Abstract: The Thomson scattering method is a widely used diagnostic method for determining the temperature and electron density of arc gases and electrons. Thomson scattering is the process of radiating electromagnetic field energy in any direction by the vibration of free electrons. The free electrons vibrate under the action of the incident wave electric field and radiate part of the incident energy in the form of dipole radiation. This method has the advantage of being able to measure local temperature, and its accuracy is not premised on the existence of local thermodynamic equilibrium. The method can effectively measure the temperature in the radial position of the welding arc; the electron temperature and electron density distribution can be directly determined by the spectral analysis of the electronic characteristics of the Thomson scattering. This paper describes recent advances in gas and electron temperature and electron density measurements in the field in recent years. The prospect of the development of this technology in China is discussed.
Key words: Thomson scattering; laser diagnosis; welding arc; spectral line analysis
0 preface [1]
With the development of industrial production, in order to adapt to the ever-increasing processing technology requirements, welding workers are constantly improving and perfecting welding equipment and processes. Arc has been used as a heat source for material connections for more than a hundred years. From the Second World War to the mid-1960s, most of the common welding methods, including lasers and electron beams, were introduced, and they also stimulated the exploration and understanding of its mechanism. In this context, it is of great practical significance to speed up the study of welding arc state, to guide the welding process with arc physics theory, to improve production efficiency and product quality. This has led to the development of a variety of welding arc diagnostic methods.
1 different arc diagnosis methods
In the past, emission spectroscopy was the most commonly used diagnostic method in determining plasma gas temperature, electron temperature, and electron density. However, the measurement results are valid only when local thermodynamic equilibrium ( LTE) is present in the arc plasma. It is generally believed that when the electron number density exceeds 10 23 m -3 , LTE [1] can be satisfied, that is, those areas where the electron density exceeds 10 23 m -3 can measure the temperature of the arc. However, the existence of LTE in free-burning arcs is controversial. At the same time, the traditional emission spectroscopy is the accumulation of radiation intensity in the direction of observation, which cannot directly diagnose the state parameters of a single point in the arc. Therefore, the effect of directly measuring the arc temperature is not ideal.
Direct determination of these parameters in the plasma, especially when it does not interfere with the surrounding conditions, a feasible method is the spectral line shape analysis of plasma laser Thomson scattering [2] . Snyder and Bentley measured the DC arc of free combustion under normal pressure by this method, and described the centerline gas temperature curve with time [3] ; Bentley applied this method to find the electron temperature and electron density in the atmospheric pressure free combustion argon arc. Radial and axial distributions deviate widely from LTE [4] .
Under the condition of free-burning argon arc, there is a big difference between the temperature values ​​measured by different scholars using spectroscopy and other methods. For example, Koyabashi and Suga [5] , Hsu, Etemadi, and Pfender [6] measured by spectroscopy that these arcs have temperature peaks in excess of 20,000K. Conversely, Seeger and Tiller [7] measured by spectroscopy, while Gick, Quigley, and Richards [8] measured by electrostatic probe method. Under the same type of arc conditions, they obtained an arc maximum temperature of about 12000K. Murphy et al. applied the laser scattering method to the same type of arc, which can prove that the temperature in the arc near the cathode is as high as 20,000 K, and the temperature of neutral atoms and ions in the radial position of the arc can be measured without depending on the presence of LTE. [9] , opened up a new way for the measurement of plasma parameters.
2 Thomson scattering principle
     Thomson scattering is an elastic scattering that occurs between photons and free electrons. It is actually Compton scattering in quantum mechanics, but occurs in the low energy region ( hω << m e c 2 ). This process can be well described by classical electrodynamics under low scattering conditions. The classic image is shown in Figure 1. When a plane electromagnetic wave is irradiated onto a free electron, the electron vibrates under the action of electromagnetic waves. Since the acceleration of the vibrating electron is not zero, the electron radiates electromagnetic waves and acts as a dipole. The form of radiation radiates a small fraction of the incident energy absorbed, which is the Thomson scattering. When a plane electromagnetic wave is incident on a group of electrons in the plasma, the scattered electromagnetic wave at this time is a coherent superposition of the scattered electron scattering waves of each motion. Therefore, the spectrum of the scattered light has a great relationship with the state of motion of the electrons in the plasma. It is for this reason that Thomson scattering is a powerful tool for diagnosing the state parameters of plasma and collective motion in plasma.
The Thomson scattering method has the advantage of being able to measure local temperature, and its accuracy does not depend on the presence of LTE. At present, Thomson scattering has been successfully applied abroad to measure the physical parameters of microscopic particles. Through the propagation of laser beams, supplemented by certain optical instruments, spectra or other signals reflecting the typical physical parameters of microscopic particles are collected and calculated by modeling and fitting. Physical parameters.
    3 Research progress of Thomson scattering
3.1 Application of Thomson scattering in temperature measurement
Among the methods for determining plasma gas and electron temperature and electron density, a laser Thomson scattering method has recently been frequently applied. The obtained temperature value is obtained from the spectral distribution of the laser-scattered light, and the kinetic temperature, electron temperature, and electron density diagnosis of heavy particles in the plasma are measured. Now, this technology has been extended to a variety of lasers, including gas lasers, solid-state lasers, semiconductor lasers and so on. When the electron density exceeds 10 22 m -3 , the scattering spectral line consists of a narrow ion distribution at the center and a broad and symmetrical electron distribution in the peripheral region. By analyzing these lines, heavy particle temperature, electron temperature, and electron density can be obtained.
The ion component line is obtained by heterodyne detection of the Thomson scattering laser [10] , which has been successfully used to determine the transfer arc temperature of low voltage and large current. By tuning a narrow-bandwidth dye laser as a laser source and performing spectroscopic analysis with a monochromator, high-resolution line gas temperatures in the transfer arc can be obtained [11] . Snyder et al. found that as the working current of the torch increases, the gas temperature keeps rising to a certain stable value and remains unchanged [12] .
The literature [4,13,14] reports the Thomson scattering measurement results of thermal plasma electrons and ions under normal pressure, and points out that the electron temperature of Ar plasma jet and transfer arc is measured by the spectral line analysis method of laser Thomson scattering under normal pressure. The peak temperature of electrons is as high as about 20000K; the temperature of heavy particles is about 12000K and about 14000K in plasma jet and transfer arc, respectively. Comparative analysis of the corresponding temperature measured by spectroscopy, as can be seen from Table 1, the spectral method is clearly overestimated at low temperatures. This results from the deviation of LTE, which is caused by atomic excitation of the outer region of the plasma from linear radiation emitted from the central region. When the temperature is higher than a certain value, the temperature values ​​measured by the spectral method and the laser scattering method are consistent [6, 7] . These results indicate that the spectroscopy method does not provide reliable electronic temperature measurements under certain conditions.
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