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Frequency linearization of voltage controlled oscillator (VCO) for 2 µs frequency modulated continuous wave (FMCW)
1Korea Institute of Fusion Energy, Daejeon 34133, South Korea.
The Review of Scientific Instruments
|October 21, 2024
Summary
This study developed a faster frequency modulation (FM) driver for KSTAR reflectometers. The new system reduces FM time to 2 µs, improving plasma density profile measurements in fusion devices.
Area of Science:
- Plasma physics
- Fusion energy research
- Diagnostic techniques
Background:
- Frequency modulated continuous wave (FMCW) reflectometers are crucial for measuring plasma density profiles in magnetic fusion devices.
- Existing KSTAR reflectometer systems with a 20 µs frequency modulation (FM) time struggle with high-frequency edge density fluctuations (20-50 kHz) in ELMy H-mode plasmas.
- These fluctuations cause significant distortion in density profile measurements due to changes occurring during the FM time.
Purpose of the Study:
- To address the limitations of current FMCW reflectometers in accurately measuring rapidly changing plasma edge densities.
- To design and implement a new voltage-controlled oscillator (VCO) driver capable of achieving a significantly reduced FM time.
- To develop an effective frequency linearization method for improved accuracy in plasma density profile diagnostics.
Main Methods:
- A new VCO driver was designed using the AD8067 operational amplifier to achieve a 2 µs FM time.
- Linear frequency modulation was implemented by predistorting the VCO tuning voltage with a 400 MSamples/s arbitrary waveform generator.
- The output frequency was measured by mixing the VCO output with a frequency synthesizer and analyzing the intermediate frequency using a 2.5 GSamples/s digitizer.
- A feedback loop adjusted the tuning voltage to minimize frequency deviation, considering the response times of the VCO driver and input interface.
Main Results:
- A new VCO driver enabling a 2 µs FM time was successfully developed.
- A frequency linearization method was implemented to improve the accuracy of density profile measurements.
- The developed system is designed to overcome the limitations caused by high-frequency plasma edge density fluctuations.
Conclusions:
- The developed VCO driver and linearization technique significantly enhance the capability of FMCW reflectometry for measuring dynamic plasma edge densities.
- This advancement is critical for accurate plasma profile diagnostics in magnetic confinement fusion research, particularly in devices like KSTAR.
- The improved FM rate allows for reliable density profile measurements even in the presence of rapid edge fluctuations.
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