Charge exchange spectroscopy on the Lithium Tokamak eXperiment-β: calibration and initial results*

The Lithium Tokamak eXperiment has recently undergone an upgrade to LTX-β which includes increased field strength and the addition of a neutral beam. Neutral beam injection will allow for active charge exchange spectroscopy for measuring ion temperature, impurity density, and plasma rotation. For this reason, a new light collection system was designed, fabricated, installed, and calibrated on and in LTX-β. Spatial calibration ensured the tangency radii of the views pointing towards the beam matched those pointing away from the beam, for background subtraction and shot-to-shot spectral calibration. The new system has tangency radii which extend from outside of the last closed flux surface to within the magnetic axis (25-59 cm) during a majority of plasma conditions. For spectral and throughput calibration, an in-vessel, positionable integrating sphere was built and utilized, which was then calibrated to a standard light source. Prior to beam operation, the system will be used for passive spectroscopy of impurity species. The Thomson scattering system has also been upgraded which will increase the reliability of the charge exchange measurements.