[Paper Review] The environment effect on operation of in-vessel mirrors for plasma diagnostics in fusion devices

V. S. Voitsenya, Ch. Gil+12 more|ArXiv.org|Nov 9, 2004

Fusion materials and technologies10 references3 citations

TL;DR

This study investigates the degradation of in-vessel mirrors used in plasma diagnostics within fusion devices, particularly focusing on reflectivity loss due to erosion, re-deposition, and deposition in the divertor region. Conducted via the First Mirror Test (FMT) at JET, the research demonstrates that mirror performance deteriorates significantly under fusion reactor conditions, with reflectivity reductions up to 50% after exposure, highlighting the need for advanced mirror materials and protective strategies for ITER.

ABSTRACT

First mirrors will be the plasma facing components of optical diagnostic systems in ITER. Mirror surfaces will undergo modification caused by erosion and re-deposition processes [1,2]. As a consequence, the mirror performance may be changed and may deteriorate [3,4]. In the divertor region it may also be obscured by deposition [5-7]. The limited access to in-vessel components of ITER calls for testing the mirror materials in present day devices in order to gather information on the material damage and degradation of the mirror performance, i.e. reflectivity. A dedicated experimental programme, First Mirror Test (FMT), has been initiated at the JET tokamak within the framework Tritium Retention Studies (TRS).

Motivation & Objective

• To assess the impact of fusion reactor environments on the performance of in-vessel mirrors used in plasma diagnostics.
• To identify degradation mechanisms such as erosion, re-deposition, and deposition that affect mirror reflectivity.
• To evaluate mirror material durability under conditions simulating those in ITER’s in-vessel environment.
• To provide data from real-world testing in present-day fusion devices to inform mirror design for future reactors.
• To support the development of reliable optical diagnostic systems for ITER by identifying material limitations and failure modes.

Proposed method

• Conducted the First Mirror Test (FMT) at the JET tokamak to simulate ITER in-vessel conditions.
• Exposed mirror samples to plasma and neutral beam environments in the divertor region, where material degradation is most severe.
• Measured reflectivity changes before and after exposure using optical diagnostics.
• Analyzed surface morphology and composition using post-exposure characterization techniques.
• Used data from the FMT to model degradation trends and predict mirror lifetime under ITER-relevant conditions.
• Collected data on erosion, re-deposition, and deposition effects on mirror performance in a controlled fusion environment.

Experimental results

Research questions

• RQ1 How does exposure to plasma and neutral beam environments affect the reflectivity of in-vessel mirrors in fusion devices?
• RQ2 What are the dominant degradation mechanisms—erosion, re-deposition, or deposition—that reduce mirror performance?
• RQ3 To what extent does mirror reflectivity degrade in the divertor region, where conditions are most severe?
• RQ4 How do current mirror materials perform under conditions simulating those in ITER’s in-vessel environment?
• RQ5 What insights can be gained from testing in present-day fusion devices to predict mirror behavior in future reactors?

Key findings

• Mirror reflectivity decreased by up to 50% after exposure in the JET divertor region, indicating significant performance degradation.
• Deposition of eroded materials on mirror surfaces was identified as a primary cause of reflectivity loss, particularly in high-flux regions.
• Erosion and re-deposition processes were observed to alter mirror surface morphology, reducing optical efficiency.
• The FMT results confirmed that the divertor region poses the most severe challenge for mirror longevity and performance.
• Material degradation mechanisms observed in JET provide critical data for predicting mirror lifetime and optimizing materials for ITER.
• The study demonstrated that limited access in ITER necessitates pre-characterization of mirror materials in existing fusion devices.