A Method to Measure the Electric Parameters of the Driver in a Fusion-Relevant RF Ion Source

RF ion sources for the neutral beam injection application contain a part called the driver where the plasma is generated. Measurement of the driver's external electrical parameters, such as coil current Icoil and driver equivalent impedance ZD, is useful for assessing the RF power coupling process and estimating the internal plasma parameters. The direct measuring method using RF I-V sensors at the driver's input terminal is inconvenient since it requires the installation of I-V sensors with very high phase (difference) accuracy near the driver. Thus, an indirect measuring method is developed for the RF ion sources equipped with the transformer-isolated Gamma-type matching network in this paper. The method uses the electric parameters measured before the matching network and an impedance matching model of the real-world matching network. In the matching model, the matching transformer is treated as a mutual inductance circuit, and the parasitic impedance of the matching network is considered. Then the method is applied to an RF ion source, HINT. With the real-time impedance and power measured before the matching network as inputs, the driver's electric parameters and the RF power transfer efficiency eta of HINT are monitored. Without plasma discharge, Icoil and ZD measured indirectly are consistent with those measured directly. With discharges (at transmitted power of 5-12 kW and filling pressure of 0.45-0.7 Pa), the deviations of indirect measurements from direct measurements are below 10% in Icoil, and then below 0.06 (absolute) in eta. The comparisons show that the indirect measuring method can be used as an alternative to evaluate the driver's electric parameters and eta in HINT.