In Negative-ion based Neutral Beam Injection systems (NNBI) for fusion, a hydrogen plasma is generated via inductive RF coupling at a frequency of 1 MHz inside the ion source in cylindrical vessels, called drivers. At low gas pressures of 0.3 Pa, electron densities and temperatures of 1e18 m⁻3 and 10 eV are reached. Only a fraction η of the generator power of up to 100 kW per driver is absorbed by the plasma, the rest is lost via eddy currents in the RF network, the internal Faraday screen and the surrounding steel structure. Since at 100 kW, the RF components work close to their technological limits, it is desirable to use lower generator powers while increasing η. To optimize the RF coupling with respect to e.g. RF frequency or geometry, a 2D cylindrically symmetric multi-species fluid model is used, which describes the coupling between the RF fields and the electrons in the stochastic heating regime self-consistently. The model is successfully validated with electrical and Langmuir probe measurements from the BATMAN Upgrade ion source testbed, where η is measured to be around 45 - 65%. At the high power low pressure regime of the ion source, effects such as neutral depletion, the ponderomotive force and a cusp magnetic field generated by permanent magnets are shown to be important for the correct simulation of the measured trends. The predictive model is then utilized to study the impact of the driver and coil geometry, as well as the RF frequency on η.
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