Sub-picosecond compression by velocity bunching in a photo-injector
  We present an experimental evidence of a bunch compression scheme that uses a traveling wave accelerating structure as a compressor. The bunch length issued from a laser-driven radio-frequency electron source was compressed by a factor >3 using an S-band traveling wave structure located immediately downstream from the electron source. Experimental data are found to be in good agreement with particle tracking simulations. 
P. Piot, L. Carr, W.S. Graves, H. Loos