A 15-point Thomson scattering diagnostic system is developed for ENN’s spherical torus experiment XuanLong-50 (EXL-50). A BeamTech laser with 3 J/pulse (1064 nm wavelength) at 50 Hz repetition rate is chosen for measurements during EXL-50 plasma operations. To enable measurements at low density (∼0.5 × 1018 m−3) plasma operations, the opto-mechanical subsystems are carefully designed to maximize the collection and transmission of the scattered light and to minimize the stray light level. In addition, the high bandwidth trans-impedance amplifiers and segmented high speed waveform digitizers allow for the application of muti-pulse averaging to further improve the signal-to-noise ratio. Details of the diagnostic system are described and initial experimental results are presented.

The Energy iNNovation’s XuanLong-50 is a spherical torus experiment with up to 10 s plasma operation duration. A 3 J/50 Hz pulsed laser is used in the Thomson scattering diagnostic system that is developed to measure the time evolutions of plasma electron temperature and density profiles. The expected laser pulse number is about 7.5 × 106/year with a power load of 150 W. To meet at least 1-year lifetime requirement, a Chevron type beam dump with polished molybdenum plates is designed and fabricated, which absorbs the laser beam energy in a 3D structure to reduce the laser fluence deposited on the material surface. To prevent the backscattered stray light from interfering with the Thomson scattering measurements, a 7.5 m beam path with folding mirrors is set between the beam dump and the plasma scattering volumes. Details of the beam dump design procedure including the laser beam profile control, multi-pulse laser damage threshold, heat dissipation, Zemax modeling, folding mirror selection, and beam path enclosure are presented together with the testing results.