Electron Beam Pumped Laser Induced Fluorescence

While laser induced fluorescence (LIF) is relatively straightforward in low temperature argon plasmas, measurements of the ion velocity distribution in low temperature helium plasmas have been problematic. Not only is the ionization energy of helium much greater than that of argon, the lowest energy metastable helium ion state (the 2s state) lies 40.8 eV above the ion ground state. Therefore, in low temperature helium plasmas (T_E < 5 eV), the metastable ion population is quite small and therefore there is only a small target population available for LIF. Another challenge is that the excited states of helium ions are degenerate and therefore, most excited states have a very strong return directly to the ground state through deep UV and soft x-ray emission. To maintain a sufficient metastable population for two photon absorption LIF (TALIF), we inject a low energy (∼100 eV) electron beam into a low temperature plasma. Here we explore the ability of an electron beam to pump the 2s helium ion state, which is reflected in the increased intensity of helium's spectral lines. Preliminary data and further discussion on electron beam pumped TALIF is also presented.