Plasma Cleaning Techniques for Diagnostic Mirrors in Fusion Systems
Summary
Diagnostic mirrors form a critical element of optical systems in magnetic confinement fusion devices. Situated at the interface between the hot plasma and detection instruments, these mirrors suffer progressive degradation in reflectivity due to the deposition of wall atoms, erosion by energetic particles and oxide formation. Plasma cleaning represents a versatile in situ method to restore mirror performance without breaking vacuum. Techniques typically employ radio-frequency discharges in noble gases or deuterium, generating ionised species that sputter away contaminant layers. Parameters such as discharge frequency, gas pressure, self-bias voltage and magnetic‐field orientation profoundly influence cleaning efficacy and uniformity. Advances, including dual-mirror cleaning schemes and high-frequency capacitively coupled plasmas, have demonstrated the ability to remove mixed‐material films, control redeposition and preserve pristine mirror surfaces over multiple cleaning cycles. As fusion experiments move towards steady-state operation, reliable plasma cleaning protocols will become indispensable for sustaining diagnostic accuracy and advancing reactor development worldwide.
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Plasma Cleaning Techniques for Diagnostic Mirrors in Fusion Systems publication trend
The graph below shows the total number of articles in plasma cleaning techniques for diagnostic mirrors in fusion systems across all publications each year (not limited to Nature Index journals).
Technical terms
Diagnostic first mirror: A metallic optical surface that relays plasma emissions to external detectors, subject to erosion and contamination in fusion environments.
Radio-frequency plasma: An ionised gas sustained by alternating electromagnetic fields, commonly used for in situ mirror cleaning via ion sputtering.
Self-bias voltage: A negative potential that develops on an electrode in a capacitively coupled plasma, controlling ion energy at the mirror surface.
Nanocrystalline coating: A mirror surface composed of grains in the nanometre scale, offering enhanced resistance to patterning and improved reflectivity retention.
Sputtering: The physical ejection of surface atoms caused by energetic ion bombardment, employed to remove contaminants from mirror substrates.
References
- Surface modification of ITER-like mirrors after one hundred cleaning cycles using radio-frequency plasma. Journal of Nuclear Materials (2023).
- Effect of 3 T magnetic field on RF plasma sputtering in an ITER-relevant first mirror unit. Nuclear Fusion (2022).
- Deuterium plasma sputtering of mixed Be-W layers. Journal of Nuclear Materials (2022).
- Mirror dual cleaning of ITER equatorial diagnostic Wide Angle Viewing System. Fusion Engineering and Design (2025).
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