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Formation of antihydrogen atoms in an ultra-cold positron-antiproton plasma

We discuss the formation of antihydrogen atoms ($\bar{\rm H}$) in an ultra-cold positron-antiproton plasma. For positron densities $n_p\agt 10^8$ cm$^{-3}$ the characteristic formation time of stable $\bar{H}$ is determined by collisional relaxation of highly excited atoms produced in the process of 3-body Thompson recombination. Relying on the mechanisms of ``replacement collisions'' and ``transverse collisional drift'' we find a bottleneck in the relaxation kinetics and analyze the physical consequences of this phenomenon.