VOLUME RECOMBINATION AND OPACITY IN ALCATOR C-MOD DIVERTOR PLASMAS

Volume recombination within the Alcator C-Mod [I. H. Hutchinson et al. , Phys. Plasmas 1, 1511 (1994)] divertor plasma is measured and found to be a significant fraction of the total ion sink under detached dive rtor conditions. The recombination occurs in regions where T-e similar to leV and n(3) similar to 10(21)m(-3). Measurements of the spatial d istribution of the recombination are presented. The determinations of the recombination rates are made by measuring the D-0 Balmer spectrum and by using a collisional radiative model describing the level popula tions, ionization, and recombination of D-0. The concept of ''recombin ations per Balmer series photon'' is developed to simplify the determi nations. Measurements of the opacity of Ly(beta) emission are presente d. It is observed that up to 50% of the Ly(beta) emission is trapped, indicating that Ly(alpha) is strongly trapped in some cases. The effec ts of Ly(alpha,beta) trapping on the ''recombinations per photon'' cur ves are calculated and considered in the recombination rate determinat ions. Observations indicating the presence of molecular activated reco mbination are discussed. (C) 1998 American Institute of Physics.