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

• This study provides experimental evidence of a transition from hydrogen molecular-activated recombination (H-MAR) to nitrogen molecular-activated recombination (N-MAR) when both hydrogen and nitrogen are seeded in the D-module of GAMMA 10/PDX, showing the coexistence of both MARs for the first time. • The ratio of H α emission line to H β emission line, the indicator of H-MAR, decreases by nitrogen seeding. This result means that the transition of H-MAR dominant phase to N-MAR dominant phase occur. • The reduction and displacement of H-MAR occur from the V-shaped target. This result indicates that W surface acts as catalyst in ammonia production on the target. • Nitrogen seeding significantly enhances plasma detachment even at low electron temperatures (∼5 eV), likely due to the increase in NH x neutral density. 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 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.