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<6.85cm) of a low pressure (0.3mTorr) helicon plasma source terminated by a current-free double layer. The source field of about 130G is generated using a Helmholtz coil pair and the radio frequency (rf) power is maintained at 250W. All EEPFs exhibit a distribution with a temperature Tebulk out to a break energy ebreak and a relatively depleted distribution for higher energies with a lower temperature Tetail. Tebulk and Tetail are about 8eV and 5eV, respectively, for r<4cm and increase up to about 14eV and 9eV near the source wall, i.e., near the rf antenna. ebreak 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.