Spatially-resolved TALIF measurements of atomic hydrogen density, temperature, and velocity in the Upgraded Pilot-PSI linear device*

Direct measurements of atomic and molecular densities in ITER-like divertor conditions are critical for constraining edge-modeling codes, like SOLPS-ITER. A Two-photon Absorption Laser Induced Fluorescence (TALIF) diagnostic was developed to make direct, spatially-resolved measurements of ground-state atomic hydrogen density (n_H) in similar conditions. A tunable dye-laser was used to excite H(n=1) to H(n=3) with λ ~ 205 nm and a chord of ~40 mm of H_α emission was imaged onto a gated ICCD. Measurements at the linear plasma device, Upgraded Pilot-PSI (UPP), over a range of plasma conditions (0.2 ≤ T_e ≤ 2 eV, 0.1 ≤ n_e ≤ 1.5 x10²⁰ m^-3), yielded n_H measurements from 7 x10¹⁸ to 2 x10¹⁹ m^-3_.  Densities were absolutely calibrated with the TALIF excitation at λ = 209 nm in xenon (5p⁶ → (²P_3/2)7f), and its associated fluorescence branch via 656.0 nm emission.

 

In addition, a two-temperature distribution of H atoms was observed with TALIF measurements in magnetized plasmas; one “hot” with T_H ~ 1 eV and one “cold” with T_H ≤ 0.1 eV. The hot population of H atoms are strongly coupled to the ion population and had rotational velocities up to ~10 km/s. The cold population showed negligible rotation. TALIF measurements were also made at various positions throughout the pre-sheath and up to the target-surface. These measurements indicate a doubling in n_H between upstream and near-target conditions, attributed to surface-recombination.