Influence of cusp magnetic field configuration on plasma parameters of radio frequency driven negative ion source for CRAFT NNBI

Abstract

Radio Frequency (RF) driven negative ion source is the preferred ion source for negative ion based Neutral Beam Injection system (NNBI). The plasma generator, which consists of RF driver(s) and an expansion chamber, is one of the key parts for NNBI. The distribution of cusp magnetic field which is generated by permanent magnets installed at the lateral wall of the expansion chamber is a common issue, due to the great influence on electron density, electron temperature and spatial uniformity of plasma density in the extraction area. Two kinds of configuration of cusp magnetic field (checkerboard type and symmetric type) and three types of arrangement of permanent magnets (with three, four and five rows of magnets) have been carried out with the approach of finite element and the experimental performance. On the RF negative ion source testbed, a moveable Langmuir probe has been adopted to investigate the characteristic of electron density and temperature near the extraction area. Since the symmetric type with three rows of permanent magnets generates the distribution of cusp magnetic field that contribute to extract the plasma from RF driver and improve the probability of particle collision easily, the higher electron density and the lower electron temperature was obtained via simulation. The experimental conclusion is that the symmetric type with three rows of permanent magnets leads to a higher electron density and lower electron temperature, showing a great agreement with simulation results. The results are helpful to optimize the structure and performance of high power ion source, and provide support for the development of large area RF negative ion sources with multi-driver.