Study On The Characteristics Of Dust Particle In The Sheath For Fusion Plasma

Dust particles,which are produced by the interaction between plasma and plasma-facing materials(PFMs)in magnetic confinement tokamak,influence on the security problems of the tokamak device and the improvement of the performance of plasma constraint.Plasma sheath is an important link with the interaction between plasma and PFMs.The study on the characteristics of dust particles is helpful for knowing dust transport in the boundary of plasma and understanding the interaction between dust particle and plasma.In this paper,the characteristics of dust particles are studied by fluid model in fusion plasma sheath.Dust particles are charged in plasma,and the charge characteristics of dust particles in the plasma are studied by the orbit motion limited(OML)theory.The results show that dust particles in bulk plasma are negatively charged in the absence of emitted electrons.The charging relaxation time of dust particle is shorter with thelarger radius,the less density plasma and the higher temperature electrons.The increase in the radius of dust particle and the mass of ions can improve the charge number of dust particle when charging is completed.Comparing the time across the sheath for dust particle with the charging relaxation time in any position in fusion plasma sheath,we can find that the charge with nanometer-sized dust particle can be constant in the plasma sheath because the time across the sheath for dust particle is less than the charging relaxation time in any position.And the dust charge with micrometer-sized dust particle meets local equilibrium in any position because the charging relaxation time is less than the time across the sheath.The characteristics of dust particles in plasma sheath are studied using one-dimensional fluid model when the time across the sheath for dust particles is much greater than the relaxation time.Among the forces acting on the dust particle in the sheath,the effect of the magnetic field force and the gravity effect on dust particles are very small.The drag force dominates at close to the sheath edge.The electric field force dominates at away from the sheath edge and most of the space in the sheath.The density increases and the velocity decreases for dust particle along the sheath.The larger velocity and the higher temperature of ions can both lead to the lower velocity and the more density for dust particles.The sheath structure and the dust characteristics are studied in plasma sheath with two kinds of positive ions.The dust particle is that the charging relaxation time is less than the time across the sheath.The ions with bigger mass relative to the other positive ions will lead to the increase of the sheath width and the critical secondary electron coefficient,and the decrease of the wall potential and the ion kinetic energy flow deposited to the wall.When the ions with smaller mass relative to the other positive ions are contained,the results are just the opposite.The dust characteristic of dust particle is studied in plasma sheath with two kinds of ions,and the ions are hydrogen and argon,respectively.The result shows that the presence of argon ions in the hydrogen plasma can increase the charge number of dust particle.The forces acting on the dust particle are analyzed.The result shows that dust particle with negative charge can be suspended in the sheath.The dust particle with smaller radius can suspended at closer to the sheath edge and the dust particle with larger radius suspends away from the sheath edge.There have not dust particle which can suspend at close to the wall.The more density of argon ion in hydrogen plasma will lead to the increase in the radius with suspending dust particle in the sheath.The size of the dust particle which can suspend in the sheath is beryllium dust,carbon dust,molybdenum dust and tungsten dust in turn.The results presented in this thesis are helpful for knowing dust particle which located in the region of divertor and near the protection limiter of lower hybrid antenna,and understanding the interaction between dust particle and plasma.The results can provide not only the theoretical support for dust particle during the pulse discharge of EAST,but also the scientific basis for the operation of ITER.