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Molecular Assisted Recombination Processes in N2 Seeded D Plasmas

  • Author(s): Abe, Shota
  • Advisor(s): Tynan, George R
  • et al.
Abstract

The new molecular assisted recombination [MAR] of hydrogen plasmas supported by ammonia is investigated and presented throughout this work. The plasma detachment scheme has been considered to be a practical approach to drastically reduce the harsh heat flux to prevent severe damage onto the divertor plates. In the detached divertor plasma operation, the plasma from the fusion core loses its energy by the radiation loss in the upstream divertor region, and is recombined before it touches the divertor plate in the downstream region, with help of impurity gas seeding. Nitrogen is the strong candidate of cooling gas species, and forms compounds with hydrogen i.e. ammonia. This hydronitrogen-enhanced MAR [HN-MAR] process would occur throughout two steps: 1) charge or ion exchange reactions between hydrogen ions and ammonia molecules, 2) recombination reactions between NH3+ or NH4+ ions and electrons. In this work, the HN-MAR process including those 2 steps is studied combining experiments and rate equation model. The PISCES-E plasma device is used to make plasmas (ne ~ 10^16-10^18 m-3, Te ~ 3-4 eV) with D2, N2, ND3, and Ar gas feeding. As the 1st step of HN-MAR, the formation processes of ND3+ and ND4+ are investigated in low density D-N plasmas (ne ~ 10^16 m-3). The model shows that dominant neutralization channel of D+, D2+, and D3+ in the volume is the creation process of ND3+ and ND4+ throughout the charge or ion exchange reactions with ND3. The 2nd step of HN-MAR is studied using high density D-N-Ar plasmas (ne 10^17=10^18 m-3), in which the recombination process of ND3+ and ND4+ cannot be negligible comparing to the wall loss. The result shows that, as the plasma density is increased to these larger values, the ND4+ density fraction undergoes a drastic decrease in good agreement with a 0-d chemical kinetics model. These results provide evidence for the dissociative recombination reactions in the HN-MAR process. Finally, it is strongly suggested that the HN-MAR process is major chemical reactions in our D-N plasmas.

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