Development of an ion beam measurement instrument for divertor simulation experiments in radio-frequency plasma

A retarding field analyzer (RFA) that consists of three grids and a collector was developed, and the measurement of an ion beam that passes through plasma was demonstrated. First, a suitable grid potential structure to allow the measurement of an ion beam in plasma was investigated. After this investigation, a helium ion beam was measured without the production of plasma. It was found that the helium ion beam current was significantly overestimated when an unoptimized potential structure was utilized. One probable reason for the overestimation is secondary electron emission. Next, ion beam measurement in low density helium ionizing plasma was conducted. Accompanying the onset of the beam extraction, the collector current clearly increased, which implies that the beam ions penetrated through the plasma and reached the RFA. Subsequently, similar measurements were conducted after the electron density of the helium plasma was changed. Since a nearly identical beam extraction condition was retained, the ion beam current obtained after plasma production was almost constant. However, the ion beam current obtained during plasma production increased as the electron density increased. A calculation of the ion beam envelope indicated that space charge neutralization by bulk electrons could account for the increase in the ion beam current.