Improvements to the ion Doppler spectrometer diagnostic on the HIT-SI experiments

Area of Science:

• Plasma Physics
• Spectroscopy
• Fusion Energy

Background:

• Spheromak devices are crucial for magnetic confinement fusion research.
• Accurate measurement of impurity ion temperature and velocity is vital for understanding plasma behavior.
• Existing diagnostic systems have limitations in spatiotemporal resolution and error margins.

Purpose of the Study:

• To enhance an ion Doppler spectrometer diagnostic system for spheromak devices.
• To achieve higher spatiotemporal resolution and lower error in measurements.
• To simultaneously observe impurity ion flow, displacement, and temperature profiles.

Main Methods:

• Implemented hardware and software improvements to an established ion Doppler spectroscopy technique.
• Utilized 72 fused-silica fiber channels and an f/8.5 Czerny-Turner spectrometer with a video camera.
• Employed biorthogonal decomposition as a novel filtering tool for data analysis.

Main Results:

• Achieved record 6.9 μs temporal and ≤2.8 cm spatial resolution in the device midplane.
• Reduced uncertainty in ion temperature from ≲13 to ≲5 eV and velocity from ≲2 to ≲1 km/s.
• Observed axisymmetric temperature and displacement profiles, with HIT-SI3 revealing toroidal current-dependent rotation up to 3 km/s.

Conclusions:

• The enhanced diagnostic system provides unprecedented data on impurity ion dynamics in spheromaks.
• Results agree with theoretical predictions and previous observations, validating the new system's capabilities.
• This advancement significantly improves the ability to study plasma behavior in fusion devices.