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| Progress of Thomson Scattering Diagnostic on HL-2A Tokamak |
| SHANG Jie1, 2, HUANG Yuan2, YANG Kai1, CHEN Bao-wei1, LIU Chun-hua2, YANG Yi1 |
1. China Institute for Radiation Protection, Taiyuan 030006, China
2. Southwestern Institute of Physics, Chengdu 610225, China |
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Abstract The diagnosis of incoherent laser Thomson scattering only needs to assume that the electron velocity satisfies the Maxwell distribution, and the measured data of plasma electron temperature and electron density are accurate and reliable. It is an important diagnostic tool in the Tokamak and other magnetically confined fusion research device, and it is developing towards the measurement requirements of high reliability, high spatial resolution and high repetition rate, while high reliability is the premise condition. The Thomson scattering cross section of electron is very small, and its total cross section is σT=6.65×10-25 cm2. Generally, the Q-switched Nd∶YAG laser is used as the scattering light source. The laser pulse width is about 10 ns, and the pulse energy is about 3 J. The scattering spectrum is measured and analyzed by a 5～8 channel polychromator. One of the key problems of laser scattering diagnosis is how to collect the scattering-related electric pulse output from photoelectric detection module. In the past, charge-sensitive analogue-to-digital converters (Q-ADCs, such as CMC080 module) were used to integrate the scattered pulse signal on the sampling capacitor over a certain time width (such as 50 ns), so as to obtain the strength value of the scattered signal. This method is difficult to eliminate circuit noise and external interference. Now, fast digitizers (vertical resolution ≥10 bits, sampling frequency f≥1 GS·s-1, such as V1742B module) are used to collect the data in the time period including the scattering signal (such as 300～500 ns), so as to obtain the data sequence superimposed by the scattering pulse signal, the disturbance of plasma light and background noise. In this paper, the least square method is used to fit the scattering pulse with Gaussian function, and then the scattering pulse is numerically integrated into 50 ns-width to get the strength value of the scattering signal. The results show that the use of high-speed synchronous acquisition technology can eliminate most of the interference with digital filtering technology, so as to improve the signal-to-noise ratio, and its amplitude can reach about 10 times. After more accurate and reliable spectral data are extracted, the error-weighted least square method is used to process the data at the confidence level of 95%. A. C. Selden scattering spectral expression is used to estimate the parameters of electron temperature, and the measured value of electron temperature is obtained. The statistical error is about 3%, which is better than the previous 10%.
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Received: 2020-01-22
Accepted: 2020-04-17
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