On dust dynamics in tokamak edge plasmas
S. I. Krasheninnikov1, Y. Tomita2, R. D. Smirnov3, and R. K. Janev3
1

University of California at San Diego, La Jolla, CA 92093, USA
National Institute for Fusion Science, Toki, Gifu 509-5292, Japan
3
The Graduate University for Advanced Studies, Toki, Gifu 509-5292, Japan2

It has been suggested long time ago [1] that dust particles might be a source of plasma impurities in the earlystages of a tokamak discharge. Later on the presence of significant amounts of dust has been observed in thechambers of fusion devices (see for example the review paper [2], and the references therein). The dust particle
impact on the performance of current fusion devices has still not been adequately understood [3, 4]. The
existence of dust particles in next -step fusion devices, however, may potentially pose a significant safety threat
due to the accumulation of hardly containable toxic and radioactive materials retaining tritium [2].
In this paper we consider some aspects of the dust dynamics in tokamak plasmas, and show thattransport of dust particles can be an important mechanism of core plasma contamination with impurities. We
assume that dust density in a tokamak plasma is rather small so that our consideration is based on a single dust
particle motion. Therefore, we ignore the collective phenomena associated with the dust [5].
Similarly as in the studies of dust behavior on the devices with weakly ionized plasmas (see [6], andreferences therein), a dust particle in a tokamak edge plasma can be confined in an effective potential wellformed in the sheath region by the perpendicular components of plasma friction and electrostatic forces.
However, unlike those studies, dust particles in a tokamak edge plasma are subject of acceleration by
unbalanced large parallel components of the plasma friction force, which appear due to: a) the plasma flow
along the shallow magnetic field lines and b) diamagnetic and E×B flows within the magnetized sheath.
Therefore, dust particles in tokamak divertor region can be quickly (for ~1 ms) accelerated to the speed ~3×103
cm/s, which can explain some of the puzzles in Ref. [4].
In the present study we have found that the motion of a dust particle along the wall surface, having
micro-inhomogeneities, is subject to resonance interactions with particle oscillations in the potential well. As aresult, the oscillation amplitude can grow drastically and dust particle can leave the sheath region and fly
outside it and, being pushed back by the plasma flow to the wall, bouncing time to time off the surface andfurther gaining speed by plasma flow acceleration. Finally, the amplitude of particle excursions becomes toolarge to be pushed back so that dust particle moves to another location at the wall, or flies toward the coreplasma.
Assessment of dust impact on core plasma contamination by impurities shows that: It is unlikely that
dust formation on the main chamber wall due to first wall recycling processes can strongly contribute to corecontamination. It is, however, quite likely that dust formation in the divertor region, with further dust transport
into main chamber, can play significant role in core contamination.
[1] T. Ohkawa, Kaku Yugo Kenkyu 37 (1977) 117[2] G. Federici, C. H. Skinner, J. N. Brooks, et al., Nucl. Fusion 41 (2001) 1967[3] D. H. J. Godall, J. Nucl. Materials 111-112 (1982) 11[4] K. Narihara, K. Toi, Y. Hamada, et al., Nucl. Fusion 37 (1997) 1177
[5] P. K. Shukla, Phys. Plasma 8 (2001) 1791
[6] V. N. Tsytovich, Physics-Uspekhi 40 (1997) 53