The injection of tenuous plasma into a magnetic guide field in the absence of wall-shorting

Abstract

A burst of plasma from a thetatron gun is allowed to expand freely as it passes along a drift tube; an aperture limiter selects a central beam of plasma which then flows axially into the field of a solenoid. The ion energy is centred about 800 eV and the maximum ion density is about 3 × 1011 cm-3. The solenoid is suspended in a large vacuum chamber so that the wall-shorting effect, described in connexion with a similar experiment, is inhibited. A removable conducting cylinder at the solenoid entrance enables a direct comparison to be made between the case when a radial space charge field exists in the plasma stream and when it is short-circuited. Measurements of azimuthal ion flux, electric field and current flow at the solenoid entrance, show that shorting can be prevented. The radius of the plasma stream is always comparable to twice the ion gyro-diameter 2Rci; it is not reduced to 2Rci(m/M)1/2, as simple theory predicts, when `shorting' is prevented.