High efficiency energy extraction from a relativistic electron beam in a   strongly tapered undulator
  We present results of an experiment where, using a 200 GW CO2 laser seed, a 65 MeV electron beam was decelerated down to 35 MeV in a 54 cm long strongly tapered helical magnetic undulator, extracting over 30$\%$ of the initial electron beam energy to coherent radiation. These results demonstrate unparalleled electro-optical conversion efficiencies for a relativistic beam in an undulator field and represent an important step in the development of high peak and average power coherent radiation sources. 
Nicholas Sudar, Pietro Musumeci, Joe Duris, Ivan Gadjev, Mikhail   Polyaniy, Igor Pogorelsky, Mikhail Fedurin, Christina Swi...
physics.acc-ph