Measurement of MAST edge ion temperatures and velocities

A novel experimental method using gas puffing of neutrals to stimulate charge exchange emission has been developed to measure ion temperatures and toroidal rotation velocities in the edge of MAST. This uses cold deuterium gas puffing as the source of neutrals in place of the more usual method of donation from neutral beams. A pre-existing spectroscopy system with 60 toroidal chords with spatial resolution of 1.5 mm and temporal resolution of 10 ms was adopted for this purpose. The C6+(n = 8 → 7) transition at 5290.5 A was identified as the optimal emission line, while a resonance in the cross section between the D0(n = 2) and C6+(n = 8) energy levels makes the measurement feasible.
In order to assess the potential of this measurement method a model of the expected performance of the system was created. All important processes in the edge regime were identified and density profiles of electrons, carbon ions and deuterium neutrals were modelled. These were combined to produce a prediction of performance.
The spectrum observed was complicated by molecular deuterium emission and a fitting routine to this complex data was developed and benchmarked. Correction factors were also
identified and calculated for the emission line. System improvements were made including a new bandpass filter, gas valve and camera.
Analysis of the measured intensity profiles showed a good response to gas puffing and toroidal velocity and ion temperature results are presented. A large reverse shear in the toroidal rotation is observed close to the separatrix in normal operation, but not in counter-rotation experiments. This is attributed to parallel Pfirsch-Schluter flow. The ion temperature results display a large difference between ion and electron temperature in the pedestal region in many H-mode discharges. This is linked to the collisionality of the ions such that flux surface coupling in the banana regime leads to small ion gradients.