Ionization Dynamics in a Pulse Disturbed Magnetically Confined Helium Plasma

Abstract

The equilibrium of a magnetized Helium plasma is disturbed by a pulsed Trivelpiece-Goiildwitvt'. The electrons obtain the energy by linear collisionless wave absorption. The relaxation phenomena of density and energy are explained in terms of two relaxation times r,, t l and a quantity giving the additional ionization. These quantities are derived from a small signal fluid model based upon energy and particle balance equations. I n the experiment they are taken from the transient curves of Langmuir-probe current, optical line radiation and the noise power at the electron cyclotron frequency.5 . loLo cm-", R = 6,5 . 10-2 T, 27 MHz rf plasma source, low frequency fluctuation level < i%, classical losses.The energy relaxation time tg = 10 ... 15 p is given by inelastic collision losses. The ionization time constant rl is related to the instantaneous ionization frequency during the transient state. It shows a high value at the very beginning of the pulse which must be explained by a tail f o r m h o n in the distribution function nnd enhanced radial losses becoming Bohm-like in the transition phnsc.The experimental conditions are: Helium-gas, p = 1 5 Pa, T, = 4 eV, n = 1