Influence of negative bias voltage on the large-area positive hydrogen ion source

Neutral beam injection is an effective way for auxiliary heating in tokamak fusion research. To enhance the efficiency of extracting hydrogen positive ions for improving the injected power, research on negative bias voltage has been conducted through numerous experiments in the testbed of an arc ion source. Experimental results of extracted positive ion density are agreed with results predicted by sheath theory. The current of extracted hydrogen positive ions increased from 7.8 A to 8.7 A with negative bias voltage. The positive ion current remains constant when negative bias voltage exceeds 15 V. It has been observed that electron density is higher and electron temperature is lower compared to no bias conditions. The uniformity of electrons in the vicinity of extraction area is better by adding negative bias voltage. However, the plasma temperature on the extraction area was detected in the experiment against results predicted by sheath theory. An imperative phenomenon, which the electron temperature and density are increasing with negative bias voltage, has been seen. Ultimately, electron density remains constant and the change in electron temperature is less. The changes in plasma parameters will be explained by electron energy distribution function approach, since the research work of plasma sheath theory for plasma characteristic is relatively less.