It is shown that the requirements for high quality electron bunch generation
and trapping from an underdense photocathode in plasma wakefield accelerators
can be substantially relaxed through localizing it on a plasma density
downramp. This depresses the phase velocity of the accelerating electric field
until the generated electrons are in phase, allowing for trapping in shallow
trapping potentials. As a consequence the underdense photocathode technique is
applicable by a much larger number of accelerator facilities. Furthermore, dark
current generation is effectively suppressed.Comment: 4 pages, 3 figure