Downramp-assisted underdense photocathode electron bunch generation in   plasma wakefield accelerators
  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. 
Alexander Knetsch, Oliver Karger, Georg Wittig, Henning Groth, Yunfeng   Xi, Aihua Deng, James Benjamin Rosenzweig, David Le...
physics.acc-ph physics.plasm-ph