Effects of ambient gas in beam drift space on performance of high brightness and low energy ion beam

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Abstract
Effects of the ambient gas in the ion drift space on the transport property of the low energy (155 eV) and high brightness beam are studied experimentally. A bucket ion source with single-aperture triode extraction is used. Beam characteristics such as the divergence angle and the current density are improved by the increase in the ambient gas pressure in the high brightness beam (high deceleration voltage, −2000 V), but not in the low brightness beam (low deceleration voltage, −600 V). The beam spreading due to the space charge of the ion beam is seen at the low ambient gas pressure when the central current density, which is measured at 35 cm downstream side from the electrode system, exceeds 5 μA/cm2. The electron density in the ion-beam plasma increases with the ambient gas pressure, but the space charge is not sufficiently neutralized by the electrons produced by the ambient gas atom ionization.