Statics of Dust Particle on Plasma Facing Wall and Dynamics in Boundary Plasma

In present fusion devices the generation of dust particles is observed and characteristics of the dust particles collected after discharges have been analyzed. Typically the collected dust particles have irregular, flake or spherical shapes with sizes from nanometers to tens of micrometers and consist of materials of divertor plates, first wall or inner structures. They are considered to appear in a plasma due to erosion of plasma-facing surfaces, condensation and adhesion of plasma-spattered materials, flaking of redeposition layers. The motion and destruction of dust particles in the high temperature plasma region can contribute to the impurity transport that makes essential to study behavior of dust particles during a discharge. It is well known that dust particles obtain large, usually negative, electrical charge (up to 10⁶ elementary charges) in a plasma and can affect electric potential distributions and change kinetic properties of the plasma due to scattering and absorption of plasma particles. The important safety issue is the ability of dust particles to accumulate large amount of radioactive tritium. The dust may be a primary radiological and explosion risk factor. In recent years dust particles also have attracted attention in growing technological applications of plasmas connected to astrophysical, space, laboratory, and processing applications. In fusion devices the dust density usually is not high enough to show the collective effects. Therefore, the aim of the present study is to investigate the behavior of a single dust particle in a plasma wall transition layer that includes sheath formation with the dust particle, analysis of releasing conditions at the wall and possible trajectories of dust particles with various sizes and masses. We consider a conductive spherical dust particle initially placed on the wall. The directions and magnitudes of the forces acting on the dust depend on the local plasma parameters, dust particle size and charge. The present problem needs to be analyzed self-consistently because the charge of the dust particle affects the surrounding plasma and mutually depends on fluxes of electrons and ions to the dust particle. This presents significant difficulties in the theoretical treatment of such problem; therefore, we used computer simulations in combination with a simplified. the oretical approach. Assuming local plasma parameters known and fixed we can theoretically find the charge, currents and forces to the dust particle that allows to analyze its behavior in a wide range of sizes, masses, spatial and time scales. <br />　　At first, we investigate the conditions allowing a dust particle to be released from the wall, when the total force is acting on the dust towards plasma. The total force in our analysis includes the electrostatic force, the drag forces due to ion absorption and scattering, the electrostatic image force due to redistribution of charges on the wall and the gravitational force. The ion drag force is obtained using the Orbital Motion Limited (OML) theory, which gives us absorption cross section of electrons and ions by the dust particle, and the charge of dust particle attached to the wall is determined by the wall surface charge density and dust radius. The condition for releasing of the dust particle is obtained analytically in respect to the dust radius. From this condition we derive the "first critical dust radius" that is the largest radius of the dust particle capable to leave the wall. Using estimations of plasma parameters near the wall according to the Bohm sheath theory, we can express the first critical radius as a function of the wall potential. <br />　　For the case of the zero gravitational force (vertical surfaces), it was shown that the first critical radius exists only when the wall potential exceeds the threshold value, below which no dust particles can be released from the wall. For the deeper wall potentials than the threshold one, the smaller dust particle than the first critical radius will be released and the bigger one will be pinned to the wall due to the large ion drug force compared to the electrostatic force. Changing the wall potential we can control the size of released dust particles or suppress motion of all dust particles. When the gravitational force is directed toward the wall, it reduces the value of the first critical radius, but does not affect the threshold potential. For the opposite direction of the gravitational force, there are two values for the first critical radius, which define two zones in the "dust radius -wall potential" space for the released dust particles. Configuration of the zones is controlled by the gravitational parameter that is a function of dust mass density and plasma parameters. One of the zones is dominated by the electrostatic force and another one by the gravitational force. For a large gravitational parameter they are merged, while for a small one they are separated by a rang ...

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