| The development of nuclear fusion energy is the focus and difficulty on current international energy research.Tokamak is currently considered as the most promising device to achieve the commercial application of fusion energy.The International Thermonuclear Experimental Reactor(ITER)currently under construction is a magnetic confinement fusion device with high parameters.In actual operation,magnetohydrodynamic(MHD)activities or control system errors will inevitably occur under high parameters,which may lead to the plasma disruption.The resulting thermal load,energetic runaway electron and electromagnetic stress during the disprution will affect the safety of the device.In order to avoid or reduce the threat as much as possible,the damage must be mitigated.At present,one of the most effective ways is to inject impurity into the plasma,and to dissipate the stored energy(magnetic energy and thermal energy)through the impurity ionizing radiation to achieve the purpose of protecting the device.The methods are closely related to the type of impurity,the depth of injection,and the impurity distribution in the plasma.Among them,Shattered Pellet Injection(SPI)has been regarded as one of the disruption mitigation methods of ITER.But the relevant experiments are not sufficient,and the numerical simulation studies are even less.Therefore,it is necessary to carry out research under SPI,including the transport process of injected impurity,radiation asymmetry and the physical mechanism.The numerical simulations,based on the Shattered Pellet Injection(SPI)with the 3D MHD code NIMROD,are performed on J-TEXT tokamak.The effect of rotation on the neon impurity transport and radiation asymmetry were studied.Current pellets injection models ignore the velocity and ablation process.For a more consistent horizontal injection of multiple pellets in the experiment,the original SPI model was modified and perfected in the simulations.In order to study the effect of rotation on impurity at different penetration depths,the pellets are deposited in the plasma core and boundary region,respectively.The results of simulations show that when the pellets are deposited on the magnetic axis,rotation inhibits the transport of ionized impurity in the radial direction,preventing the impurity escaping from the core.However,if the pellets are in the edge area,rotation promotes the mixing of impurity from the boundary into the 2/1 surface.The former is because the stabilizing effect of rotation reduces the MHD instability,making the magnetic topology structure more complete,the magnetic surface restricts the outflow of impurity.The greater possibility of the latter comes from the influence of rotation on the flow field.The electromagnetic force generated by the flow field on the ionized state causes the impurity to move to the core,resulting in a similar effect as the former.In addition,the rotation increases the collision rate between particles in the plasma,strengthens the toroidal transport of impurity,and makes the distribution of impurities in the toroidal space more uniform.At the same time,the maximum radiated power and radiated toroidal peaking factor(TPF)under rotation are also significantly reduced.From the results,rotation(45 km/s)during impurity injection can significantly improve the mitigation effect,which has a positive guiding significance for the experiment.Considering that impurity enter the plasma deeper under SPI,the pellets can be deposited at any position in the poloidal plane in code.In the future,the effect of the pre-existing 2/1 or 3/1 magnetic islands on the transport of impurities is worth studying.The deposition position can be set at the X or O point of the pre-existing magnetic island to study the mixing efficiency of impurities and toroidal radiation asymmetry.These studies have important reference value for understanding the physical mechanism of mitigating disruption by impurity injection. |
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