|Phase-merging Enhanced Harmonic Generation for Seeded Free-electron Lasers
Funding: This work was partially supported by the Major State Basic Research Development Program of China (2011CB808300) and the National Natural Science Foundation of China (11175240, 11205234, and 11322550)
The recently proposed phase-merging enhanced harmonic generation (PEHG) mechanism [*, **] holds promising prospect for seeded free-electron lasers (FEL) in short-wavelength. A remarkable bunching efficiency at high harmonics can be accomplished by using PEHG mechanism. In this paper, the theoretical descriptions and numerical simulations of PEHG will be presented, including the more recent theoretical progresses and a potential proof-of-principle experiment at Shanghai deep ultraviolet FEL test facility.
* Haixiao Deng, Chao Feng, Phys. Rev. Lett. 111 (2013) 084801.
** Chao Feng, Haixiao Deng, Dong Wang, Zhentang Zhao, New Journal of Physics,16 (2014) 043021.
|Slides FRB01 [29.808 MB]
|A Collinear Wakefield Accelerator for a High Repetition Rate Multi-beamline Soft X-ray FEL Facility
Funding: Supported by U.S. Department of Energy under Contract No. DE-AC02-06CH11357 and by the U.S. Department of Energy Laboratory LDRD program at Los Alamos National Laboratory.
A concept is presented for a multi beamline soft x-ray free-electron laser (FEL) facility where several FEL undulator lines are driven by an equal number of high repetition rate single-stage collinear wakefield accelerators (CWA). A practical design of the CWA, extending over 30 meters and embedded into a quadrupole wiggler, is considered. The wiggler’s structure of alternating focusing and defocusing quadrupoles is used to control single-bunch breakup instability. It is shown that practical restrictions on the maximum attainable quadrupole field limit the maximum attainable charge in the drive bunch whose sole purpose is to produce a high accelerating field in the CWA for the following main bunch. It is also pointed out that the distance between drive and main bunches varies along the accelerator, causing a measurable impact on the energy gain by the main bunch and on the energy spread of electrons in it. Means to mitigate these effects are proposed and results are presented for numerical simulations demonstrating the main bunch with plausible parameters for FEL application including a relatively small energy spread. Finally, results are presented for the expected FEL performance using an appropriately chosen undulator.
|Slides FRB02 [6.512 MB]
|Tapering Enhanced Stimulated Superradiant Amplification
|The electrical to optical conversion efficiency of FELs is typically limited to less than 1 percent. Efforts to improve conversion efficiency have generally involved undulator tapering to drive the interaction beyond saturation in combination with focusing the electron beam to compensate gain guiding losses within undulator. Here we propose a scheme whereby a coherent radiation seed is focused into a strongly tapered undulator to violently decelerate electrons, thereby converting as much as 70 percent of e-beam energy to coherent radiation. By tapering the undulator to accommodate the radiation growth, a modest input seed may be used to drive the FEL interaction far beyond saturation in order to achieve high electrical to optical conversion efficiency. The scheme relies on a prebunched beam and a seed laser focused into a strongly tapered undulator and is therefore called tapering enhanced stimulated superradiant amplification (TESSA).
|Slides FRB03 [1.100 MB]
|Divergence Reduction and Emittance Conservation in a Laser Plasma Acceleration Stage
Plasma accelerators promise a compact source of highly relativistic electron beams. Driven by high-intensity lasers or high-energetic electron beams, the longitudinal and transverse electric fields inside the plasma cavitiy support the generation of GeV electron beams over m-scale distances, while measured emittances on the order of 0.1 mm.mrad have been reported from plasma-driven accelerators. However, it remains challenging to conserve this excellent emittance when coupling from the plasma into vacuum and a subsequent beam optics, especially when considering the large energy spread, typically accumulated during the off-crest acceleration inside the plasma. Recently, we presented an analytical solution  to describe an adiabatic matching from the plasma into vacuum. Further elaborating this concept , we will discuss the generation of low-divergence electron beams from a tailored plasma target in order to preserve the emittance generated within the plasma. We will apply our concept to an externally injected electron bunch, that is matched in and out of a tailored plasma target, generating a GeV-level electron beam with low divergence and good emittance.
* K. Floettmann, Phys. Rev. ST - Accel. Beams 17, 054402 (2014)
** I. Dornmair, K. Floettmann, and A. R. Maier, submitted (2014)