S. J. Karttunen

Two‐stage electron acceleration by simultaneous stimulated Raman backward and forward scattering

The coexistence of stimulated Raman forward and backward scattering of intense electromagnetic radiation, which can occur, for instance, in laser fusion plasmas, is investigated. The simultaneous Raman forward and backward scattering is shown to create an electrostatic field structure which is exceptionally efficient in producing highly relativistic electrons. The mechanism of the electron acceleration is analyzed both by Vlasov–Maxwell simulations with self‐consistent fields and by test particle calculations with prescribed electrostatic fields. The Vlasov–Maxwell simulations reveal that the two plasma waves generated by the backward and forward scattering are spatially separated, and thus form a two‐stage electron ‘‘accelerator.’’

ITB formation in terms of ωE×B flow shear and magnetic shear s on JET

A linear empirical threshold condition ωE×B/γITG>0.68s-0.095 has been found for the onset of the ion internal transport barriers in the JET optimised shear database. Here, s is the magnetic shear, ωE×B the flow shearing rate and γITG is an approximate of the linear growth rate of the ion temperature gradient instability. The present empirical threshold condition for the ITB formation will provide a first clear indication of the strong correlation of s and ωE×B at the ITB transition. The empirical analysis consists of ITB discharges from a wide plasma parameter range; the toroidal magnetic field varies between 1.8-4.0 T, the auxiliary heating power between 10-30 MW and the diamagnetic energy between 3-12 MJ. The predictive simulations of several ITB discharges with the empirical ITB threshold condition reproduce the experiments with time averaged prediction errors of the order of 10-25% in Ti and Te profiles and 10-15% in ne profiles as well as the toroidal flow velocity with errors of the order of 10-20%. The simulated times of the onset of the ITB compared to the experimental ones are typically within 0.4 s and the simulated ITB widths within 0.1 in r/a throughout the whole simulations.

Progress in LHCD : a tool for advanced regimes on ITER

The recent success in coupling lower hybrid (LH) waves in high performance plasmas at JET together with the first demonstration on FTU of the coupling capability of the new passive active multijunction launcher removed major concerns on the possibility of using LH on ITER. LH exhibits the highest experimental current drive (CD) efficiency at low plasma temperature thus making it the natural candidate for off-axis CD on ITER where current profile control will help in maintaining burning performance on a long-time scale. We review recent LH results: long internal transport barrier obtained in JET with current profile sustained and controlled by LH acting under real time feedback together with first LH control of flat q-profile in a hybrid regime with T e ∼ T i . Minutes long fully non-inductive LH driven discharges on Tore Supra (TS). High CD efficiency with electron cyclotron in synergy with LH obtained in FTU and TS opening the possibility of interesting scenarii on ITER for MHD stabilization. Preliminary results of LH modelling for ITER are also reported. A brief overview of ITER LH system is reported together with some indication of new coming LH experiments, in particular KSTAR where CW klystrons at the foreseen ITER frequency of 5 GHz are being developed.

Bright spots generated by lower hybrid waves on JET

Observations of bright spots on the JET divertor aprons during lower hybrid current drive experiments are described. These bright spots are important because they can potentially cause damage to large tokamaks. The bright spots arise due to the impact of a fast particle beam. This beam originates from the front of the lower hybrid launcher, where thermal particles are accelerated according to theory by interaction with the high spatial harmonics of the lower hybrid wave. The bright spots are clearly related to the lower hybrid power as they disappear when the lower hybrid power is switched off. According to the analysis versus various parameters, the brightness of the spots clearly decreases with increasing plasma–wall distance, i.e. the distance between the last closed flux surface and the poloidal limiter. This is clearly beneficial for ITER, as it is designed to operate at a large plasma–wall distance.

Particle-in-cell simulation of ion Bernstein wave excitation

Ion Bernstein wave excitation is investigated with the self-consistent two-dimensional particle-in-cell method. The real ion to electron mass ratio is used in simulations in high harmonic frequency bands. The simulation results are compared with linear theory and ray tracing. Successful excitation of the ion Bernstein wave has been demonstrated with the particle-in-cell method. In some cases, the excited wave temporarily propagates in the opposite direction and slows down permanently due to complicated dispersive behavior, which makes it very difficult to use the particle-in-cell method. The excitation is studied as a function of temperature and frequency, i.e., it is determined how the dispersive behavior varies in the parameter space. The simulations indicate that there is a temperature-and-frequency-dependent critical level of coupled energy flux above which excitation fails. Possible effects causing the failure of excitation at high power intensity are identified.

Generation of hot spots by fast electrons in lower hybrid grills

Generation of hot spots on plasma facing components magnetically connected to the grill is a main limiting factor in high-power operation of a lower hybrid system. A possible explanation for the hot spots is the sputtering caused by fast electrons generated by parasitic absorption of lower hybrid power near the grill mouth. The behavior of the edge plasma near the grill mouth is investigated with a new tool in this context: the self-consistent particle-in-cell (PIC) code XPDP2 [V. Vahedi et al., Phys. Fluids B 5, 2719 (1993)]. The PIC simulations provide the key parameters of the problem: the absorbed power, the radial deposition profiles, and the energy spectrum of the accelerated particles close to the grill.

Particle-in-cell simulation of parasitic absorption of lower hybrid power in edge plasmas of tokamaks

In lower hybrid (LH) current drive experiments at Tore Supra and Tokamak de Varennes (TdeV), hot spots and the generation of impurities have been observed. Melting of the grill mouth has also occurred in LH current drive experiments. A possible explanation for these observations is parasitic absorption of the short-wavelength part of the LH spectrum close to the grill mouth. In this work, we investigate the parasitic absorption of the LH power in the edge plasma of Tore Supra and JET with particle-in-cell (PIC) simulations. The LH spectra are calculated with the SWAN coupling code and used in the self-consistent electrostatic PIC code XPDP2 which calculates the absorption. The absorption was calculated for different edge densities and density scale lengths when the coupled LH power densities varied between 25 MW m-2 and 50 MW m-2. The high-n|| part of the LH spectrum was found to be absorbed near the edge within a few millimetres. The absorbed power density varied from 65 kW m-2 to 420 kW m-2 which corresponds to 0.2-0.8% of the coupled power. In an edge plasma having a temperature of 25 eV, the maximum energies of the fast electrons generated by the parasitic absorption were between 0.4 keV and 1.6 keV.

Parasitic Particle Acceleration and Rf Power Absorption in Edge Plasmas

Understanding edge plasma phenomena in the presence of radio-frequency heating is crucial to constructing launchers that can survive long-pulse or steady-state high-power operation. In some experiments, significant fractions of lost power for ion cyclotron range-of-frequency (ICRF) antennas and for lower hybrid (LH) grills have been observed with adverse effects like impurity generation and hot spots. Theoretical aspects of parasitic power absorption in the edge plasma for both high-power ICRF and LH launching are reviewed.

Momentum diffusion in current drive by electron plasma waves

Fast electron acceleration and velocity diffusion are investigated in wavepackets formed by large-amplitude waves with relativistic phase velocities. In tokamaks, such waves are generated, e.g. in current drive by free-electron lasers where large-amplitude electron plasma waves or lower hybrid waves are generated. At small field intensities, the velocity diffusion in wavepackets with Gaussian-like envelopes is found to be well described by the quasilinear approximation. At large field intensities, trapping effects become important, and deviations from quasilinear diffusion are found. It is demonstrated that these deviations may have a significant effect on the velocity distribution of the electrons. A wave-induced runaway-like phenomenon of relativistic electrons is discussed.

Electromagnetic particle-in-cell simulations of a lower hybrid grill

The coupling of lower hybrid waves to the plasma is a crucial issue for efficient current drive in tokamaks. In this work, the coupling problem is attacked with a new tool in this context: an electromagnetic particle-in-cell (PIC) code, XOOPIC (Verboncoeur J P et al 1995 Comp. Phys. Comm. 87 199). A model for a grill with 32 waveguides is constructed using perfectly conducting walls. The wave propagation in the waveguides and the coupling to plasma is followed. The wave–plasma interaction is studied and the evolution of the launched spectrum is resolved in the near-field of the grill. The reflection coefficients in the individual waveguides are determined.