Laser calibration of the DIII-D coherence imaging system

Area of Science:

• Plasma Physics
• Spectroscopy
• Fusion Energy Research

Background:

• Coherence Imaging Spectroscopy (CIS) is crucial for diagnosing plasma properties.
• Accurate calibration of CIS is essential for reliable ion flow measurements.
• Previous calibration methods may lack precision or in situ capabilities.

Purpose of the Study:

• To develop and validate an in situ calibration technique for CIS on DIII-D.
• To enable precise 2D measurement of ion flows in fusion plasmas.
• To improve the accuracy and reliability of plasma diagnostic tools.

Main Methods:

• Utilized a tunable CW diode laser (464-468 nm) and a high-resolution wavemeter.
• Characterized interferometer phase response as a function of wavelength near C iii and He ii lines.
• Employed mechanical and thermal stabilization, optical stirring, and an integration sphere for uniform calibration images.

Main Results:

• Achieved high-quality calibration data enabling phase vs. wavelength measurement over ~10 fringes.
• Established coefficients related to interferometer optics and birefringent crystal properties.
• Implemented an automated procedure on DIII-D for in situ calibration with a zero-velocity reference.

Conclusions:

• The developed in situ calibration technique provides accurate phase characterization for CIS.
• This method enhances the capability for precise 2D ion flow measurements in fusion devices.
• The automated calibration process improves the efficiency and reliability of plasma diagnostics.