Stable propagation of a modulated particle beam in a crystal channel
  The propagation of a modulated beam of charged particles in a planar crystal channel is investigated. It is demonstrated that the beam preserves its modulation at sufficiently large penetration depths to ensure the feasibility of using a crystalline undulator as a coherent source of hard X rays. This finding is a crucial milestone in developing a new type of lasers radiating in the hard X ray and gamma ray range. 
Andriy Kostyuk, Andrei Korol, Andrey Solov'yov, and Walter Greiner