Beam energy recovery for fusion: Secondary electrons problem study for the proposed experimental test

The Experimental Fusion Reactor ITER will use fast Neutral Beams (NBs) injectors to increase the plasma temperature in order to drive the plasma current necessary for stability. The NBs are produced by D- beams, which are generated by negative ion sources and neutralized by gas cells with an efficiency of 60%. In order to improve the electrical efficiency of the NB production, a beam recovery system of the residual charged particles could be foreseen. In particular, we have already proposed an energy recovery device with an axisymmetric cylindrical ion collector. A scaled ITER type negative ion source, NIO1 (Negative Ion Optimization 1), has been developed at RFX (Padua) for studying and improving the ion extraction efficiency. An experimental test on beam energy recovery by using the NIO1 ion beam has been recently proposed. However, one important issue related to the beam energy recovery are the secondary electrons generated by the interaction of the ions with the collector walls which may escape from the device, reducing energy recovery efficiency and cause damages. In this contribution, the problem of minimizing the secondary electrons production in the energy recovery device has been studied and simulated. Although a fraction of the secondary electrons escapes from the ion collector, they are greatly stopped in the axisymmetric geometry cup. A simple test bench for secondary electron emission to verify the simulation results is in preparation at INFN-BA laboratory.