Electron space charge effects in ion sources for residual gas analysis

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Abstract
An electron impact ionization source suitable for residual gas analysis (RGA) with a quadrupole mass spectrometer has been studied both experimentally and by computer simulation. The electronic space charge is shown to play a role in limiting the extracted current from RGA ion sources and the simulation treats this aspect of the problem self-consistently. Under certain source conditions, the ion extraction efficiency is observed to decrease for electron currents above approximately 1 mA-well below the current at which space charge limiting of the electron emission is expected. The observed effects are well reproduced by the simulation. It is shown that whereas the electron trajectories are only weakly perturbed by space charge effects, the loss of ionization efficiency can be attributed to drastic changes in the ion trajectories.

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