Simulation Of Runaway Electrons During Disruptions In HL-2A Tokmak

The simulation of runaway electrons and electric field evolution during HL-2A disruption is presented using a self-consistent model, which is constituted by Maxwell’s equations and generation rate equation. The generation rate equation includes Dreicer and avalanche generation mechanisms of runaway electrons. The physical model was solved numerically by finite difference, and a study about runaway electrons was made from two aspects below:Firstly, we explored the runaway electrons and toroidal electric field evolution during plasma disruption, most of the input parameters in the simulation are taken from the HL-2A disruption experiment discharge (No.15335). The simulation shows that the electric field increases rapidly from0.15ms after the disruption, reaches the maximum about0.15ms later, and lasts about2.5ms. The temporal and spatial distribution of electric field is different from JET. The simulation results also show that runaway electrons are produced mostly in the region around the magnetic axis, which is in accordance with the results observed by soft x-ray camera. Besides, the runaway electron beam arises in lms after disruption, which demonstrates tens of milliseconds steady-state current appearing after disruption is carried mainly by runaway electrons. However, the runaway electron beam formation starts in7ms in experiment because of magnetic perturbation, which is not included in the modelling, thus, there is a time delay between simulation and experiment. Further, it is found that42%of the Ohmic current is converted into runaway current in HL-2A, which is in approximate agreement with experimental data55%. Although the avalanche generation rate of runaway electrons is weaker than Dreicer generation mechanism in HL-2A, it plays a key role because of its longer action time than Dreicer generation mechanism, which is consistent with its small toroidal current observed in the experiment.Secondly, some parameters are poorly diagnosed during disruptions in experiment, huge obstacles emerge when making the research about runaway electrons by simulation, so the impact on runaway electrons caused by different post-disruption temperatures and different thermal quench time is researched in this work. The study shows that the lower post-disruption temperature is taken, the more runaway electrons are produced, and larger runaway current is converted from ohm current, because larger toroidal electric field is induced when the post-disruption temperature is lower; At the same time, the runaway current reaches steady state earlier for the shorter-lived toroidal electric field caused by lower post-disruption temperature. Similar results about thermal quench time is presented:the shorter quench time is chosen, the more runaway electrons are produced, but if the quench time is chosen within one millisecond, significant changes can’t be obtained in the simulation.The results in this work provide the evolution of the runaway electrons, plasma toroidal current and electric field in detail, which may be applied to the controlling of runaway electrons and referential to laboratory diagnosis in HL-2A.