MEASUREMENTS OF NEUTRAL DENSITY IN THE ALCATOR C-MOD TOKAMAK

摘要

In magnetically confined plasmas, the study of edge dynamics has many important implications which include overall confinement and performance, and, in particular, calls for a quantitative understanding of the particle inventory and its sources. In fact, when studying the particle, momentum and energy balance of the edge plasma, one must include the contribution of neutral particles. However, direct measurements of the neutral population, i.e. neutral density and associated ionization rate, are particularly arduous in that region. Based on a simple measurement of the excitation of neutral deuterium (and hydrogen), we installed a series of diagnostics in the Alcator C-Mod tokamak, from which we can obtain those two quantities with high radial resolution. In addition we installed a series of modified Penning-type gauges to measure the neutral pressure in close proximity of the discharge itself. The Alcator C-Mod tokamak is based on a compact design, with high magnetic field, high density and high power density (ohmic and RF heating). Typical densities of ～2xl0~(20) m~3 and ～5x10~(19) m~3 are found at the center and at the edge of the plasma, respectively. Measured neutral pressures, as obtained by standard ion gauges located away from the plasma at the outer wall, range from 0.01 to 1mTorr. Since the various relevant edge scale-lengths typically are found to be smaller than 1cm, it is important that we measure with sufficient resolution the ionization rate and the neutral density in that region. By integrating the ionization rate over the volume at the edge, we obtain directly the neutral particle influx in the scrape-off layer and into the main discharge, and we can obtain directly the local effective diffusion of the main ions.