A plasma channel beam conditioner for a free electron laser

By "conditioning" an electron beam, through establishing a correlation between transverse action and energy within the beam, the performance of free electron lasers (FELs) can be dramatically improved. Under certain conditions, the FEL can perform as if the transverse emittances of the beam were substantially lower than the actual values. After a brief review of the benefits of beam conditioning, we present a method to generate this correlation through the use of a plasma channel. The strong transverse focusing produced by a plasma channel (chosen to have density 10(16)/cm(3)) allows the optimal correlation to be achieved in a reasonable length channel, of order 1 m. This appears to be a convenient and practical method for achieving conditioned beams, in comparison with other methods which require either a long beamline or multiple passes through some type of ring.