研究目的
To develop and validate a Phase Contrast Imaging (PCI) diagnostic system on the HL-2A tokamak for measuring plasma density fluctuations over a broad wavenumber range, enabling research into broadband turbulence and MHD instabilities.
研究成果
The PCI diagnostic successfully measures plasma density fluctuations on HL-2A with a broad wavenumber range (2-15 cm?1) and high temporal resolution (<2 μs). Calibration with sound waves and comparisons with magnetic probe data confirm its reliability for studying turbulence and MHD instabilities. The system provides valuable insights into multi-scale plasma phenomena, with potential for future enhancements in sensitivity and application to other fusion devices.
研究不足
The wavenumber resolution is limited by the port size (2 cm?1), and temporal resolution is constrained by detector response (better than 2 μs). Phase scintillation effects and integration path finiteness can affect measurements, and the system's sensitivity decreases at higher frequencies due to signal-to-noise ratio issues.
1：Experimental Design and Method Selection:
The PCI diagnostic uses a CO2 laser (10.6 μm wavelength) to measure phase shifts caused by plasma density fluctuations. The system includes a phase plate for π/2 phase delay, optics for beam expansion and focusing, and a 32-channel HgCdTe detector array for signal detection. Data is analyzed using two-dimensional Fourier transformation to obtain frequency-wavenumber spectra.
2：6 μm wavelength) to measure phase shifts caused by plasma density fluctuations. The system includes a phase plate for π/2 phase delay, optics for beam expansion and focusing, and a 32-channel HgCdTe detector array for signal detection. Data is analyzed using two-dimensional Fourier transformation to obtain frequency-wavenumber spectra. Sample Selection and Data Sources:
2. Sample Selection and Data Sources: Experiments were conducted on the HL-2A tokamak during plasma discharges, with specific shots (e.g., #32,841) analyzed. Calibration was performed using sound waves to simulate density fluctuations.
3：List of Experimental Equipment and Materials:
CO2 laser (wavelength 10.6 μm), HeNe laser (632.8 nm wavelength for debugging), ZnSe lenses and windows, off-axis parabolic mirrors, HgCdTe detectors (MCT), phase plate with gold coating on ZnSe, PXIe-5105 digitizer (National Instruments), safety shutter, and various optical components.
4：6 μm), HeNe laser (8 nm wavelength for debugging), ZnSe lenses and windows, off-axis parabolic mirrors, HgCdTe detectors (MCT), phase plate with gold coating on ZnSe, PXIe-5105 digitizer (National Instruments), safety shutter, and various optical components. Experimental Procedures and Operational Workflow:
4. Experimental Procedures and Operational Workflow: The laser beam is expanded and passed through the plasma; after scattering, it is focused onto the phase plate, and the resulting intensity is measured by the detector array. Data acquisition is synchronized with plasma discharges, and signals are processed to compute spectra.
5：Data Analysis Methods:
Two-dimensional Fourier transformation is applied to the detector signals to generate frequency-wavenumber spectra. Power spectra and time-frequency analyses are performed to study turbulence and MHD instabilities, with comparisons to magnetic probe and soft X-ray data.
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CO2 laser
10.6 μm wavelength
Used as the beam probe to measure phase shifts caused by plasma density fluctuations.
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HeNe laser
632.8 nm wavelength
Used as a debugging beam since CO2 laser is invisible.
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HgCdTe detectors
MCT
32-channel detector array for measuring light intensities in the PCI system.
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Digitizer
PXIe-5105
National Instruments
Used for high-speed data acquisition with a maximal sample rate of 60 MHz.
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Phase plate
ZnSe substrate with gold coating
Key component for generating π/2 phase delay in the optical path.
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ZnSe lenses
Focal length 500 mm and 250 mm
Used for beam control and image reduction in the optical system.
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Off-axis parabolic mirror
Focal length 1250 mm
Used for beam expansion and focusing in the optical system.
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Concave spherical mirror
Focal length 200 mm
Used for beam expansion in the optical system.
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Safety shutter
Used to cut off the laser beam when plasma discharge is over.
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