Effects Of Sheath On Plasma-Wall Interactions In EAST Tokamak

For physical and engineering reasons,the tokamak magnetic field can not per-fectly confine the plasma.These unconfined plasmas are eventually lost to the plasma-facing components on tokamak,leading to heat flux and plasma-wall interactions on the plasma-facing components.The Plasma-wall interactions have a significant impact on the discharge performance of the fusion plasma and the lifetime of the plasma-facing components.Therefore,the plasma-wall interaction,which has a significant impact on the steady-state operation of tokamaks,is a challenging physics problem that requires focused attention.Different simulation programs(such as SOLPS,BOUT++,JOREC)based on fluid models have been developed to study the heat flux and plasma-wall interactions,how-ever,these programs only include the quasi-neutral region of plasma,taking the Bohm criterion of classical Debye sheath as a boundary condition.But the properties of the sheath are related to numerous factors such as plasma-wall interactions,external mag-netic field,components of plasma,whcih can also have a completely different effect on the heat flux and plasma-wall interactions.Therefore,the treatment of the sheath only as a boundary condition is not self-consistent in all physical studies involving the sheath.In this thesis,we study the effect of the sheath on heat flux and plasma-wall interaction at two high heat flux components(limiter and divertor)on the Experimental Advanced Superconducting Tokamak(EAST)by taking the sheath into account self-consistently through particle-in-cell(PIC)simulations.Hot spots were observed on the guard limiters of the lower hybrid wave antenna on EAST tokamak during the high power current driven discharge experiments.Hot spots not only damage the first wall components and limit the increase of the power of lower hybrid wave,but also greatly degrade the plasma discharge performance by generating excessive impurities.In order to find the real reason of the hot spot,we calculated the heat flux to the surface of the guard limiter using a one-dimensional simulation consis-tently.Using the velocity distribution of electrons after interaction with low spurious waves given by quasi-linear theory,it is found that due to the presence of fast elec-trons,the floating potential in the sheath is greatly enhanced,and at the same time,the energy flux on the surface of the guard limiter is greatly increased so that the surface temperature of the guard limiter rises to the temperature of the hot spot.The presence of fast electrons and the high floating potential make the plasma-wall interactions such as secondary electron emission and physical sputtering important.Therefore,in our simulations,we consider the effect of secondary electron emission on the energy flux at the surface of the limiter for two different wall materials:graphite(the EAST limiter material at the time of this work)and tungsten wall(future modification of EAST).In the long pulse discharge experiments on EAST,most of the energy carried by charged particles is transported along the magnetic lines to the divertor region and de-posited on the divertor target plates.Therefore,the energy flux deposited on the divertor target plates is naturally an important parameter for the safe operation of the EAST toka-mak.Similarly,a sheath is formed between the divertor target plates and the plasma.Because of the small angle between the divertor target plates and the magnetic field,the magnetic sheath at this point has very different properties from the classical Debye sheath.In order to accurately evaluate the energy flux in the divertor target plates,the effect of the magnetic sheath has to be reconsidered.For this purpose,we again in-vestigated the energy flux deposited onto the divertor target plates with a small angle between the magnetic field and the target plates using a particle-in-cell simulation pro-gram.It was found that within the sheath,the velocity distribution of electrons parallel to the magnetic field is a truncated Maxwellian distribution.Therefore,the electron energy flux deposited on the target plates of the divertor can be given by the analytical equation.However,the velocity distribution function of the ions strongly depends on the angle between the magnetic field and the target plates.This is because,as the ions move along the open orbit inside the sheath,the Lorenz force converts the potential en-ergy of the sheath more towards the kinetic energy in the direction normal to the plane where the magnetic field and the electric field of the sheath are located.Therefore,the ion energy flux on the target plates of the divertor can only be given as an approximate formula based on the simulation results,and it is found that the approximate formula agrees well with the simulation results.Finally,since we found that the electron temperature has an important influence on heat flux.Therefore,the hot plasma ejected from the boundary plasma during the edge-localized mode burst will certainly increase the heat flux on the divertor target plates significantly.In addition,the multi-component plasma will change the properties of sheath,which will in turn change the heat flux and plasma-wall interactions on the di-vertor target plates.For this reason,we investigated the heat flux and sputtering yield on the divertor target plates during the edge-localized modes burst using PIC simulations.It is found that the hot plasma released from the edge-localized modes significantly increases the electric potential within the sheath and the heat flux on the divertor tar-get plates.Since the concentration of background ions is much larger than that of hot ions,the heat flux on the divertor target plates is mainly contributed by the background ions.The heat flux of hot electrons becomes important only when the temperature of hot plasma is very high.And at that time,the background electrons cannot enter the sheath because the sheath potential is too high and thus background electrons do not contribute to the heat flux on divertor target plates.The sputtering yield on the target plates is mainly from hot ions because of their higher temperature and thus higher inci-dent energy.Especially for tungsten sputtering,the incident energy of the background ions is always lower than the sputtering energy threshold of tungsten and cannot strip tungsten from the material surface.