Effects of sheath potential of source plasma on characteristics of low energy beam extraction

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Abstract
In order to obtain a high brightness ion beam from a plasma ion source in the low energy region (150 eV), the effects of sheath potential on the ion beam characteristics such as beam divergence and brightness have been studied experimentally. A bucket-type ion source (magnetic multicusp ion source) with an accel–decel triode extraction system is used for the experiment. Bias voltage between a discharge chamber and a positive (plasma) electrode is applied for sheath potential control. Two types of electrode systems have been examined, such as straight hole aperture (S type) and recessed hole aperture (R type), and have found that the effects of the bias voltage on the beam characteristics depend on the type of aperture and the deceleration voltage. For an S-type system, the application of bias voltage improves the beam performance in a low deceleration voltage (−600 V), but not in a high deceleration voltage (−1600 V). For an R-type system, almost no improvement is observed in all deceleration voltages. Although the highest brightness is obtained in a deceleration voltage of −1600 V without the application of bias voltage in the S-type system, the significant fraction of the intersecting beam to the electrodes is present in this case.