Digital Polychromator

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Spectrum analyzing system for Thomson scattering (TS) signals registration and processing (hereinafter referred to as “system ”) is intended for laboratory plasma investigation by the Thomson scattering method in order to register local values of electrons temperature and density in plasma with detailed temporal and spatial resolution, as well as for recording, storing and processing the obtained data. Application fields: scientific research.

Detailed description

​For more detailed overview of the diagnostic system please refer the whitepaper

 

Technical specifications

#Operating conditions

General Principles of Operation

The system operation is based on analysis of electron component of laboratory plasma by the method of Thomson scattering of laser radiation. Scattered laser radiation is collected by the collection lens, the focal surface of which comprises input ends of optical fiber bundles, distributing radiation across the filter polychromators (hereinafter - “polychromator”). Each polychromator measures electron temperature and density at one spatial point. Electron temperature is determined on the basis of scattered radiation spectrum analysis. Density value is determined by intensity of the recorded scattering signals. Scattering signals registration is carried out by a small number of spectral channels (up to 6). Each spectral channel is equipped with transimpedance amplifier with avalanche photodiodes (APD). The used amplifier comprises two outputs: high-frequency (for measuring scattering signals) and low-frequency (for calibration), capable to measure quasi-permanent signals. The signals are digitized by the ADC embedded in the polychromator.

Prior to measurements, the system should be calibrated. Spectral and absolute calibrations should be carried out for the Thomson scattering diagnostics. Spectral calibration, the essence of which is to estimate relative sensitivity of spectral channels, is necessary for electron temperature determination. A set of calibration equipment supplied with the system is used for spectral calibration. Absolute calibration is necessary toSpectrum analyzing system for Thomson scattering signals registration and processing determine electrons density; its purpose is to determine the coefficient of correspondence between the number of scattered photons and magnitude of the detected signal for each spectrometer. Absolute calibration is performed using the Raman scattering method.