Sherwood 2015

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Dust transport in tokamaks: beyond the Orbital-Motion-Limited theory

Author: Gian Luca Delzanno
Requested Type: Poster Only
Submitted: 2015-01-19 20:54:15

Co-authors: X.Z. Tang

Contact Info:
Los Alamos National Laboratory
MS: B284
Los Alamos, NM   87545

Abstract Text:
Long-pulse tokamaks like ITER or DEMO are characterized by much higher energy fluxes to the wall than present-day short-pulse tokamaks, implying stronger plasma-material interaction and higher quantities of dust produced in the chamber. Indeed, ITER is the first tokamak with dust safety and administrative limits, setting the maximum amount of dust that can be present at any given time in the machine [1]. For this reason, dust transport in tokamaks and dust destruction/survival have received a lot of attention in recent years [2-5].

Studies of dust transport typically involve solving simultaneously the following time-dependent equations: (1) the dust charging equation; (2) the dust equation of motion; (3) the dust heating equation; and (4) an equation for dust mass loss. The expressions for the dust charging currents, forces and energy fluxes necessary to solve the equations are normally expressed analytically in the framework of the Orbital-Motion-Limited (OML) theory [2-5].

In this work, we explore the regimes of validity of OML. We perform self-consistent Particle-In-Cell (PIC) simulations of dust charging for tokamak relevant conditions, with and without thermionic emission from the dust grain. We study micron-sized tungsten dust, in a regime where the plasma Debye length is comparable (or smaller) than the dust radius. First, without thermionic emission, we compare PIC simulations with a recently revised OML theory that can calculate the plasma response [6] and find good agreement up to relatively large dust grains. However, we find that OML can become inaccurate when the grain becomes positively charged via thermionic emission: it can miss the transition between negatively and positively charged dust, and it can significantly overestimate the dust charge, currents and the energy fluxes. This behavior is associated with the development of a non-monotonic potential (a potential well) near the dust grain [7].

[1] J. Roth, E. Tsitrone, A. Loarte, et al., J. Nucl. Mat. 390, 1 (2009).
[2] R.D. Smirnov, A.Y. Pigarov, et al., Plasma Phys. Control. Fus. 49, 347 (2007).
[3] M. Bacharis, M. Coppins, J.E. Allen, Phys. Plasmas 17, 042505 (2010).
[4] S. Ratynskaia, L. Vignitchouk, P. Tolias, et al., Nucl. Fusion 53, 123002 (2013).
[5] G.L. Delzanno, X. Tang, Phys. Plasmas 21, 022502 (2014).
[6] X. Tang , G.L. Delzanno, Phys. Plasmas 21, 123708 (2014).
[7] G.L. Delzanno, X. Tang, Phys. Rev. Lett. 113, 035002 (2014).


March 16-18, 2015
The Courant Institute, New York University