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 recombination at temperatures below 6-8 eV. The mechanism is based on the proton charge exchange with CxHy and the subsequent recombination of CxHy+ with plasma electrons, while its catalytic property derives from the simultaneous creation of new hydrocarbon molecules (CxHy')-H-' in each charge exchange-dissociative recombination cycle. The plasma recombining efficiency of CH4, C2H6, and C3H8 molecules is for a factor of about 10, 30, and 40 higher than the efficiency of the similar ion conversion-dissociative recombination mechanism based on the molecule hydrogen. A 2.5% of C3H8 in the divertor molecular 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].