Simulation And Research On Properties Of Plasma Sheath Near The Divertor Plate

In fusion reactor, plasma-material interaction will critically influence discharge duration and plasma energy, so it is more important to make research on the characteristics in the region between the plasma and wall, in order to keep the corrosion of the wall material within reasonable limits. Four models are generally used to study the solution of plasma-wall transition problem.Taking into account collisions and ion temperature, and the parameters chosed close to the fusion device, one-dimensional fluid model was used to investigate the interaction between plasma and floating-wall in a uniform oblique magnetic field, and discuss the dependence of each physical quantity on some crucial parameters. The simulated charged particles include hydrogen ions and electrons. There are two main approaches in the research. The first method is full-region solution, obtaining smooth transition from plasma to sheath. The results illustrate that the potential, ion density, electric field, the velocity parallel to magnetic field, the angle between ion velocity vector and the wall are significantly affected by the magnitude and the direction of the magnetic field, charge-exchange collision frequency, ionization collision frequency and ion temperature, moreover, the impact of these parametes is regular expect charge-exchange collison frequency. However, this method can not modeled separately the quasi-neutral region and sheath to obtain the characteristics in each region.Patching is the alternative approach. First, investigate magnetic quasi-neutral presheath and magnetic non-neutral sheath respectively, considering different physical processes for different local scales, then, patch the plasma and sheath to approximate full solution. It is important to identify the location of the patching point as the sheath-edge, which must fulfill requirement and obtain right solution in sheath. Owing to the application of magnetic field and electric field, the direction of ion velocity changes greatly in the plane of magnetic field in the presheath, whose scale is a few ion Larmor radiis. The plasma boundary determined by Bohm criterion can be assumed to specify the point where the quasi-neutrality is violated, whose characteriatics are very sensitive to the relationship between the ion cyclotron frequency and collision frequency. An propriate patching point is needed as sheath-edge which locates before the plasma boudary, due to the electric field is discontinuous and quasi-neutrality is break down near plasma boundary. It is found that the patching point is affected by ion temperature, magnetic magnitude and its direction. Because of nonzero electric field and the non-negligible ion velocity parallel to wall, the ion velocity perpendicular to wall is smaller than ion acoustic speed at the patching point. The magnetic sheath is a region of positive charge and most affected by magnetic field and ion temperature, whose ion-flux is conservative and thickness is in Debye length’s order. At last, it is illustrated that the curves of each quanlity obtained in the first method wre in good qualitative agreement with those gained in the second, which has proven the correctness of the patching approach.