Standard
rf-photoguns lose orders of magnitude in RMS brightness due to curvature
and twist in the 6 dimensional phase-space. Because uniformly filled
ellipsoidal ('waterbag') bunches have linear self-fields in all dimensions,
they do not suffer from these deteriorating effects. The combination
of waterbag electron bunches and ballistic bunching leads to a new type
of rf-photoinjector where the first half-cell is separated from the
second cell(s). Detailed GPT simulations of such a device, including
realistic fields, 3D space-charge effects, path-length differences and
image charges at the cathode will be presented. It will be shown that
with a single 10 MW S-band klystron and fields below 100 MV/m, 100 pC
bunches with a peak-current of 1 kA can be created with a FWHM duration
below 100 fs and an emittance below 1 micron.