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 tempera tures in hydrogenic plasmas. The experiments were performed in the PISCES-A linear plasma experiment with electron densities n approximate to 10(11)-1 0(12) cm(-3), ion temperatures T-i approximate to 2-8 eV, electron temperat ures T-e approximate to 3-30 eV, and magnetic fields B = 0.8-1.9 kG. The re sulting 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 d ominant. 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 di fferent ions are found to vary as a function of ion repeller voltage, proba bly due to differences in ion temperature. (C) 2001 American Institute of P hysics.