Tokamak fusion test reactor poloidal rotation diagnostic (invited)

Abstract

A new spectroscopic diagnostic was developed to measure poloidal velocity profiles of Tokamak fusion test reactor (TFTR) plasmas. Carbon poloidal velocities were measured using the Doppler shift of the C VI 5291 Å impurity line of both intrinsic emission and charge exchange emission from neutral beams. Poloidal velocities are typically small (vθ⩽104 m/s) requiring small wavelength shifts (Δλ⩽0.2 Å) to be measured. However, the high central ion temperatures in TFTR required the use of a low dispersion spectrometer to view the entire linewidth (full width at half maximum ⩽25 Å). A very high throughput spectrometer/detector system was assembled to achieve the necessary precision in vθ. Statistical errors in the chord-averaged poloidal velocity less than 100 m/s have been obtained. The short focal length spectrometer features f/1.8 input optics, a transmission grating, and refractive optics. A thinned back-illuminated charge coupled device detector provided a high quantum efficiency (QE=75%). The diagnostic had 20 ms time resolution and a radial resolution of ⩽3.5 cm after an inversion. Two symmetric up/down views were used to eliminate atomic physics effects that would otherwise distort the measured rotation velocity. This diagnostic complemented the measurements of Ti, vφ, and ni from a charge exchange spectroscopy system, allowing the determination of radial electric field profiles using the force balance equation.