Spectroscopic Characterization of Ablation Plasmas in the Olmat HHF Facility

Abstract

OLMAT (Optimization of Liquid Metal Advanced Targets) is a new High Heat Flux (HHF) Facility for testing solid and liquid metallic targets at DEMO-relevant power densities that has been installed, commissioned and first operated at the National Fusion Laboratory in Madrid in 2021 [1] . A Neutral Beam Injector (NBI) is used as a high-power source providing 100 ms pulses at maximum repetition rate of 1p/30s and delivering power densities at the target (Mo, W, liquid Sn) above 50 MW/m 2 [2] . A calibrated survey spectrometer covering the UV-visible-near IR ranges provided information about the impinging hydrogenic species, plasma composition and its microscopic parameters, while a Balmer Ha (656.3 nm) monitor, a 16-Channel PMT array and a fast-frame imaging camera were used for the characterization of particle recycling and the plume’s spatial structure. Moreover, the floating voltage and saturation current developed at the target was monitored on a shot-to-shot basis. In addition to the above, nitrogen injection was performed through a dedicated piezoelectric valve for active spectroscopy. Plasma temperatures between 1 and 2 eV were deduced from the ratios of H Balmer lines, in line with the recorded values of floating potential, but higher, inconsistent values were obtained from the ratio of molecular N 2 and N 2 + bands. The facility and the plasmas therein generated will be described.