Commissioning a high-repetition rate, 1 J neodymium-doped yttrium aluminum garnet laser for the helically symmetric experiment Thomson scattering system

The Thomson scattering diagnostic on the Helically Symmetric eXperiment (HSX) stellarator has been upgraded with a high-repetition-rate laser, enabling time-resolved measurements of electron temperature and density. The newly acquired InnoLas SpitLight EVO IV neodymium-doped yttrium aluminum garnet laser supports burst operation with 80 consecutive pulses at 400 Hz or four sets of ten pulses at 2 kHz, separated by a 25 ms cooldown period. The EVO IV also allows for continuous operation at 100 Hz. This newly accessible burst-mode operation enables the investigation of fast plasma phenomena on HSX, including plasma breakdown, magnetohydrodynamic activity, impurity injection behavior, and density profile evolution. To maximize beam energy, the laser was installed on an elevated optical table, eliminating the requirement for beam-steering periscope optics and reducing the optical path to include a single mirror interaction between the laser and the vacuum vessel. Beam polarization was adjusted using quarter- and half-waveplates, in conjunction with a Brewster exit window aligned to the magnetic axis. We characterized energy across the three operating modes, including continuous-mode warm-up behavior and differences between warm-rod and cold-rod operation in burst-mode, particularly regarding beam divergence caused by thermal lensing. We monitored per-pulse energy using a coherent energy monitor, which served as both a beam dump and a shot-to-shot diagnostic. Under the standard operational burst-mode of 400 Hz, we measured a mean of 852 mJ of through-vessel energy, corresponding to 89.0% of the available 957 mJ output energy.