Plasma End Losses and Heating in the ``Low-Pressure'' Regime of a Theta Pinch

A neutron-producing plasma with ion energy ∼3–4 keV has been produced at filling densities 10–50 μHg without negative bias magnetic fields in a 570-kJ theta pinch. Axial interferograms, taken with a ruby-laser-illuminated Mach—Zehnder interferometer show that a stable compressed plasma core exists throughout the magnetic half cycle with no ionized impurities outside the core, and no drift toward the wall. The interferograms give peak plasma densities of 2 to 5 × 1016 cm-3, and also indicate a loss of particles as a function of time. Plasma containment times (e-folding times of N) before peak compression are 6 to 30 μsec. The observed loss rates are approximately in agreement with predictions of free flow through an orifice whose radius is equal to an ion Larmor radius. Soft x-ray measurements yield ∼300 eV electron temperature for all filling pressures. Absolute intensities of the soft x-ray emissions show the impurity level to be <0.1%. The ion energy for the low-pressure regime deduced from pressure balance between plasma and magnetic field (assuming β = 1) is about a factor two higher than the ion energy deduced from the measured neutron yield for a Maxwell distribution. The discrepancy suggests that the distribution is more nearly monoenergetic than Maxwellian.