Studying the influence of nitrogen seeding in a detached-like hydrogen plasma by means of numerical simulations

Abstract

The leading candidate for impurity seeding in ITER is currently nitrogen. To date, there are only a few studies on the plasma chemistry driven by N 2 /H 2 seeding and its effect on the molecular-activated recombination of incoming atomic hydrogen ions in a detached-like scenario. Numerical simulations are needed to provide insights into such mechanisms. The numerous amount of plasma chemical reactions that may occur in such an environment cannot be entirely included in a 2 or 3-dimensional code such as Eirene. A complete global plasma model, implemented with more than 100 plasma chemical equations and 20 species, has been set up on the basis of Plasimo code. This study shows two main nitrogen-included recombination reaction paths resulted to be dominant, i.e. the ion conversion of NH followed by dissociative recombination and a proton transfer between H 2 + and N 2 , producing N 2 H +. These two processes are referred to as N-MAR (nitrogen-molecular activated recombination) and have subsequently been implemented into Eunomia, a spatially-resolved Monte Carlo code, designed to simulate the neutrals inventory in linear plasma machines such as Pilot-PSI and Magnum-PSI. To study the effect of N 2 on the overall recombination, three cases of study have been set up: from a defined puffing location with a constant total seeding rate of H 2 + N 2 , three N 2 ratios have been simulated, i.e. 0, 5 and 10%. The parameter monitored is the density of atomic hydrogen, being the final hydrogenic product of any recombination mechanism in the scenario considered. The difference in H density between the 0% case and the 10% case is about a factor 3. The importance of NH as electron donor is highlighted and N-MARs confirmed as reaction routes enhancing the conversion of ions to neutrals, making the heat loads to the divertor plate more tolerable. This work is a further step towards the full understanding of the role of N 2-H 2 molecules in a detached divertor plasma.

Plasma-assisted catalytic formation of ammonia in N2–H2plasma on a tungsten surface

“…Another possible reason of this decrease can be a molecular assisted recombination process of nitrogen molecules in the presence of hydrogen. This process is proposed by Perillo et al 53 through a numerical simulation study of the nitrogen recombination mechanisms in N 2 /H 2 plasma. One of the possible mechanisms they presented (specifically for the plasma edge) has the following reaction paths: from 12% (no catalyst) to 32% (W) and from 5% to 24% for 25 and 50% of N 2 , respectively.…”

Plasma radiation studies in Magnum-PSI using resistive bolometry

“…As a result, the large atomic H density increases the fraction of line emission in addition to recombination radiation in the overall radiation yield. We thus conclude that experimentally obtained increase in radiated power loss as a result of N 2 seeding in H can be attributed to MAR processes which are intensified by H 2 -N 2 chemistry [33]. Figure 6a shows the time evolution of steady-state plasma operation in H while the background pressure in the target chamber is varied.…”

“…The mechanism driving the increase of radiation during H exposure in presence of N 2 was elucidated by using Eunomia [32] Figure 5 shows that increasing the background pressure under the influence of N 2 strongly increases the neutral H density. The formation of atomic H is facilitated via the presence of NH molecules which is identified as the main driver of so-called Nitrogen Molecular Assisted Recombination (N-MAR) causing enhanced recombination [33]. As a result, the large atomic H density increases the fraction of line emission in addition to recombination radiation in the overall radiation yield.…”

Investigating the effect of different impurities on plasma detachment in linear plasma machine Magnum-PSI

“…Such a mechanism is referred to as N-MAR, 27 and is characterized by an ion conversion promptly followed by dissociative recombination. This process effectively converts ion to neutral, thus dissipating energy from the plasma via volumetric recombination.…”

“…33 A wide and detailed global model for N 2 þ H 2 divertor relevant plasmas, together with a full description of the code, can be found in Ref. 27.…”