Development of electromagnetic pellet injector for disruption mitigation of tokamak plasma

Disruption remains to be a serious threat to large tokamaks like the International Thermonuclear Experimental Reactor(ITER).The injection speed of disruption mitigation systems(DMS)driven by high pressure gas is limited by the sound speed of the propellant gas.When extrapolating to ITER-like tokamaks,long overall reaction duration and shallow penetration depth due to low injection speed make it stricter for plasma control system to predict the impending disruptions.Some disruptions with a short warning time may be unavoidable.Thus,a fast time response and high injection speed DMS is essential for large scale devices.The electromagnetic pellet-injection(EMPI)system is a novel massive material injection system aiming to provide rapid and effective disruption mitigation.Based on the railgun concept,EMPI can accelerate the payload to over 1000 m/s and shorten the overall reaction time to a few milliseconds.To verify the injection ability and stability of the EMPI,the prototype injector EMPI-1 has been designed and assembled.The preliminary test has been carried out using a 5.9 g armature to propel a dummy pellet and the results suggest that the EMPI configuration has a great potential to be the DMS of the large scale fusion devices.

Comparison of different noble gas injections by massive gas injection on plasma disruption mitigation on Experimental Advanced Superconducting Tokamak

Massive gas injection(MGI)is a traditional plasma disruption mitigation method.This method directly injected massive gas into the pre-disruption plasma and had been developed on the Experimental Advanced Superconducting Tokamak(EAST).Different noble gas injection experiments,including He,Ne,and Ar,were performed to compare the mitigation effect of plasma disruption by evaluating the key parameters such as flight time,pre-thermal quench(pre-TQ),and current quench(CQ).The flight time was shorter for low atomic number(Z)gas,and the decrease in flight time by increasing the amount of gas was insignificant.However,both pre-TQ and CQ durations decreased considerably with the increase in gas injection amount.The effect of atomic mass on pre-TQ and CQ durations showed the opposite trend.The observed trend could help in controlling CQ duration in a reasonable area.Moreover,the analysis of radiation distribution with different impurity injections indicated that low Z impurity could reduce the asymmetry of radiation,which is valuable in mitigating plasma disruption.These results provided essential data support for plasma disruption mitigation on EAST and future fusion devices.

Development of a Fast Valve for Disruption Mitigation and its Preliminary Application to EAST and HT-7

In large tokamaks, disruption of high current plasma would damage plasma facing component surfaces （PFCs） or other inner components due to high heat load, electromagnetic force load and runaway electrons. It would also influence the subsequent plasma discharge due to production of impurities during disruptions. So the avoidance and mitigation of disruptions is essential for the next generation of tokamaks, such as ITER. Massive gas injection （MGI） is a promising method of disruption mitigation. A new fast valve has been developed successfully on EAST. The valve can be opened in 0.5 ms, and the duration of open state is largely dependent on the gas pressure and capacitor voltage. The throughput of the valve can be adjusted from 0 mbar·L to 700 mbar·L by changing the capacitor voltage and gas pressure. The response time and throughput of the fast valve can meet the requirement of disruption mitigation on EAST. In the last round campaign of EAST and HT-7 in 2010, the fast valve has operated successfully. He and Ar was used for the disruption mitigation on HT-7. By injecting the proper amount of gas, the current quench rate could be slowed down, and the impurities radiation would be greatly improved. In elongated plasmas of EAST discharges, the experimental data is opposite to that which is expected.

Research on High Pressure Gas Injection As a Method of Fueling, Disruption Mitigation and Plasma Termination for Future Tokamak Reactors

High-pressure gas injection has proved to be an effective disruption mitigation tech- nique in DIII-D tokamak experiments. If the method can be applied in future tokamak reactors not only for disruption mitigation but also for plasma termination and fueling, it will have an attractive advantage over the pellet and liquid injection from the viewpoint of economy and engineering design. In order to investigate the feasibility of this option, a study has been carried out with relevant parameters for conveying tubes of different geometrical sizes and for different gases. These parameters include pressure drop, lagger time after the valve＇s opening, gas diffusion in an ultra-high vacuum condition, and particle number contour.

Mitigation and prediction of disruption on the HL-2A Tokamak

Injection of high-Z impurities into plasma has been proved to be able to reduce the localized thermal load and mechanical forces on the in-vessel components and the vacuum vessel, caused by disruptions in Tokamaks. An advanced prediction and mitigation system of disruption is implemented in HL-2A to safely shut down plasmas by using the laser ablation of high-Z impurities with a perturbation real-time measuring and processing system. The injection is usually triggered by the amplitude and frequency of the MHD perturbation field which is detected with a Mirnov coil and leads to the onset of a mitigated disruption within a few milliseconds. It could be a simple and potential approach to significantly reducing the plasma thermal energy and magnetic energy before a disruption, thereby achieving safe plasma termination. The plasma response to impurity injection, a mechanism for improving plasma thermal and current quench in major disruptions, the design of the disruption prediction warner, and an evaluation of the mitigation success rate are discussed in the present paper.

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.