Neutralization of negative hydrogen and deuterium ion beams using non-resonance adiabatic photon trap

Abstract

Experimental results on neutralization of negative ion beams in a photon stripping target are presented. For the energy of beams used in these studies (6–10 keV), the maximum neutralization efficiency is 95%. In contrast with gas or plasma neutralizers, the generation of positive ions from the negative ion beam was negligible. A non-resonance photon trap with highly reflecting mirrors of special shape was used as a stripping target. Due to a special mirror shape, in the process of photons reflections and propagation inside the trap there are some adiabatically conserved invariants that limits the volume accessible for photon motion even if the mirror surface is not closed. This approach makes it possible to overcome limitations typical for the Fabry–Perot cells, which require high radiation quality, high quality of the optical elements, and very high mechanical stability of the cell. The trap was pumped by commercially available fiber laser (λ = 1070 nm, Δλ = 7 nm, P = 2.1 kW) through a small hole in the mirror. The experiments were carried out with H- and D-beams. The observed neutralization efficiency depended mainly on the reflectivity of the mirrors, on defects on their surface, and on the laser pumping power.

Cavity-Enhanced Photodetachment of H− as a Means to Produce Energetic Neutral Beams for Plasma Heating

“…Investigations are carried out, on the other hand, on the possibility of having a H − or a D − beam efficiently photodetached by incoherent light recycling, in a so-called "photon cell" [17]. Both ways appear worthy of being investigated further.…”

A conceptual system design study for an NBI beamline for the European DEMO

“…Photo-(laser-)neutralisation was proposed for the DEMO NBI beamline [11,12] but only demonstrated at small scale far from the parameters required for neutral beam heating [13,14]. The idea is to pass the negative ion beam through an intense laser beam in an optical cavity with the laser wavelength chosen such that the photons can detach the loosely bound second electron, but cannot further ionize the resulting hydrogen atom.…”

“…Hence, the neutralisation efficiency is theoretically unlimited and in practice efficiencies in excess of 90 % are deemed achievable. Both resonant [11,13] and non-resonant [12,14] cavities for trapping the laser beam have been proposed and proof-of-principle tested. Besides the high neutralisation efficiency an additional significant advantage is that, as opposed to a gas or plasma neutraliser, a photoneutraliser does not introduce additional background gas into the beamline, reducing (re)ionisation and stripping losses and relaxing the requirements for the pumping system.…”

High power multiple mirrors optical path design and simulation of photo-neutralization

“…In 2018, a French research team conducted desktop experiments using a foldable resonant cavity and verified the stability of the cavity [6] . Due to the typical limitations of resonant cavities, many research teams have also developed non resonant cavities that can effectively reduce the laser flux that needs to be fed, but fine tuning the beam divergence becomes very difficult [7][8][9] . Theoretical calculations indicate that the neutralization efficiency mainly depends on reflectivity, surface defects of the reflector, and laser pump power.…”