171. a study of a new recombination process of d2 plasma mediated by nd3 molecules

Department: Mechanical & Aerospace Engineering
Research Institute Affiliation: Center for Energy Research (CER)
Faculty Advisor(s): George R. Tynan

Primary Student
Name: Shota Abe
Email: sabe@ucsd.edu
Phone: 858-822-3672
Grad Year: 2017

Abstract
Impurity seeding of N2, Ne, or Ar will be mandatory to protect the tungsten divertor from the heat loads in future Tokamak devices. It has been reported that N2 seeding enhances radiation loss and improves plasma confinement efficiently rather than other species. However, nitrogen forms compounds with hydrogen, i.e. ammonia, in H2-N2 plasmas. Processes in N2 seeded hydrogen plasmas are not fully understood yet. There are studies of a volume recombination process of hydrogen plasma mediated by CxHy, which has attracted interest because carbon was one of probable candidates of the reactor wall material. The volume recombination has been recognized as being relevant to the plasma detachment phenomenon, which drastically reduces heat fluxes to the wall. In this research, a similar recombination process supported by ammonia is suggested. The process is initiated with charge/D+ exchange reactions between deuterium atomic/molecular ions Dx+ (x = 1-3) and ND3 molecules. After the reactions, the formed NDy+ (y = 3 or 4) ions are recombined with electrons. The rate of this electron-ion recombination is much faster than the e-i recombination rate of Dx+. Therefore, ND3 catalyzes recombination of hydrogen plasmas throughout those two steps. The initial step of the ND3-enhanced recombination process is studied by experimental and simulation methods in this research. The Etcher plasma machine is used to make low density plasmas (Ne∼10^16 m-3, Te∼4 eV) with D2 and N2 gas feeding. Total pressure is kept constant as 3.0 mTorr while partial gas pressure fraction is changed. A rate equation model is used to understand the atomic and molecular processes in the plasma. The model consists of 20 equations for 10 ion and 10 neutral species. The solution gives densities of those 20 species. To check applicability of the model to our experimental configuration, calculation results are compared with experimental density fractions, which is measured by a mass spectrometer combined with ion energy analyzer. The comparison shows good agreement qualitatively so that the model seems to capture main production/loss processes of ions in the plasma. The model shows that dominant neutralization channel of Dx+ in the volume are the creation process of NDy+ throughout the charge/D+ exchange reactions with ND3. This process is the initial step of the ND3-enhanced recombination process as predicted. Thus, the ammonia would strongly enhance neutralization of Dx+ in the divertor plasmas.

Industry Application Area(s)
Energy/Clean technology | Materials

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