  Photo-electric absorption of blackbody photons is an important process which
limits the acceleration of ions under the space-charge limited flow boundary
condition at the polar caps of pulsars with positive corotational charge
density. Photo-electric cross-sections in high magnetic fields have been found
for the geometrical conditions of the problem, and ion transition rates
calculated as functions of the surface temperatures on both the polar cap and
the general neutron-star surface. The general surface temperature is the more
important and, unless it is below 10^5 K, limits the acceleration electric
field in the open magnetosphere to values far below those needed either for
electron-positron pair creation or slot-gap X-ray sources. But such ion beams
are unstable against growth of a quasi-longitudinal Langmuir mode at rates that
can be observationally significant as a source of coherent radio emission.
