High resolution measurements of neutral density and ionization rate in the main chamber of the Alcator C-Mod tokamak

Recent theoretical and experimental work has focused on the importance of neutrals in the edge dynamics of Alcator C-Mod plasmas. Two new high resolution detectors have been installed on the Alcator C-Mod tokamak to measure the neutral density and ionization rate near the edge of the discharge in the main chamber. The detectors consist of a 20 channel photodiode array which views the plasma tangentially 12.5 cm below the outer midplane, and 10 cm above the inner midplane, with a nominal radial resolution of 2 and 3 mm, respectively. The spectral bandwidth is limited by a filter with a very narrow band around the neutral deuterium Lyman alpha wavelength (1215 Å). The local emissivity is then obtained via a standard Abel inversion of the absolutely calibrated brightness profile. Employing well-known branching ratios, and using measured local electron density and temperature, we therefore, infer the neutral density and ionization rate with similar radial resolution. We have observed that both Lyman alpha emissivity and ionization rate are usually peaked near the separatrix with a full width, half-maximum between 1 and 2 cm. The neutral density was found to drop rapidly with decreasing minor radius, from 2–3×1017/m3 (5–10 mm outside the separatrix) to 1–2×1016/m3 (5–10 mm inside the separatrix) for a line averaged density of 2.0×1020/m3. Variations in ionization rate and neutral density from low to high confinement mode (L and H mode) are also discussed.