Achieving high amplifications in a cw-driven optical cavity relevant for photoneutralization of negative ion beams

In order to overcome the drawback of the limited neutralization efficiency of a gas neutralizer, laser neutralization is discussed for negative ion based neutral beam injection systems for future fusion devices. Those could easily deliver neutralization efficiencies far beyond the 60% constraint of conventional systems, in principle only limited by the provided optical power. For fusion-relevant ion beams, optical powers beyond tens of MW would be required, which poses the challenge of reliably providing such high powers in cw operation. Measures to amplify the optical power are thus mandatory and high-finesse optical cavities wrapped around the ion beam can be applied for this purpose. The target is to reduce the initially required laser power by three to four orders of magnitude and maintain stable coupling of the laser and the enhancement cavity in resonance. In order to confirm that such high amplifications can be reliably obtained in cw, an independent optical test bench was set up. The setup proved that using a two-fold feedback system (Pound–Drever–Hall locking scheme), stable resonance locking of laser and cavity for more than an hour is feasible. Furthermore, amplifications of 7’000 could be achieved experimentally, reaching in-cavity powers in the range of several kW, driven by only 240 mW input power. Approaches to integrate such a laser neutralizer setup at the negative ion beam facility Batman Upgrade and options for cavity folding are discussed in terms of the achievable neutralized fraction of the ion beam.