Research On The Suppression And The Dissipation Of Runaway Current During Plasma Disruption On The J-TEXT Tokamak

Plasma disruptions are one of the major challenges for tokamak devices.Heat Loads,halo currents,and runaway electrons(RE)produced during disruptions can deal large damages to the first wall and inner structures of tokamaks.Impurity injections are the main methods to mitigate the disruptions in the world for now.For the future large-scale tokamaks like International Tokamak Experimental Reactor(ITER),the RE generation may be hard to suppress by a single impurity injection due to the high avalanche multiplication factor,and therefore leads to the formation of runaway currents.The runaway currents can carry large energy and also do harm to the devices,therefore,need to be significantly dissipated.The experiments about the suppression and dissipation of runaway currents during the plasma disruption have been done on the Joint-Texas Experimental Tokamak(J-TEXT)tokamak.Massive gas injection system(MGI)with argon is used to trigger the disruption and to form the runaway current in the experiments.The runaway currents are about 50% of the target plasma current.A one-dimensional model based on the J-TEXT parameters for the RE generation during disruption has been built.Dreicer,Hot-tail,and avalanche mechanism are included in the model.The model uses the finite-difference method to calculate the onedimensional Maxwell equations with a given evolution of electron temperature.The simulation results show that,the Dreicer generation mechanism dominates in the phase of RE generation on J-TEXT.Raising the electron density and critical field during the phase of RE seed generation can decrease the RE generation rate.Therefore,it is feasible to suppress the formation of runaway current by impurity injection on J-TEXT.Different species and quantities of impurity are simulated to dissipate the runaway current.The runaway current dissipation rate increases with the increase of impurity,and the dissipation efficiencies of high Z impurities are better than others.The experiments of suppressing runaway current by MGI with argon and krypton during the RE generation phase have been done.The electron densities during the thermal quench(TQ)and the current quench(CQ)were raised by injecting argon or krypton impurities in different time.Experiments result show that,the runaway current can be totally suppressed when enough quantity of impurities arrived at the plasma during TQ;the runaway current can be reduced to20-60% of the reference value when the impurities arrived at the plasma during CQ.Analysis shows that,the electron density can be raised by impurity injection during TQ to suppress the RE generation basing on the Dreicer mechanism;both of the electron density and effective critical electric field for RE generation were raised by impurity injection during CQ to suppress the RE generation basing on the Avalanche mechanism.The experiments of dissipating runaway current by MGI and shattered pellet injection(SPI)system have been done.Up to 28 MA/s of runaway current dissipation rate and 15% of the energy dissipation rate have been achieved by injecting large quantity of argon or krypton impurities by MGI into the runaway current beam.The experiment results show that,the dissipation efficiency of runaway current increased with the increase of the quantity of injected impurities,however,it reached to saturation when the quantity is large.This saturation is caused by the limit of valve on the impurity injection rate,and the decrease of impurity assimilation rate in the runaway current beam when the RE current beam moved away from the tube of valve.The increasing trend of runaway current dissipation rate may be slowed down by the increase of inner inductance during the dissipation phase.The experiments of dissipating runaway current by using SPI system also have been done.Runaway current dissipation rate achieved by shattered argon pellet was lower than that achieved by argon gas with similar quantity of argon particles.It may be caused by the low assimilation efficiency in cold plasma for solid,and needs further researches.Analysis about the asymmetry of thermal radiation during the dissipation phase shows that,the peaking factor of thermal radiation near the impurity injection port was high.The distribution of thermal radiation may be improved by changing the layout of injection systems in the future.