Energy Loss from a Theta Pinch

Abstract

Electron temperatures measured in a megajoule theta pinch are found to be limited to ∼300 eV over the pressure range 10–50 mTorr and to vary during the discharge time in a nonadiabatic manner. The plasma diamagnetism shows a rapid decay at the time of maximum Poynting flux and since there is no particle loss at this time there must be an energy loss of the order of the energy input rate. Variation of the concentration of oxygen, which is the major impurity, by addition of known amounts or by reduction of the intrinsic 1.5% an order of magnitude with a new pre-ionization technique, indicates an energy loss rate of 2 × 1013 erg cm−3 sec−1 at zero oxygen concentration. It is not possible to account for this loss by radiation in the continuum spectrum. A simple model of axial thermal conduction is developed in which the energy is transported to cold regions beyond the ends of the theta-pinch coil. The model predicts a limiting temperature due to the steep temperature dependence of the conductivity coefficient K = αT5/2. Other predictions of the model concerning the variation of diamagnetism are found to be valid. Finally the results of other theta-pinch experiments are examined and some are found to fit the model.