Table of contents

The implosion of neon-filled glass microballoons has been examined via numerical simulations using a one-dimensional Lagrangian hydrodynamic code. Comparison has been made to experiments performed on a four-beam neodymium glass laser system using an array of diagnostics including X-ray pinhole cameras and soft-X-ray spectrometers. Interpretation of the results obtained here leads to the conclusion that the electron thermal transport is strongly inhibited and that energy transport is dominated by suprathermal electrons producing an explosive pusher implosion.

Absolute-brightness measurements and spatial scans in the vacuum ultraviolet have been used to investigate the presence and spatial density distribution of various charge states of carbon and aluminium (the major impurities) and neutral hydrogen in the ELMO Bumpy Torus. The impurity concentrations are quite low; the total fractional carbon concentration in the toroidal core (compared to the electron density) is less than 6 × 10⁻⁵. Sources for the C and Al impurities are examined. Experimental evidence and arguments based on the space potential and drift surfaces which are presented indicate that the surface blanket plasma plays a role analogous to a divertor, shielding the core plasma from impurities.

The results of a study on the non-linear stability of tearing modes for hollow current profiles in cylindrical geometry are presented. Their non-linear evolution which, through magnetic island formation, leads either to saturation of the tearing mode (a new non-axisymmetric equilibrium is reached) or to a re-distribution of the flux and field line reconnection in Kadomtsev's sense (a new axisymmetric equilibrium is obtained) has been studied. An empirical prediction for the accessibility of the reconnected state is given. In particular, tearing modes with low poloidal mode number (m = 2, 3) are considered in order to interpret some minor disruptions observed in tokamaks, i.e. m = 3 minor disruptions in the Princeton Large Torus. Tearing modes with higher poloidal mode number have also been examined

Target designs using low-energy monoenergetic ion beams with a range of up to several × 10⁻³ g⋅cm⁻² delivering energies of tens of kilojoules to a megajoule have been developed. Minimal pulse shaping produces yield ratios of 10–100. Applications to laser-produced ions show that if the ions are produced from an isothermal expansion, there is a high degree of inefficiency in converting laser energy to useful ion energy in driving an implosion.

Low-Z impurity transport in tokamaks was simulated with a one-dimensional impurity transport model including both neoclassical and anomalous transports. The neoclassical fluxes are due to collisions between the background plasma and impurity ions as well as to collisions between the various ionization states. The evaluation of the neoclassical fluxes takes into account the different collisionality regimes of the background plasma and the impurity ions. A limiter scrape-off model is used to define the boundary condition for the impurity ions in the plasma periphery. To account for the spectroscopic measurements of power radiated by the lower ionization states, fluxes due to anomalous transport are included. The sensitivities of the results to uncertainties in rate coefficients and plasma parameters in the periphery are investigated. The implications of the transport model for spectroscopic evaluation of impurity concentrations, impurity fluxes, and radiated power from line emission measurements are discussed.

The results from a spherical-symmetry hydrodynamic calculation are adapted to describe the production of large plasmas by the laser ablation of solid-density pellets. Expressions are given for the rate of ablation, plasma temperature and ion energy. The upper limit on the ionized particle number is set by pellet disintegration following heating by laser-driven shock. Scaling laws are derived for the total plasma energy and particle number. Plasma conductivity transforms one-sided illumination into an approximately spherical ablation, allowing a discussion of the scaling laws for laser-induced acceleration. The reduction in laser acceleration due to quasi-spherical ablation has negative implications for laser acceleration as a method of reactor re-fuelling.

A model for the rate of density rise observed when neutral gas is fed into a plasma-containing chamber is presented for regimes where known collisional transport processes do not provide an adequate explanation. A dense layer of cold plasma produced at the edge of the plasma column and the resulting relatively sharp ion temperature gradient, as compared with the local density gradient, can lead to the excitation of electron temperature fluctuations driven by ion drift modes. The net inflow of electrons and ions that is produced by these modes has been included in a one-dimensional transport code used to simulate experiments performed by the Alcator device. The linear and quasi-linear theories of these modes are given for the regimes of interest. The cold-plasma-layer model is also consistent with the presence of an outflow of impurity ions, due to impurity driven modes, that balance the inflow produced by discrete collisions.

The plasma density profile modification by radiation pressure near the critical surface has been extensively studied for normally incident light within the context of the hydrodynamic equations and the wave equation. In one-dimensional spherical geometry, the radial stability of these profiles to density perturbations is studied and it is shown that, except for a small range of parameters, the only stable profiles are accompanied by a transition from subsonic flow inside the critical-density surface to supersonic flow outside the critical-density surface. The long-time behaviour of the class of solutions that do not admit such transitions has been studied and that class exhibits repetitive, unsteady flow.

Neoclassical transport coefficients for a bumpy torus have been calculated by solving a bounce-averaged drift kinetic equation locally in radius. In contrast with previous work, where the ambipolar field was assumed to be large relative to the plasma thermal energy (e_jϕ/kT_j ≫ 1), finite electric fields are considered here [e_jϕ/kT_j = 0(1)]. Both the Landau operator and the particle and energy-conserving BGK model operator have been used, and a comparison of the two sets of results indicates reasonable agreement. The resulting transport coefficients exhibit a strong dependence on the ambipolar field and on plasma collisionality; in the large-field limit, the present results correlate closely with earlier work.