  In the fusion experiment ITER powerful neutral beam injection (NBI) systems
will be used. The NBI's core component is a negative hydrogen ion source, which
is based on a modular concept. Eight cylindrical drivers, each having a volume
of several liters, are attached to one common expansion and extraction region.
Within the drivers an inductively coupled plasma is sustained by an external
cylindrical coil at filling pressures not larger than 0.3 Pa. Radio frequency
(RF) generators operating at a driving frequency of 1 MHz feed the coils via a
matching network with powers of up to 100 kW per driver. These high powers
entail high voltages, which make the ion source prone to electrical breakdowns
and arcing, wherefore its reliability is reduced. Moreover, at the ITER
prototype RF ion source not more than 60% of the power is absorbed by the
plasma, whereas the rest is lost for heating the RF coil and conducting
structures of the driver. To optimize the power coupling in the prototype
source, a previously validated self-consistent fluid-electromagnetic model is
applied. The optimization studies reveal a complex interplay between network
losses (mainly caused by the skin effect and eddy currents), and nonlinear
plasma phenomena, such as the RF Lorentz force. The model demonstrates
promising optimization concepts for the RF coupling in future NBI ion sources.
In particular, by increasing the axial driver length and the driving frequency
it is possible to enhance the fraction of absorbed power to values around 90%.
