Operation of a Theta Pinch at Extremely Low Filling Pressures

Observations are reported that by adding methane to the hydrogen or deuterium filling in the discharge tube of a theta pinch the absolute density at which it is possible to achieve breakdown in the preheater phase can be lowered. Plasma conditions have been investigated for a filling gas pressure of 0.8-m Torr pure CH 4 or CD 4, respectively, using light scattering and neutron emission. Measurements on impurity concentrations indicate that for low filling pressures and low absolute impurity concentrations, selective injection and loss of impurities to the walls during the preheater phase of a theta pinch may become important. Large errors can then arise in determining the concentration of elements with physically stable condensable forms (e.g., carbon and boron) when using the method involving the measurement of the intensity of an emitted spectral line as a function of added impurity concentration.