Ohmic heating of plasma in closed magnetic systems

An analysis is carried out of experiments for studying the electron energy lifetime in stellarators with ohmic plasma heating. It is shown that when the ohmic heating current is increased, the convective losses associated with particle diffusion become important. For low plasma densities and small dimensions of the current channel, the current drift velocity can exceed the threshold for the excitation of the ion-acoustic instability when the input power to the plasma is raised. The electrical and thermal conductivities of the plasma are then of the turbulent variety, and this leads to a sharp decrease in the electron energy lifetime. Experiments on the Uragan, Wendelstein IIb, Wendelstein VIIA, L-2 and CLEO stellarators are in satisfactory agreement with the calculations. (author)