Study Of Synchrotron Radiation Of Runaway Electrons In EAST Tokamak

In tokamak plasma,when the electron energy exceeds a certain threshold,the collision resistance will be less than the acceleration force,so the electrons will be continuously accelerated to tens of MeV or even higher energy.This phenomenon is called electron runaway,and these high-energy electrons are called runway electrons.Once these high-energy runaway electrons are lost,they will cause serious damage to the plasma facing components and even make them melt locally.At present,runaway electrons are still an important and unsolved key scientific problem that threatens the safety of tokamak-type magnetic confinement fusion reactors.Therefore,it is urgent to seek ways to mitigate the threat of runaway electrons or suppress runaway electrons.The key factors that determine the threat of runaway electrons are their density distribution,energy distribution and loss area distribution,which are determined by the distribution function of runaway electrons,including their spatial distribution,energy distribution and θp(the angle between the velocity direction of runaway electrons and the magnetic field line)distribution,so it is very important to study the distribution function of runaway electrons and its evolution.Based on the synchrotron radiation characteristics of high-energy runaway electrons in Tokamak,studying the distribution function of runaway electrons confined in plasma is the most direct,important and even the only research method,which is the main research content of this paper.Firstly,the influence of the runaway electron parameters on the synchrotron radiation image is studied in this paper.Our research is based on the analytical formula of I.M.Pankratov,our own two-dimensional simulation program and M.Hopper’s three-dimensional simulation program SOFT(Synchrotron-detecting Orbit Following Toolkit),the simulation results show that the synchrotron radiation characteristics of runaway electrons are obviously affected by their spatial distribution,energy distribution and θp distribution.The spatial distribution of runaway electrons has the most obvious influence on the spatial distribution of synchrotron radiation.The intensity of synchrotron radiation becomes weaker in the region with relatively low density of runaway electrons.Especially when the density of runaway electrons is low in the plasma core,the synchrotron radiation image will show hollow structure.The energy of runaway electrons mainly affects the decay rate of synchrotron radiation intensity from high field side to low field side.When the energy is low,the runaway electron synchrotron radiation will rapidly decay to approach 0,so that synchrotron radiation can only be observed on the high field side.However,in both two-dimensional and three-dimensional models,only a narrow range of synchrotron radiation of runaway electrons can be observed when θp is very small,and the observed range of synchrotron radiation will increase with the increase of θp.Then,the runaway electron distribution function and its evolution with time are studied.The main method is to import different distribution functions into SOFT program for simulation,by comparing the simulation results with the experimental results,we find the runaway electron distribution that best matches the experiment.For the runaway electron distribution function,first assume that the spatial distribution is Gaussian distribution,ξ=cosθpis exponential distribution and monoenergetic.After obtaining the runaway electron θp distribution and energy which are most consistent with the experiment,the energy distribution of the runaway electrons is explored.The results show that the error between the simulation and the experimental results is small when the runaway electrons are distributed in a narrow range near the optimal energy.This part of the high-energy runaway electrons that dominate the synchrotron radiation is quasi-monoenergetic.At the same time,using the synchrotron radiation spectrum of runaway electrons as a supplementary condition,the energy of runaway electrons can be obtained more accurately,which provides an important basis for the development of spectral infrared camera diagnostic systems.Based on this,the evolution of runaway electron parameters with time is studied.The research shows that the runaway electron energy increases gradually in the current ramp-up phase,and begins to decrease after reaching the flat-top,the average θp of runaway electrons has been on the rise in the current ramp-up phase and early flat-top phase.Also,the behavior of trapped runaway electrons observed in EAST experiment is studied.The theoretical and simulation results show that the runaway electrons can be trapped in the plasma to form the trapped runaway electrons.We observed and proved the existence of the trapped runaway electrons for the first time in the EAST experiment.This is also the first time in the world that trapped runaway electron synchrotron radiation has been observed experimentally.The results show that although the proportion of trapped runaway electrons is very low,the actual synchrotron radiation power of single trapped runaway electron is several orders of magnitude higher than that of passing runaway electrons,so the trapped runaway electrons may still dominate synchrotron radiation.When the trapped runaway electrons dominate the synchrotron radiation,because most of the trapped runaway electrons cannot reach the high field side,the intensity of synchrotron radiation is weak on the high field side.In this paper,we estimate the energy and θp of trapped runaway electrons according to the variation trend of runaway electron synchrotron radiation with R and the test particle model.According to these parameters,KORC(Kinetic Orbit Runaway electron Code)program is used to simulate the trapped runaway electron synchrotron radiation image in EAST.Both the shape of synchrotron radiation and the change of synchrotron radiation with plasma major radius are in good agreement with the experiment,these results verify previous numerical calculations.Finally,this paper introduces the development of high frame rate spectral infrared diagnostic system for observing runaway electrons.Based on the above research results,a medium wave,high-speed and spectrum infrared camera diagnosis system specially used for the study of runaway electron synchrotron radiation characteristics is developed on EAST.The medium wave infrared band is the band with the most concentrated power of the runaway electron synchrotron radiation spectrum on EAST.The high-speed data can be used to study the instability and runaway electrons produced during plasma disruption,and the spectrum function of infrared camera can be used to study the runaway electron synchrotron radiation spectrum.At present,the establishment of the whole system on EAST device has been completed and preliminary data have been obtained,which provides an important basis for subsequent research.In conclusion,based on the synchrotron radiation characteristics of high-energy runaway electrons in Tokamak,this paper studies the synchrotron radiation characteristics of runaway electrons,the distribution function of runaway electrons and its evolution,and the behavior of trapped runaway electrons,and develops a set of high frame rate spectrum infrared diagnosis system.The relevant research results provide important support for mitigate the threat of runaway electrons or suppress runaway electrons.