Isotope Effects in Molecule Assisted Recombination and Dissociation in Divertor Plasmas

Abstract

A burning fusion plasma will necessarily contain a mixture of D and T iso-topes, in the atomic, molecular and ionised components. It is demonstrated that molecule assisted recombination (MAR) and dissociation (MAD) mech-anisms expected to be strong in typical low-temperature hydrogenic diver-tor plasmas exhibit a kinetic isotope effect. The effect originates from the mass dependence of rate coeffcients of dissociative electron attachment and atomic-to-molecular ion conversion reactions, that are common precursors for MAR and MAD mechanisms. MAR favours a faster recombbination of the lighter ion with plasma electrons, while MAD favours a faster dissociation of the lighter molecule. The effect is cumulative, and during the ion residence time in the divertor it may produce significant differences in the recombined electron-ion pairs of the two isotopes, and in the amounts of dissociation products of the two isotopic molecules. For example a factor of 3 - 9 in MAR, and 22 - 400 in MAD in the concentrations of recombination and dissociation products results, for T = 3 eV, n$_{e}$ = 3 $\cdot$ 10$^{14}$ cm$^{-3}$ and [H$_{2}$]:[D$_{2}$]:[HD] in the range 1:1:0 - 1:1:2. Some implications of MAR and MAD isotope effects for divertor plasma physics are discussed