Effect of accelerating electrode rotation on ion beam optics

Abstract

The effect of a rotation of the electrode used to accelerate a positive ion beam is treated analytically for the Joint European Torus (JET) Positive Ion Neutral Injector (PINI) tetrode accelerator. The rotation is taken to be about an axis in the plane of the electrode and perpendicular to the beam axis, so the effect is that of a tilt in the alignment of the affected electrode. The effect of such a tilt in either of the two multiaperture accelerating electrodes is to introduce an additional steering of the beam, the magnitude of which depends upon the ratio of the potentials across the first and second accelerating gaps and the position of the beamlet with respect to the tilt axis. The direction of the steering is dependent upon which electrode is displaced. Steering angles of the order 10 mrad can be generated by this effect, which is significant compared to the alignment precision required from high power neutral injectors such as those on JET. The presence of a grid rotation also affects the total perveance of a multiaperture system and this can be used to obtain information on the position of the tilt axis. Typical values are calculated for the JET PINI geometry and compared to observation.