Influence of accelerator grid misalignment on multi-aperture particle beam properties

Accurate accelerator grid alignment is a major factor governing the properties of multi-aperture particle beams. It is particularly important for JET positive ion neutral injectors (PINIs), where accelerators are constructed of two grid halves. Grid misalignment can lead to increased power loading of beam intercepting components, to reduced beam transmission, and, finally, to reduced neutral beam power injected into plasma. A new PINI accelerator assembly procedure, which utilises a robotic arm measuring device, was recently developed at JET. PINI alignment data are used as input to a beam simulation code to predict beam properties prior to testing at the Neutral Beam Test Bed. Since the new PINI assembly procedure and the associated computer code have been implemented the alignment of JET PINIs has been considerably improved.