Improved edge charge exchange recombination spectroscopy in DIII-D

Area of Science:

• Plasma physics
• Fusion energy research
• Spectroscopy diagnostics

Background:

• The edge of tokamak plasmas is crucial for confinement.
• Accurate measurement of edge plasma parameters is challenging.
• Charge exchange recombination spectroscopy (CXRS) is a key diagnostic.

Purpose of the Study:

• To enhance the radial resolution of the CXRS diagnostic at the DIII-D tokamak edge.
• To improve measurements of impurity behavior in steep gradient, H-mode plasmas.
• To investigate non-Maxwellian ion distributions in low collisionality regimes.

Main Methods:

• Upgraded the CXRS diagnostic with additional high radial resolution view chords.
• Utilized side-by-side fiber optics at spectrometer entrance to avoid crosstalk.
• Increased edge view chords from 16 to 38 with 8 mm radial separation.
• Employed new fused silica fibers for improved light throughput.

Main Results:

• Achieved higher radial resolution measurements near the plasma edge (r/a > 0.8).
• Successfully diagnosed steep gradient, H-mode plasmas with diverse shapes.
• Observed non-Gaussian spectra, indicating non-Maxwellian ion distributions.
• Enhanced light throughput improved spectral clarity.

Conclusions:

• The upgraded CXRS diagnostic provides improved measurements of edge plasma parameters.
• The diagnostic is effective for characterizing complex H-mode plasma conditions.
• Evidence of non-Maxwellian ion distributions in low collisionality plasmas was obtained.