Beam energy recovery for fusion and collector design for tests on compact sources

Abstract

The next fusion project DEMO, which will be the evolution of the experimental fusion reactor [International Tokamak Experimental Reactor (ITER)], would require a high efficient energy production. As in ITER, DEMO will use fast Neutral Beam (NB) injectors to increase the plasma temperature needed for the fusion reaction. A way to recover the electric energy production efficiency in DEMO could be the beam energy recovery in the NB production, which is produced by a D− beam, neutralized by a gas cell with 60% efficiency. A compact energy recovery device with an axisymmetric cylindrical ion collector that uses only decelerating electric fields combined with the beam space charge effect has been recently proposed. It can be used for a test on the beam of the NIO1 (Negative Ion Optimization 1) source, a compact ion source (scaled down from ITER size sources) that has been developed at INFN-LNL and Consorzio RFX (Padua). The detailed collector design to be used on one of the beamlets of the NIO1 source within typical space limitation is presented and discussed here. Furthermore, a preliminary trajectory simulation for a beam with a rectangular geometry similar to the beam used in ITER to verify the beam recovery for a nonaxial symmetric geometry is also shown.

Latest experimental and theoretical advances in the production of negative ions in caesium-free plasmas

“…Study of new concept for energy recovery (see Fig. 20) was recently adapted to space limitation typical of NIO1 [126] or other similar sources; note that these energy recovery studies are not funded by Eurofusion. Other upgrades of beam diagnostic (including a strongly cooled pepper pot calorimeter and removable faraday cup) and of ion source (Faraday screen, dual gas injection) are foreseen.…”

“…The magnetic deflection may require considerable energy and large yokes, so electrostatic deflection is usually preferred (with possible issues of sheath stability). Our concept joins deceleration and ion removal, achieved by the defocusing due beam deceleration itself [126]: so the secondary electron emission (SEE) due to stopped H − impact is minimized (to Eurofusion request, we repeat that this concept is not funded by them, as noted before). To fit inside a CF200 side port (many ports are available in NIO1, see Fig.…”

“…To fit inside a CF200 side port (many ports are available in NIO1, see Fig. 20a, especially B or C), the proposed prototype is limited to 20 kV beams, with typical collector voltage V c = −19.9 kV and repel voltage V r = −20.3 kV; SEE back streaming is suppressed by low impact energy and radial geometry, as shown by simulations [126].…”

“…Note that H + beam recovery is possible with a subsequent similar electrode set, with much easier SEE trapping; moreover, the plugin can be tested also on different sources [126]. Moreover, the NIO1 cooled calorimeter installation is slowly progressing, allowing some isolator addition for testing some energy recovery capability; calorimeter also integrates a pepper pot hole array for possible emittance measurement.…”