RF power transfer efficiency of inductively coupled low pressure H2 and D2 discharges

Abstract

The RF power transfer efficiency and the relevant power absorption mechanisms of inductively heated hydrogen and deuterium plasmas are investigated in the low-pressure region between 0.25 and 10 Pa. The discharges are generated in a cylindrical vessel via a helical coil applying a frequency of 1 MHz and delivered RF powers up to 800 W. The power transfer efficiency η is quantified by a subtractive method that relies on the measurement of the delivered RF power and of the RF current through the plasma coil both with and without discharge operation. By means of optical emission spectroscopy and electrical probe measurements, the key plasma parameters are obtained. For both H2 and D2, the relative behavior of the power transfer efficiency is well comparable, which increases with increasing delivered RF power and describes a maximum at pressures between 1 and 3 Pa where more than of the provided power are absorbed by the plasma. The observed relative dependencies of η on the operational parameters are found to be well explained by an analytical approach that considers the power absorption by the plasma via evaluating the RF plasma conductivity based on the measured plasma parameters. At the parameters present, non-collisional stochastic heating of electrons has to be considered for pressures , while collisional heating dominates at higher pressure. Molecular dissociation is found to have a significant influence on the power transfer efficiency of light molecular discharges. The direct comparison of H2 and D2 identifies the higher atomic density in deuterium to cause a systematically increased power transfer efficiency due to an increased ionization rate in the present electron temperature region.