Energetic electrons in inductively coupled RF plasma*

Radio frequency (RF) plasma sources are attractive as neutralizers for propulsion and industry applications due to their long device lifespans and high purity operation [1]. To maximize neutralizing efficiency, the electron population produced by the neutralizer should be dominantly low energy.  We measure the electron energy distribution functions of plasmas generated by an inductively coupled RF cathode using a Langmuir probe and observe significant populations of relatively high energy electrons under certain discharge conditions. The transition between regimes with and without these high energy electrons occurs suddenly as certain discharge parameters are changed.  Among possible origins of these energetic populations are secondary electron emission and non-local potential structures (e.g., [2 - 4]) which will be discussed in this presentation. In particular, we evaluate the possibility of a double layer structure formed near the front of the cathode under certain conditions, which may explain the presence of energetic electrons and the sudden change between regimes. 

[1] B. Longmier et al., Rev. Sci. Instrum. 77, 113504 (2006).

[2] V. A. Godyak et al., IEPC-2007-266, 30th Int. Elec. Prop. Conf., Sept 17-20, 2007, Florence, Italy.

[3] V. A. Godyak and V. I Kolobov, Phys. Rev. Lett. 81, 369–372 (1998)

[4] I. V. Schweigert et al., Phys. Plasmas 20, 101606 (2013).