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.
Characteristics of steady-state plasma flow in the tokamak limiter scrape-off layer