Optimization design of magnetic filter for the prototype RF negative ion source at ASIPP

For a prestudy of the key science and technology of the RF negative ion source for fusion application, a negative RF ion source test facility was developed at the Institute of Plasma Physics, Chinese Academy of Science (ASIPP). The magnetic filter field in front of the extraction system plays an important role in reducing the loss of negative hydrogen ions and inhibiting coextraction of electrons. The existing filter field of the prototype ion source is generated by permanent magnets arranged on both sides of the expansion chamber; the gradient and the uniformity of the field are poor, resulting in a large plasma distribution unevenness in the experiment. In order to reduce the B→×∇B drift and the beam deflection, the plasma nonuniformity, and the beam alignment, its gradient should be as low as possible, especially near the Plasma Grid (PG), while its strength should be as low as possible inside both the driver and the extraction region. Hence, the magnetic filter field generated by the permanent magnet and the PG current with return wires is proposed. A finite element analysis method is used to calculate the distribution of the magnetic field throughout the ion source, especially the filter profile along the centerline perpendicular to the PG and the section parallel to the PG. Several cases were compared and the final design provides a more uniform magnetic field in the region within 70 mm above the plasma grid, while the field strength is around 5 mT and the integral BdL quantity is greater than 1.2 mTm.