Numerical study of RF power coupling in fusion-relevant single- and multi-driver H$^-$ ion sources

ITER's large and powerful neutral beam injection system is based on an ion source utilizing a modular concept, where eight cylindrical drivers are attached to one common expansion and extraction region. In each driver, a plasma is sustained via inductive coupling with powers of up to 100 kW at a driving radio frequency (RF) of 1 MHz to produce fusion-relevant hydrogen beams. These high powers impose great stress on the electric system. Recent measurements at the single-driver test bed BATMAN Upgrade showed that the RF power transfer efficiency $\eta$, which measures the ratio of power absorbed by plasma to total RF power, is only around 0.5, leaving room for optimization. In multi-driver test beds such as ELISE with four drivers $\eta$ is found to be even further decreased to around 0.4. To explain this difference, a previously validated self-consistent 2D RF power coupling fluid model is applied. For the same absorbed power per driver, the model shows virtually the same spatial distributions of plasma parameters and power absorption in single- and multi-driver sources. However, the coil current is slightly increased in the multi-driver model due to a changed spatial distribution of the magnetic RF field in the region surrounding the drivers. Typically, in multi-driver sources conductive shields are applied to cancel the electromagnetic interference between individual drivers. These shields are found to affect the spatial distribution of the RF fields more severely, the effect being highly dependent on the distance between the RF coil and the shield. In the case of the ELISE ion source a further decrease of $\eta$ is calculated by the model being in good agreement with experimental measurements.