High energy X-ray measurements during lower hybrid current drive on the Alcator C tokamak

High energy X-ray emission (E_γ> 20 keV) from superthermal plasma electrons during lower hybrid current drive on the Alcator C tokamak has been measured using sodium iodide (NaI) scintillation spectroscopy. The X-ray spectra are generally linear on a semi-log plot of count rate versus photon energy and extend out to several hundred kiloelectronvolts. For the range of densities (_e≈(0.3–0.8) × 10¹⁴ cm⁻³) over which current drive was performed on Alcator, there was negligible emission before the injection of radiofrequency wave power. The radial profiles of the emission were also measured and indicate that the current carrying high energy electrons exist primarily within the inner half (r/a < 1/2) of the plasma column. Plasma parameter scans produced variations in the X-ray emission profiles that are consistent with changes in the launched Fourier power spectrum and the conditions imposed by lower hybrid wave accessibility. In addition, the velocity space distribution function of the energetic tail electrons has been determined using the angular variation in the X-ray emission.