Effect of electron emission on the charge and shielding of a dust grain in a plasma: A continuum theory
- PDF / 250,234 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 61 Downloads / 188 Views
, NONLINEAR, AND SOFT MATTER PHYSICS
Effect of Electron Emission on the Charge and Shielding of a Dust Grain in a Plasma: A Continuum Theory L. G. D’yachkova*, A. G. Khrapaka, and S. A. Khrapakb a
Joint Institute for High Temperatures, Russian Academy of Sciences, Moscow, 125412 Russia b Max-Planck-Institut für extraterrestrische Physik, D-85741 Garching, Germany *e-mail: [email protected] Received June 13, 2007
Abstract—The continuum approximation is used to analyze the effect of electron emission from the surface of a spherical dust grain immersed in a plasma on the grain charge by assuming negligible ionization and recombination in the disturbed plasma region around the grain. A parameter is introduced that quantifies the emission intensity regardless of the emission mechanism (secondary, photoelectric, or thermionic emission). An analytical expression for the grain charge Zd is derived, and a criterion for change in the charge sign is obtained. The case of thermionic emission is examined in some detail. It is shown that the long-distance asymptotic behavior of the grain potential follows the Coulomb law with a negative effective charge Zeff , regardless of the sign of Zd. Thus, the potential changes sign and has a minimum if Zd > 0, which implies that attraction is possible between positively charged dust grains. PACS numbers: 52.27.Lw, 52.20.Hv DOI: 10.1134/S1063776108010147
1. INTRODUCTION Generally, a dust grain (a micrometer-sized disperse-phase particle) immersed in a plasma acquires a negative charge, because electrons have a much higher mobility as compared to positive ions [1, 2]. The negative charge increases until the electron and ion fluxes to the grain surface balance out because of increasing repulsion of electrons and attraction of ions. This occurs regardless of whether the charging is collisionless or highly collisional. It is well known that any charge is screened by the surrounding plasma. However, the grain charge in a collisional plasma is incompletely screened if volume ionization and recombination are negligible in the vicinity of the grain (the ionization and recombination lengths are much larger than the screening length λD). The long-distance asymptotic behavior of the grain potential follows the Coulomb law with an effective charge lower than the actual grain charge. This result has been obtained both numerically [3] and analytically [4–6]. It is explained by the balance of the electron and ion fluxes to the grain surface. A positive dust-grain charge can result from secondary, photoelectric, or thermionic emission of electrons from the grain surface. The steady-state charge is then determined by the balance of three fluxes: the ion and electron fluxes to the grain and the emission flux. When the effect of emission is weak, the charge remains negative, but is lower in absolute value. As emission increases, the charge becomes positive.
In this paper, we examine the effect of electron emission on the charging and shielding of a spherical dust grain in the continuum approximation, assumin
Data Loading...
