in the next step device. The major concern is localized heat loads on the plasma facing components produced by alpha particle loss, which might affect their lifetime in a tokamak reactor.

Dynamical modelling of tearing mode stabilization by RF current drive

Abstract: The theory of tearing mode stabilization in toroidal plasmas by RF driven currents that are modulated in phase with the island rotation is investigated. A time-scale analysis of the phenomena involved indicates that transient effects, such as finite time response of the driven currents, island rotation during the power pulses and the inductive response of the plasma, are intrinsically important. A dynamical model of such effects is developed, based on a 3-D Fokker-Planck code coupled to both the electric field diffusion equation and the island evolution equation. Extensive applications to both ECCD and LHCD in ITER are presented.

Physics design of the national spherical torus experiment

Abstract: The mission of the National Spherical Torus Experiment (NSTX) is to prove the principles of spherical torus physics by producing high-beta toroidal plasmas that are non-inductively sustained, and whose current profiles are in steady-state NSTX will be one of the first ultra low a[P(input) up to 11 MW] in order to produce high-beta toroidal (25 to 40%), low collisionality, high bootstrap fraction (less than or equal to 70%) discharges Both radio-frequency (RF) and neutral-beam (NB) heating and current drive will be employed Built into NSTX is sufficient configurational flexibility to study a range of operating space and the resulting dependences of the confinement, micro- and MHD stability, and particle and power handling properties NSTX research will be carried out by a nationally based science team

Bulk ion heating with ICRH in JET DT plasmas

Abstract: Reactor relevant ICRH scenarios have been assessed during DT experiments on the JET tokamak using H mode divertor discharges with ITER-like shapes and safety factors Deuterium minority heating in tritium plasmas was demonstrated for the first time For 9% deuterium, an ICRH power of 6 MW gave 166 MW of fusion power from reactions between suprathermal deuterons and thermal tritons The Q value of the steady state discharge reached 022 for the length of the RF flat-top (27 s), corresponding to three plasma energy replacement times The Doppler broadened neutron spectrum showed a deuteron energy of 125 keV, which was optimum for fusion and close to the critical energy Thus, strong bulk ion heating was obtained at the same time as high fusion efficiency Deuterium fractions around 20% produced the strongest ion heating together with a strong reduction of the suprathermal deuteron tail The ELMs had low amplitude and high frequency and each ELM transported less plasma energy content than the 1% required by ITER The energy confinement time, on the ITERH97-P scale, was 090, which is sufficient for ignition in ITER 3He minority heating, in approximately 50:50 D:T plasmas with up to 10% 3He, also demonstrated strong bulk ion heating Central ion temperatures up to 13 keV were achieved, together with central electron temperatures up to 12 keV The normalized H mode confinement time was 095 Second harmonic tritium heating produced energetic tritons above the critical energy This scheme heats the electrons in JET, unlike in ITER where the lower power density will allow mainly ion heating The inverted scenario of tritium minority ICRH in a deuterium plasma was demonstrated as a successful heating method producing both suprathermal neutrons and bulk ion heating Theoretical calculations of the DT reactivity mostly give excellent agreement with the measured reaction rates

Characteristics of Alfvén eigenmodes, burst modes and chirping modes in the Alfvén frequency range driven by negative ion based neutral beam injection in JT-60U

Abstract: The excitation and stabilization of Alfv?n eigenmodes and their impact on energetic ion confinement were investigated with negative ion based neutral beam injection at 330-360?keV into weak or reversed magnetic shear plasmas on JT-60U. Toroidicity induced Alfv?n eigenmodes (TAEs) were observed in weak shear plasmas with ?h ? 0.1% and 0.4 ? vb||/vA ? 1. The stability of TAEs is consistent with predictions by the NOVA-K code. New burst modes and chirping modes were observed in the higher ? regime of ?h ? 0.2%. The effect of TAEs, burst modes and chirping modes on fast ion confinement has been found to be small so far. It was found that a strongly reversed shear plasma with internal transport barrier suppresses AEs.

Overview of ITER physics deuterium-tritium experiments in JET

Abstract: An overview of JET experimental results in DT plasmas directly relevant to ITER modes of operation is presented. Experiments in D:T mixtures varying from 100:0 to 10:90 and those carried out in hydrogen plasmas show that the H mode threshold power has an approximately inverse isotope mass dependence. Matching some of the key dimensionless parameters to the ITER values, the ITER similarity experiments with ITER shape and safety factor q show that the global energy confinement time is practically independent of isotopic mass (~A0.03±0.08), where A is the atomic mass of the hydrogenic species. Subtracting the edge pedestal energy (which scales as ~A0.57±0.2) from the total stored energy leads to a ~A-0.17±0.1 dependence of confinement in the plasma core, very similar to that expected from the gyro-Bohm transport (~ A-0.2) model. The observed scaling of the edge pedestal energy is consistent with a model in which the edge pressure gradient saturates at the ballooning limit over a region of width that scales as the ion poloidal Larmor radius governed by the average energy of the fast ions in the edge. The steady state total stored energy for a given input power in both ICRH and NBI discharges is the same despite the lower edge pedestal in the ICRH case, which is compensated for by more peaked power deposition profiles in ICRH. The ELM frequency is smaller with NBI; it decreases with isotopic mass in both NBI and ICRH discharges. A steady state, type I ELMy H mode discharge with ITER shape and q at 3.8 T/3.8 MA with an input power of 22 MW produced a Q ≈ 0.18 for 3.5 s and extrapolates well to ignition with ITER parameters. Here, Q is the ratio of fusion output power to input power. The thermal ELMy H mode confinement in both deuterium and tritium gas fuelled plasmas decreases significantly when the plasma density exceeds 0.75 of the Greenwald (nGW) limit, and the maximum density achieved is 0.85nGW. In L mode, the density limit decreases with increasing isotope mass roughly in accordance with code predictions. ITER reference ICRH scenarios have been evaluated. Second harmonic heating of tritium at the densities available in JET produces strong tails and heats electrons predominantly as expected. The 3He minority in 50:50 D:T and tritium dominated plasmas showed strong bulk ion heating leading to ion temperatures up to 13 keV with ICRH alone. Deuterium minority ion cyclotron heating in tritium plasmas at a power level of 6 MW produced steady state record values of Q ≈ 0.22 for more than 2.5 s.

Reduced transport and E r shearing in improved confinement regimes in JT-60U

Abstract: The global confinement and the local transport properties of improved core confinement plasmas in JT-60U were studied in connection with Er shear formation In the improved core confinement mode with internal transport barriers (ITBs), these are roughly classified into `parabolic type' ITBs and `box type' ITBs The parabolic type ITB has a reduced thermal diffusivity χ in the core region; however, the Er shear, dEr/dr, is not as strong The box type ITB has a very strong Er shear at the thin ITB layer and χ decreases to the level of neoclassical transport there The estimated E × B shearing rate, ωE × B, becomes almost the same as the linear growth rate of the drift microinstability, γL, at the ITB layer in the box type ITB Experiments with hot ion mode plasmas during the repetitive L-H-L transition showed that the thermal diffusivity clearly depends on the Er shear and the strong Er shear contributes to the reduced thermal diffusivity

Turbulent Dynamos and Magnetic Helicity

Abstract: It is shown that the turbulent dynamo alpha-effect converts magnetic helicity from the turbulent field to the mean field when the turbulence is electromagnetic while the magnetic helicity of the mean-field is transported across space when the turbulence is elcetrostatic or due to the elcetron diamagnetic effect. In all cases, however, the dynamo effect strictly conserves the total helicity expect for a battery effect which vanishes in the limit of magnetohydrodynamics. Implications for astrophysical situations, especially for the solar dynamo, are discussed.

Stability of alpha particle driven Alfvén eigenmodes in high performance JET DT plasmas

Abstract: The stability of alpha particle driven Alfven eigenmodes (AEs) is analysed in high fusion power DT discharges on JET. Both hot ion H mode and shear optimized discharges are considered. Unstable AEs are not observed in hot ion H mode DT discharges even at the highest fusion power with alpha particle beta βα (0) ≈ 0.7%. Theoretical analysis shows that the AE stabilization is caused by the large plasma pressure, which prevents the existence of core localized AEs at peak fusion performance. Kinetic toroidal AEs (KTAEs), which persist at high plasma pressure, are found to be radially extended and subject to strong damping. The stability analysis based on the CASTOR-K code confirms that AEs cannot be driven unstable by alpha particles in high performance hot ion H mode discharges performed at JET. Alfven eigenmodes in shear optimized regimes are more unstable than those in the hot ion H mode mainly due to the elevated central safety factor q, which increases the efficiency of AE interaction with energetic ions. As a consequence, AEs are observed in shear optimized DT discharges when ion cyclotron heating as low as 1 MW is applied.

A kinetic‐fluid model

Abstract: A nonlinear kinetic-fluid model for high β plasmas with multiple ion species which can be applied to multiscale phenomena is presented. The model embeds important kinetic effects due to finite ion Larmor radius (FLR), wave-particle resonances, magnetic particle trapping, etc., in the framework of simple fluid descriptions. When further restricted to low-frequency phenomena, with frequencies less than the ion cyclotron frequency, the kinetic-fluid model takes a simpler form in which the fluid equations of multiple ion species collapse into one-fluid density and momentum equations and a low-frequency generalized Ohm's law. The kinetic effects are introduced via plasma pressure tensors for ions and electrons which are computed from particle distribution functions that are governed by the Vlasov equation or simplified plasma dynamics equations such as the gyrokinetic equation. The ion FLR effects provide a finite parallel electric field, a perpendicular velocity that modifies the E × B drift, and a gyroviscosity tensor, all of which are neglected in the usual one-fluid MHD description. Eigenmode equations are derived, which include magnetosphere-ionosphere coupling effects for low-frequency waves (e.g., kinetic/inertial Alfven waves and ballooning-mirror instabilities).

The design of the KSTAR tokamak

Abstract: The Korea Superconducting Tokamak Advanced Research (KSTAR) Project is the major effort of the Korean National Fusion Program (KNFP) to develop a steady-state-capable advanced superconducting tokamak to establish a scientific and technological basis for an attractive fusion reactor. Major parameters of the tokamak are: major radius 1.8 m, minor radius 0.5 m, toroidal field 3.5 Tesla, and plasma current 2 mA with a strongly shaped plasma cross-section and double-null divertor. The initial pulse length provided by the poloidal magnet system is 20 s, but the pulse length can be increased to 300 s through non-inductive current drive. The plasma heating and current drive system consists of neutral beam, ion cyclotron waves, lower hybrid waves, and electron–cyclotron waves for flexible profile control. A comprehensive set of diagnostics is planned for plasma control and performance evaluation and physics understanding. The project has completed its conceptual design phase and moved to the engineering design phase. The target date of the first plasma is set for year 2002.

Behaviour of electron and ion transport in discharges with an internal transport barrier in the DIII-D tokamak

Abstract: The article reports results of experiments to further determine the underlying physics behind the formation and development of internal transport barriers (ITBs) in the DIII-D tokamak. The initial ITB formation occurs when the neutral beam heating power exceeds a threshold value during the early stages of the current ramp in low density discharges. This region of reduced transport, made accessible by suppression of long wavelength turbulence by sheared flows, is most evident in the ion temperature and impurity rotation profiles. In some cases, reduced transport is also observed in the electron temperature and density profiles. If the power is near the threshold, the barrier remains stationary and encloses only a small fraction of the plasma volume. If, however, the power is increased, the transport barrier expands to encompass a larger fraction of the plasma volume. The dynamic behaviour of the transport barrier during the growth phase exhibits rapid transport events that are associated with both broadening of the profiles and reductions in turbulence and associated transport. In some but not all cases, these events are correlated with the safety factor q passing through integer values. The final state following this evolution is a plasma exhibiting ion thermal transport at or below neoclassical levels. Typically the electron thermal transport remains anomalously high. Recent experimental results are reported in which RF electron heating was applied to plasmas with an ion ITB, thereby increasing both the electron and the ion transport. Although the results are partially in agreement with the usual E × B shear suppression hypothesis, the results still leave questions that must be addressed in future experiments.

Physics Design of a High-beta Quasi-axisymmetric Stellarator

Abstract: Key physics issues in the design of a high- quasi-axisymmetric stellarator configuration are discussed. The goal of the design study is a compact stellarator configuration with aspect ratio comparable to that of tokamaks and good transport and stability properties. Quasi- axisymmetry has been used to provide good drift trajectories. Ballooning stabilization has been accomplished by strong axisymmetric shaping, yielding a stellarator configuration whose core is in the second stability regime for ballooning modes. A combination of externally generated shear and non-axisymmetric corrugation of the plasma boundary provides stability to external kink modes even in the absence of a conducting wall. The resulting configuration is also found to be robustly stable to vertical modes, increasing the freedom to perform axisymmetric shaping. Stability to neoclassical tearing modes is conferred by a monotonically increasing profile. A gyrokinetic f code has been used to confirm the adequacy of the neoclassical confinement. Neutral beam losses have been evaluated with Monte Carlo codes.

High harmonic ion cyclotron heating in DIII-D: beam ion absorption and sawtooth stabilization

Abstract: Combined neutral beam injection and fast wave heating at the fourth cyclotron harmonic produce an energetic deuterium beam ion tail in the DIII-D tokamak. When the concentration of thermal hydrogen exceeds ~5%, the beam ion absorption is suppressed in favour of second harmonic hydrogen absorption. As theoretically expected, the beam absorption increases with beam ion gyro-radius; also, central absorption at the fifth harmonic is weaker than central absorption at the fourth harmonic. For central heating at the fourth harmonic, an energetic, perpendicular, beam population forms inside the q = 1 surface. The beam ion tail transiently stabilizes the sawtooth instability but destabilizes toroidicity induced Alfv?n? eigenmodes (TAEs). Saturation of the central heating correlates with the onset of the TAEs. Continued expansion of the q = 1 radius eventually precipitates a sawtooth crash; complete magnetic reconnection is observed.

Confinement physics study in a small low aspect ratio helical device: CHS

Abstract: Variation of the plasma position relative to the centre of the helical coil winding is a very effective means of controlling the MHD stability and the trapped particle confinement in heliotron/torsatron systems, but improving one of these two characteristics with this parameter simultaneously has a detrimental effect on the other. The inward shifted configuration is favourable for drift orbit optimization but is predicted to be unstable according to the Mercier criterion. Various physics problems, such as electric field structure, plasma rotation and MHD phenomena, have been studied in the Compact Helical System (CHS) with a compromise intermediate position. With this standard configuration, CHS has yielded experimental results that contribute to the understanding of general toroidal confinement physics and low aspect ratio helical systems. In the recent experiments, it was found that a wide range of inward shifted configurations give stable plasma discharges without any restriction to the special pressure profile. Such an enhanced range of operation made it possible to study experimentally the drift orbit optimized configuration in heliotron/torsatron systems. The effect of configuration improvement was studied with plasmas in a low collisionality regime.

Physics of energetic particle-driven instabilities in the START spherical tokamak

Abstract: The recent use of neutral beam injection (NBI) in the UKAEA small tight aspect ratio tokamak (START) has provided the first opportunity to study experimentally the physics of energetic ions in spherical tokamak (ST) plasmas. In such devices the ratio of major radius to minor radius R0/a is of order unity. Several distinct classes of NBI-driven instability have been observed at frequencies up to 1 MHz during START discharges. These observations are described, and possible interpretations are given. Equilibrium data, corresponding to times of beam-driven wave activity, are used to compute continuous shear Alfven spectra: toroidicity and high plasma beta give rise to wide spectral gaps, extending up to frequencies of several times the Alfven gap frequency. In each of these gaps Alfvenic instabilities could, in principle, be driven by energetic ions. Chirping modes observed at high beta in this frequency range have bandwidths comparable to or greater than the gap widths. Instability drive in START is provided by beam ion pressure gradients (as in conventional tokamaks), and also by positive gradients in beam ion velocity distributions, which arise from velocity-dependent charge exchange losses. It is shown that fishbone-like bursts observed at a few tens of kHz can be attributed to internal kink mode excitation by passing beam ions, while narrow-band emission at several hundred kHz may be due to excitation of fast Alfven (magnetosonic) eigenmodes. In the light of our understanding of energetic particle-driven instabilities in START, the possible existence of such instabilities in larger STs is discussed.

Influence of charge-exchange processes on x-ray spectra in TEXTOR tokamak plasmas: experimental and theoretical investigation

Abstract: Time-resolved high-resolution soft x-ray spectra from gas-puff injected Ar impurity ions have been investigated for neutral beam heated and ohmically heated discharges in the TEXTOR tokamak. The experimental spectra show systematic deviations from corona model calculations for the line intensities of the forbidden He-like lines x, y, z and the Li- and Be-like dielectronic satellite spectra: theoretical corona model calculations predict intensities significantly too low. High-intensity Li-like inner-shell excited satellites correlate with the neutral beam injection. The discrepancies could also be observed in the stationary phase of an inductively heated discharge. In the heating phase the discrepancies are even larger. We propose charge-exchange processes between the neutral atoms and the impurity ions as an explanation of the experimental findings. Good agreement with the experimental observations can then be obtained without the need for invoking large (anomalous) diffusion coefficients. A self-consistent coupling of the population kinetics of the neutrals and the impurity ions, also taking into account charge-exchange processes from excited states of hydrogen/deuterium permit the determination of the neutral fraction and of the electron lifetime on the sole basis of impurity spectra analysis. Independent Monte Carlo simulations of neutral gas transport also provides the ionization degree in the centre and the electron lifetime. These calculations are also in good agreement with the spectroscopic results.

A Lagrangian analysis of advection-diffusion equation for a three dimensional chaotic flow

Abstract: The advection-diffusion equation is studied via a global Lagrangian coordinate transformation. The metric tensor of the Lagrangian coordinates couples the dynamical system theory rigorously into the solution of this class of partial differential equations. If the flow has chaotic streamlines, the diffusion will dominate the solution at a critical time, which scales logarithmically with the diffusivity. The subsequent rapid diffusive relaxation is completed on the order of a few Lyapunov times, and it becomes more anisotropic the smaller the diffusivity. The local Lyapunov time of the flow is the inverse of the finite time Lyapunov exponent. A finite time Lyapunov exponent can be expressed in terms of two convergence functions which are responsible for the spatio-temporal complexity of both the advective and diffusive transports. This complexity gives a new class of diffusion barrier in the chaotic region and a fractal-like behavior in both space and time. In an integrable flow with shear, there also exist fast and slow diffusion. But unlike that in a chaotic flow, a large gradient of the scalar field across the KAM surfaces can be maintained since the fast diffusion in an integrable flow is strictly confined within the KAM surfaces.

Flaking of co-deposited hydrogenated carbon layers on the TFTR limiter

Abstract: Flaking of co-deposited layers on the inner limiter tiles was recently observed in TFTR. This phenomenon was unexpected and has occurred since the termination of plasma operations on 4 April 1997. Flaking affects approximately 15% of the observable tiles and appears on isotropic graphite but not on carbon fibre composite tiles. Photographic images of the flakes and precise measurements of the limiter geometry are reported. The mobilizability of tritium retained in co-deposited layers is an important factor in safety analyses of future DT reactors. A programme to analyse the flakes and tiles is underway.

Energetic-ion-driven toroidal alfven eigenmodes observed in a heliotron/torsatron plasma

Abstract: Toroidal Alfv{acute e}n eigenmodes (TAEs) of low toroidal mode number, n=1 and 2, are observed in neutral-beam-heated plasmas in the compact helical system heliotron/torsatron. The observed frequency is proportional to the computed TAE frequency and lies near the lower bound of the innermost TAE gap. The modes are excited only when the beam velocity exceeds about half the central Alfv{acute e}n velocity and when the net plasma current induced by coinjected neutral beams is in the required range. The modes are localized in the plasma core region, between 0.2 and 0.6 of the plasma minor radius. {copyright} {ital 1999} {ital The American Physical Society }

Progress towards sustainment of advanced tokamak modes in diii-d

Abstract: Improving confinement and β limits simultaneously in long pulse ELMy H mode discharges is investigated. The product βN H98y serves as a useful figure of merit for performance, where βN ≡ β/(I/aB) and H98y is the ratio of the thermal confinement time to the most recent ELMy H mode confinement scaling established by the ITER confinement database working group. In discharges with q0 ≈ 1 (no sawteeth) and discharges with qmin > 1.5 and negative central magnetic shear, βN ≈ 2.9 and H98y ≈ 1.4 are sustained for up to 2 s. Although peaked profiles are observed, steep internal transport barriers are not present. Further increases in βN in these discharges are limited by neoclassical tearing modes (NTMs) in the positive shear region. In another recently developed regime, βN ≈ 3.8 and H98y ≈ 2 have been sustained during large infrequent ELMs in non-sawtoothing discharges with q0 ≈ 1. This level of performance is similar to that obtained in ELM free regimes such as VH mode. The limitation on βN and pulse length in these discharges is also the onset of NTMs.

High fusion power steady state operation in JET DT plasmas

Abstract: Because of its large size, single null divertor and flexible magnetic geometry, JET is capable of producing the most reactor relevant plasmas of any present generation tokamak. In recent DT experiments, the fusion performance of these plasmas was tested for the first time. Over 4 MW of fusion power was produced in a high power, steady state pulse of 5 s, limited by the duration of the heating power. The fusion QE, defined simply as the fusion energy produced divided by the input energy over this 5 s interval, was 0.18. These DT ELMy H mode discharges performed up to expectations based on DD preparation pulses and thus establish a firm basis for extrapolating to a next step machine. Operation at low q95 is possible in JET with no degradation in the confinement enhancement factor and provides an improved margin to ignition when extrapolated to ITER. Considerable uncertainties remain, nonetheless. In particular, access to high density, relative to the Greenwald limit, and operation in close proximity to the H mode threshold may both result in a degradation of the confinement in the next step machine.

Neoclassical islands, β-limits, error fields and ELMs in reactor scale tokamaks

Abstract: An assessment is presented of the impact of recent magnetohydrodynamic research results on performance projections for reactor scale tokamaks as exemplified by the ITER Final Design Report (ITER/FDR) facility. For nominal ELMy H mode operation, the presence and amplitude of neoclassical tearing modes governs the achievable β value. Recent work finds that the scaling of β at which such modes onset agrees well with a polarization drift model, with the consequence that, with reasonable assumptions regarding seed island width, the mode onset β will be lower in reactor scale tokamaks than in contemporary devices. Confinement degradation by such modes, on the other hand, depends on relative saturated island size which is governed principally by β and secondarily by ν* effects on bootstrap current density. Relative saturated island size should be comparable in present and reactor devices. DT ITER demonstration discharges in JET exhibited no confinement degradation at the planned ITER operating value of βN = 2.2. Theory indicates that electron cyclotron current drive can either stabilize these modes or appreciably reduce saturated island size. Turning to operation in candidate steady state, reverse shear, high bootstrap fraction configurations, wall stabilization of external kink modes is effective while the plasma is rotating but (so far) rotation has not been maintained. Recent error field observations in JET imply an error field size scaling that leads to a projection that the ITER/FDR facility will be somewhat more tolerant to error fields than thought previously. ICRF experiments on JET and Alcator C-Mod indicate that plasmas heated by central energetic particles have benign ELMs compared with the usual type 1 ELM of NBI heated discharges.

Generation of periodic accelerating structures in plasma by colliding laser pulses.

Abstract: A mechanism for generating large (>1 GeV/m) accelerating wakes in a plasma is proposed. Two slightly detuned counterpropagating laser beams, an ultrashort timing pulse and a long pump, exchange photons and deposit the recoil momentum in plasma electrons. This produces a localized region of electron current, which acts as a virtual electron beam, inducing intense plasma wakes with phase velocity equal to the group velocity of the short pulse. Modulating the pumping beam generates periodic accelerating structures in the plasma ("plasma linac") which can be used for particle acceleration unlimited by the dephasing between the particles and the wake. An important difference between this type of plasma accelerator and the conventional wakefield accelerators is that this type can be achieved with laser intensities I<<10(18) W/cm(2).

X-Ray Spectroscopy at the TEXTOR-94 Tokamak

Abstract: Spectra of the 1s-2p resonance line of helium-like argon, Ar XVII, and the associated satellites in the narrow wavelength range from 3.9494 A to 3.9944 A have been observed at TEXTOR-94 with two high-resolution x-ray crystal spectrometers at Bragg angles near 54 degrees. Together, these instruments can be used as an x-ray polarimeter. The instruments are very sensitive to small amounts of argon; thus, a count rate of 4 × 105 photons/s, which corresponds to the maximum count rate capability of the presently used detectors, can be obtained for argon densities of less than 10-3 times the electron density, if the central electron temperature is about 1 keV. Up to 8192 spectra per discharge can be recorded allowing for the investigation of fast events during MHD activities. The instruments have been used to study the electron and ion behavior during internal sawtooth oscillations, which affect the confinement properties in the core of the plasma. The x-ray polarimeter at TEXTOR-94 is a unique diagnostic tool for the measurement of non-Maxwellian electron-velocity distributions, which can be produced in tokamak discharges in the presence of electric fields. First polarization measurements were performed during the flat-top phase in discharges with both pure ohmic heating and additional neutral-beam heating. These measurements showed that inhomogeneities in the emissivity profile can produce effects which look like polarization effects. No polarization was found when these source inhomogeneities were taken into account. The polarimeter is presently upgraded by an imaging x-ray crystal to facilitate polarization measurements. This will be important for the upcoming experiments with electron cyclotron heating at TEXTOR-94.

Electromagnetically induced guiding of counterpropagating lasers in plasmas

Abstract: The interaction of counterpropagating laser pulses in a plasma is considered. When the frequencies of the two lasers are close, nonlinear modification of the refraction index results in the mutual focusing of the two beams. A short (of order of the plasma period) laser pulse can also be nonlinearly focused by a long counterpropagating beam which extends over the entire guiding length. This phenomenon of electromagnetically induced guiding can be utilized in laser-driven plasma accelerators. {copyright} {ital 1999} {ital The American Physical Society}

Transition from L mode to high ion temperature mode in CHS heliotron/torsatron plasmas

Abstract: A high ion temperature (Ti) mode is observed for neutral beam heated plasmas in the Compact Helical System (CHS) heliotron/torsatron. The high Ti mode plasma is characterized by a high central ion temperature, Ti(0), and is associated with a peaked electron density profile produced by neutral beam fuelling with low wall recycling. Transition from L mode to high Ti mode has been studied in CHS. Ti(0) in the high Ti mode discharges reaches 1 keV, which is 2.5 times higher than that in the L mode discharges. The ion thermal diffusivity is significantly reduced by a factor of more than 2-3 in the high Ti mode plasma. The ion loss cone is observed in neutral particle flux in the energy range 1-6 keV with a narrow range of pitch angle (90° ±20°) in the high Ti mode. However, the degradation of ion energy confinement due to this loss cone is negligible.