The plasma discharge power acts as a control parameter for the plasma density, and the beam optics can be optimized by properly controlling it. In negative ion sources for fusion, a plasma grid (PG) is positively biased with respect to an ion source chamber in order to control the ratio of negative ions to electrons in the extracted beam. In our recent studies, it was confirmed that the negative ion and electron densities near the PG decrease with increasing the bias voltage. Hence, the bias voltage is expected to be one of the other key parameters to determine the beam optics, and we have examined a relation of the beam optics to the bias voltage thoroughly.
Systematic scan of the bias voltage was carried out for different discharge powers with fixed extraction and acceleration voltages. It was observed that the beam optics sensitively changes with the bias voltage. This indicates that the bias voltage plays an important role for production of a high energy and high current beam.
In the paper, we will compare dependences of the beam optics on the bias voltage and the discharge power for hydrogen and deuterium plasmas in detail. Moreover, we will discuss the effect of the plasma grid bias on the meniscus formation.