The role of photon self-absorption on the H (n= 2) density determination by means of VUV emission spectroscopy and TDLAS in low pressure plasmasTo cite this article before publication: Frederik Merk et al 2020 Plasma Sources Sci.

The production of H- ions in negative ion sources relevant for particle accelerator facilities and neutral beam injection systems is based predominantly on the surface conversion of H atoms at a low work function surface covered with caesium (the plasma grid). Therefore, the H atom density n H and energy distribution function (EDF) close to the plasma grid determine the amount of surface produced H- ions. As a direct method for the density and EDF determination, two-photon absorption laser induced fluorescence (TALIF) on H atoms was implemented at the ion source of the teststand BATMAN Upgrade being the first time that this was accomplished at an H- ion source. Several challenges had to be overcome concerning the application of the diagnostic at the complex facility and the evaluation of the fluorescence signals against a bright Hα background. The observed line profiles suggest a Maxwellian EDF with an H atom temperature of (2000 ± 500) K. The presence of highly energetic H atoms (measured by optical emission spectroscopy, OES) could not be resolved by the TALIF system due to the insufficient signal-to-noise ratio. Atomic densities were measured for H2 and D2 plasmas for varying ion source parameters at BATMAN Upgrade resulting in values between 3×1018 m-3 and 1.1×1019 m-3 for hydrogen. For the operation with deuterium, 30% higher atomic densities are observed for similar ion source parameters which agree well with the previous results obtained with OES.

The large ion source of ITER’s neutral beam injection (NBI) systems (0.9 m×1.9 m) with 1280 apertures has to deliver 57 A D- for 3600 s (286 A/m2) and 66 A H- for 1000 s (329 A/m2). The RF ion source test facilities ELISE and BUG at IPP are aimed to demonstrate the ion source parameters, the homogeneity of large beams (up to 1 m×1 m) and to perform beam optic studies. While the ITER parameters could be demonstrated in hydrogen, the achievement in deuterium for long pulses is still pending due to the large fraction of co-extracted electrons, their temporal dynamics, and inhomogeneity in vertical direction, limiting the ion source performance. Biasing of the bias plate in the vicinity of the plasma grid improves the symmetry of the co-extracted electrons and contributes to its stabilzation being thus a promising alternative to potential rods formerly used to achive high performance. With the replacement of the high-voltage power supply at ELISE first 100 s steady state extraction is demonstrated increasing the relevance of the test facility for ITER and DEMO studies. For the latter, the IPP contributions focus on improvement of the RF coupling, the caesium management and conceptual studies of a beam driven plasma neutraliser as alternative to the gas neutraliser system.

Laser neutralization of an accelerated negative ion beam is a promising alternative to the gas neutralizer of current NNBI systems. It has the prospect to boost the overall wall-plug efficiency of the NNBI system from currently about 28 % at ITER to about 60 % for the next step after ITER, a DEMO tokamak. Laser neutralization requires resonant coupling of a cw laser to an amplification cavity built around the ion beam. In the final stage, the amplified laser radiation of several hundreds of MW needs to intersect an ion beam cross section of ∼ m 2 at energies up to 1 MeV. Such a sophisticated concept needs to be tested in small scale at laboratory experiments. Challenges include achieving coverage of the entire ion beam cross section, thermal lensing of the cavity mirrors due to the huge optical power loads as well as coupling to a cavity under vibrations due to vacuum pumps and possible interferences during ion beam extraction. Using a medium power laser (8 W), investigations regarding the influence of vibrations are performed on an optical table, while in a second step the application to an ion beam with reduced cross section (∼ mm 2 ) and beam energy (∼ keV) will be studied.