It is shown that the ablation of a solid hydrogen pellet subject to a plasma is likely to produce a quasi-steady dense neutral gas cloud. The total integrated density of the cloud is such that the plasma electrons lose essentially all their energy in the cloud. The electron energy flux is degraded by inelastic collisions and elastic backscattering with the neutral molecules, providing local heating and acceleration of the neutral gas. Only a small fraction of the energy flux reaches the surface of the pellet, raising the pellet's surface temperature to a point where the energy flux at the pellet's surface is in balance with the energy lost through vaporization. The vaporization rate, in turn, determines the total integrated neutral gas cloud density. The scaling laws derived from the model indicate that the pellet lifetime varies as: where τp is the lifetime of the pellet and Te, ne, and rp0 are the electron temperature, density of the plasma, and initial pellet radius, respectively. A good agreement is found between this model and the ORMAK pellet injection experiment.

No abstract

High-speed pellet fuelling experiments have been performed on the ISX-B device in a new regime characterized by large global density rise in both Ohmically and neutral-beam heated discharges. Hydrogen pellets of 1 mm in diameter were injected in the plasma midplane at velocities exceeding 1 km·s−1. In low-temperature Ohmic discharges, pellets penetrate beyond the magnetic axis, and in such cases a sharp decrease in ablation is observed as the pellet passes the plasma centre. This behaviour can be accounted for by an ablation model that includes dynamic cooling of the target plasma while the ablation proceeds. Complete penetration can be prevented by operation in low-density regimes where runaway electrons are thought to be responsible for high ablation. A similar effect is observed with moderate to large amounts of neutral-beam injection. There is a strong enhancement of the ablation rate in the outer 10-cm plasma region even for short heating intervals, which can be explained by the presence of multi-kilo-electron volt ions in the discharge. Density increases of ∼300% have been observed without degrading plasma stability or confinement. Energy confinement time increases in agreement with the empirical scaling τE ∼ ne and central ion temperature increases as a result of improved ion-electron coupling. Laser-Thomson scattering and radiometer measurements indicate that the pellet interaction with the plasma is adiabatic. The low level of power emission from the pellet-plasma interaction region is consistent with negligible charge-exchange losses; within the experimental accuracy, nearly all of the pellet mass can be accounted for in the initial plasma density rise. Penetration to r/a ∼ 0.15 is optimal, in which case large-amplitude sawtooth oscillations are observed and the density remains elevated. Gross plasma stability is dependent roughly on the amount of pellet penetration and can be correlated with the expected temporal evolution of the current density profile.

NOTICE This document contains information of a preliminary nature. I t is subject to revision or correction and therefore does not represent a final report.

No abstract