Langmuir probe array for the small angle slot divertor in DIII-D

Area of Science:

• Fusion energy research
• Plasma physics
• Divertor technology

Background:

• The DIII-D small angle slot (SAS) divertor requires specialized diagnostics due to its closed geometry, hindering optical measurements.
• Understanding plasma conditions within the SAS divertor is crucial for enhanced neutral confinement and target geometry studies.

Purpose of the Study:

• To implement and refine a multipurpose Langmuir probe array for in-situ plasma measurements within the DIII-D SAS divertor.
• To overcome diagnostic challenges posed by the SAS divertor's design, including optical access limitations and probe placement constraints.

Main Methods:

• A novel spring-loaded probe and tile design was developed to accommodate space limitations at the bottom of the slot.
• Langmuir probes were strategically spaced for high-resolution measurements of the near scrape-off layer.
• A laser scanning alignment tool was employed to precisely align custom tiles with the measured strike point toroidal surface.

Main Results:

• Initial measurements revealed tile edge shadowing and misalignment issues between probe positions and the divertor's slot geometry.
• Resolved shadowing and alignment issues by redesigning tiles with centered probes and utilizing precise laser alignment.
• Post-alignment measurements showed close agreement between Langmuir probe data and plasma behavior at multiple toroidal locations.

Conclusions:

• The adapted Langmuir probe system effectively measures plasma conditions in the challenging DIII-D SAS divertor environment.
• Improved probe design and alignment resolved initial measurement discrepancies, enhancing the reliability of divertor physics studies.
• The refined diagnostic approach provides valuable data for optimizing divertor performance in fusion devices.