|Circular Polarization Control by Reverse Undulator Tapering
|In order to produce circularly polarized light at X-ray FEL facilities one can consider an installation of a short helical (or cross-planar) afterburner, but then one should have an efficient method to suppress powerful linearly polarized background from the main undulator. We propose a new method for such a suppression: an application of the reverse taper in the main undulator. We discover that in a certain range of the taper strength, the density modulation (bunching) at saturation is practically the same as in the case of non-tapered undulator while the power of linearly polarized radiation is suppressed by orders of magnitude. Then strongly modulated electron beam radiates at full power in the afterburner. Considering SASE3 undulator of the European XFEL as a practical example, we demonstrate that soft X-ray radiation pulses with peak power in excess of 100 GW and an ultimately high degree of circular polarization can be produced. The method can be used at different X-ray FEL facilities, in particular at LCLS after installation of the helical afterburner in the near future.
|Slides MOC01 [1.545 MB]
|Correlated Energy Spread Removal with Space Charge for High Gain Harmonic Generation
We study the effect of longitudinal space charge on the correlated energy spread of a relativistic beam that has been microbunched for the emission of coherent, high harmonic radiation. We show that, in the case of microbunching induced by a laser modulator followed by a dispersive chicane, longitudinal space charge forces can act to significantly reduce the induced energy spread of the beam without a reduction in the harmonic bunching content. This effect may be optimized to significantly increase the harmonic number achievable in seeded HGHG free electrons lasers, which are otherwise limited by the induced energy spread from the laser.
The work presented here has been published in Reference .
 E. Hemsing et al., to appear in Phys. Rev. Lett. (2014)
|Slides MOC02 [4.092 MB]
|Radiation Properties of Tapered Hard X-ray Free Electron Lasers
|We perform an analysis of the transverse coherence of the radiation from a TW level tapered hard X-ray Free Electron Laser (FEL). The radiation properties of the FEL are studied for a Gaussian, parabolic and uniform transverse electron beam density profile in a 200-m undulator at a resonant wavelength of 1.5 Angstrom. Simulations performed using the 3-D FEL particle code GENESIS show that diffraction of the radiation occurs due to a reduction in optical guiding in the tapered section of the undulator. This results in an increasing transverse coherence for all three transverse electron beam profiles. We determine that for each case considered the radiation coherence area is much larger than the electron beam spot size, making X-ray diffraction experiments possible for TW X-ray FELs.
|Slides MOC03 [3.797 MB]
|Chirped and Modulated Electron Pulse Free Electron Laser Techniques
Funding: We acknowledge STFC MoA 4132361; ARCHIE-WeSt HPC, EPSRC grant EP/K000586/1; John von Neumann Institute for Computing (NIC) on JUROPA at Jlich Supercomputing Centre (JSC), under project HHH20
A potential method to improve the free electron laser's output when the electron pulse has a large energy spread is investigate and results presented. A simplified model is the first given, in which there are a number of linearly chirped beamlets equally separated in energy and time. By using chicanes, radiation from one chirped beamlet is passed to the next, helping to negate the effect of the beamlet chirps and maintaining resonant interactions. Hence the addition of chicane allow the electrons to interact with a smaller range of frequencies (Δ ω <2 ρ γr), sustaining the FEL interaction. One method to generate such a beamlet structure is presented and is shown to increase FEL performance by two orders of magnitude.
|Slides MOC04 [6.777 MB]