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(EX3/3) Detached Divertor Plasmas in ALCATOR C-Mod: A Study of the Role of Atomic Physics

B. Lipschultz¹, C. Boswell¹, J. Goetz¹, C. S. Pitcher¹, J. Terry¹, J. Weaver², B. Welch^2*, A. Hubbard¹, S. Krasheninnikov¹, B. LaBombard¹, D. Pappas¹

¹ M.I.T. Plasma Science and Fusion Center, Cambridge, Ma., 02139, USA
² Institute for Plasma Research, U. Maryland, Maryland 20742, USA
^* present address Applied Research Center, 12050 Jeffereson Ave., Newport News, Va. 23606.

Abstract.  Detailed profiles of the volumetric recombination occurring in Alcator C-Mod plasmas are presented. During detachment the recombination sink is compared to the divertor plate sink as well as the divertor ion source. Depending on plasma conditions, volume recombination removes between 10 and 75% of the ions before they reach the plates. A second, equally important process that leads to a drop in plate ion current is inferred to be a reduction in divertor ion source, which is correlated with a drop in power flowing into the ionization region and the pressure loss of detachment. For high $\overline{n}_{e}^{}$ the divertor recombination can cross the separatrix near the x-point, cool the core and lead to a disruption. Experimental measurements show a difference in ion and neutral velocities for H-mode detached plasmas. The resulting ion-neutral collisions are found to be more efficacious than recombination in removing momentum from the ions. The neutral component of volumetric power emission from the divertor has been measured by means of a novel filtering technique to be substantial ($\sim$ 20% of the total divertor volumetric emission).

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