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 Te bulk out to a break energy εbreak and a relatively depleted distribution for higher energies with a lower temperature T#1e tail#2. T#1e bulk#2 and T#1e tail#2 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. ε#1break#2 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.