Wavelength calibration of the charge exchange recombination spectroscopy system on the DIII-D tokamak

Abstract

A wavelength calibration of all the detectors on the charge exchange recombination spectroscopy system is performed after every plasma discharge on the DIII-D tokamak. This is done to insure that the rest of the wavelength position of the C VI 5290.5 Å charge exchange line on the detector is accurately known so that the Doppler shift of the spectral line emitted during the discharge can be used for measurements of plasma rotation. In addition, this calibration provides a check on the spectral dispersion needed to determine the ion temperature. The reference spectra for the calibration are Ne I lines created by neon capillary discharge lamps contained within specially designed diffuse reflectors. The Ne I lines at 3520.4720, 5274.0393, 5280.0853, 5298.1891, and 5304.7580 Å are used in this work. The location of these lines on the linear detectors can be determined to an accuracy of 0.1 pixel, which corresponds to a plasma rotation accuracy of 1.2 and 0.7 km/s for the central and edge rotation measurements, respectively. Use of oppositely directed views of the plasma at the same major radius have been used to verify that the nominal 5290.5 Å wavelength of the C VI (n=8–7) multiplet is the correct wavelength for the line emitted owing to charge exchange excitation.