Upper limit to Landau damping in helicon discharges

The uncommonly high rf absorption efficiency of helicon discharges has been thought to be caused by Landau damping of helicon waves and the concomitan t acceleration of primary electrons. By constructing an energy analyzer tha t accounts for rf fluctuations in plasma potential, it is shown that Landau -accelerated electrons are too sparse to explain the ionization efficiency. Instead, rf absorption and ionization are found to be consistent with the mechanism of mode coupling to Trivelpiece-Gould modes at the plasma boundar y, [S0031-9007(99)08827-4].