Efficient production of Ps and progress towards high densities of Ps*

In the last two decades, the development of techniques to store and manipulate large numbers of positrons\footnote{R. G. Greaves \& C. M. Surko (1997) \emph{Phys. Plasmas} 4 {\bf 4}, 1528.} has made it possible to study many interesting related phenomena.\footnote{C. M. Surko \emph{et al.} (2005) \emph{J. Phys. B} {\bf 38}, R57}$^,$\footnote{G. F. Gribakin \emph{et al.} (2010) \emph{Rev. Mod. Phys.} {\bf 82}, 2557}$^,$\footnote{D. B. Cassidy \& A. P. Mills, Jr. (2007) \emph{Nature} {\bf 449}, 195} As the number of trapped positrons increases, new experiments are made possible. Recently Cassidy \emph{et al.}\footnote{D. B. Cassidy \emph{et al.} (2011) \emph{Phys. Rev. Lett.} {\bf 106}, 133401} have demonstrated that positronium (Ps) is emitted from clean p- and n-type Si surfaces, with very high efficiency. The discovery of an efficient mechanism for producing Ps (as much as $\sim$~70\% of the incident positrons are converted to Ps at high sample temperatures) paves the way for studies with high densities of Ps. Here I will discuss recent efforts to better characterize the Ps emitted from both p- and n-type Si surfaces, and describe other experiments that are either planned or have been conducted with such Ps beams.