Neutralisation of intense ion beams at JET

Neutralization is an important issue in the design of neutral beam injection (NBI) systems for heating and fueling fusion plasmas. It has long been established that the neutralization efficiency of the JET injectors falls short of the theoretical value as determined by the expected gas target thickness and the well-known reaction cross-sections. This problem is even more acute in the recently upgraded 130 kV/60 Amp injectors at JET. An illustration shows the measured deuterium power delivered to JET as a function of extracted beam power. It has been proposed that the efficiency deficit is caused by a reduction in the line integrated gas density in the injector's neutralizer. This density reduction could be due to a number of factors including wall pumping and modification of the flow regime, but recent spectroscopic measurements of the neutralization system suggest that the density depletion is due to heating of the neutralizer gas. The mechanism responsible for this is thought to be indirect energy transfer from the beam to the neutral gas via the low temperature plasma that is formed in the neutralization process inside the neutralizer cell. This paper presents measurements of neutralization efficiency based on calorimetric data from the JET injectors. We also present some results from an experimental investigation at the JET Neutral Beam Test Bed including measurements of the low temperature neutralizer plasma. The plasma measurements are used as input parameters for a gas heating model and the outputs of this model are compared to a spectroscopic measurement of the neutralizer gas temperature. Overall, the results support hypothesis that the neutralization deficit is caused by gas density depletion due to indirect heating of the neutralizer gas.