Sheath over a finely structured divertor plate

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The surface of a divertor plate typically has fine structure. Depending on the material - and the duration of exposure to the plasma, the characteristic size of the surface imperfections may vary over a broad range. In this paper, we consider the case where these structures have scale h that is much smaller than the ion gyroradius {rho}{sub i} but greater than the electron gyroradius {rho}{sub e}. The magnetic field intersects the divertor plate at a shallow angle {alpha}<<I. The present paper demonstrates that the combination of these two factors, fine surface structures and strongly tilted magnetic field, gives rise ... continued below

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16 p.

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Cohen, R. H., LLNL May 15, 1998.

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The surface of a divertor plate typically has fine structure. Depending on the material - and the duration of exposure to the plasma, the characteristic size of the surface imperfections may vary over a broad range. In this paper, we consider the case where these structures have scale h that is much smaller than the ion gyroradius {rho}{sub i} but greater than the electron gyroradius {rho}{sub e}. The magnetic field intersects the divertor plate at a shallow angle {alpha}<<I. The present paper demonstrates that the combination of these two factors, fine surface structures and strongly tilted magnetic field, gives rise to many interesting new phenomena in the sheath. We consider only the plasma part of the problem: given the presence of some structure, what are the consequences in terms of the plasma properties in the vicinity of the surface? We are not addressing the issue of what process has caused the appearance of the structure. However, once the plasma part of the problem is solved, on could return to the analysis of the wall erosion problem, based on the solution obtained. For the environment of the divertor region of a medium-size tokamak (plasma density n{approximately}4x10{sup 13} cm{sup -3}, plasma temperature T{approximately}50 eV, the magnetic field strength B{approximately} 2T), one has: {rho}{sub i} {approximately}500 {micro}m (hydrogen), {rho}{sub e}{approximately}10 {micro}m. We, therefore, are going to analyze the scales of imperfections in the range 10 {micro}m <h<500 {micro}m. These scales are quite typical for a number of fusion devices. For reference purpose, one can mention that the Debye radius {rho}{sub D} is {approximately} 10 {micro}m. We will use the following model of the surface: we assume that it is formed by the randomly distributed cones of height h, with base radius and the distance between neighboring cones both also of order h. This latter assumption means that the number of cones per unit surface area is {approximately}1/h{sup 2}. With minor modifications, the analysis can be extended to bumps with smooth tops. The approach used in this paper is based on scaling arguments and order-of magnitude estimates. Despite its simplicity, it allows one to obtain basic scaling relations and reveal some interesting phenomena. After presenting a theoretical analysis of the plasma interaction with a finely structured surface, we present a brief description of a small-scale dedicated device for experimental studies of this problem.

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16 p.

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INIS; OSTI as DE98058598

Other: FDE: PDF; PL:

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  • 13. international conference on plasma surface interactions, San Diego, CA (United States), 18-22 May 1998

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  • Other: DE98058598
  • Report No.: UCRL-JC--128903
  • Report No.: CONF-980560--
  • Grant Number: W-7405-ENG-48
  • Office of Scientific & Technical Information Report Number: 304510
  • Archival Resource Key: ark:/67531/metadc677401

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  • May 15, 1998

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  • July 25, 2015, 2:20 a.m.

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  • April 10, 2017, 2:02 p.m.

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Cohen, R. H., LLNL. Sheath over a finely structured divertor plate, article, May 15, 1998; California. (digital.library.unt.edu/ark:/67531/metadc677401/: accessed September 25, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.