Plasma-surface interactions in all-metal-wall tokamaks

Gas seeding is often used in tokamaks to reduce the power load onto the divertor target plates. Nitrogen is the preferred seeding species because of its favourable radiative properties as well as its apparent beneficial effect on plasma confinement.
However, nitrogen molecules are chemically reactive with hydrogen and its isotopes to form stable tritiated ammonia. The last studies and observations show that with a 5% conversion into ammonia, 0.2 g of associated tritium could be trapped per pulse in the future ITER D-T operation. The formation of large quantities of tritiated NH3 has consequences for several aspects of the ITER operation and maintenance. In particular, cryopump would need more frequent regeneration that would limit ITER operational cycle. Since NH3 is a polar molecule, it can be easily adsorbed on metallic surfaces and in particular on ITER first-wall material beryllium (Be), divertor material tungsten (W) and on the vacuum vessel and pipework made of stainless steel (SS). The in-vessel T inventory in ITER is limited to 1 kg for safety reasons and the formation and sticking of large quantities of tritiated ammonia could contribute to the overall inventory while the recovery of T from ND2T is still an open issue.
This thesis seeks to develop a fundamental understanding of the ammonia formation process, to explore possible ways to decrease it and to investigate the formed ammonia interaction with materials inside the tokamak. This study will, therefore, address the following questions:
1- Where and how does the ammonia formation occur i.e: in the plasma and/or on the surface and how does it depend on the surface material?
2- What is the impact of specific parameters from the fusion reactor environment, in particular temperature and presence of other gases, on the ammonia formation and is there possible ways to decrease the formed quantity?
3- How much of the ammonia produced quantity can stick on fusion device relevant materials (tungsten, stainless steel, beryllium and boron) and is ammonia adsorbed as a molecule or does it undergo dissociative adsorption?
According to these objectives, the three chapters following the theory chapter 1 dealing with the basics of nuclear fusion will be the following:
Chapter 2 begins by laying out the theoretical dimensions of plasma-assisted catalysis and dominant reactions pathways for ammonia synthesis. The experimental part includes a detailed description of a newly built setup and the experimental procedure developed for the ammonia production study. The effect of the presence of tungsten and stainless steel surfaces on ammonia production for different nitrogen-hydrogen plasma composition will then be presented. The last part of this chapter gives fundamental information about the nature of the reactive processes occurring during RF plasma-assisted ammonia synthesis will be presented based on RGA results and surface chemistry analysis carried out using X-ray photoelectron spectroscopy (XPS).
Chapter 3 is concerned with the effect of two parameters on the ammonia production from nitrogen-hydrogen plasma including the sample surface temperature and He or Ar addition. The results will be shown for temperature values ranging from room temperature (RT) to 1270 K, a value that the ITER tungsten divertor can reach in the active areas where the plasma impact occurs. On the other hand, the main drive of the noble gas admixture effect is to determine a process that reduces the ammonia production in the nuclear device without modifying the positive effects of nitrogen seeding on plasma performance. While He will be present as an intrinsic plasma impurity in ITER, Ar gas was identified as the best candidate for the simultaneous enhancement of core and divertor radiation in the case where elevated main chamber radiation is desired as well.
In the chapter 4, ammonia sticking on different fusion-relevant materials will be presented. In particular, the interaction of NH3 molecules with W, SS and Be surfaces using QMB and XPS techniques will be shown. In addition to these materials used for the tokamak components, boron and gold surfaces will also be investigated as well. The former element is largely used in tokamaks to decrease the oxygen (O) content (by boronization) while the latter can be used as a reference for the QMB technique. NH3 adsorption/desorption study will be presented by examining the effect of both pressure and surface material on sticking along with an XPS study to analyze the residual NH3 molecules sticking on the surface.
The last two chapters of this dissertation include further material related topics for the fusion research namely metallic first mirror plasma cleaning and tokamak wall reflection measurements and simulation. The reliability of optical diagnostic systems is a key element for successful ITER operations.
Particular attention has to be given to first mirrors, involved in almost 40 diagnostics, and whose reflectivity might change due to erosion and deposition from the first wall. Mirror surface recovery techniques will be required and in situ plasma cleaning is considered as the most promising solution. Chapter 5 presents the findings on the removal of relevant ITER contaminants namely beryllium deposits with deuterium gas. The use of deuterium is of crucial interest for ITER and the fusion community as it possesses a unique set of advantage in regard to cleaning: (i) effective on beryllium, (ii) harmless for the mirror material and (iii) fully compatible with other ITER systems (Neutral Beam Injection, cryo-pumping . . . ). Mirrors with a Be deposited films, as well as mirrors exposed in JET-ILW heavily contaminated with beryllium, are employed in this study and two sputtering regimes, at low and high deuterium energy are studied.
The performances of next step fusion facilities, such as ITER, will strongly depend on the ability to monitor and protect the vessel walls from excessive heat loads coming from the plasma power deposition. Infra-red (IR) thermography systems are commonly used in tokamaks to fulfill such requirements by providing thermal images of the plasma facing components (PFCs) under plasma exposure. However, with the introduction of all-metal walls in fusion devices, the significant contribution of reflected flux in the collected flux by the cameras will affect the interpretation of IR measurements, leading to inaccurate PFC temperature estimation. This could lead to excessive interruptions of the plasma shots and limitations on scenario development towards high performances.
The development of a photonic simulation taking into account the contribution of the reflected flux in the collected flux for surface temperature evaluation is, therefore, essential to discriminate the parasitic light-reflection to other thermal events with a real risk for the machine protection. Furthermore, in order to get accurate results, this simulation will certainly have to be based on experimental data set for different PFC materials. Chapter 6 start by describing a photonic simulation carried out using the ray tracing software SPEOS taking into account the multiple inter-reflections of the ray in the vacuum vessel. In the following experimental section, a new redesigned Basel University Laboratory Goniometer (BULGO) will be presented with a description of the apparatus, the measurement and calibration procedure, and the assessment of the accuracy of the device. Directional and total emissivity are also deduced by indirect measurements. The results are presented for tungsten samples at different roughness.
Tungsten is the material chosen for the most critical component in tokamak (divertor) and which is exposed to highest heat loads and for which the roughness can be changed during experimental campaign (erosion/deposition phenomenon). The relation between reflectance/emittance and roughness will also be discussed in this chapter. Experimental results are then used as input of photonic simulation and the resulting IR synthetic image is compared with the experimental image of WEST tokamak. In the conclusion, a brief summary of the findings and areas for further research will be presented.