Impact Of The Corrugated Divertor Target On Dust Escape Capability

Dust moving in the tokamak can bring kinds of effects on the tokamak steady operation. On the one hand, as a dust particle contains massive impurities (a carbon dust with radius of1μm contains about1012atoms), its impact on the transport of impurity atoms cannot be ignored, which would greatly affect the profile of impurities in tokamak. On the other hand, dust itself has a series of effects on the plasma and the wall, such as tritium retention, co-deposition and so on, and even results in the termination of the discharges. As the first wall related issues for ITER became more and more important, the interest in the role of dust in fusion plasmas drastically increased. The international fusion community have compiled the DTOKS and DUSTT codes to study the dust dynamics. Both codes feature real tokamak geometry and can simulate the dynamics of dust moving in tokamak plasma, dust-plasma interactions, dust-wall collisions and so on. Although these codes are very powerful, both of them didn’t consider the impact of sheath along the wall on the dust motion.This paper focuses on the change of the trajectory of the dust caused by dust and rough wall collisions. Because the dust mainly resides in the divertor region, thus the study areas can be limited to the divertor in the model. By setting the initial position of the dust near the strike point of the divertor target plate, the movement of dust can be simulated in the vicinity of the divertor target plate. Nowadays, there are some theoretical/computational studies of dust dynamics and transport in tokamak divertor. The model in this paper is built on the basis of the Krasheninnikov’s theoretical model on dust moving in the divertor region, and has further development. In this paper, we improve the theoretical model and add the component of friction force which is introduced by the asymmetry of electric field, as it causes an additional ion drift velocity, which can drag the dust in the sheath region, and consider the Lorentz force.By compiling the model, the results are analyzed and discussed in the chapter3. The results show that the interactions of dust particle with corrugated surface can cause it to exit the recycling region and fly towards the null "X" point and even the tokamak core. The energy of dust in (xyz) three respective directions can transform partly between each other when dust collides with the wall, and this energy conversion relationships are expressed with mathematic formulas. It can be seen from the simulation results and formulas that the degree of the corrugated surface along the toroidal direction of tokamak can have a great impact on dust trajectory and increase the escape capability of dust from recycling region highly, but the corrugated surface along the poloidal direction of tokamak can’t have this effect. Simultaneously, the results also confirm that the dust grain, being dragged by plasma flows in tokamak, can be accelerated to speeds of～104cm s-1. It is conceivable that toroidal configuration of tokamak (including the roughness of toroidal surface of wall and toroidal geometry of the device) can have an important impact on the dust grain dynamics and play an important role to increase the opportunity of dust grain moving to the core plasma. These results would be important for understanding the dust motion in tokamak and the increase of impurities in the core plasma.