Introduction

ITER has recently decided to replace the beryllium (Be) First Wall with tungsten(W). In order to get efficient start-up by gettering impurities and to avoid excessive W core contamination this wall will be regularly coated with a layer of boron (B) by glow-discharge boronization. This boron will be eroded and co-deposit with hydrogen isotopes (D and T) in deposition regions of the machine. As a result several open questions exist regarding the safe operation of ITER in such a scenario, for example regarding the trapping of the scarce T inventory, including:

• What is the expected T/B co-deposition ratio as a function of B and T fluxes, and surface temperature?
• At what temperature will T be removed from T/B co-deposits?
• What is the T retention rate in extant B layers?
• What is the sputtering rate of different depositedB layers with differing T and O content?
• What is the O gettering capability of the B layer?
• What role does oxygen play in theT/B co-deposition, sputtering, trapping and de-trapping?
• What is the physical stability of (T+)B layers – are flakes or dust readily formed? How well does it adhere to the deposition surface?
• Can T beremoved via isotope exchange? On what timescale and at what temperature? With what flux of D ions/radicals/molecules? As a function of layer thickness?

This project aims to tackle some of these questions, particularly focusing on the questions related tritium retention and safety. For this we use D as a proxy for T.

Your Project

As a first step deposition methods for use in Magnum-PSI and/or UPP need to be developed. B readily oxidises in air which strongly alters its gettering and other physical properties so this should be done in-situ. Possible options to be explored include:

• Upstream sputtering of Btargets leading to downstream co-deposition with D
• Injection of diborane gas into the plasma stream
• Pulsed laser deposition of boron onto the target surface
• Boron powder dropping into the plasma with downstream deposition
• Evaporative atomic beam source (molecular beam epitaxy (MBE) effusion cells)

Subsequently the boron layer thickness and D content should be assessed using Nuclear Reaction Analysis. Different conditions should be determined such as substrate temperature, B deposition rate and ion energy. D trappingcan also be determined using Thermal Desorption Spectroscopy. From this an empirical relationship similar to that developed for co-deposition of Be and D can be determined [1]. A post-mortem analysis of the layers by e.g. SEM and nano-indentation can give information on their properties.

[1]. G. D. Temmerman, M. J. Baldwin,R. P. Doerner, D. Nishijima, and K. Schmid,“An empirical scalingfor deuterium retention in co-deposited beryllium layers,” Nucl. Fusion, vol. 48, no. 7, p. 075008, Jun. 2008, doi: 10.1088/0029-5515/48/7/075008.

What we offer

• Access to top class facilities and instrumentation at DIFFER like Magnum-PSI and the Ion Beam Facility
• Supervision and guidance that aims to support you while letting you be in charge of the project
• A PMI team of enthusiastic and knowledgeable students around you
• Limitless free coffee!