Study of volume recombination and radiation opacity effects in Alcator C-Mod

Abstract

Observations of significant volume recombination within the Alcator C-Mod divertor plasma and in the edge plasma (MARFE) are described. The recombination occurs in regions where Te≲1 eV and ne≳1×1021 m−3. The determinations of the recombination rates are made by measuring the D0 Lyman and/or Balmer spectra and by using a collisional radiative model describing the level populations, ionization and recombination of D0. In regions of strong recombination the upper levels (n≳4) populations are close to those determined by Saha-Boltzmann distribution and are independent of the ground state density. Thus the intensities of lines from these levels are related to the recombination rate, and curves determining the number of ‘recombinations per photon’ are calculated. Lyβ line emission is shown to be trapped in some cases, meaning that Lyα can be strongly trapped. Since opacity affects the recombination rates, the effects of the trapping of Lyα,β photons on the ‘recombinations per photon’ curves are calculated and considered in the recombination rate determinations. Total recombination rates in the detached divertor plasma and in MARFEs located at the periphery of the main plasma are determined. Recombination can be a significant sink for ions.