Beam formation in a triode ion gun.

Beam formation in a triode ion gun has been investigated. The gun consisted of a plasma ion source operated with argon, a low-potential cup extractor and an acceleration tube (diameter D). The ion optics of the system were studied by recording the current density profile of focussed 40 keV beams with the beam current as a parameter. Currents up to about 200 μA were obtained by increasing the plasma density in the ion source. Focussing was achieved by suitably adjusting the extractor potential. To get a comprehensive view of beam formation the following gun parameters were varied: extraction-gap distance (0.5≤d≤7 mm, adjustable in situ), extractor length (0.59≤ L D≤1.65), acceleration-gap distance (0.05≤ g D≤0.8), length of ion-source outlet (0.1≤l≤1 mm), diameter of extractor entrance opening (2.5≤b≤5 mm) and monitor position (z = 28 and 70 cm from the midplane between the electrodes). The focussing extractor voltage was found to increase linearly with increasing extractor length for L D{succeeds above single-line equals sign}1. A minimum extractor voltage of only 150 V was observed at L D = 0.7. The beam profiles at z = 70 cm were found to be broadened markedly by space-charge expansion in the field-free drift region. The influence of extraction-gap distance and extractor length was much less pronounced. Profile distortions observed under well-defined conditions at either low or high beam currents were found to be due to a particular shape of the plasma boundary or strong beam divergence in the extraction region, respectively. The divergence of beams from convex plasma boundaries could be described quantitatively by applying the space-charge law between concentric spherical electrodes. The width of the profiles at z = 28 cm could be interpreted using standard beam-transport theory. Combined profile measurements at either monitor position provided an estimate of the beam emittance. Using recently reported properties of a two-tube electrostatic lens a self-consistent picture of beam formation in the triode ion gun could finally be obtained