Theoretical Study On The Effects Of Synchrotron Radiation And Stochastic Magnetic Fluctuations On Runaway Electrons

Magnetic confinement fusion is one of the ways to solve the human energy problem in the future.Tokamak is the most studied and the most promising device to realize magnetic confinement fusion.However,plasma disruption is almost inevitable in the operation of the tokamak.One of the hazards of plasma disruption is the generation of large amounts of highenergy runaway electrons(REs).Without control,these energetic REs will eventually hit the first wall of the device,posing a serious threat to the safe operation of the device.Therefore,the theoretical study on the generation of REs and finding a practical mitigation method are of great significance.Whether the electrons run away or not depends on their velocity space structure.When the electron velocity exceeds the thermal velocity,the collision friction force decreases with the increase of the velocity.When the electrons get enough acceleration force from the electric field,which overcomes the collision friction force,the electrons can be accelerated continuously and run away from the plasmas.However,in the electromagnetic field,the synchrotron radiation of electrons increases with increasing electron velocity,so the energy of REs is limited by synchrotron radiation.In addition,the trajectory of the REs is constrained by the configure space,and the "survival" time of the REs is determined by their transport mechanisms.Magnetic fluctuation is an important mechanism affecting the transport of electrons.Therefore,we focus on the combined effects of synchrotron radiation and stochastic magnetic fluctuations(SMFs)on the production of REs.The method of statistical description of plasma is a powerful tool to solve the plasmarelated problems.According to the different time scales in the process of REs generation,we use the method of time scale separation to solve the gyrokinetic relativistic FokkerPlanck equation including synchrotron radiation and SMFs.It is found that whether an electron can run away or not is closely related to the relationship between its acceleration rate and diffusion rate.Electrons can run away only when the acceleration rate is faster than diffusion rate.From the balance between the acceleration rate and the diffusion rate,we can obtain the sustainment electric field for the REs,which is a positive correlation function of the SMFs level.For a fixed level of magnetic fluctuations,if the electric field is smaller than the sustainment electric field,any existing REs will be lost due to the acceleration rate less than the diffusion rate.REs only occur when the electric field is higher than the sustainment electric field.Meanwhile,it is found that the minimum runaway momenta of electrons are extremely sensitive to the change of SMFs.In contrast,the energy limit is almost not affected by SMFs.In addition,the sustainment electric field is not enough for avalanche growth of REs,and a stronger electric field is required.Furthermore,the avalanche threshold electric field is enhanced and the runaway growth rate is reduced due to the combined effects of SMFs and synchrotron radiation as compared to the case for only synchrotron radiation being considered.The runaway growth rate decreases significantly with the increase of the level of magnetic fluctuations.These theoretical results are qualitatively consistent with a large number of experimental and simulation results,which indicates the possibility of suppressing REs by artificially enhancing the level of magnetic fluctuations in the future International Thermonuclear Experimental Reactor(ITER).