Removal of fusion-relevant deposits from metallic surfaces using low-temperature plasmas

• Main Author: Shaw, David
• Other Authors: Wagenaars, Erik
• Published: University of York 2018
• Subjects: 530
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.762605

Optical diagnostics on fusion devices are important for both research and real time control. All of these diagnostics depend on reflective optics in the form of metallic mirrors. Etching and re-deposition during fusion operation from the beryllium inner wall onto the mirrors can cause severe degradation in the reflectivity. Using the mirror as the powered electrode to form a capacitively coupled plasma above the surface is seen as the most favourable method for recovery of the mirror reflectivity. The ions created within the plasma can bombard the surface and remove the deposit. This method has been tested experimentally in various ways and in various geometries and has been proven to work in these cases. However, in order to optimise the system modelling efforts are carried out within this thesis. The Hybrid Plasma Equipment Model (HPEM) is configured to simulate the etching plasma and is benchmarked against experimental results. After successful benchmarking parameters are varied in an attempt to find optimum settings for the successful implementation of this method on ITER. Results concluding that individual mirrors require individual modelling efforts as trends cannot necessarily be applied to each mirror geometry. A beryllium/argon/oxygen gas chemistry set is created to more accurately model the ITER environment which is compared with a published aluminium/argon/oxygen set. Aluminium is currently used as a proxy for beryllium in the majority of experimental work. They are shown to be dissimilar in their behaviour within a bulk Ar plasma which will have knock on effects for the etching process. The bulk plasma properties remain identical at low fractions of Be or Al. Also presented is work involving understanding the mechanism behind modification of polypropylene using an atmospheric-pressure plasma jet. A two stage process is identified involving atomic oxygen from the jet and nitrogen from the surrounding atmosphere.