Ion source backplate loading due to backstreaming electrons and the arc discharge in the JET EP2 neutral beam injectors

The neutral beam power into JET can be increased by increasing the neutraliser gas flow (and gas pressure), hence ensuring a higher neutralisation efficiency. This has the potential to increase the loading on the ion source backplate due to backstreaming electrons. Measurements of the total backplate power loading due to backstreaming electrons and also the arc discharge are presented for the JET EP2 neutral beam injectors as the neutraliser gas flow rate is varied. The measurements are carried out for normal gas delivery operation and for the gas delivery method (grid gas delivery) that is required for the production of tritium beams at JET. Modelling of the backstreaming electron power load and the power distribution is also carried out and the former is compared to the experimental measurements. The backplate loading is more sensitive to the gas flow rate in the case of grid gas delivery. The measurements and calculations can be used to understand the thermo-mechanical performance of the system to balance the benefit of increased neutral beam power against potential increased fatigue and reduced lifetime of the backplate.