Evaluation of the background signal for mass spectrometry under high ambient hydrogen pressures in plasma discharges

The background signal of a quadrupole mass spectrometer (QMS) has been evaluated under high ambient hydrogen pressures. Hydrogen gas is injected into a vacuum chamber and the resultant change of the QMS signal is investigated for dominant gaseous impurities with mass numbers from M=1 to M=50. The partial pressures of impurity gases (H2O, CO, CH4) are observed to increase with the quantity of the hydrogen gas injected. It is found that each increase of partial pressure is in proportion to the hydrogen pressure in the range 4×10−6–4×10−5 Torr. A method to compensate for suppressing the background signal is considered on the basis of experimental results and is applied to mass spectrometry of residual gases in plasma discharges. The partial‐pressure measurements in the GAMMA 10 tandem mirror device, which is used for fusion plasma studies, were performed and it was confirmed that this method substantially improves the accuracy of the measurement. A production mechanism of the impurity gases inside the QMS is...