Generation of 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]. The electron population generated by the neutralizer should be dominantly low in energy for maximal neutralizing efficiency. We measure the electron energy distribution functions of plasmas generated by an inductively coupled RF cathode using a Langmuir probe. Under certain conditions, we observe significant populations of relatively high energy electrons, and we find that there is a sudden transition between regimes with and without these high energy electrons as certain discharge parameters are varied. 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).