Study of Back Streaming Ion using a Slot-type Grounded Grid in Hydrogen Negative-ion Source

The properties of cesium (Cs) recycling due to back-streaming ions have been investigated using an optical emission spectroscopy in the hydrogen negative ion (H-) source with a slot-type grounded grid (GG). The slot-type GG performed well to enhance the beam performance, and to reduce the thermal loading on GG by high transparency. We clearly observed increase of Cs optical emission intensity during beam extraction owing to the increase of the Cs ions sputtered from the back plate of the source due to the back-streaming positive hydrogen ions. Increase of Cs is closely related with the extracted H- current, but it does not depend on the beam energy. Recycling Cs from the back plate is deeply relevant to the perveance condition of the H- beam, and is minimized at the optimum perveance. Strong Cs recycling from the back plate owing to the back-streaming ions is promoted in the high perveance condition with large divergence, which is consistent with the numerical calculation for the distribution of back streaming ions. This high Cs recycling condition, however, is not suitable for safety beam operation with high energy beam. The output of beam power is saturated by the space charge limitation, and divergent beam is trapped in the grounded grid (GG), which may cause damage on the slot-type GG surface by high thermal loading as large as in the aperture-type GG.