Ionization Dynamics in a Pulse Disturbed Magnetically Confined Helium Plasma

Abstract

AbstractThe equilibrium of a magnetized Helium plasma is disturbed by a pulsed Trivelpiece‐Gouldwave. 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 τE, τ1 and a quantity giving the additional ionization. These quantities are derived from a small signal fluid model based upon energy and particle balance equations. In 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.The experimental conditions are: Helium‐gas, p = 1 …︁ 5 Pa, Te = 4 eV, n = 1 …︁ 5 · 1010 cm−3, B = 6,5 · 10−2 T, 27 MHz rf plasma source, low frequency fluctuation level < 1%, classical losses. The energy relaxation time …︁E = 10 …︁ 15 μs is given by inelastic collision losses. The ionization time constant τ1 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 formation in the distribution function and enhanced radial losses becoming Bohm‐like in the transition phase.