Runaway Electrons And Cold Front Propagation During Fast Shut-down Experiments On J-TEXT Tokamak

Disruption is a common fast uncontrolled event in tokamaks，and its damage to thedevice mainly results from three aspects: the local heat loads that can ruin the first wall;the halo current that brings the device a mechanical load through j B-forces; a largenumber of runaway electrons with high energy that threaten the operation of the tokamak.Many disruption mitigation methods are effective to the heat loads and the halo current,but still can’t suppress the generation of runaway electrons. Therefore, it is very importantto study the related physical mechanism of the runaway electrons, especially during thedisruption phase. Nowadays, when changing different kinds of injected gas, the massivegas injection experiment have been succeed in reducing the damage of all these aspects.Thus, massive gas injection experiment is most likely to be the disruption mitigationscheme of the next generation device. However, the mixing efficiency of the injected gasand the plasma is very low, and the penetration depth is limited by the position of the q=2magnetic surface. Hence, it is necessary to find an optimization scheme in order toimprove the mixing efficiency and the penetration depth.In tokamak plasmas, electrons with energies higher than some critical energy arecontinuously accelerated by the electric field and become runaway electrons. Classicaltheory of the runaway electron generation mechanism includes two parts: the primaryDreicer mechanism and the secondary avalanche generation mechanism. We canreproduce the runaway current platform in the experiment through a numerical modelingincluding both generation mechanisms and some loss mechanisms. Another theory aboutrunaway electrons considers the phase-space analysis. The relaxation equations of testparticles including the acceleration along the toroidal electric field, collisions with theplasma particles, and deceleration due to synchrotron radiation losses is used here todescribe the motion of runaway electrons in phase-space. There are two singular points inthe phase-space, which correspond to the generation threshold and the energy limit,respectively. We can get lots of information about runaway electrons generation thresholdand energy limit through the analysis of the singular points. Moreover, we can add some other losses term to the model in order to investigate the relationship between this lossterm and the behavior of runaway electrons, here we take the stochastic magneticfluctuations as an example of loss term.During the discharge experiments, when the runaway electrons lose control and hitthe limiter, hard X-ray due to the thick target bremsstrahlung will be produced. We usehard X-ray diagnostic system to investigate the experiments about runaway electrons anddisruption. This diagnostic system has been upgraded several times in these years,including the detectors installed at the forward, backward and radial direction ofmid-plane, a set of space detector array installed at several spatial symmetry points ofdevice recently, part of which have already been in used.SMBI triggering the fast plasma shut-down to investigate the cold front process ofthe impurities is useful for us to understand the relationship between gas penetration depthand the position of the q=2magnetic surface. The experiment results indicate that whenthe q=2magnetic surface get closer to the plasma edge, both current quench time and thetime interval between SMBI trigger and the major disruption get shorter. By theinvestigation of the cold front propagation process using ECE signal, we find the coldfront stops around the q=2magnetic surface, no matter howqa changes. In another word,the injected gas can’t penetrate further than q=2magnetic surface before the disruption.Recently, it’s obviously to see that the cold front stops around q=2magnetic surface byusing High-speed camera.