Huang, Z.
(Zhirong Huang)

TUBIS01 Attosecond X-ray Pulses in the LCLS using the Slotted Foil Method
Paul J Emma, Zhirong Huang (SLAC, Menlo Park, California), Michael Borland (ANL/APS, Argonne, Illinois)

A proposal has been made to generate femtosecond and sub-femtosecond x-ray pulses in the Linac Coherent Light Source (LCLS) SASE FEL by using a slotted spoiler foil located at the center of the second bunch compressor chicane. This previous study highlighted a simple case, using the nominal LCLS parameters, to produce a 2-fsec fwhm, 8-keV x-ray pulse. The study also pointed out the possibility of attaining sub-femtosecond pulses by somewhat modifying the LCLS compression parameters, but did not undertake a full tracking simulation for this more aggressive case. We take the opportunity here to study this `attosecond' case in detail, including a full tracking simulation, pushing the limit of the technique.

WEBOS01 Current-Enhanced SASE Using an Optical Laser and its Application to the LCLS
Alexander A Zholents, William M. Fawley (LBNL/CBP, Berkeley, California), Paul J Emma, Zhirong Huang, Gennady Stupakov (SLAC, Menlo Park, California), Sven Reiche (UCLA/DPA, Los Angeles - California)

We propose a significant enhancement of the electron peak current entering a SASE undulator by inducing an energy modulation in an upstream wiggler magnet via resonant interaction with an optical laser, followed by micro-bunching of the energy-modulated electrons at the accelerator exit. This current enhancement allows a considerable reduction of the FEL gain length. The x-ray output consists of a series of uniformly spaced spikes, each spike being temporally coherent. The duration of this series is controlled by the laser pulse and in principle can be narrowed down to just a single, ~100-attosecond spike. Given potentially absolute temporal synchronization of the x-ray spikes to the energy-modulating laser pulse, this scheme naturally makes pump-probe experiments available to SASE FEL’s. We also study various detrimental effects related to the high electron peak current and discuss potential cures. We suggest a possible operational scenario for the LCLS optimized with respect to the choice of the modulating laser beam and electron beam parameters. Numerical simulations are provided.

MOPOS59 Coherent Radiation Effects in the LCLS Undulator
Sven Reiche (UCLA/DPA, Los Angeles - California), Zhirong Huang (SLAC, Menlo Park, California)

For X-ray Free-Electron Lasers, a change in the electron energy while amplifying the FEL radiation can shift the resonance condition out of the bandwidth of the FEL. The largest sources of energy loss is incoherent undulator radiation. Because the loss per electron depends only on the undulator parameters and the beam energy, which are fixed for a given resonant wavelength, the average energy loss can be compensated for by a fixed taper of the undulator. Coherent radiation has a strong enhancement proportional to the number of electrons in the bunch for wavelengths comparable to or longer than the bunch dimension or bunch sub-structures. If the coherent loss is comparable to that of the incoherent the required taper depends on the bunch charge and the applied compression scheme and a change of these parameters would require a change of the taper. This imposes a limitation on the operation of FELs, where the taper can only be adjusted manually. In this presentation we analyze the coherent emission of undulator radiation and transition undulator radiation for LCLS, and estimate the effect of the energy spread by the coherent synchrotron radiation within the undulator.

MOPOS63 Generation of GW-level, sub-Angstrom Radiation in the LCLS using a Second-Harmonic Radiator
Zhirong Huang (SLAC, Menlo Park, California), Sven Reiche (UCLA/DPA, Los Angeles - California)

Electron beams are strongly microbunched near the high-gain FEL saturation with a rich harmonic content in the beam current. While the coherent harmonic emission is possible in a planar undulator, the third harmonic radiation typically dominates with about 1% of the fundamental power at saturation. In this paper, we discuss the second harmonic emission in the main undulator induced by effects of finite beam size and angular spread. We show that by a suitable design of an second-stage undulator with its fundamental wavelength tuned to the second harmonic of the main undulator, coherent second harmonic radiation much more intense than the third harmonic is emitted. Numerical simulations and applications to the LCLS project aiming at generating GW-level and sub-Angstrom x-ray pulses are presented.