Rapid compression heating of a plasma in the linear z-pinch

Abstract

The electrical characteristics of discharges with high and low initial rates of current rise are briefly described and the electrical measurements are used to determine the work done on the plasma by rapid compression. The particle and the magnetic energy in the discharge are discussed for both high and low initial rates of current rise (dl/dt(0)) and for discharges with and without an axial magneticfield (Hz). It is shown that the gas is heated much more efficiently both with and without Hz at high dl/dt(0). For dl/dt(0) ∼ 1.5× 1012 amps/sec (Hz = 0) 15% of the initial bank energy goes into heating the gas and 10% into the magneticfieldat the first maximum contraction; for dl/dt ∼ 1011 amps/sec (Hz = 0) 11% goes into the field and only 6.5% into the gas at this time. The energy input to the gas is found to become limited well before peak current, and the initial implosion is not followed by much of the subsequent isentropic compression heating which might be expected, so that the temperature never exceeds about 300 eV; it follows that most of the observed neutron yield (∼ 106) is not thermonuclear. Data obtained from the electrical waveforms and from high-speed photographs are presented and analysed, and it is shown that large currents build up outside the central plasma column after the first maximum contraction. Further evidence suggests that these currentsflowin wall material boiled off by radiation from the central column. These wall effects account for the absence of further isentropic compression heating and also contaminate the central plasma column; they are such as to prevent any significant gain in heating by increasing the power input to the discharge. It is concluded that similar effects probably account for the failure to achieve a thermonuclear temperature in many high power pinch experiments.