Abstract
We describe a method for producing high power, coherent x-ray pulses from a free electron laser with femtosecond scale periodic temporal modulation of the polarization vector. This approach relies on the generation of a temporal intensity modulation after self seeding either by modulating the seed intensity or the beam current. After generating a coherent temporally modulated $s$-polarization pulse, the electron beam is delayed by half a modulation period and sent into a short orthogonally oriented undulator, serving as a $p$-polarization afterburner. We provide simulations of three configurations for realizing this polarization switching, namely, enhanced self seeding with an intensity modulation generated by 2 color self seeding, enhanced self seeding of a current modulated bunch, and regular self seeding of a current modulated bunch. Start to end simulations for the Linac Coherent Light Source-II are provided for the latter.
Highlights
The x-ray Free Electron Laser (FEL) has provided the scientific community with a tunable source of coherent, high peak power x-rays capable of probing phenomena at the atomic scale [1]
This is mathematically equivalent to a pulse comprised of two frequencies separated by Δω 1⁄4 ω1 − ω2 as would be radiated by an electron beam coherently bunched at these two frequencies
The previous method can be simplified in practice by using the current modulated long bunch to generate its own seed in a standard self seeding configuration, as shown in Fig. 4, row III
Summary
The x-ray Free Electron Laser (FEL) has provided the scientific community with a tunable source of coherent, high peak power x-rays capable of probing phenomena at the atomic scale [1]. FEL amplification of the first temporally modulated x-ray pulse with s-polarization is accompanied by periodic modulation on the electron beam bunching profile. The preservation of the initial bunching between stages gives a stable phase relationship between the two pulses This allows for fine adjustment of the phase shift between pulses, potentially producing a smooth temporal rotation of the polarization vector across the pulse. III, we discuss the three methods for generating the requisite intensity modulation in greater detail This includes simulations with idealized conditions and uses undulator and electron beam parameters relevant to the Linac Coherent Light Source-II (LCLS-II) using the 3-D time dependent FEL code GENESIS [19].
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