Study Of Runaway Electrons And Characteristics Of Current Quenches In J-TEXT

Plasma disruption is a common phenomenon during discharges in tokamaks, which isthe sudden loss of magnetic confinement and thermal energy. The disruption can causesevere damage by three means: thermal loads, electro-magnetic (EM) forces and runawayelectrons (REs). There have been effective techniques to mitigate the first two damages,such as massive gas injection (MGI) and killer pellet injection (KPI). However, none ofthem can completely suppress the generation of REs in next generation device-ITER.Sometimes the plasmas will disrupt with a stable long live runaway current plateau, whichreduces the life time of first wall materials during the wall interaction as a result of theirhigh energy. So it’s very important to study the behavior of runaway electrons anddisruptions.The HXR emissions come from the thick target bremsstrahlung when runway electronsinteract with the vessel walls. In order to study the behavior of REs, a Hard X-Ray (HXR)detector array (0.5~10MeV) has been developed in J-TEXT tokamak, which is arranged onthe equatorial plane. Based on HXR diagnostic system and other signals, a series ofexperiments on REs and disruptions have been performed in J-TEXT.The influence of magnetic turbulence on runaway transportation has been studied.Being sensitive to magnetic turbulence, runaway electrons are always studied as a uniquetool to probe the magnetic perturbations. Two types of magnetic turbulence have beeninvested in J-TEXT tokamak discharges: magnetic turbulence in controlled inward/outwardplasma displacement experiment and Resonant Magnetic perturbations. Both of them canmake an impact on runaway transportation since a drop or rise in the HXR signals wereobserved in the experiments.The characteristics of current quenches have been evaluated by90-10%and80-20%ofthe pre-disruption current in J-TEXT. For disruptions with the fastest current quench, thewaveforms of plasma current can usually be fitted well by linear fitting, while in slow current quench cases which have two decay phases the exponential fittings are suit for bothtwo phases.Provoked shutdown by gas puffing (GP) and supersonic molecular beam injection(MGI) has been investigated in J-TEXT. The experiments include two parts: single pulseinjection and double pulses injection. The type, pressure and pulse width of injected gaseswill have influences on current quench time. Sometimes the double pulse injection canaccelerate the current quench rates compared with single pulse injection. High Z impurityinjection is easier to accelerate decay rates, while low Z atoms can move faster in plasma.The regime of runaway current generation has been studied in J-TEXT. It is found thatno runaway current generated when the toroidal magnetic field is lower than2.2T. If theplasma current is higher than200kA, there will be no generation of runaway current eventoroidal magnetic field is2.2T. The plasma disrupted without runaway current with a largeamount of GP while with a small amount of GP the plasma disrupted with runaway currents.So the runaway current may be avoided by GP in J-TEXT.