Analysis of discharge characteristics of EAST NBI hot cathode ion source coupled with external circuit

Hot cathode ion sources are widely used in large-scale neutral beam injection (NBI) systems, but in practical engineering, there are still some problems that affect their stable operation, so further research is needed on the mechanism of arc initiation and the operation of hot cathode ion sources. In a complex electromagnetic field environment, it is challenging to measure the initial discharge process at the ns scale. This research focuses on this transient process in the Experimental Advanced Superconducting Tokamak NBI hot cathode ion source, exploring its mechanism through a 3D multiphysics coupled model, and further highlights the function of designing a ‘floating plate’. The effects of the main parameters of the ion source are discussed, including floating plate size, external circuit parameters, and magnetic field. Results show that, in the initial arcing process, the floating plate controlled by the external circuit has a slightly stronger space charge limitation on the filaments compared to the discharge chamber structure with pure anode side wall. But the entire arcing process can reach the steady state faster, and the wall loss of the emitted electrons in the subsequent steady state is less. Compared with the conventional assumption that the floating plate is fixed at 50 V, in the case of considering the external circuit, the space charge limitation is smaller, and the filament can also quickly transfer to the required thermionic emission limited mode. When the floating plate area accounts for 15% of the side wall area of the discharge chamber, the above characteristic is more obvious. The arc voltage almost determines the energy of electrons at different positions and has an important influence on ionization efficiency. The divider resistor of the external circuit can affect the stability of the discharge process. The magnetic field generated by filament current seriously affects the electron trajectory and restricts the emission of primary electrons. The multiphysics coupled model in this research offers analysis methods and optimization ideas for similar ion sources, providing references for the design of hot cathode ion source.