AN EXPERIMENTAL-STUDY OF BREAKDOWN IN A PULSED HELICON PLASMA

Fast time-resolved measurements of ion and electron energy distributions in the first 200 mu s of a discharge in a 20 cm diameter glass Helicon source show that, 10 mu s after the RF is turned on, an intense burst of electrons is produced with average energy 200 eV and lasting about 1 mu s. This initial pulse of high-energy electrons can be explained in terms of resonant secondary electron multiplication (the multipactor effect) of electrons which are accelerated in the RF fields produced by the Helicon antenna. These electrons ionize the background gas very rapidly and a plasma with high electron temperature and hence high plasma potential can be created in about 1 mu s. Subsequently, further ionization occurs and, after 20 mu s, the plasma potential has fallen to about 200 V and the electron temperature to about 20 eV. The electron temperature continues falling and reaches a steady state value of 8 eV in about 30 mu s, The density and plasma potential reach steady state after about 70 mu s.