The Key Technology Of Continuous Operating Interferometer Diagnosis For Fusion Reactor

Plasma electron density is one of the key physical parameters of tokamak device physics experiment.Fast and accurate electron density measurement and density feedback control are important bases for continuous operation of high parameter fusion plasma physics experiment.Far infrared(FIR)laser interferometer is the most important and reliable density diagnosis system for tokamak devices,which can provide support for evaluating the energy constraint time of the device,and can also help realize real-time density control.At present,optically pumped terahertz laser is the most suitable light source for far infrared laser polarization interferometer.With the continuous increase of operating parameters of Tokamak fusion device,the demand for high electron density measurement is increasingly urgent.The self-developed shortwavelength optically pumped terahertz dual-frequency laser is very important for the continuous operation of future fusion reactors.Moreover,with the development of physics experiments,most of the devices have the requirement of spatial measurement range from the plasma center to the boundary of the interferometer system.To meet the requirement of wide density measurement range of the fusion reactor,the dualwavelength composite interferometer system can be developed using two light sources.In recent years,the domestic hardware technology in the field of FIR laser interferometer has gradually matured,and the successful implementation of domestic interferometer equipment and device applications has far-reaching significance for the development of far-infrared interferometer diagnosis technology in China.In this paper,a CO2 pumped HCOOH dual laser which can run continuously for 24 hours is developed to meet the demand of future polarization interferometer system.On the other hand,in order to meet the requirement of wide density diagnosis range,a microwave-far infrared composite laser interferometer system was developed on the EXL-50 spherical tokamak device,which consists of two sets of interferometers in different bands:The 3-channel HCN interferometer and 3-channel 340 GHz microwave solid source interferometer,two systems running at the same time,can provide 6channel line integral electron density measurement for the EXL-50 device,and realize the application of domestic devices such as light source,phase comparator,detector and Angle mirror on the composite interferometer system.This paper first describes the detailed design of the optically pumped HCOOH dual laser,including the parameters and structure design of the terahertz resonator,the selection of the infrared pump source and auxiliary system,the design of the pump optical path,and finally the construction of the CO2 pumped HCOOH dual laser.A 9.27μm(9R20 spectral line)CO2 laser pump resonator length 1.6 m and filled with high purity formic acid terahertz cavity,output frequency 0.69 THz,power up to 20 mW terahertz wave,and the output stability test was carried out.The laser can run continuously for 24 hours.The output power is maintained at more than 14 mW,and the difference frequency drift is less than 5%in a short time.A dual-laser interferometer table was built,and its phase measurement capability was tested by using rotating wedge and ICP sources.The test results show that the dual-laser system has excellent output performance as the light source of the far-infrared laser interferometer.On this basis,the concept design of output stability feedback control system for dual lasers is presented.Then,the measuring principle of microwave-far-infrared composite laser interferometer is described,and the characteristics and measuring principle of heterodyne interferometer with rotating grating modulation and double laser method are introduced.The design,selection and testing of two sets of key components of the interferometer are carried out.The optical and mechanical design of the whole system is introduced in detail.The microwave-far infrared laser composite interferometer system is a Mach-Zehnder interferometer,with 6 detection channels all measuring vertically and the density measuring range covering 1×1016～1×1020 m-3.Angular mirrors are used on the upper windows of the device to achieve double optical path measurement,avoiding large support structures.The HCN laser interferometer uses the HCN laser with the frequency of 890 GHz as the light source,and uses the rotating grating for phase modulation,which can generate the beat signal of 100 kHz,and the time resolution can reach 10 μs.The solid-source interferometer uses two independent 340 GHz solid-state frequency-doubling sources as light sources.By setting the frequency difference between the two sources to 1 MHz,the time resolution of the solidsource interferometer can reach 1μs.The main optical path of the two interferometers is compactly installed on a set of double-layer optical platforms directly below the EXL-50 device.At present,the RMS of phase noise of HCN interferometer and solid source interferometer are 4.1 deg and 2.1 deg respectively,and the corresponding linear integral electron density is 0.75×1017 m-2 and 1.5×1016 m-2,respectively.The composite interferometer system obtains the measurement results of 6 channels.The highest density measured so far is about 1×1019 m-2.The measurement data of the composite interferometer are compared with the original 140 GHz microwave interferometer,and the results are in good agreement with each other,which confirms the accuracy of the measurement system.Finally,the problems of mechanical vibration and electromagnetic interference encountered in the development of the composite interferometer are described,and effective solutions are put forward.