USE OF 1D D_α CAMERA TO MEASURE EDGE NEUTRAL AND ELECTRON DENSITY EVOLUTIONS

摘要

One of the clearest signatures of H-mode transitions in START and MAST plasmas is the sharpening of the edge visible radiation, indicating decrease in the edge neutral density and an increase in the edge electron density gradients. These densities are also very important factors in understanding the fuelling efficiency, H-mode access and dynamics, NBI edge loses, as well as interpretation and cross-calibration of a number of diagnostics. The neutral densities are obtained from the absolute D_α emissivity profiles unfolded from a linear camera and employing a collisional-radiative modelling of emissivity rates from the ADAS rate coefficients database and using T_e and n_e from the multipoint Thomson scattering. The near insensitivity of the D_α emissivity profiles to electron temperatures can be turned to good advantage in extracting electron densities. Here we describe that simulations and observations show an almost reciprocal dependence of the D_α spatial FWHM on the electron density at the position of the D_α peak. These values and the absolute D_α emissivity allow the neutral density to be determined at the D_α peak. This is being used in H-mode threshold studies as well as to optimise the fuelling and heating efficiencies. We also outline some anticipated developments and applications.