Physics design for the ata tapered wiggler 10.6 μ FEL amplifier experiment

We are presently designing and constructing a high-gain, tapered wiggler 10.6 μ FEL amplifier to operate with the 50 MeV ATA e-beam. The initial experiments will be done with a constant period (λ_w = 8 cm), 5 m-long linear wiggler. For an input laser power of 800 MW and electron beam brightness of 2.105 A/(rad-cm)2, we hope to achieve a trapped particle fraction ˜0.5 and an energy extraction efficiency of ˜2% with a ˜10% taper in the wiggler magnetic field. This taper corresponds to decelerating the trapped particle approximately two full ponderomotive well (i.e. bucket) heights. In this talk, we will discuss the physics motivations behind our tapered wiggler design and initial experimental diagnostics.