Table of contents

Report on the 6th IAEA Technical Committee Meeting held at Naka-machi, Naka-gun, Ibaraki, Japan 12-14 October 1999

The behaviour of runaway electrons in three types of magnetic turbulence in tokamak discharges is reviewed: (a) micromagnetic turbulence, (b) low-m/n magnetic islands in a sea of stochasticity, (c) macroscale magnetic turbulence. The confinement of runaway electrons is much better than that of bulk thermal electrons in (a) and (b), but is greatly degraded in (c). Spontaneous and intrinsic termination of runaway current, which will be favourable for tokamak fusion reactors in order to reduce the heat flux on the first wall, was first found in JT-60U by decreasing the safety factor at the plasma surface q_s to around 2 or 3 by three different methods: (i) controlled inward plasma shift, (ii) a vertical displacement event, (iii) plasma current rampup.

Issues of Alfvén mode stability in advanced tokamak regimes are addressed on the basis of recent developments in theory, computational methods and progress in experiments. The instability of toroidal Alfvén eigenmodes (TAEs) is analysed for spherical tokamaks, such as the NSTX using the NOVA-K code. Modes in the Alfvén frequency range observed in JT-60U during negative ion based NBI heating at energies E_b0 = 360 keV, and in TFTR during ICRH experiments with chirping frequency, on various timescales, are analysed using the kinetic non-perturbative code HINST, which is able to resolve new resonant branches of the toroidal Alfvén modes called resonant TAEs (RTAEs). However, as is shown the mechanisms for frequency chirping may be different in different experiments. In TFTR, frequency chirping results from the slow variation of q profile between sawteeth. In JT-60U experiments some frequency chirping modes have very short timescales, which suggests the cause is the change in fast particle distribution.

The results are summarized of studies on energetic ion transport induced by sawtooth oscillations in tokamaks. The main attention is paid to a description of the physical mechanisms responsible for this transport. In addition to an overview, new material is also presented. The new results concern the resonant interaction of the particles and the electromagnetic field of sawtooth crashes. In particular, it is discovered that the dominant harmonic of the crashes (m = n = 1) can lead to stochastic motion of particles with large orbit widths (`potato' orbits). The regular motion of potato orbits and quasi-stagnation particles in the presence of an n = 1 mode is studied, and their characteristic displacements associated with quick switching on and off of the mode are found.

Alfvén eigenmodes (AEs) with intermediate toroidal mode numbers are modelled using the global gyrokinetic PENN code to determine the stability of high performance tokamak discharges in the presence of energetic particles. A large plasma pressure and a weak magnetic shear in the core give rise to radially extended kinetic AEs, which are stabilized by the high shear at the edge of a divertor (X point) configuration. Large values for the safety factor and the ion Larmor radius in reversed shear operation may however trigger drift kinetic Alfvén eigenmode instabilities that could affect the alpha particle confinement in a reactor.

Recent results of energetic ion driven MHD instabilities observed in the heliotron/torsatron devices Compact Helical System (CHS) and Large Helical Device (LHD) are presented. Alfvén eigenmodes (AEs) and fishbone-like burst modes (FBs) destabilized by energetic ions were observed in NBI heated plasmas of CHS. The AEs are toroidicity induced Alfvén eigenmodes (TAEs) and global Alfvén eigenmodes (GAEs), where the identified toroidal mode numbers are n = 1 and 2 for TAEs and n = 0 for GAEs. The frequencies of the FBs are less than, at most, half of the minimum TAE gap frequency and do not exhibit the obvious density dependence related to Alfvén velocity. The modes have characteristic features of the energetic particle modes or the resonant TAEs excited by circulating energetic beam ions produced by NBI. Bursting amplitude modulation is observed in TAEs as well as in FBs. Rapid frequency chirping is observed in each burst, by a factor of 2-6 in FBs and about 25% in TAEs. In several shots, the power spectrum of the TAEs is split into multiple peaks having the same toroidal mode number through non-linear evolution of TAEs. A pulsed increase in energetic ion loss towards the wall is induced by m = 3/n = 2 FBs, but so far not by m = 2/n = 1 FBs, TAEs and GAEs, where m is the poloidal mode number. This research has been extended to LHD plasmas heated by neutral hydrogen beams with about 130 keV energy. Similar to CHS, TAEs and FBs were observed in relatively low density plasmas at low toroidal magnetic field (B_t = 1.5 T).

Results achieved on JET during the 1997-1999 experimental campaigns in the physics of energetic ions and runaway electrons are reviewed. Heating of deuterium-tritium (DT) plasmas by fusion born alpha particles is found to be similar to that achieved by comparable ICRF heating of deuterium plasmas. The stability of alpha particle driven Alfvén eigenmodes (AEs) in the highest fusion power ELM-free H mode discharges is shown to be consistent with the existing theoretical analysis for AEs. Direct measurements of the trapped alpha particle and knock-on deuteron distribution functions by a neutral particle analyser (NPA) are described. New ICRF heating scenarios tested in JET DT plasmas are presented in view of the possible use of the ICRF heating of ITER-like DT plasmas on route to ignition. The energetic ion pinch in the presence of toroidally asymmetric ICRF waves is studied experimentally on JET. Recent experimentally observed effects of ICRF accelerated ions on sawteeth in JET are reviewed. Detailed time and space resolved X ray images of the electron runaway beam in flight spontaneously generated by disruptions on JET are described.

Results of fast particle experiments in JT-60U are presented. The fast particles were created with ICRF heating and negative ion based neutral beam injection (N-NBI) heating. With both heating systems Alfvén eigenmodes (AEs) were excited. The AEs that were excited with ICRF during sawtooth stabilization experiments are used to extract information about the magnetic safety profile in the plasma centre. These measurements are compared with motional Stark effect measurements and with sawtooth models. The first results on the radial mode structure of TAEs as measured with an X mode reflectometer system are also given. When the N-NBI beam was injected into a sawtoothing plasma, a significant increase in sawtooth period was observed. A survey is given of the bursting, chirping and stationary frequency modes that have been found in the Alfvén range of frequencies when N-NBI was injected. Most, but not all, of these modes scale with the Alfvén velocity. One group of modes that starts in the continuum and chirps up to the toroidicity induced Alfvén eigenmode (TAE) gap in typically 200 ms can be identified as resonant TAEs, whereas another group of modes appears as short bursts (up to 20 ms) in the Alfvén continuum.

Positive radial electric fields have been created at the edge of the TEXTOR tokamak plasma using an electrode. The electric field induces a thin (δr∼1.5 cm), E × B driven layer at the edge rotating poloidally at 12-20 km/s and featuring high shear. Concomitant changes in the density and poloidal electric field fluctuations and their cross-phase in the shear layer result in suppression of radial turbulent particle transport, even at low radial electric field strength. Temperature fluctuations are reduced, resulting in diminished turbulent heat flux. As turbulent particle transport is quenched, the particle confinement time τ_p increases by a factor of 2 and the energy confinement time τ_E by 20%. Turbulent transport accounts for ∼50% of the total particle flux. Both the cross-phase and the density fluctuations are sensitive to the sign of ∇E_r.

The frequency and amplitude of edge localized instabilities in DIII-D tokamak H mode discharges are shown to depend on the discharge shape squareness. An abrupt increase in instability frequency and decreases in instability amplitude and edge pressure gradient are observed when the squareness is changed to a sufficiently large or a sufficiently small value. The observed changes in the instability frequency and amplitude correlate with a predicted loss, at high and low values of discharge shape squareness, of access in the H mode edge region to the ideal infinite toroidal mode number ballooning mode second regime of stability. When no edge region second stability regime access is predicted, the measured pressure gradient is in agreement with the predicted ballooning mode first stability regime threshold. When the second stability regime is predicted to be locally accessible, the edge region pressure gradient increases to values well above the first stability regime limit.

The electromagnetic power in the ion cyclotron frequency range radiated by a tokamak plasma with energetic ions is considered. It is found that the radiation may result from plasma fluctuations which are associated with the edge localized fast magnetoacoustic eigenmodes and strongly enhanced by interaction with the energetic ions. In this case, almost all the energy transferred from the energetic particles to the waves is emitted by the plasma and the system is marginally stable due to the presence of small wave damping. In particular, this picture is consistent with the power measurements of superthermal ion cyclotron emission in the JET preliminary tritium experiment.

Corrections to Nuclear Fusion 39 (1999) 2495

Corrections to Nuclear Fusion 39 (1999) 1133