  We report the application of evaporative cooling to clouds of trapped
antiprotons, resulting in plasmas with measured temperature as low as 9~K. We
have modeled the evaporation process for charged particles using appropriate
rate equations. Good agreement between experiment and theory is observed,
permitting prediction of cooling efficiency in future experiments. The
technique opens up new possibilities for cooling of trapped ions and is of
particular interest in antiproton physics, where a precise \emph{CPT} test on
trapped antihydrogen is a long-standing goal.
