Positron transport and thermalization - the plasma-gas interface

Low energy positrons are now used in many fields including atomic physics, material science and medicine [1]. Plasma physics is providing new tools for this research, including Penning-Malmberg buffer-gas traps to accumulate positrons and the use of rotating electric fields (the ``rotating wall'' technique) to compress positrons radially and create tailored beams [1]. These devices (now available commercially), which rely in key instances on positron-neutral interactions, are a convenient way to create plasmas and beams for a variety of applications. A deeper understanding of the relevant cooling and loss mechanisms is required to take full advantage of this technology. This talk focuses on a recent study of positrons in such a tenuous gaseous environment in the presence of an applied electric field [2]. Energy-resolved collision cross sections and a Monte Carlo code modified to include positrionium (Ps) formation are used to obtain transport coefficients and the thermalization and Ps-formation rates. A markedly different type of negative differential conductivity is observed (i.e., not seen in electron systems), due to the non-conservative nature of the Ps-formation process. It is particularly prominent in gases with large, highly energy dependent Ps-formation cross sections. The relevance of these calculations to other positron applications will also be discussed, including a currently planned study of positrons in gaseous water. It is hoped that these calculations will inspire a new generation of positron transport experiments.\\ *Work done in collaboration with Z.Lj. Petrovi\'c, A. Bankovi\'c, M. \v{S}uvakov, G. Malovi\'c, S. Dujko, S.J. Buckman. \\ 1. C. M. Surko and R. G. Greaves, \textit{Phys. Plasmas} \textbf{11}, 2333-2348 (2004).\\ 2. A. Bankovi\'c, J. P. Marler, M. \v{S}uvakov, G. Malovi\'c, and Z. Lj. Petrovi\'c, \textit{Nucl. Instrum. and Meth. in Phys. Res. B} \textbf{266}, 462-465 (2008).