Development And Preliminary Application Of The Shattered Pellet Injection System On The J-TEXT

Plasma disruption is a common phenomenon in tokamak operation and can lead to serious damage to large tokamaka device.In the magnetic confinement fusion,the plasma contains huge thermal energy and magnetic energy.Especially in the International Thermonuclear Experimental Reactor(ITER),its plasma current will reach 15 MA,and the thermal and magnetic energy will both reach 500 MJ.Large plasma disruption can cause three detrimental effects to the tokamak devices: First,the massive heat loads have the potential to damage the first-wall components;additionally,the large electromagnetic force caused by current quench and halo currents will destroy the structure of the device;finally,the runaway current generated by the disruption will hit the first wall.Therefore,the avoidance and mitigation of large plasma disruption is very important for large tokamak devices.Injecting a large amount of impurity gas before disruption to raise the density of the plasma core is one of the main means to mitigate the disruption.The shattered pellet injection(SPI)is one of the basic measures used to disruption mitigation and runaway current dissipation on ITER.It can increase radiation,reduce halo current and suppress runaway electrons acceleration by increasing plasma density.Compared to the means of massive gas injection(MGI)and "killer" pellet injection(KPI),the impurities can be injected to the position which is closer to the plasma core.What’s more,it will give no damage to the tokamak devices.In this paper,an argon SPI system has been designed and developed on the J-TEXT for the first time.It is used to carry out a research of disruption mitigation.The system includes: an injector and its gas circuit system,a diagnostic system,a buffer chamber,a strike tube and a control system.It can produce a pellet with a 5 mm diameter and a 4-10 mm length at a temperature of about 60 K.The pellet is injected with a speed of 100-300 m/s,then shattered by a curved tube and fly into the plasma.Through the platform test,the reliability of the system is verified and the main factors affecting the speed,size and flight speed of the pellet were determined.The system was officially installed on the J-TEXT tokamak in the spring experiment of 2018.The related experiments were carried out to explore the effect of the pellet size and speed on SPI rapid shutdown.It is found that the increasing pellet size and speed will reduce the penetration time and increase the penetration depth.At the same time,the experiments of plasma rapid shutdown and runaway current dissipation based on SPI were carried out.Compared with the results of MGI,it was found that SPI has deeper penetration depth and the impurities are deposited closer to the core.But in dissipation,SPI does not achieve the dissipation effect of MGI on the runaway current.