  We report the results of a recent beam dynamics study, motivated by the need
to redesign the LCLS photoinjector, that lead to the discovery of a new
effective working point for a split RF photoinjector. We consider the emittance
compensation regime of a space charge beam: by increasing the solenoid
strength, the emittance evolution shows a double minimum behavior in the
drifting region. If the booster is located where the relative emittance maximum
and the envelope waist occur, the second emittance minimum can be shifted to
the booster exit and frozen at a very low level (0.3 mm-mrad for a 1 nC flat
top bunch), to the extent that the invariant envelope matching conditions are
satisfied. Standing Wave Structures or alternatively Traveling Wave Structures
embedded in a Long Solenoid are both candidates as booster linac. A careful
measurement of the emittance evolution as a function of position in the
drifting region is necessary to verify the computation and to determine
experimentally the proper position of the booster cavities. The new design
study and supporting experimental program under way at the SLAC Gun Test
Facility are discussed.
