Table of contents

Laser blow-off impurity injection in PLT yields conspicuous modifications of the soft and ultrasoft X-ray signals. the 2 omega _CE signal giving temperature fluctuations is unaffected. A transport code is used to deduce transport coefficient as functions of time and radius. The significance of the results for MHD behaviour is discussed.

A linearized MHD system with a resistive wall has been studied. The stabilizing effect of a resistive wall against MHD instabilities has been estimated. Plasma rotation makes a resistive wall act as a conductive wall for helical perturbations.

A force-free magnetic field satisfies a relation Del *B= alpha B, where alpha is assumed a constant in space. For a surface current cylindrical main plasma confined by force-free fields, with tenuous plasma including small resistivity, the stability is investigated in detail numerically by evaluating the stable region and the growth rate of the unstable mode for parameters k, m, R₀/r₀, alpha r₀, beta and ( mu ₀r₀)^-1 from dispersion relations where k is a real wave number, m is an integer, r₀ and R₀ are the radii of the main plasma and the conducting wall, respectively, and beta and mu ₀ are the plasma beta and reciprocal pitch on the main plasma surface, respectively. For negative ( mu ₀r₀)^-1, positive k and m=1, the stable region is expanded by introducing the force-free magnetic field. The marginal stability curve of the main plasma with the force-free field approaches the curve for the vacuum field with the wall of R₀/r₀=1 as alpha r₀ decreases from 0 to a minimum value. To stabilize the unstable mode by the force-free field and wall, the values of beta and ( mu ₀r₀)^-1 must be chosen for the mode to vanish at k=- mu ₀. The instability for the positive ( mu ₀r₀)^-1 can be sufficiently suppressed by using the wall of R₀/r₀ approximately 1.5.

The stability against resistive tearing and interchange modes for an inviscid incompressible plasma confined by a resistive rigid wall is studied. The limiting case of a non conducting rigid wall is also examined in some detail. Results for equilibrium configurations of the reversed field pinch type are presented for both cases. In particular the high beta pitch and pressure model and experimental profiles for the Eta Beta II RFP are considered and the results are compared with the classical (i.e. infinite conducting wall) case.

Inverse bremsstrahlung has been incorporated into an analytical model of the expanding corona of a laser-irradiated spherical target. Absorption decreases slowly with increasing intensity, in agreement with some numerical simulations, and contrary to estimates from simple models in use up to now, which are optimistic at low values of intensity and very pessimistic at high values. Present results agree well with experimental data from many laboratories; substantial absorption (>0.5) is found up to moderate intensities, say below 10¹⁵ W cm^-2 for 1.06 mu m light. Anomalous absorption, when included in the analysis, leaves practically unaffected the ablation pressure and mass ablation rate, for given absorbed intensity. Universal results are given in dimensionless form.

Conditions for the existence of ion-acoustic solitons have been derived and studied for a warm multi-ion component plasma with two electron temperatures. The models: (a) reductive perturbation method (using KdV equation), (b) warm ion fluid theory based on a method by Sakanaka (1972) and (c) kinetic theory approach have been compared.

The equations describing short wavelength ballooning modes in a Tokamak are derived by retaining hot species finite Larmor radius effects and permitting the response of the hot populations to be evaluated for frequencies on the order of and below the hot particle magnetic drift frequency. The derivation relies on the high beta Tokamak ordering of a general system of equations that has been derived to investigate the role of the hot species in tandem mirrors and bumpy tori. Bounce averaging of the background ion species is shown to be unnecessary.

Parametric instability of lower hybrid and ion sound waves in the vicinity of the resonance cone arising from a point source that oscillates at a frequency slightly higher than lower hybrid resonance, is considered. Estimates of the dimension of the effective interaction region are given.

The ablative, quasisteady expansion flow of a plasma generated by impinging a pellet by an intense light ion beam pulse is analysed. Profile of beam density and velocity and plasma density, velocity, and temperatures as functions of the radius are obtained from the analysis. The ablation pressure P_a and the ablated mass rate, for a given pellet radius r_a, are found as functions of both the beam energy per nucleon E₀ and the current intensity I₀. The results may be used for determining the instantaneous E₀ (or I₀), for a given I₀ (or E₀), required to generate given values of P_a and r_a; consequently, if the time laws P_a(t) and r_a(t), for optimal pellet compression, are obtained independently, the results yield the light ion beam energy history E₀(t) I₀(t) for a given or (I₀(t) for E₀(t) given).

Flux loop and line density measurements in the reversed-field pinch ZT-40M indicate the existence of poloidally symmetric excitations in the edge that move in the direction anti-parallel to the toroidal current with a velocity of the order of the thermal ion speed. A recently proposed mechanism of momentum transfer between the plasma and the wall explains these observations by showing that plasma flow near the wall can be sustained over magnetohydrodynamic time scales as a result of unidirectional transfer of momentum during a collision with the wall of an ion with finite gyroradius. The present hybrid-kinetic simulations substantiate the proposed mechanism by showing that within a few to ten microseconds flow of the required direction and order of magnitude results from collisions of ions with a momentum-absorbing wall.

The possibilities of applying optical scattering techniques to the determination of H^- concentrations in plasma sources relevant to negative ion beam generation are considered. Rayleigh scattering measurements for incident wavelengths just below the H^- photoionization limit appear to be only just feasible experimentally. A more promising possibility is observation of the modification in a plasma containing negative ions of the collective ion-feature in Thomson scattering. Numerical predictions of the effects of H^- concentration on the spectral distribution of the ion-feature are presented.

Metal impurity (Ni, Fe, Cr) line emission from TFR plasmas with graphite limiters has been studied during both the ohmic heating phase and supplementary heatings (either neutral beam injection or ion-cyclotron resonance heating). The experimental results have been compared with the predictions of an impurity transport numerical simulation code. It has thus been possible to evaluate the central metal ion density the central radiated power density, and the total radiated power. It is concluded that, at the present time, metal impurity radiation losses can be neglected in the discharge power balance.