Assessment of plasma uniformity in the extraction region of an RF-driven negative ion source for CRAFT NNBI

The uniformity of beam, for large RF negative ion source, is closely related to the plasma uniformity in the extraction region. The six electrostatic probes were installed above the plasma grid (PG) 5 mm to evaluate the plasma uniformity. The influence of RF power, source pressure, magnetic filter field, and bias voltage on the plasma uniformity in the extraction region were analyzed. In the absence of magnetic filter field and bias voltage, the plasma density exhibits good uniformity (>90%), but the uniformity of electron temperature (Te) is poor (>60%). It is mainly caused that the magnetic field was generated by the permanent magnet inside extraction grid (EG). The plasma density and Te can be increased by increasing RF power, but it has little effect on the plasma uniformity. Increasing source pressure can effectively lower Te and slightly improve its uniformity, but the uniformity of plasma density has deteriorated by increasing source pressure. While increasing the PG current effectively reduces the Te in the extraction region, it simultaneously degrades the uniformity of both Te and plasma density due to drift effects. Increasing the bias voltage can increase Te in the extraction region when the PG current is 1200 A, but Te at the bottom increases more significantly, thereby worsening the uniformity of Te. In addition, the electron density (Ne) at the top decreases, while the Ne at the bottom increases, thereby improving the uniformity of Ne. The influence of the magnetic filtered field and bias voltage on the non-uniformity of Te in the extraction region, as observed in the manuscript, is similar to the effect on the non-uniformity of the extracted beam current reported in the literature. These results provide theoretical foundations for optimizing the beam uniformity in large RF negative ion sources.