Initial Kinetic Energy Discrimination Effects in Crossed-Field Ion Sources

Abstract

Previously developed differential equations for crossed-field trajectories are particularly suitable for investigating the characteristics of the orbits in crossed-field ion sources of mass spectrometers. At low repeller voltages there can be enormous initial kinetic energy discrimination due to the cycloidal orbits turning around before reaching the plane of the first slit. Energy relations for collectible orbits will be presented. A transformation of angle may be made between the isotropic distribution of initial velocity components to the anisotropic distribution of orbits that applies in the beam defined by the first slit. The latter will depend upon the operating conditions and may be studied by deflecting the beam across the exit slit by varying the deflection voltages. The signal reaching the detector then represents a progressive lateral sampling of the beam. The width and maximum intensity in this beam are functions of operating conditions and it is shown how very misleading results may be obtained depending upon how the exit slit samples this beam. Collection efficiency can be defined and set up as an integral of a function of the projected initial kinetic energy components on the plane of motion. The area under the curve obtained by progressively pushing the beam from the first slit across the exit slit represents the flux of ions collected by the first slit. This is dependent on operating conditions and may be conveniently used to investigate initial kinetic energies.