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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.

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A new mode of reversed-field pinch (RFP) startup has been tested in the RFP experiment ZT-40M. Instead of increasing the toroidal current almost sinusoidally to its maximum, as is normally done, the current is initially raised to ~ 70 kA in 0.8 ms and subsequently slowly ramped up to-170 kA in 14 ms. The behaviors of toroidal flux, particle density, temperature, and radiated power for the ramped discharges are compared with fast current rise, flat-topped operation. The successful demonstration of ramped toroidal discharges has an important impact on future large RFP experiments since the voltage necessary to drive the toroidal current for slow (~ 10 to ~ 20 ms) startup is reduced. I.