The electron impact ion source: Distortion of its product ion velocities

In an electron impact ion source (EIIS), the electric field in the cloud of ionizing electrons alters the velocities of its product ions, distorting their kinetic energy and angle distributions and, if large, completely erasing the ion signal. Thus, it is a problem for the EIIS to provide sufficient electrons for ion detection together with minimal distortion in ion energy and angle. Evidence of this problem arose in satellite measurements of the neutral wind and temperature in Earth's thermosphere with an EIIS providing the neutral-to-ion conversion in the neutral spectrometer. Analysis indicates that the problem with neutral observations was likely due to large ion energy distortions (>10 eV) by the ionizing electrons, so large that the product ions were not detected. Space charge analysis shows that it is possible for the EIIS to provide ions with minimal distortions and sufficient ion signal, e.g., a 40 μA electron beam 0.1 mm thick allows precision better than 0.013 eV, as required for the neutral wind uncertainties of ±10 m/s in the Wind-Ion-Neutral-Composition-Suite. The analysis quantifies the benefits of proper selection of electron beam current and geometry with expressions given to minimize distortion in ion products. This offers precise knowledge as closely as desired within the known limits of the state of the neutral target gas prior to ionization, a new capability in thermosphere and laboratory applications. Additional effects important to the EIIS performance are described and discussed.