Experiments on the Ohmic Heating and Confinement of Plasma in a Stellarator

Abstract

The basic concepts of confinement and ohmic heating in a figure‐eight stellarator are briefly reviewed and experimental data in these areas summarized. The production of energetic x‐rays by runaway electrons, up to ten milliseconds after the disappearance of the accelerating field, has indicated effective single‐particle confinement by magnetic fields of 20 000 to 30 000 gauss. However, the plasma confinement time during ohmic heating appears to be limited by unknown processes to approximately 100 microseconds for hydrogen, more than three orders of magnitude shorter than predicted from classical collisional diffusion. The ionization level in a helium discharge, with an initial pressure of about 1 μ of Hg, becomes virtually complete during the heating pulse, and the electric resistivity corresponds to a kinetic temperature of about 100 volts, or 10⁶ degrees. After the heating pulse, the electron density decays with a time constant as great as 6 milliseconds under some conditions. The hydromagnetic kink instability predicted by Kruskal has been clearly observed.