Amplified coherent emission from electron beams prebunched in a masked chicane

We present a new FEL approach based on amplified coherent emission from electron beams that are prebunched in a masked chicane or a circular buncher. In this approach, a series of wires in the middle of the buncher modulates the transverse density of a chirped electron bunch. At the end of the chicane, the compressed beam exhibits longitudinal density modulations at the optical wavelength. The bunched beam is then injected into a short wiggler with period ∼2γ2 times the beam modulation wavelength. Due to the prebunched nature of the beam, the emitted undulator radiation will grow exponentially from coherent spontaneous emission. Because the coherent emission is orders of magnitude larger than spontaneous emission, prebunching the beam effectively reduces the number of gain lengths needed to reach saturation. This technique thus reduces the required undulator length.