Experimental study of radio-frequency plasma surface interactions on diagnostic mirrors under ITER-relevant environments

Nuclear fusion is a nuclear reaction involving the fusing of two or more light atoms into heavier ones, a process that releases a tremendous amount of energy. The knowledge of achieving fusion-based energy has come a long way thanks to the development of devices like the International Thermonuclear Experimental Reactor (ITER). For successful operations of ITER, they will be equipped with diagnostic systems to monitor fusion reactions, many of which are optical. Most optical diagnostic systems will be equipped with metallic first mirrors (FMs) with the objective of directing the light from the fusion plasma towards the diagnostics through an optical labyrinth in order to prevent neutron leakage. Being the initial elements in the optical diagnostics, however, the FMs will be subject to erosion from the charge-exchange neutrals. Moreover, they will also be deposited with reactor first wall materials (beryllium and tungsten) which would significantly degrade their optical properties. The FMs would thus require periodic cleaning to maintain their optical efficiency, which is foreseen to be achieved by the means of an in-situ cleaning technique. In-situ Plasma cleaning with capacitively coupled radio-frequency (CCRF) discharges is one of the techniques adopted for achieving FM cleaning. In addition to the parasitic deposition, the FMs would also be exposed to high thermal loads owing to energetic neutrons and gamma radiation, for which they would require to be actively water-cooled. The electrical contact between the grounded water cooling lines with the FMs would lead to their electrical grounding as well, disabling RF discharge generation. Additionally, ceramics are not welcome between the FMs, vacuum, and the water cooling pipes in ITER to decouple the ground from the RF. As a coping strategy to achieve RF cleaning, the water cooling lines are expected to be implemented in the form of a quarter-wavelength (λ/4) filter, commonly known as “notch-filter”. This would DC-ground the FMs while allowing the RF to propagate through them, in spite of their physical contact with the grounded water cooling lines. DC-grounding the powered electrode has a significant impact on the properties of the RF plasma and leads to an increase of the plasma potential to several hundred volts depending on the applied RF power. This can lead to substantial sputtering of the walls surrounding the FMs, which can get deposited on the FMs impeding the mirror cleaning process. As the fusion plasma is composed of charged species and is at a temperature of over 150 million degrees, it is confined using a strong magnetic field which will also be experienced at the FMs. This would significantly influence the properties of RF plasma for the FM cleaning and require thorough investigation.
The main objective of this thesis is to provide an exhaustive study of capacitively coupled RF plasma for FM cleaning with the addition of a notch filter as well as in the B field to emulate the conditions of future fusion reactors. Experimental and simulation work was performed with numerous international collaborations such as the Swiss Plasma Center in Lausanne, the Wigner Institute of Physics in Budapest, the National Institute for Laser, Plasma, Radiation Physics (NILPRP) in Bucharest, University Hospital, Basel as well as ITER Organisation to obtain results valuable to the fusion community. The experimental investigations undertaken in this thesis have led to numerous outcomes. To begin with, experiments were conducted with CCRF to investigate the cleaning of mirror samples with ITER relevant contaminants: 1. oxides that appear on FM surface as a result of steam exposure originating from a prospective coolant leak in a vacuum vessel and 2. mixed Be and W contaminants in varied proportions. Cleaning tests with different process gases and ion energies in high vacuum conditions in Basel (steam ingress experiments) as well as NILPRP (mixed Be-W experiments) to study the physical or chemical sputtering regime in the removal of contaminants showed very promising results. After that, an experimental study was carried out in the HV chamber in Basel investigating the properties of plasma when the electrodes (representing FMs) are DC-grounded via a λ/4 filter. The experimental work was bench-marked with Particle in Cell/Monte Carlo Collision simulations. The dielectric properties of the electrode surface significantly influenced the plasma potential which ranged from 150V for conducting electrode to 20V for an insulating electrode. Further, the plasma surface interactions and impact of wall sputtering on DC-grounded FMs were studied on a First Mirror Unit (FMU) mock-up developed in Basel. The FMU is a real size box to reproduce the configuration of most diagnostics at the front end and is composed of a first mirror (M1) and a second mirror (M2) to direct the light further into the diagnostic assembly. Plasma cleaning with λ/4 filter on the FMU resulted in significant wall sputtering and deposition on FMs. Hence mitigation strategies were developed in order to control wall sputtering and deposition, so as to achieve FM cleaning with a λ/4 filter. Firstly, experiments were conducted with different wall materials (tungsten, aluminum, and copper walls) to understand the impact of varying physical properties (sputtering energy threshold and erosion yield) on deposition. The physical properties of the metal as well as their oxides were found to significantly influence the wall sputtering and their deposition on FMs. Secondly, we investigated the influence of varying electrical properties of the walls (grounded versus floating) on their sputtering and deposition on the mirrors. Using floating wall components significantly minimized the wall deposition on the mirrors and allowed for an efficient plasma cleaning of the FMs. Lastly, the influence of positioning a steel mesh between the FM and the walls was investigated to confine the plasma and minimize ion flux on the walls. The resulting ion flux measured at the wall decreased by over 60% and the FM was sputtered with a relatively higher flux allowing for its efficient cleaning.
Thorough experimental investigations were conducted studying the plasma surface interactions in presence of an external B field using a superconducting magnet at SPC, Lausanne. The presence of the B field led to a non-homogeneous etching and increased the rate of surface erosion by over 15 times at 0.1T and over 80 times at 3 T. After, the distribution profile (directionality) of the particles sputtered from the electrode surface was studied in similar configuration at 3 T. This was done in order to study the deposition of particles upon sputtering as a function of B-field as well as surface topography (flat versus sawtooth). It was found that the deposition of the sputtered particle increased by over 10 times and the direction of deposition modified significantly in the B field. The overall deposition could be reduced by structuring the surface in a sawtooth shape, which leads to the capture of the ejected particles. Finally, to investigate a large-scale FMU mock-up in a high magnetic field, experiments were conducted in the 3T Magnetic Resonance Imaging (MRI) facility at University Hospital, Basel. B field confined the plasma in a cylindrical column and modified the wetted area on the FMU wall. M1 was observed to be efficiently cleaned without wall deposition, while M2 had wall deposits 4 to 54nm thick. The net deposition on M2 was dependent on the angle between the electrode (M1) and the B field.