Omegatron mass spectrometer for analysis of ion concentrations in hydrogenic plasmas

We describe an omegatron-type mass spectrometer used for measurement of the molecular and atomic ion concentrations as well as the ion species temperatures in hydrogenic plasmas. The experiments were performed in the PISCES-A linear plasma experiment with electron densities n approximate to 10(11)-10(12) cm(-3), ion temperatures T-i approximate to 2-8 eV, electron temperatures T-e approximate to 3-30 eV, and magnetic fields B = 0.8-1.9 kG. The resulting mass spectra show clear evidence of atomic hydrogen H+, as well as of the molecular ions H-2(+) and H-3(+), with H+ and H-3(+) usually being dominant. It is found that large electron repeller voltages (approximate to -200 V) are important for preventing electrons from entering the diagnostic region and ionizing the residual gas. The relative peak heights for the different ions are found to vary as a function of ion repeller voltage, probably due to differences in ion temperature. (C) 2001 American Institute of Physics.