Extended discharge plasma channels for ion beam transport

Discharge channel transport of heavy ion beams through the reactor chamber of an IFE reactor is considered as an alternative approach with significant potential advantages compared to more conventional final transport schemes. To approach the requirements of an reactor application the channel length was recently extended from a length of 50 cm to 106 cm in experiments at GSI. A careful potential grading of the metallic discharge chamber was important to ignite the discharge across the extended electrode gap. Different initiation mechanisms for gas discharges of up to 50 kA were studied and compared for these long discharge channels. While in previous experiments laser gas heating of ammonia gas by a CO₂ laser was the standard ignition mechanism, currently ion beam induced gas ionization is used to initiate the discharge. This allows a wide choice of discharge gases and reduces MHD instabilities which were seeded by inhomogeneities of the CO₂ laser intensity distribution. The ion beam transport properties of the extended channel were studied using a 11.4 MeV/amu probe beam from the Unilac accelerator and a beam emittance measurement of the beam behind the transport channel is currently set-up.