Investigations on Caesium Dispersion and Molybdenum Coating on SPIDER Components.

Area of Science:

• Nuclear Fusion Engineering
• Plasma Physics
• Materials Science

Background:

• The SPIDER (Source for the Production of Ion of Deuterium Extracted from plasma) is a prototype negative ion source for the ITER Neutral Beam Injector.
• It operates at Consorzio RFX, generating 100 keV ions from RF-driven plasma for fusion applications.
• SPIDER is the world's largest negative ion source, crucial for ITER's heating systems.

Purpose of the Study:

• To investigate and characterize unusual coatings observed on SPIDER components during a major shutdown.
• To determine the composition and origin of these coatings, potentially linked to operational issues.
• To inform future improvements and ensure optimal performance of the negative ion source.

Main Methods:

• Disassembly and meticulous cataloging of SPIDER beam source components.
• Surface characterization using Scanning Electron Microscopy (SEM).
• Analysis of material composition and structure via X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS).

Main Results:

• Discovery of non-uniform red, white, and green coatings on the Plasma Grid, Extraction Grid, and Bias Plate.
• Evidence of caesium evaporation and deposition on molybdenum-coated components.
• Formation of oxides and hydroxides identified as the nature of the coatings.

Conclusions:

• The observed coatings are attributed to back-streaming positive ions, electrical discharges, and caesium evaporation.
• Surface characterization confirmed caesium presence and compound formation, impacting component integrity.
• Ongoing studies aim to fully understand and mitigate these depositional effects for improved ion source reliability.