#68
Crossed atom beam experiment for study of ammonia production in surface
reactions of neutral hydrogen and nitrogen atoms
A. Drenik, R. Zaplotnik, G. Primc, M. Mozetič, S. Markelja
a
Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia
[email protected]
Following the elimination of carbon from plasma facing components (PFCs) in AUG, JET and
especially the upcoming ITER, impurity seeding became necessary to maintain the divertor
heat loads within the limits of the power handling capabilities of the metallic PFCs, and to
replace carbon as the intrinsic radiator. Among the impurities tested so far, nitrogen seeding has
given the best results, however puffing of nitrogen can lead to in-vessel ammonia formation.
Ammonia is a hydrogen-containing molecule with a significantly higher boiling point than that
of molecular hydrogen, and is therefore very efficiently pumped by cryopumps at temperatures
at which hydrogen is otherwise not pumped. The formation of ammonia then becomes a
mechanism of fuel retention and, in the case of DT operation, contributes to the in-vessel
tritium inventory.
While the complete mechanism of ammonia formation in fusion devices is not fully
understood, results from AUG [1] and TEXTOR [2] indicate that processes in plasma-shaded
areas could significantly contribute to the amount of produced ammonia. This hypothesis is
supported by laboratory results which show that ammonia is dissociated by adsorption on
surfaces at temperatures above 300 °C, or by electron impact in the plasma phase already at
plasma densities and energies which are considerably lower than in the divertor plasma.
Therefore, ammonia formation happens in a succession of surface reactions between neutral N
and H atoms, however the role of the surface parameters is not yet sufficiently well known.
In this contribution, we present an experimental system which allows for the study of the
impact of surface parameters on the ammonia formation, as well as the impact of H and N atom
densities. The set-up features two plasma sources, each of which provides a beam of neutral H
and N atoms respectively. The beams are crossed in the reaction chamber. This set-up allows
for the individual characterization and control of each of the atom densities. Moreover, it
prevents plasma-phase dissociation of the surface produced ammonia, and restricts the
interaction of N and H atoms to a very well defined area. The ammonia production on glass and
metallic surfaces (Cu and Co) is measured with a differentially pumped mass spectrometer, and
the neutral atom densities are measured with catalytic probes.
Results so far show that the NH3 production is proportional to both N and H atom density
throughout the whole explored parameter range, and no clear saturation was observed with
either of the atom density dependencies. The fraction of N atoms converted to NH3 reaches to
around 7 %, however higher conversions could still be obtained with higher H atom densities.
In the general case, the impact of surfaces was always observable in an increased rate of
reactions, compared to the glass surface of the reaction chamber, however not every type of
surface considerably changes the fraction of N atoms converted to NH3.
[1] Neuwirth et al, Plasma Phys. Control. Fusion 54 (2012)
[2] Carrasco et al, Journal of Nuclear Materials 463 (2015)
This work has been carried out within the framework of the EUROfusion Consortium, WPPFC, and has received funding from the Euratom
research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not
necessarily reflect those of the European Commission.