Table of contents

The anomalously fast relaxation of the perturbations to the electron temperature profile caused by a sawtooth crash has been studied extensively on TFTR. It is shown that on a short time-scale the heat pulse is not simply diffusive as has been generally assumed, but that modelling of the heat pulse requires a transient enhancement in χ_e following the sawtooth crash. It is also shown that the time dependent enhancement in χ_e predicted by non-linear thermal transport models, i.e. incremental chi models or the Rebut-Lallia-Watkins transport model, is much smaller than that required to explain the anomalies in the heat pulse propagation

An experimental study of the generation of runaway electrons in TEXTOR has been performed. From the infrared synchrotron radiation emitted by relativistic electrons, the number of runaway electrons can be obtained as a function of time. In low density discharges (n_e < 1 × 10¹⁹ cm^-3) runaways are created throughout the discharge and not predominantly in-the startup phase, From the exponential increase in the runaway population and the ongoing runaway production after the density is increased, it is concluded that the secondary generation, i.e. the creation of runaways through close collisions of already existing runaways with thermal electrons, provides an essential contribution to the runaway production. The effective avalanche time of this secondary process is determined to be t_eff = 0.9 ± 0.2 s

A distinct poloidal asymmetry is observed in the response of the density fluctuations at the H mode transition on the CCT tokamak. The fluctuations are located on the same density layer and are monitored by using reflectometers at four poloidal positions (θ = 0°, 90°, 180°, 270°). On the outboard midplane the fluctuations decrease significantly at the transition, while those on the inboard midplane generally show little or no reduction. The fluctuations located at the top and bottom of the tokamak exhibit an intermediate response. All poloidal positions show different time histories even for the same shot. These observations may indicate a poloidal symmetrization of the fluctuations at the H mode transition. A poloidal variation in the E × B velocity induced Doppler shift is also observed, indicating a poloidally asymmetric E × B velocity profile. The significance of these observations is discussed, and comparison is made with the transition properties on the larger neutral beam heated DIII-D tokamak

For a low frequency drift wave helical structure to be embedded in a strongly inhomogeneous plasma edge, a self-consistent radial electric field must have built up such that δ(ω_E + ω*_e)/δx ≡ 0 (ω ≡ ω_E + ω*_E is the drift mode angular frequency, where ω_E is the E × B Doppler shift and ω*_e is the electron diamagnetic frequency). Further, at marginal stability, the destabilizing action of the wave through neutrals ionization must be balanced by electron Landau damping (rather than collisional damping, as the latter would lead to a thermally unstable equilibrium constraint). These two conditions imply certain relationships between the profiles which are shown to be apparently satisfied in DIII-D (this experiment affording a high spatial resolution). The marginal stability condition, in which Landau damping is proportional to the toroidal mode number (l), suggests that only the l=1 mode may persist in the equilibrium: this is the case with the Toi mode observed in the low confinement regime of the ASDEX tokamak. The marginal stability condition closes the system of quasi-linear cross-field transport equations, in which the square of the modulus of the eigenfunction is-by comparison with neoclassical transport theory - an extra unknown entering the expressions of the anomalous particle and heat fluxes. The quasi-linear particle flux, however, is directed inward: quasi-linear theory, therefore, is not compatible with a stationary equilibrium edge model in which ionization of the neutrals produced by recycling would trigger the anomalous return flow of ionized particles to the material structure. In the plasma edge, quasi-linear cross-field transport theory is inappropriate if 'open' field lines-impinging on the target plates - penetrate the plasma to a depth of a few centimetres, as will be the case if the mode develops a sufficiently broad magnetic island

For pt.I, see ibid, p.1799-1816 (1993). To the ionization instability described in Part I correspond odd ϕ, even b_r eigenfunctions leading, as for the tearing mode, to a magnetic island belt centred about the rational magnetic surface q = m < q_a (q is the safety factor; m is the mode number). Plasma dumping on the target plates, along the island magnetic field lines, releases the neutrals, the ionization of which drives the instability. This self-consistent model of the plasma edge yields the electron temperature on the last closed equilibrium magnetic surface and the particle confinement time, which are compared with the values measured in TEXTOR and other tokamaks; interestingly, the value obtained for τ_p is very reminiscent of the heuristic energy confinement time expression proposed by Kaye and Goldston(1985). Theory also predicts an equilibrium bifurcation at high power, corresponding to a reduction, and then a collapse, of the island width. The hypothesis that the (L mode) island belt be hooked up to the machine's structure is briefly discussed

Enhancement of lower hybrid current drive in the presence of wave converted ion-ion slow modes is discussed using a one-dimensional quasi-linear equation. The special case in which the slow mode resulting from conversion in a deuterium-hydrogen mixture is absorbed by low energy electrons is considered in detail. In this scheme, the effect of the ion-ion modes is to increase the electron tail density, and the current enhancement is given by the ratio of the parallel velocity distributions with and without mode converted slow modes, calculated at the lower p// limit of the lower hybrid (LH) tail. The method of synergy is suited for remote control of the LH fast tail population, and is useful for current drive optimization. The computation also gives some insight into current drive by ion-ion slow modes in the absence of LH waves, a situation of relevance in a high temperature plasma where LH accessibility is severely reduced

Measurements of the toroidal magnetic field inside the JET tokamak plasma by means of the motional Stark effect are reported. In the vicinity of the resonance layer ion cyclotron resonance heating (ICRH) generates a population of energetic ions. For the first time, the diamagnetic change in the toroidal magnetic field due to these ICRH fast ions in combination with a pressure increase of the thermal population has been measured. A magnetic field decrease of up to 4% has been observed. Oscillations of the toroidal magnetic field caused by sawtooth oscillations of the thermal plasma pressure have been resolved. First results of the local β increase due to ICRH and the slowing-down time of the fast particles are presented.

The observed characteristics of toroidicity induced Alfven eigenmodes (TAE) in DIII-D tokamak discharges are compared in detail with predictions of various theories of TAE mode stability, and good qualitative agreement is found. The observed range of unstable toroidal mode numbers (n approximately 3-6) is consistent with theoretical predictions. For DIII-D parameters, low mode numbers are damped by coupling to the stable Alfven continuum, while high mode numbers are clamped by electron kinetic effects including coupling to kinetic Alfven waves. A threshold for destabilization is observed experimentally at a fast ion beta of approximately 1%. The predicted driving and damping rates, estimated from experimental data, balance within about a factor of two for discharges at the threshold. It is demonstrated experimentally that the damping of TAE modes can be increased by current profile control, in this case with a peaked current profile produced by a negative current ramp, in qualitative agreement with theoretical predictions. A possible stabilizing effect of discharge elongation is also observed

The effect of quasi-linear diffusion of alpha particles in an external high frequency (HF) field on the continuum Alfven wave instability driven by the pressure gradient of the alpha particles is investigated theoretically in the limit of weak 'collisions' (ν_QL < ω_A < ω_bα), where ν_QL, = r/R, ω_A and ω_bα are the characteristic frequency of the quasi-linear process, inverse aspect ratio of the tokamak, Alfven frequency and alpha particle bounce frequency, respectively. It is shown that in this limit efficient instability suppression is possible. The HF electric field strength needed for stabilization is available from modem HF power sources

High frequency pressure driven modes have been observed in high poloidal beta discharges in the Princeton Beta Experiment Modification (PBX-M). These modes are excited in a non-axisymmetric equilibrium characterized by a large, low frequency m_t = 1/n_t = 1 island, and they are capable of expelling fast ions. The modes reside on or very close to the q = 1 surface and have mode numbers with either m_h = n_h or (less probably) m_h/n_h = m_h/(m_h-1), with m_h varying between 3 and 10. Occasionally these modes are simultaneously localized in the vicinity of the m_l = 2/n_l = 1 island. The high frequency modes near the q = 1 surface also exhibit a ballooning character, being significantly stronger on the large major radius side of the plasma. When a large m_t = 1/n_t = 1 island is present, the mode is poloidally localized in the immediate vicinity of the X point of the island. The modes occur exclusively in high beta beam heated discharges and are likely to be driven by the beam ions. They can thus be a manifestation of either a toroidicity induced shear Alfven eigenmode (TAE) at q = (2m_h+1)/2n_h, a kinetic ballooning mode, or some other type of pressure driven (high β) mode. Most of the data are consistent with the theoretical predictions for the TAE gap mode. Since the high frequency modes in PBX-M, however, are found exclusively on or in the immediate neighbourhood of magnetic surfaces with low rational numbers (q = 1, 2,...), other possibilities are not excluded

The local ideal three dimensional (3-D) magnetohydrodynamic (MHD) stability for the Wendelstein VII-X (W VIl-X) configuration is studied. A volume averaged beta limit of 5% is confirmed with a nearly optimal pressure profile using two methods to calculate the parallel current density: the magnetic method that uses magnetic information about the configuration (in particular, the condition of charge conservation, ∇.j = 0, is explicitly used in the resolution) and the geometric method that uses the geometry of the configuration itself. It is shown that the ballooning stability does not depend on the method of calculating the parallel current. In contrast, the value of the Mercier criterion depends sensitively on which method is used. Not only is the geometric method not sensitive to resonant surfaces (in particular, the surface ι_p = 1/6), but there is a systematic error in the Mercier criterion for nonresonant surfaces when an insufficient number of modes is used to calculate the equilibria numerically with a spectral method. However, this systematic error does not change the averaged beta limit of W VIII-X because the ballooning stability is more stringent than the Mercier stability for this configuration

Report on the Third IAEA Technical Committee Meeting on Alpha Particle Physics in Thermonuclear Reactors held at Trieste, Italy 10-14 May 1993.

Corrections to Nuclear Fusion32 (1992) 611.