Study Of CO₂ Dispersion Interferometer On HL-2A

The electron density is a very basic and important parameter in plasma experiments and physics.Physical behaviors of plasma such as MHD instability,transport,and confinement are all related to density.Plasma density measurement in real-time is also used for density feedback control during tokamak discharges.Laser interferometers are widely used in density diagnosis.However,in future larger devices such as ITER,the condition is approaching burning plasma.Therefore the refraction and fringe jumps caused by plasma density will have a great impact on widely used far-infrared laser interferometers,while short-wavelength laser interferometers suffer from mechanical vibrations.In this thesis,the development of the first dispersion interferometer based-on frequency-doubling technology in China is described.The system uses a 10.6μm wavelength CO₂ laser as the light source.The signal modulation method eliminates the effects of mechanical vibration.The interferometer was applied on HL-2A device and SCU-PSI linear device.Our work mainly includes the principle design of the system,the study of the key component characteristics,the construction of the interferometer system,and the application on HL-2A device.In the frequency-doubling experiment,we successfully generated a second harmonic of 5.3μm by an AgGaSe2 crystal.The experimental results on the HL-2A device show that the measured static phase resolution of the system reaches 1.2°,and the average density measurement resolution reaches 1.74×10¹⁷/m³.The DI system measured the complete density curve in the pellet injection experiment of HL-2A device without fringe jump.On the SCU-PSI linear device of Sichuan University,high-density plasma with a density exceeding 10²¹/m³ was measured,demonstrating that the density measurement range can satisfy the requirement of measuring burning plasma.The work has the following innovations:The system uses a RF excitation laser as the light source.This kind of laser has the advantages of high power and compact structure compared with the DC-excited continuous laser used in similar systems.At the same time,several sets of data processing systems including FPGA real-time data processing systems,offline software data processing systems,and lock-in amplifiers have been designed.It can meet the needs of providing real-time feedback signals,real-time observation of system phase and stability,and off-line data analysis.In the future work,a multichannel dispersion interferometer system for HL-2M tokamak devices to provide a density feedback signal is under designing.At the same time,it is planned to apply the wedge beam offset compensation technology and the heterodyne modulations to improve the system’s accuracy and time resolution.