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At present, the instruments for direct plasma measurement are divided into two major categories: one is to measure the plasma density and temperature, and the methods are divided into two types: one is based on the current generated by charged particles that fall on the sensor, such as Faraday cups, decelerating potential analyzers and ion traps, the other being probes, which are extrapolated by the power supply variations caused by the addition of different voltages to the probes; the other major class is the characteristic spectrum of the plasma (spectroscopy) The use of optical fiber to detect plasma signals, through the spectrometer for data acquisition and analysis, such as the Dutch Avantes company's micro, high resolution, multi-channel fiber optic spectrometer. The following describes the plasma measurement using the spectroscopic method.
2. Experimental 2.1 Instrument principle The AvaSpec-2048FT fast-acting spectrometer from Avantes in the Netherlands adopts a symmetrical optical design with an incident focal length and a dispersive focal length of 75mm. It includes fiber connectors (standard SMA interfaces, and other types of interfaces are also available). , Collimating mirror, diffraction grating, focusing lens and Sony ILX554B type 2048 pixel linear array CCD detector, wavelength range 200-1100nm, maximum resolution 0.04nm, interface provides USB1.1 or USB2.0 interface, RS-232 interface and I/O digital/analog interface.
Figure 1 Optical Platform Figure 2 AvaSpec-2048FT Fast-Acting Spectrometer 2.2 Functions and Features 2.2.1 External Triggering The AvaSpec-2048FT fast-acting spectrometer can only sample after an external trigger signal is delayed by 1.3 s; this spectrometer can also be issued. A TTL signal triggers the pulsed laser, and the AvaSoft software can set the delay between the start of sampling and the laser pulse (-42ns to 2.7ms in 42ns steps) and the negative delay of -42ns for the sampling time characteristic of the measurement pulse. Lasers are very useful.
These characteristics of the AvaSpec-2048FT make it suitable for applications that require fast response and require precise synchronization with external trigger signals (it has only ±21ns of time jitter), such as product inspection on the conveyor belt. The characteristics of the output TTL signal can be used for laser induced breakdown spectroscopy (LIBS) measurement or fluorescence measurement. These applications require the laser pulse to start sampling after a period of time. 
2.2.2 History Channel Function The AvaSpec-2048FT fast-acting spectrometer can simultaneously monitor changes of up to 8 output parameters over time, such as user-defined functions, integrals, peaks (wavelength, relative intensity), and more. Functions can be written in Visual-Basic code. The results of timing measurements can be stored, loaded or printed. The zoom and panning functions can quickly zoom in on the part of interest in the timing measurement to full-scale display.
2.2.3 Independent Measurement Function The AvaSpec-2048FT fast-acting spectrometer can be configured to perform independent measurements without using a computer.
The AvaSpec-2048 spectrometer on the USB2 platform adds Bluetooth (-BT) communication and SDRAM card options for storing spectra on the board, enabling long-distance measurement and transmission (greater than 300 meters) and long-term spectral data storage.
2.2.4 High resolution over a wide spectral range For plasma and LIBS measurements, the spectrometer is generally required to have a high resolution over a wide spectral range.
The highest resolution of the AvaSpec-2048FT fast-acting spectrometer is up to 0.04nm, while the multi-channel spectrometer can meet the spectral range of 200-900nm with a resolution of better than 0.1nm. At the same time, it uses a fiber bundle for detection (a fiber connector at one end, Used to detect the signal; the other end is a plurality of connectors, connecting each channel), the operation is simple, but also can guarantee to detect the signal in the same position.
2.2.5 Symmetrical light path design The symmetric light path has a strictly 1:1 relationship between the incident focal length and the dispersive focal length ratio. A perfect linear image is formed on the CCD, and the influence of stray light is reduced because there is no light path crossing. To the minimum.
2.2.6 Simultaneous sampling of data between multiple channels The AvaSpec-2048FT fast-acting spectrometer can be configured as dual channels or multiple channels (up to 8 on the USB1.1 platform and up to 128 on the USB2.0 platform), controlled by the master and slave boards, and each channel Realize synchronous data acquisition in real sense. Each channel includes a separate optical platform. 
2.2.7 Good temperature stability and low thermal drift The optical components and the bottom plate of the AvaSpec-2048 spectrometer are assembled without stress and processed through a special process before leaving the factory. Therefore, the ambient temperature has minimal influence on the spectrometer, and the ambient temperature only drifts by 1°C for each degree of change. 0.1 pixels; excellent temperature stability ensures long-term measurement accuracy and repeatability.
2.3 Measurement method In fact, the method of measuring the characteristic line of plasma using the AvaSpec-2048FT spectrometer is very simple, as shown in Figure 5. The first step: install AvaSoft-Full full-featured software; the second step: connect the fiber, spectrometer and computer, one end of the fiber is fixed and aligned with the measured plasma, and then run the software; the third step: the software will collect The data arrived is plotted as a spectrum.
Because the spectral intensity measured by the spectrometer is a relative value (Electronic value, in units of Counts), if absolute intensity measurement is to be performed, the absolute calibration of the spectrometer in the calibration laboratory of the Avantes company in the Netherlands can be selected. Wavelength range 200- 1100nm. In order to make the entire measuring device more flexible, it is also possible to purchase a radiation calibration light source for calibration on site. 
2.4 Test Data and Devices 
Above is the spectrogram data of a plasma measured by the AvaSpec-2048FT fast-acting spectrometer. 
Figure 6 shows how the AvaSpec-2048FT fast-acting spectrometer measures the plasma of an inert gas during discharge.
3. Typical applications and users in China, AvaSpec-2048FT fast trigger spectrometer for plasma measurement units and applications are as follows:
1. Institute of Plasma Physics, Chinese Academy of Sciences, based on TOKAMAK research;
2. Department of Engineering Physics, Tsinghua University, based on research in low temperature plasma physics and material growth;
3. School of Chemistry and Molecular Engineering, Peking University; Research on low temperature plasma synthesis based on nanomaterials;
4. Institute of Modern Physics, Fudan University, plasma spectrum measurement of gas discharge process;
5. Plasma Propulsion Laboratory of Harbin Institute of Technology, based on research of plasma propulsion measurement technology in outer space environment;
6. Northwest Institute of Nuclear Technology, plasma research on high pressure discharge process;
7. Plasma Physics Chemistry Laboratory, Dalian University of Technology, Plasma-catalytic and Plasma Chemical Preparation of Nanocatalysts;
8. Institute of Electronic Engineering of PLA, research on solid combustion plasma and plasma stealth technology;
9. School of Physics, Soochow University, research based on plasma deposition;
10. School of Materials Science, North University of China, based on arc plasma research;
11. Beijing Institute of Clothing Technology, based on research of low temperature plasma modification technology;
12. Institute of Plasma Physics and Materials, Beijing Institute of Printing, based on plasma deposition;
13. Peking University, Beijing Institute of Geology, Beijing Institute of Geology, Northern Jiaotong University, South China University of Technology, China University of Geosciences, Shanxi University, Zhejiang Normal University, etc., based on laser induced breakdown spectroscopy (LIBS) technology;
4. Conclusion The plasma measurement using the AvaSpec-2048FT fast trigger spectrometer is cost-effective, high-sensitivity, high-precision, fast measurement, and simple operation. It is compact and easy to integrate into the system for on-line measurement.
Application of Micro-resolution Fiber Optic Spectrometer in Plasma
1. Introduction As the temperature rises, the general material in turn appears as a solid, a liquid, and a gas, collectively referred to as the three states of matter. As the gas temperature rises further, many, even all, of its molecules or atoms will dissociate into electrons and positive ions due to intense mutual collisions. At this point the material will enter a new state, which is mainly composed of electrons and positive ions (or positively charged cores). The substance in this state is called plasma, which can be called the fourth state of matter.
Figure 3 external trigger timing diagram
Figure 4 AvaSpec-2048FT-8-RM octal spectrometer
Fig. 5 Experimental layout of plasma profile measurement