Table of contents

The paper discusses various possibilities for reducing neoclassical transport in stellarator systems. Attention is drawn in particular to the possibility of achieving a very large reduction of transport in the low-collisionality region by decreasing the effective depth of the local magnetic wells.

The paper describes a successful proof-of-principle experimental determination of tokamak ion temperature using cw far-infrared (FIR) collective laser scattering from externally excited ion Bernstein waves. It is shown that a viable wave excitation technique for tokamak plasmas is mode conversion of an externally launched fast Alfvén wave. A fit of the experimentally determined ion Bernstein wave dispersion to the temperature-dependent theoretical dispersion yields the local ion temperature. Partial ion temperature profiles (chord-averaged) have been obtained with temperature values consistent with charge-exchange measurements.

Plasma-induced arcs and their ignition conditions have been studied in the divertor chamber of the ASDEX tokamak by introducing arc probes and Langmuir probes in front of the divertor plates. Arcs are rare events and are always correlated with unstable phases of the plasma, such as at disruptions or during neutral-beam injection. Arcs require high ignition potentials (100–500 V) and burning voltages of the order of 100 V. In addition, an internal limitation of the rate of current rise to values of about 10⁷ A·s⁻¹ has been observed. The probability of arcing during short transient voltages is thus reduced. Arcs are generally preceded by diffuse discharges carrying currents of a few amperes. Since diffuse discharges occur more frequently than arcing events, a significant contribution of diffuse discharges to surface erosion is suspected.

Integral relations for the average poloidal beta _p and the plasma internal inductance ℓ_i are derived from the magnetohydrodynamic (MHD) equilibrium equation for an axisymmetric torus. The volume-dependent parameters that appear depend only weakly on the actual current density distribution inside the plasma and can be evaluated approximately, given the plasma shape and boundary poloidal magnetic field. In practice, these can be accurately and efficiently obtained for both diverted and limited plasmas from measured external poloidal magnetic field and flux values by approximating the plasma current distribution using a few filaments or distributed sources. For a tokamak plasma with a non-circular cross-section of sufficient elongation, _p and ℓ_i can then be approximately determined separately. This is demonstrated for analytic equilibria of known shape as well as for actual Doublet III (D-III) plasmas for which _p and ℓ_i have been determined by using other methods. Results of a sensitivity study are described.

The empirical scaling of the electron thermal diffusivity, χ_e, is investigated for more than 100 beam-heated discharges. These discharges include two major features: 1) high-temperature and high-density plasma (T_e(0)≈T_i(0)≈2.5–3 keV at _e≈(5–7) × 10¹³ cm⁻³, P_NBI ⪅ 4 MW), which will be the basis for the breakeven experiments in the next-generation tokamaks, 2) three types of discharges, i.e. good and poor confinement divertor discharges and limiter discharges. – All kinds of discharges (good heating and poor heating divertor discharges, limiter discharges) have fhe same functional form in χ_e within ∼ 40% at 0.25 a < r < 0.65 a, where χ_e in the simplest expression scales as χ_e ∝ 1/n_e. The ion thermal diffusivity, χ_i, is consistent with the assumption that the neoclassical χ_i can be applicable to our three kinds of discharges. For discharges with different heating efficiency, there is no systematic difference, in the adjustable multiplier, to the neoclassical theory by Hinton-Hazeltine for an ion collisionality of ν_i* = 0.02–0.5.

The effect of pressure anisotropy on the stability of ideal-MHD ballooning modes is studied for a large-aspect-ratio tokamak of circular cross-section. Significant increase of the perpendicular energy of electrons or ions can cause strong modifications of the stability boundary, especially at low shear.

An approximate solution to the Fokker-Planck equation for the alpha-particle distribution function in an open-field-line fusion plasma with electrostatic confining potential is presented. Particle and energy pitch-angle scattering losses are computed. The electrostatic confining potential is found to substantially reduce particle and energy losses.

Ion neoclassical transport near the separatrix of a divertor tokamak is investigated. In the high-collisionality regime, large ion temperature gradients may develop with single-null operation if the ion grad-B drift is toward the x-point, and if the power is comparable to a 'critical' value. This may explain the observed dependence, of the power threshold for the H-mode in ASDEX, on the number and location of the x-points. In the low-collisionality regime, achieved experimentally with higher heating power, the ion temperature gradient is large and does not depend on x-point number or location.

Equilibrium and stability results are presented for the Advanced Toroidal Facility (ATF) device. The results of three-dimensional equilibrium calculations and free-boundary average method calculations are shown to be in good agreement with previous fixed-boundary average method results. These favourable comparisons serve as a valuable validation of the simple and computationally efficient fixed-boundary average method. Stability calculations for the free-boundary average method equilibria are also in good agreement with fixed-boundary calculations, showing instability only when the plasma is shifted inwards with an applied vertical field.

A centrifuge injector that repetitively fires 1.3 mm deuterium pellets (1 torr⋅L per pellet) at a rate of 32 pellets per second was used to build up and maintain a Doublet III 2.4 MW neutral-beam-heated limiter discharge at a line-averaged density of 1 × 10¹⁴ cm⁻³. When compared to a conventional gas-fuelled plasma at similar density, the pellet-fuelled plasma was characterized by a factor-of-three reduction in edge neutral density and limiter recycling, a centrally peaked profile, a 70% increase in global energy confinement, and a tenfold increase in the fusion reaction rate.

Nitrogen has been puffed into the Impurity Study Experiment (ISX-B) tokamak as a test gas for evaluating the effect of pump limiters on impurity control. The gas is introduced either through a nozzle at the limiter tip or from a location at the wall of the vacuum chamber. The interior nitrogen content is monitored by charge-exchange excitation of N VII lines and depends only slightly on the location of the N₂ source or on whether the pumps are activated or not. The edge radiation from NIV is also affected little by activating the pumps when the source is at the limiter tip, but is strongly influenced when the source is at the wall location. The spectral data suggest that the reservoir of nitrogen in the scrape-off layer is efficiently pumped while the nitrogen content of the main plasma is not affected. The implication is that, in the quasi-steady interval of a discharge, radial diffusion is very slow compared to parallel transport in the vicinity of the limiter tip.

The first experimental results obtained on plasma confined in a toroidal heliac are reported. A simple method of generating highly ionized, weakly collisional plasma is described. It is found that the geometry of the plasma in general and the measured pressure profiles in particular conform closely with the calculated helical axis magnetic surfaces. Preliminary indications of plasma confinement are favourable.

The H-mode develops in conditions of low and controlled recycling. The electron and ion collisionality parameters at the plasma periphery are close to, but above, one in the L-phase and drop below one in the H-phase. The realization of the H-phase is easier in the single-null than in the double-null configuration which has higher power and density thresholds. The increased H-mode accessibility of the single-null configuration requires the ion grad B-drift direction to be towards the stagnation point. The experimental observations are in agreement with a recent theory of the H-transition by Hinton.

For the first time, the feasibility has been demonstrated of measuring local ion temperatures from spectral line Doppler broadening caused by the excitation of plasma ions, charge-exchanged on an injected neutral-atom beam.

The particle and energy reflection coefficients are calculated for a plasma incident on a wall with an obliquely incident magnetic field. The salient result of these calculations is that the reflection coefficients can approach unity when the magnetic field is incident at grazing angles. This reflection of particles and energy will be an important process in determining the particle and energy balance in the edge plasma.