Heating energy and effects of initial filling pressures for theta-pinch plasmas operated at low energy

Heating conditions of theta-pinch plasma were studied by the time variations of the energy supplied to the plasma Ps, Joule heating energy P sub(J), and magnetic energy P sub(B). In the Ar-plasma of 25 mToor, P sub(J) was larger than P sub(B) till 1.2 μsec after the beginning of the second half period of discharged current, but after that, P sub(J) became smaller than P sub(B), and the most part of the electro-magnetic energy changed into the kinetic energy of pinching. During the pinching process of every half period, P sub(J) first reached the maximum, then 0.2 - 0.4 μsec later, P sub(B) came to the maximum. At 11 to 40 mTorr, the magnetic diffusion time increased as the pressure decreased. The approximated solution of the magnetic diffusion equation proved that is due to the growth of a layer of high electric conductivity with long diffusion time in the outer plasma. The relation between the initial pressure and the time for the electric current layer to reach the central axis was such that in the range from 11 mTorr to 125 mTorr, the time increased as the pressure increased. This result agreed well with the pinch time characteristic of the plasma obtained by the snow-plow model. (Fujii, T.)