A photocathode system includes a plurality of photocathodes, and at least one combining device. The photocathodes have individually adjustable voltages, and each photocathode generates an individual electron bunch at an emission period. The combining device combines the individual electron bunches, generated at each emission period, into a combined bunch along a combined axis. The timing of the individual electron bunches is independently adjustable, so that an electron bunch with a lower energy arrives at the combined axis earlier in time compared to another electron bunch with a higher energy, thereby allowing the combined beam of electron bunches to be longitudinally compressed. The photocathodes may be distributed along a 1D column, or a 2D array, or a 3D array, or any arbitrary configuration. A linac is located near a longitudinal focusing point to boost beam energy and therefore freeze bunch length and emittance.