HELIUM PARTIAL-PRESSURE MEASUREMENT IN A DEUTERIUM ENVIRONMENT

To study the removal of helium ash in present day, medium sized tokama ks, experiments with deuterium plasmas and helium puffs are conducted. Because D-2 and He have almost the same mass, a spectroscopic method had been developed at TEXTOR where the He partial pressure in the exha ust line was measured by exciting the neutral gas in a Penning gauge a nd observing the characteristic emission lines of helium and deuterium . This method was Limited to low pressures and concentrations because broad spectral Lines from the deuterium molecules superimpose the He l ines. By using a spectrometer and an intensified charge coupled device camera instead of optical filters and photomultipliers, it is now pos sible to record the interesting part of the spectrum with a time resol ution of 20 ms. By subtracting the molecular spectrum of deuterium, he lium concentrations from 2% on can be detected over about three orders of magnitude in pressure down to 2x10(-6) mbar. He and D-2 pressures are measured simultaneously. By another method the 58 nm resonance lin e of helium is observed and the crosstalk of the deuterium gas could b e eliminated. Pressure measurements as low as 5x10(-8) mbar and 1% He concentration are possible. A detection system utilizing a vacuum ultr aviolet monochromator in combination with a channeltron has been teste d. (C) 1996 American Institute of Physics.