Comparative experiments of different excitation frequency in a large-area RF negative ion source for NBI application

Abstract The excitation frequency is a critical parameter for the radio frequency (RF) ion source employed in the Neutral Beam Injection (NBI) system for the magnetic confinement fusion devices. Previous numerical studies indicated that a higher excitation frequency than 1 MHz, which is commonly used for the NBI RF ion sources, can significantly increase the plasma equivalent resistance and consequently improve RF coupling efficiency. In this work, the influence of excitation frequency was experimentally investigated in a large-area RF negative ion source designed for NBI applications. Comparative plasma discharge experiments were performed on a dual-driver RF negative ion source with a source size of 85 × 45 cm 2 , operated at excitation frequencies of 1 MHz and 1.5 MHz using the same RF power supply. Plasma parameters in the vicinity of the extraction region were measured using a set of Langmuir probes, while the equivalent resistance of the ion source was evaluated from RF current measurements obtained by an online current monitor. The results reveal that at 1.5 MHz, the equivalent resistance of the ion source exhibited an approximately twofold increase. The electron density near the extraction region increased, while the electron temperature dropped by approximately threefold and the plasma potential decreased by more than half. These findings provide valuable insights for enhancing ion source performance in NBI systems of fusion devices.