Radial characterization of the electron energy distribution in a helicon source terminated by a double layer

Electron energy probability functions (EEPFs) are measured across the radius (0 <= r < 6.85 cm) of a low pressure (0.3 mTorr) helicon plasma source terminated by a current-free double layer. The source field of about 130 G is generated using a Helmholtz coil pair and the radio frequency (rf) power is maintained at 250 W. All EEPFs exhibit a distribution with a temperature T-e bulk out to a break energy epsilon(break) and a relatively depleted distribution for higher energies with a lower temperature T-e tail. T-e bulk and T-e tail are about 8 eV and 5 eV, respectively, for r < 4 cm and increase up to about 14 eV and 9 eV near the source wall, i.e., near the rf antenna. epsilon(break) is found to correspond to the potential drop of the double layer for the central part of the source and to the sheath potential near the wall. (C) 2008 American Institute of Physics.