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

•A prototype negative ion source is under construction at ASIPP, the present filter field generated by permanent magnets is described and numerical analyzed.•An additional filter field concept based on an electrical current driven through the plasma grid is discussed.•The FEM analyses indicated the distribution of current flux density across the plasma grid.•Filter field topology parallel to PG is revealed in the two distances (2mm and 22mm) of the planes above the PG. In order to accumulate the research experience and technical reserves for future negative ion sources of nuclear fusion reactors, a prototype negative ion source is being designed and constructed at ASIPP with a reduced scale extraction area (166cm2 on the opening of 12×48cm2). Based on the existing negative ion sources having the similar width, an external magnetic filter with permanent magnets is applied to the prototype negative ion source during the initial experiments. For the specific experiments of magnetic field dependence, an adjustable magnetic filter is required. Hence, the plasma grid current filter for short-pulse or long-pulse source and a combination filter using permanent magnets and plasma grid current are designed. The main design purpose is to produce a homogenous and sufficient filter field in front of the plasma grid, with the supply current as low as possible. The filter field distributions of these different magnetic filters are studied in detail by finite element method, especially on the plane paralleling with the plasma grid.