Catalytic mechanism of divertor plasma recombination provided by hydrocarbon impurities

It is shown that the hydrocarbon molecules CxHy, when present in a divertor plasma, provide a powerful catalytic mechanism for volume plasma recombina tion at temperatures below 6-8 eV. The mechanism is based on the proton cha rge exchange with CxHy and the subsequent recombination of CxHy+ with plasm a electrons, while its catalytic property derives from the simultaneous cre ation of new hydrocarbon molecules (CxHy')-H-' in each charge exchange-diss ociative recombination cycle. The plasma recombining efficiency of CH4, C2H 6, and C3H8 molecules is for a factor of about 10, 30, and 40 higher than t he efficiency of the similar ion conversion-dissociative recombination mech anism based on the molecule hydrogen. A 2.5% of C3H8 in the divertor molecu lar gas has the same effect on divertor plasma recombination as the entire content of molecular hydrogen in the divertor. (C) 2000 American Institute of Physics. [S1070-664X(00)04611-5].