Optical Emission Spectroscopy of Electron-Cyclotron- Resonance-Heated Hel ium Mirror P lasmas

In this experiment emission spectroscopy in tire 3000-5000/ ~ range has been utilized to determine the electron temperature (15-60 e V) and ion densiO ' (2-5 × 10 I r cm- ~) qf helium plasmas produced by the Michigan mirror machine ~rJ (MIMI). The plasma is generated and heated by whistler-mode lectron.cyclotron resonance ( ECR) wat,es at 7.43 G Hz with 400-900 W power in SO-ms-long pulses. Gas lkwling is provided at tire midplane region by a leak valve with a range in pressure q/" 3 × 10 ~ to 2 × 10 " ~ Tort. Emission line intensities are interpreted using a model of tire important eollisional and radiative processes occurring in tire plasma. Tire model examines seconda O " processes uch as radiation trapping, excitation tran.~fer between levels qf the same principle quantum number, and excitation l~'om metastable states fnr plasmas in the parameter ange of MIMl In~. = l-6x1Otr cm ~). From tire analysis q/ " line intensity ratios lor neutral helium, tire electron temperature is measured and its dependence upon tire gas pressure and microwave power is determined. These temperatures agree with those obtained b) ' Langmuir probe measurements. An analysis of the line intensity ratio between singly ionized helium and neutral helium yields a measurement qf tire ion densit.v which is in good agreement with electron densit) ' measurements made by a microwave interferometer. KEY WORDS: Emission spectroscopy; helium; ECR mirror plasma. 1