  We report on the development of a highly directional, narrow energy band,
short time duration proton beam operating at high repetition rate, suitable for
measurements of stopping power in high energy density plasmas as well as other
applications. The protons are generated with an ultrashort-pulse laser
interacting with a solid target and converted to a pencil-like narrow-band beam
using a compact magnet-based energy selector. We experimentally demonstrate the
production of a proton beam with an energy of 500 keV and energy spread well
below 10%, and a pulse duration of 260 ps. The energy loss of this beam is
measured in a 2 ${\mu}$m thick solid Mylar target and found to be in within 1%
of theoretical predictions. The short time duration of the proton pulse makes
it particularly well suited for applications involving the probing of highly
transient plasma states produced in laser-matter interaction experiments, in
particular measurements of proton stopping power.
