Synergistic effect of nitrogen and hydrogen seeding gases on plasma detachment in the GAMMA 10/PDX tandem mirror

We have investigated the synergistic effect of a combination of various impurity gases and hydrogen gas on plasma detachment of high temperature plasma, equivalent to scrape-off layer plasma of tokamaks in the GAMMA 10/PDX end region, utilizing an open magnetic field configuration. A small puff of an impurity gas (N₂, Ne, Ar, Kr, Xe) in combination with a puff of H₂ gas is examined to evaluate their synergistic effect on the formation of detached plasma; the following results are obtained. (i) A combination of N₂ and H₂ puffs showed a clear decrease of electron density and ion flux; (ii) N₂ and H₂ puffs form a strong density gradient along the axial direction; and (iii) other noble impurity gases showed an insufficient synergistic effect. The new results indicate the possibility of achieving a reliable divertor operation scheme and the importance of a deeper understanding of the H₂ and N₂ assisted recombination process. (paper)