Overview of runaway current suppression and dissipation on J-TEXT tokamak

The main works on disruption mitigation including suppression and mitigation of runaway current on the J-TEXT tokamak are summarized in this paper.Two strategies for the mitigation of runaway electron(RE) beams are applied in experiments.The first strategy enables the REs to be completely suppressed by means of supersonic molecular beam injection and resonant magnetic perturbation which can enhance RE loss,magnetic energy transfer which can reduce the electric field,and secondary massive gas injection(MGI) which can increase the collisional damping.For the second strategy,the runaway current is allowed to form but should be dissipated or soft landed within tolerance.It is observed that the runaway current can be significantly dissipated by MGI,and the dissipation rate increases with the injected impurity particle number and eventually stabilizes at 28 MA s^(-1).The dissipation rate of the runaway current can be up to 3 MA s^(-1)by ohmic field.Shattered pellet injection has been chosen as the main disruption mitigation method,which has the capability of injecting material deeper into the plasma for higher density assimilation when compared to MGI.Moreover,simulation works show that the RE seeds in the plasma are strongly influenced under different phases and sizes of 2/1 mode locked islands during thermal quench.The robust runaway suppression and runaway current dissipation provide an important insight on the disruption mitigation for future large tokamaks.

Suppression of runaway current by electrode biasing and limiter biasing on J-TEXT

The avoidance of runaway electrons(REs) generated during plasma disruption is of great concern for the safe operation of tokamak devices.Experimental study on the suppression of runaway current by electrode biasing(EB) and limiter biasing(LB) has been performed on the J-TEXT tokamak,which could be an alternative way to suppress the runaway current.The experimental results show that the higher the voltage value,the smaller the runaway current in both EB and LB experiments.The runaway current can be completely suppressed at an electrode biased voltage of +450 V and a limiter biased voltage of +300 V.The comparison of the energy spectra during the runaway plateau phase shows that the maximum energy max(E_(RE)) and radiation temperature T_(HXR)hard x-rays(HXRs)are significantly reduced after the application of +200 V limiter biased voltage.The electric field generated by the biased voltage may be the key factor to suppress the runaway current,and the measured radial electric field increases obviously after the voltage is applied.This may result in an increase in the loss of REs to realize the suppression of runaway current.

Synergetic Effects of Runaway Electron and Vertical Instability on the Wall Damage on HL-2A Tokamak

1 Generation of vertical instability Tokamak experiments show that the energy confinement time and performance are better, and the larger plasma current can be achieved for non-circular cross-sectional shape of plasma than circular cross-section. However, the external magnetic fields which are used to produce the non-circular cross sectional shape also cause the confined plasma to become unstable to small vertical displacement. In general, the ratio of elongation k is larger, the possibility of instability is more. In practice, this vertical displacement mode stabilized by feedback control system or other provided external radial magnetic field to balance out the plasma motion. Under some of situations the control system may be fail due to rapid growth rate of instability exceeding the ability of controlling. The plasma will then move vertically upwards or downwards depending upon the characteristics of instability and control failure.

Overview of the J-TEXT progress on RMP and disruption physics

The J-TEXT tokamak has been operated for ten years since its first plasma obtained at the end of 2007. The diagnostics development and main modulation systems, i.e. resonant magnetic perturbation （RMP） systems and massive gas injection （MGI） systems, will be introduced in this paper. Supported by these efforts, J-TEXT has contributed to research on several topics, especially on RMP physics and disruption mitigation. Both experimental and theoretical research show that RMP could lock, suppress or excite the tearing modes, depending on the RMP amplitude, frequency difference between RMP and rational surface rotation, and initial stabilities. The plasma rotation, particle transport and operation region are influenced by the RMP. Utilizing the MGI valves, disruptions have been mitigated with pure He, pure Ne, and a mixture of He and Ar （9：1）. A significant runaway current plateau could be generated with moderate amounts of Ar injection. The RMP has been shown to suppress the generation of runaway current during disruptions.

Recent progress of the ECRH system and initial experimental results on the J-TEXT tokamak

In order to broaden the range of the plasma parameters and provide experimental conditions for physical research into high-performance plasma,the development of the electron cyclotron resonance heating(ECRH) system for the J-TEXT tokamak was initiated in 2017.For the first stage,the ECRH system operated successfully with one 105 GHz/500 kW/1 s gyrotron in 2019.More than 400 kW electron cyclotron(EC) wave power has been injected into the plasma successfully,raising the core electron temperature to 1.5 keV.In 2022,another 105 GHz/500 kW/1 s gyrotron completed commissioning tests which signifies that the ECRH system could generate an EC wave power of 1 MW in total.Under the support of the ECRH system,various physical experiments have been carried out on J-TEXT.The electron thermal transport in ECRH plasmas has been investigated.When ECRH is turned on,the electron thermal diffusivity significantly increases.The runaway current is elevated when a disruption occurs during ECRH heating.When the injected EC wave power is 400 kW,the conversion efficiency of runaway current increases from 35% to 75%.Fast electron behavior is observed in electron cyclotron current drive(ECCD) plasma by the fast electron bremsstrahlung diagnostic(FEB).The increase in the FEB intensity implies that ECCD could generate fast electrons.A successful startup with a 200 kW ECW is achieved.With the upgrade of the ECRH system,the J-TEXT operational range could be expanded and further relevant research could be conducted.