The unfolding of multichord Doppler spectrometry

A 20-chord Doppler spectrometer is used on the START spherical tokamak to record charge-exchange recombination (CXR) spectra (C5+ 5291 Angstrom, n=8-->7). "Passive" radiation, mostly from near the plasma edge, is also present, and comes from electron-impact excitation of C5+ as well as C6+ CXR due to thermal neutrals. The relatively wide neutral beam, together with the blending of the passive and active emission, present difficult unfolding problems. These are overcome by imposing physics constraints such as constant ion temperatures on equilibrium magnetic flux surfaces. It is also necessary to model the beam-neutral atom density throughout the viewed equatorial plane. A nonlinear least-squares fitting code, using suitable profile parametrization, gives unfolded T-i(r) and upsilon(phi)(r) profiles. These are presented along with estimates of the errors on the profiles. (C) 1999 American Institute of Physics. [S0034-6748(99)69801-X].