Ultraviolet laser transverse profile shaping for improving x-ray free electron laser performance

It has been well known that the resonant interaction of an ultrarelativistic electron beam and the radiation field in the single-pass high-gain free electron laser (FEL) amplifier leads to the optical gain guiding. The transverse Laplacian term of the slowly varying wave equation in the linear regime can be approximated as a constant detuning parameter, i.e., j∇ 2 ⊥ j ∼ k R =z R where k R is the resonant wave number and z R is the Rayleigh range of the laser. In the post-saturation regime, the radiation power begins to oscillate about an equilibrium for the untapered case while continues to grow by undulator tapering. Moreover, in this regime the gain guiding decreases and the simple constant detune is no longer valid. In this paper we study the single-pass high-gain FEL performance in the post-saturation regime with inclusion of diffraction effect and undulator tapering. Our analysis relies upon two constants of motion, one from the energy conservation and the other from the adiabatic invariant of the action variable. By constructing a two-dimensional axisymmetric wave equation and the coupled one-dimensional electron dynamical equations, the performance of a tapered FEL in the postsaturation regime can be analyzed, including the fundamental mode profile, the power efficiency and the scaled energy spread. We begin the analytical investigation with two different axisymmetric electron beam profiles, the uniform and bounded parabolic ones. It is found that the tapered FEL power efficiency can be smaller but close to the taper ratio provided the resonant phase remains constant and the beam-wave is properly matched. Such a tapered efficiency is nearly independent of transverse electron beam size before significant electron detrapping occurs. This is essentially different from the untapered case, where the power extraction efficiency is around the essential FEL gain bandwidth (or ρ, the Pierce or FEL parameter) and depends on the beam size. It is also found that the power enhancement due to undulator tapering is attributed more by the field increase outside the transverse electron beam than that inside the transverse electron beam. Several scaling properties on the taper ratio and the transverse electron beam size are also discussed in this paper.

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“…[54] and references therein). The analysis here may provide further insights for its application to maximize the tapered FEL performance.…”

Section: Summary and Discussionmentioning

confidence: 99%

Single-pass high-gain tapered free-electron laser with transverse diffraction in the postsaturation regime

Tsai

Yang

et al. 2018

Phys. Rev. Accel. Beams

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“…The use of dynamical and digitally controllable laser shaping techniques for the drive laser has been demonstrated in photoinjectors, as in Refs. [10,18,19]. In our case, the DMD enables quantitative comparison between the traditional scanning method and our proposed ghost imaging method.…”

Section: Experimental Demonstrationmentioning

confidence: 98%

“…DMD with a modified window to allow ultraviolet (UV) transmission, as discussed in [19]. A 3 mm full width laser spot fully illuminates 240 x 240 pixels on the device.…”

Section: A Pegasus: Experimental Setupmentioning

confidence: 99%

Mapping photocathode quantum efficiency with ghost imaging

Kabra

Cropp

et al. 2020

Phys. Rev. Accel. Beams

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Measuring the quantum efficiency (QE) map of a photocathode injector typically requires laser scanning, an invasive operation that involves modifying the injector laser focus and rastering the focused laser spot across the photocathode surface. Raster scanning interrupts normal operation and takes considerable time to setup. In this paper, we demonstrate a novel method of measuring the QE map using a ghost imaging framework that correlates the injector laser spatial variation over time with the total charge yield. Ghost imaging enables passive, real-time monitoring of the QE map without manually modifying the injector laser or interrupting injector operation. We first demonstrate the method at the UCLA Pegasus photoinjector with the help of a digital micromirror device (DMD) and a piezoelectric mirror to increase our control of the overall transverse variance of the illumination profile. The reconstruction algorithm parameters are fine-tuned using simulations and the results are validated against the ground truth map acquired using the traditional rastering method. Finally, we apply the technique to data acquired parasitically from the LCLS photoinjector, showing the feasibility of this method to retrieve a QE map without interrupting normal operation.

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“…For example, in [18] a spatial light modulator was employed to pattern the wavefront, and this known pattern was correlated to the bucket detector without any additional measurement. Similar control is possible with electrons; modulating the shape of a photocathode drive laser controls the emitted electron beam [19,20], enabling computational GI with electrons. Using a relativistic multi-MeV electron beam has the significant advantage of reducing space charge effects, improving preservation of the electron beam structure during transport from the cathode to the sample.…”

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confidence: 99%

“…The initial electron beam profile at the cathode can be controlled by applying a mask to the transverse profile of the drive laser pulse. For the shaping we used a TI DLP-7000 DMD, with a modified window to allow ultraviolet (UV) transmission, as discussed in [20]. An 80 cm focal length lens imaged the DMD onto the cathode with demagnification factor 0.2 ( Fig.…”

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confidence: 99%

Electron Ghost Imaging

et al. 2018

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In this Letter we report a demonstration of electron ghost imaging. A digital micromirror device directly modulates the photocathode drive laser to control the transverse distribution of a relativistic electron beam incident on a sample. Correlating the structured illumination pattern to the total sample transmission then retrieves the target image, avoiding the need for a pixelated detector. In our example, we use a compressed sensing framework to improve the reconstruction quality and reduce the number of shots compared to raster scanning a small beam across the target. Compressed electron ghost imaging can reduce both acquisition time and sample damage in experiments for which spatially resolved detectors are unavailable (e.g., spectroscopy) or in which the experimental architecture precludes full frame direct imaging.

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Ultraviolet laser transverse profile shaping for improving x-ray free electron laser performance

Cited by 12 publications

References 25 publications

Single-pass high-gain tapered free-electron laser with transverse diffraction in the postsaturation regime

Single-pass high-gain tapered free-electron laser with transverse diffraction in the postsaturation regime

Mapping photocathode quantum efficiency with ghost imaging

Electron Ghost Imaging

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