Use of 1D D_α Camera to Measure Edge Neutral and Electron Density Evolutions

Abstract

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 gradients¹. 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 camera¹ and employing a collisional-radiative modelling of emissivity rates from the ADAS² rate coefficients database and using T _e and n _e from the multipoint Thomson scattering³. 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.