| Paper | Title | Page |
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| WEP041 | Feasibility of Single-Shot Microbunching Diagnostics for a Pre-Bunched Beam at 266 nm | 408 |
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Funding: Work supported by U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357. Co-propagating a relativistic electron beam and a high-power laser pulse through a short undulator (modulator) provides an energy modulation which can be converted to a periodic longitudinal density modulation (or microbunching) via the R56 term of a chicane. Such pre-bunching of a beam at the resonant wavelength and the harmonics of a subsequent free-electron laser (FEL) amplifier seeds the process and results in improved gain. We describe potential characterizations of the resulting microbunched electron beams using coherent optical transition radiation (COTR) imaging techniques for transverse size (50 micron), divergence (sub-mrad), trajectory angle (0.1 mrad), spectrum (few nm), and pulse length (sub-ps). The transverse spatial alignment is provided with near-field imaging and the angular alignment is done with far-field imaging and two-foil COTR interferometry (COTRI). Analytical model results for a 266-nm wavelength COTRI case with a 10% microbunching fraction will be presented. COTR gains of 7 million were calculated for an initial charge of 300 pC which enables splitting the optical signal for single-shot measurements of all the cited parameters. |
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| DOI • | reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP041 | |
| About • | paper received ※ 20 August 2019 paper accepted ※ 29 August 2019 issue date ※ 05 November 2019 | |
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| WEP042 | Observations of Short-Range Wakefield Effects in TESLA-Type Superconducting RF Cavities | 412 |
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Funding: This manuscript has been authored by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the U.S. Department of Energy, Office of Science, Office of High Energy Physics. The accelerators for high power X-ray free-electron laser (FEL) facilities such as the European XFEL and planned LCLS-II X-ray FEL are employing TESLA-type SCRF cavities. Beam propagation off axis in these cavities can result in both short-range and long-range transverse wakefields which can lead to emittance dilution within the micropulses and macropulses, respectively. The Fermilab Accelerator Science and Technology (FAST) facility has a unique configuration of a photocathode RF gun beam injecting two TESLA-type single cavities (CC1 and CC2) in series prior to the cryomodule. To investigate short-range wakefield effects, we used a vertical corrector between these two cavities to steer the beam off axis at an angle into CC2. A Hamamatsu synchroscan streak camera viewing a downstream OTR screen provided an image of y-t effects within the micropulses with resolutions of ~10-micron spatial and 2-ps temporal. At 500 pC/b, 50 b, and 4 mrad off-axis steering, we observed an ~100-micron head-tail centroid shift in the streak camera image. This centroid shift is consistent with a calculated short-range wakefield effect. Additional results for kick-angle compensation will be presented. |
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| DOI • | reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP042 | |
| About • | paper received ※ 20 August 2019 paper accepted ※ 28 August 2019 issue date ※ 05 November 2019 | |
| Export • | reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml) | |
| THP069 | Observations on Microbunching of Electrons in Laser-Driven Plasma Accelerators and Free-Electron Lasers | 722 |
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Funding: This manuscript has been authored by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the U.S. Department of Energy, Office of Science, Office of High Energy Physics. The periodic longitudinal density modulation of relativistic electrons at the resonant wavelength (microbunching) is a fundamental aspect of free-electron lasers (FELs). In one case, microbunching fractions reached 20% at saturation of a self-amplified spontaneous emission (SASE) FEL resulting in gains of 1 million at 530 nm [1]. In that experiment the z-dependent gain of coherent optical transition radiation (COTR) was also measured. In laser-driven plasma accelerators (LPAs), microbunching at visible wavelengths has also been recently reported as evidenced by significant COTR enhancements measured in near-field and far-field images on a single shot for the first time [2]. An analytical model for COTR interferometry (COTRI) addresses both cases. In the FEL, one identified microbunched transverse cores of 25-100 microns while in the LPA the reported transverse sizes at the exit of the LPA were a few microns. In the latter case, signal enhancements of nearly 100, 000 and extensive fringes out to 30 mrad in angle space were recorded. The broadband microbunching observed in the LPA case could act as a seed for a SASE FEL experiment with tunability in principle over the visible regime. [1] A.H. Lumpkin et al., Phys. Rev. Lett. 88, No.23, 234801 (2002). [2] A.H. Lumpkin, M. LaBerge, D.W. Rule et al., Proceedings of AAC18, (IEEE), (2019). |
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| DOI • | reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-THP069 | |
| About • | paper received ※ 20 August 2019 paper accepted ※ 28 August 2019 issue date ※ 05 November 2019 | |
| Export • | reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml) | |