Pre-ionization and pre-heat conditions for a compact toroidal plasma experiment in the millitorr pressure range

Abstract

The production of a highly ionized deuterium plasma at initial gas pressures of 0.5 to 2 mTorr has been investigated in a compact toroidal geometry (major radius of the torus vessel 20 cm, minor radius 10 cm). The gas, pre-ionized by a capacitively coupled RF-discharge, is further ionized (pre-heated) by toroidal and poloidal plasma currents which are induced by the discharge of a condenser bank (7.5 kJ maximum energy) into a helical coil system wound around the torus. At the end of the ionization phase, the current in the coil is crowbarred. Radial plasma density profiles are measured by using a movable 4-mm microwave probe. The plasma current, its energy, and the toroidal and poloidal magnetic field distributions are deduced from magnetic measurements, and the electron temperature from the Dβ-line-to-continuum-light-intensity ratio.At the low initial gas pressures studied here, the plasma formation occurs when the toroidal electric field amplitude on the torus axis lies below a critical value (< 6 V/cm·mTorr). Using a programmed auxiliary perpendicular magnetic field, a plasma which is both highly ionized and well detached from the walls of the torus is obtained. Plasma equilibrium can be maintained during the time of observation (up to ≈ 40 μs after crowbar). Electron densities greater than 6 × 1013 cm−3 (cutoff density for 4-mm waves), a degree of ionization higher than 60%, and electron temperatures of 5 eV are reached starting from a pressure of 1 mTorr D2. The toroidal plasma current and the toroidal magnetic field are, respectively, 7 kA and 0.45 kG at the time of crowbar. The measured density and magnetic field profiles indicate that the magnetic axis lies significantly off the axis of the torus vessel.