Table of contents

The two-dimensional flow pattern of the ablation cloud surrounding a solid pellet embedded in a magnetized plasma is analysed. In the reference frame of the pellet, the magnetic field confines the ablation cloud in the transverse direction. Transverse force balance then determines the width of the parallel to flow channel. The ablation flow is assumed to have spatially uniform temperature and electrical conductivity σ, with magnitudes prescribed by their values at the sonic radius r_*, near the pellet where the flow is essentially radial. It is found that the transverse half-width of the cloud — the stagnation radius r_b — depends on two key dimensionless parameters: N_* = σ_*, r_* B_∞² /ρ_* ν_* and p_∞/p_*, where B_∞ is the magnetic field strength in the background plasma. For most representative cases, the cloud width is 10–20 pellet diameters. The implications of these results for pellet ablation studies and studies oriented towards the goal of using (impurity) pellets for fusion plasma diagnostic purposes are discussed.

Anomalous ion temperature and plasma resistance have been studied in reversed Held pinch discharges of the REPUTE-1 device. The ion temperature measured by Doppler broadening of carbon V (2271 Å) has been observed to be anomalously high in hydrogen discharges with low electron density (_e ≤ 0.5 × 10¹⁴ cm⁻³); also the plasma resistance is anomalously large in these discharges. The ion temperature and the plasma resistance are shown to increase with the MHD fluctuation level. The profiles of ion temperature and soft X-ray emission have been found to be more peaked in the lower electron density regime than in the higher one. A high ion temperature and a large plasma resistance are observed simultaneously when the profile of the soft X-ray emission Isx is more peaked. If I_sx is mainly affected by the electron temperature profile(I_sx ∝ n_e²T_e^3.5),the experimental observations are in qualitative agreement with a calculation showing that the heating power of ions is larger when the electron temperature has a more peaked profile in a magnetic configuration.

A simple linear diffusive/convective model is proposed for helium transport in the edge plasma of a tokamak reactor. Kinetic effects related to weak collisionality of the plasma are assessed. Results of two-dimensional numerical modelling of helium transport in the edge plasma are presented. Issues of ITER divertor optimization with respect to helium exhaust are considered, with a realistic geometry of the divertor plates taken into account.

Electrostatic turbulence has been investigated in the edge region of the Advanced Toroidal Facility (ATF). A reversal in the poloidal phase velocity of the fluctuations has been observed (velocity shear) which determines a characteristic plasma radius. The location of this shear layer depends on the magnetic configuration, the limiter radius and the plasma conditions. Using the shear position as a reference point, the density fluctuation levels in ATF (currentless stellarator) are very similar to those previously reported in TEXT (ohmically heated tokamak), suggesting that the plasma current is not an important drive for the edge turbulence. The drives for the turbulence appear to be different inside and outside the shear location (a_shear), with _e/T_e < ñ/n in the scrape-off layer (r/a_shear > 1) and possibly larger _e/T_e in the plasma edge edge (r/a_shear < 1). There is a spatial decorrelation in the fluctuations at the shear location; this suggests that the poloidal shear flow has an important influence on the edge turbulence. The poloidal correlation length depends on local plasma parameters (e.g. velocity and temperature). When neutral beam injection is added, the high frequency components of ̃n increase.

The paper considers the stability of the energy and particle balance in a tokamak plasma with non-inductive current drive. The necessary stability conditions deriving from the zero-dimensional plasma model are obtained. They are used to study the stability of thermonuclear burn in a tokamak where the current consists of the electron and alpha particle bootstrap components. The stability of the bootstrap current in a thermonuclear plasma is analysed using a one-dimensional model.

Confinement properties of lower hybrid current driven (LHCD) plasmas and electron cyclotron heated (ECH) plasmas in the WT-3 tokamak are studied. The outward particle and energy fluxes are estimated from parameters of the scrape-off layer (SOL) plasma. During the LHCD period the density and temperature of the SOL plasma decrease drastically, while the density of the core plasma increases, suggesting a decrease of the outward flux. These results show that particle confinement is improved by LHCD. On the contrary, during ECH the density of the SOL plasma increases and the density of the core plasma decreases, suggesting a deterioration of particle confinement. Improved confinement is not obtained near the density limit of LHCD; the improvement is not related with MHD activities during LHCD.

Recent advances in electrostatic particle analysers enable a detailed analysis of impurity and fuel ions in the tokamak boundary. The paper presents measurements of impurity content, charge state mix, ion temperature, ion energy distribution functions, sheath voltage and secondary electron yields obtained in the DITE tokamak. The consequences of these results for the understanding of impurity production processes are discussed.

Applied error fields raise the H-mode power threshold, but do not significantly affect energy confinement in the JFT-2M tokamak. Magnetic perturbations of magnitude _n.m/B_T ≈ (2-5) × 10⁻⁴ resonant near the plasma edge, raise the H-mode power threshold from 350 kW to 550 kW of neutral beam injection power. The global energy confinement is not affected once the H-mode is achieved. These measurements give an indication of the accuracy to which tokamak coils should be designed and maintained in order to ensure optimal access to the H-mode.

The enhancement of the D-³He fusion yield by combined ³He beam injection and higher harmonic ICRF heating is investigated on the basis of the local Fokker-Planck calculation. With the fourth harmonic heating of 100 keV ³He beams the fusion yield is enhanced most efficiently when the ratio of the beam power to the radiofrequency power is 1/5. The maximum of the fusion power multiplication factor is roughly given by ΔQ_max ∼ 0.043(T_e[10 keV])(n_e [10²⁰ m⁻³])^1.5. A comparison is made with the case of fundamental minority heating of ³He beams and the case of injection of 500 keV D beams.

For high Q operation in feedback controlled subignited tokamak fusion plasmas, the operating temperature must be close to the ignition temperature. The authors discuss the technological and physical effects which may restrict this temperature difference. The investigation is based on a simplified, yet accurate, 0-D analytical analysis of the maximum Q of a subignited system. Particular emphasis is put on sawtooth oscillations which complicate the interpretation of diagnostic neutron emission data for determining the plasma temperatures and may imply an inherent lower bound on the deviation of the temperature from the ignition point. For a tokamak fusion reactor, the estimated maximum Q is found to be marginal (Q = 10-20).

The resistance anomaly in ultra-low q (surface safety factor q(a) < 1) discharges has been studied experimentally. The effective resistance (loop voltage/plasma current) is much higher than the kinematic resistance calculated with the electron temperature. The effective resistance is inversely proportional to the applied toroidal magnetic field. This scaling applies universally to devices with different dimensions.

The authors propose a scaling of the sawtooth repetition time for Ohmic discharges. This scaling has been obtained from simulations with reduced magnetohydrodynamic equations together with a simple profile consistent transport model. The choice of a profile consistent perpendicular thermal diffusion coefficient allows the scaling of the sawtooth repetition time to be obtained in terms of the diffusion coefficients and the safety factor at the edge, thus generalizing the scaling given by Vlad and Bondeson (Nucl. Fusion 29 (1989) 1139). The scaling is in good agreement with the experimental results for Ohmic discharges in the Frascati Tokamak.

The effect of the contribution of zero frequency (stationary) energetic trapped particles on magnetohydrodynamic modes and the resonant excitation of these particles is studied on the basis of the dispersion relation which is applicable to both the internal kink mode and the ballooning mode in tokamaks. The stability window is found to be significantly enlarged by the steady contribution of energetic trapped particles.

Sawtooth induced heat and density pulse measurements reported in the literature for the JET and TEXT experiments are discussed. In JET the heat pulse travels ten times faster than the density pulse, but in TEXT both pulses travel at the same speed. The measurements are analysed using coupled transport equations for energy and particles. It is shown that the different behaviour of the density pulse in the two experiments can be attributed to differences in the off-diagonal elements of the transport matrix. If the perturbed fluxes of heat and particles are expressed as linear combinations of the thermodynamic forces ∇p and ∇T (rather than ∇n and ∇T), the corresponding transport matrices are remarkably similar. However, minor differences in this transport matrix between JET and TEXT account for the qualitative difference in the density pulses.

Field line tracing is used to study the effects of poloidal field coil irregularities on the diverted field lines of a typical single-null divertor in the DIII-D tokamak. Poloidal and toroidal asymmetries result from the field errors with a radial layer Δr ≈ 1 cm of otherwise closed flux surfaces inside the separatrix connected to the divertor 'floor'.

Experimental observations of the amplitude and frequency of low frequency magnetic field and soft X-ray fluctuations in the reversed field pinch (RFP) ZT-40M, which has toroidal and poloidal gaps in its stabilizing shell, have shown that for high values of the pinch parameter these fluctuations can be strongly affected by the configuration of the toroidal field circuit. Parallel connection of coil sections improves the toroidal flux uniformity, which results in a reduction of the fluctuation level, the toroidal loop voltage and the implied energy losses compared with those of coils connected in a series or series-parallel configuration. These results indicate that the operation of RFP experiments with shell gaps and field coils well removed from the plasma can be further improved by the addition of close fitting, parallel connected, low current coils.

In a recent paper, primarily concerned with comparing the so-called gyroviscous theory with experimental data on JET, Stacey also reviewed the theoretical basis for the ad hoc assumptions underlying the theory and commented on the criticisms of Connor et al. He mainly reiterated arguments discussed at some length previously, but a particular point was emphasized, namely a claim for support of the theory from a paper by Hsu and Sigmar. The purpose of this comment is to address this new claim, not to repeat points adequately covered in Ref. [2]

In a recent paper, I cited the work of Hsu and Sigmar as an illustration that the choice of collisionality ordering can affect the results of a calculation of poloidal density and velocity asymmetries. In their Comment, Connor et al. argue that the Hsu-Sigmar strong collisionality regime (Δ ∼ 1) is irrelevant for JET and that, even if it were relevant, _I ∼ O() would not lead to significant gyroviscous momentum damping because V_θ ∼ O(δV_th).

The 1990 International Symposium on Heavy Ion Inertial Fusion (HIF) was the fifth in a series, following meetings in Darmstadt (1982), Tokyo (1984), Washington, DC (1986) and Darmstadt (1988). The meeting was sponsored by the American Physical Society and the United States Department of Energy (USDOE). It was hosted by the Lawrence Berkeley Laboratory (LBL). The Proceedings of the symposium will be published as a refereed special issue of Particle Accelerators, edited by D. Judd (LBL).