Table of contents

There is a large region of density-temperature space in which the effects of a magnetic field on heat transport and alpha-particle mobility are significant and the magnetic pressure is small compared with the pressure of a deuterium-tritium plasma. Spherical fusion burn in this regime is examined. It is found that for volume burn, magnetic fields can greatly increase the yield. In regimes where propagating burn does not occur, the burn can be enhanced by a magnetic field. In regimes where propagating deflagration would normally occur in the absence of a magnetic field, magnetic fields actually degrade the cross-field propagation. A detonation wave is harder to ignite in the presence of a magnetic field. Once a detonation wave is ignited, no change in the propagation speed is produced by applying a magnetic field.

Previous studies of Alfven wave heating which employed kinetic theory are extended in order to take into account antenna configurations, in cylindrical geometry, consisting of arbitrary shell currents and their associated radial feeders. For each Fourier component of the form exp [-i(lζ-mθ)], the shell current consists of a divergence-free part having the helicity of the mode (l, m), plus an orthogonal part requiring the feeders. It is shown, both analytically and by including the full current in the numerical code, that only the divergence-free part of the current contributes significantly to the plasma response and antenna loading. The important effect of the feeders is to cancel the contribution from the surface current perpendicular to the helicity. This explicitly verifies results reported previously.

In a study of the structure of propagation and absorption of waves in the ion cyclotron range of frequencies (ICRF) in stellarator/heliotron devices, the full Maxwell equation, formulated as a stationary boundary-value problem, is solved numerically in a straight helical configuration. A cold plasma approximation is employed to obtain the conductivity tensor. The shapes of plasma boundary, vessel wall and antenna can be chosen arbitrarily by using a finite-element method. Two-ion hybrid resonance heating by the fast wave is studied. The wave field solution shows tunnelling across the evanescent layer, formation of the cavity resonance and absorption near the hybrid resonance surface. The dependence of the loading resistance on the axial wave number shows a shift due to the helical pitch. The dependence of the loading resistance and the deposition profile on the plasma parameters is also studied. In Heliotron-E-grade plasmas, the fast wave can propagate into the central region of the plasma and the antenna loading resistance is as large as it is in tokamaks. A model of the modular torsatron ATF is studied and good coupling of the fast wave to the plasma is also predicted in this device.

An expansion method is developed for long-wavelength heliac and stellarator equilibrium. Time-dependent equations of motion are solved numerically to give equilibria which can include magnetic islands and stochastic field regions. Analytic low-beta equilibria are found which are in good agreement with computations.

The observation of H_α-radiation extending along the magnetic field lines during pellet injection in ASDEX suggests a renewed discussion of ablation models and, in particular, an investigation of the additional shielding of the pellet by the high-density, medium-temperature plasma generated by the ablated material. In the present paper the authors have simulated the frozen-in gas flowing along the field lines with a 1-D hydrodynamic code, coupled to the neutral-gas shielding model describing the ablation. To take into account long-mean-free-path effects on the electron heat transport in the plasma, it was necessary to replace the classical Spitzer-Harm conductivity by a non-local transport model. – A plasma hose with a density of up to several hundred times that of the background plasma and extending over several metres was found to form along the field lines. As the temperature of this plasma is much lower than that of the background one (in a computation for ASDEX it dropped from 500 to approximately 40 eV), this effect should lead to a distinct decrease in the resulting ablation rate.

It is shown that electron cyclotron emission can be used for detailed diagnosis of mildly relativistic electron distribution functions, provided the plasma can be viewed along a line of constant magnetic field. Calculations of the emissivity are performed, both for tenuous and finite-background-density plasmas whose results allow observations of emission to be deconvolved in terms of the density and anisotropy of the distribution as a function of total electron energy. For prolate distributions different harmonics have to be measured, while for oblate distributions different polarizations have to be measured. Absorption can also be treated using these results. Some of the issues concerning experimental application are discussed.

Turbulent transport due to the ion pressure gradient (or temperature drift) instability is thought to be significant when η_i= d(ln T_i)/d(ln n) > 1. The invariance properties of the governing equations under scale transformations are used to discuss the characteristics of this turbulence. This approach not only clarifies the relationships between earlier treatments but also, in certain limits, completely determines the scaling properties of the fluctuations and the consequent thermal transport.

Results obtained from experiments in PDX with neutral beam injection show the emission of several harmonics of a frequency centred in the ion cyclotron frequency range. The structure of the magnetic signals registered is suggestive of an electromagnetic ion cyclotron type instability. A theory is developed for explaining the generation of neutral-beam-driven ion cyclotron instability (ion Bernstein mode) and the results are compared with the experimental data. An increase in beam density towards the outside of the plasma is possible if the beam ion population in that region increases owing to either counter-injection orbits or 'fishbone' effects. A calculation is made of the threshold density necessary to drive such an instability in a plasma with anisotropic beam ion distribution, and the effects due to the curvature drift of hot ions are included. The quasi-linear evolution of the beam is studied in order to understand possible beam isotropization and spreading in velocity space.

Over the past three years, extensive energy confinement experiments have been performed on the Doublet III tokamak with up to 8 MW of both hydrogen and deuterium neutral beam heating power. These experiments covered a wide range of plasma parameters and almost a continuum of magnetic configurations from limited to fully diverted. It has been found that the global energy confinement time is linearly proportional to the plasma current and decreases with the application of neutral beam power for all configurations and beam species studied. The majority of our confinement experiments were conducted with discharges limited toward the outside of the vacuum vessel and heated with hydrogen neutral beams. With respect to this standard operating mode, several operating regimes were found with improved values of energy confinement time. Generally, deuterium neutral beam heating is superior to hydrogen beam heating and divertor operation yields superior confinement as compared to limiter operation. Our experiments also indicate that discharges limited toward the inside of the vacuum vessel exhibit somewhat better confinement relative to discharges limited toward the outside. Accordingly, the highest energy confinement values are obtained in high current, well diverted, deuterium beam heated discharges. Deuterium beam heated discharges limited toward the inside yield normalized energy confinement values similar to those from hydrogen beam heated divertor discharges.

An analytic expression is derived in which the current profile shape in tokamaks is kept constant during the current buildup phase. The required conductivity profile is parametrized by two externally controllable parameters, İ_p and _p, in the case of a Gaussian current profile. It is shown that a Gaussian current profile can be maintained, even under the condition of high İ_p, for a realistically broad conductivity profile by using the constant q_a current buildup method.

The power from 60 GHz gyrotrons was squarewave modulated to produce periodic temperature perturbations in a Doublet III discharge. The modulated component of the resulting soft-X-ray signals was isolated by digital processing and synchronous detection using the modulation as reference. The phase shift of the signals as a function of radius agrees well with a model calculation based on the heat balance equation, allowing thermal diffusivity to be determined and demonstrating the efficacy of this technique.

Helicon is a new helical-axis configuration of the Heliac type in which the number of coils is minimized by replacing the numerous toroidal field coils by a single ℓ = 1 helical coil. Helicon has flexibility and potential high-beta properties similar to those of the Flexible Heliac as well as some of the characteristics of an ℓ = 1 torsatron. Devices with four, eight and twelve periods are discussed.

Compression of low-temperature spheromak plasmas has been studied with the aid of a zero-dimensional two-fluid computer code. The major direct effect of compression is to raise the current and plasma densities, keeping their ratios constant. Above critical pre-compression values of the electron temperature, T_e, and the product of electron density and particle confinement time, n_eτ_p, the rising plasma density shifts the impurity ions to higher ionization states, with correspondingly lower radiative losses. High post-compression temperatures may thus be achieved. In oxygen radiation-dominated plasmas with pre-compression values above the critical ones, and with constant τ_p, the electron temperature can be increased by up to a factor of seven for a compression of a factor of two. This can be compared with a factor-of-four temperature rise expected for adiabatic compression. If the energy balance is dominated by particle confinement losses rather than radiation losses, the effect of compression is to raise the temperature as T_e ∼ C^6/5, for constant τ_p.