Coexistence of H-MAR and N-MAR in divertor simulation experimental module of GAMMA 10/PDX

Abstract

In a divertor simulation experimental module (D-module) of GAMMA 10/PDX, additional hydrogen seeding induces molecular-activated recombination (H-MAR). However, when seeding of hydrogen and nitrogen is combined, Nitrogen MAR (N-MAR) is observed in D-module. In this work, we experimentally investigate plasma detachment processes in D-module by seeding hydrogen-only and hydrogen + nitrogen in order to de-couple MARs contribution to the measured particle flux reduction. Nitrogen is seeded after the electron density, measured with Langmuir probes installed in D-module, begins to decrease due to hydrogen seeding. In both cases, the electron temperature near the V-shaped target decreases from ∼23 eV to ∼2 eV. A clear rollover is observed in the ion flux to the V-shaped target. When only hydrogen is seeded, the Balmer lines emission ratio (i.e. the intensity of H_α emission line (I_Hα)/the intensity of H_β emission line (I_Hβ)), an indicator of H-MAR, is observed to increase between V-shaped targets installed in D-module as the amount of hydrogen gas increases. However, when both hydrogen and nitrogen are seeded, I_Hα/ I_Hβ increase is suppressed, and the region of the largest I_Hα/ I_Hβ signal shifts upstream of D-module, as observed by a high-speed camera. These results show for the first time the transition from H-MAR-dominated phase to N-MAR-dominated phase and the coexistence of both MARs in D-module. It is found that nitrogen seeding increases plasma detachment even at already relatively low T_e (∼5 eV). The high-speed camera measurements suggest this effect to be related to the increase of NH_x neutral density, in line with previous modeling works.