Reducing the Energy Distribution in a Plasma-source Sector-field Mass Spectrometer Interface
Ion-beam kinetic-energy distributions were measured with several sector-field plasma-source mass spectrometer interface configurations. The original interface design produced relative energy distributions of 23% (with an ICP source) and 13% (with a GD source). These relative energy distributions were independent of the interface potential. Accelerating more evenly and over a shorter distance (with a grid installed) in the second vacuum stage dramatically reduced the relative energy distribution but also the signal level. The addition of an ICP torch shield also helped to reduce the relative energy distribution, presumably by lowering the plasma offset voltage. New ion optics were designed with the distance between the apertures leading to the second and third vacuum stages decreased (to roughly 20 mm) and with the majority of the ion-beam acceleration moved from the second to a short distance into the third vacuum region. This configuration reduced the relative ion-beam energy distribution to roughly 5% for both sources but also resulted in some loss of signal.