VISA IB: Ultra-High Bandwidth, High Gain SASE FEL

Gerard Andonian, Ronald Agustsson, Pedro Frigola, Alex Murokh, Claudio Pellegrini, Sven Reiche, James Rosenzweig, Gil Travish
UCLA
Marcus Babzien, Ilan Ben-Zvi, Vladimir Litvitenko, Vitaly Yakimenko
BNL
Jung Yun Huang
POSTECH
Ilario Boscolo, Simone Cialdi, Alessandro Flacco
INFN Milano
Massimo Ferrario, Luigi Palumbo, Carlo Vicario
INFN/ LNF Frascati

The results of a high energy-spread SASE FEL experiment, the intermediary experiment linking the VISA I and VISA II projects, are presented. A highly chirped beam (~1.7 %) was transported without correction of longitudinal aberrations in the ATF dogleg, and injected into the VISA undulator. The output FEL radiation displayed an uncharacteristicly large bandwidth (~11 %) with extremely stable lasing and measured energy of about 2 microJoules. Start-to-end simulations reproduce key features of the measured results and provide an insight into the mechanisms giving rise to such a high bandwidth. These analyses are described as they relate to important considerations for the VISA II experiment.

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 (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 FELÕs. 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.

Spontaneous Radiation Background Calculation for LCLS

Sven Reiche (UCLA/DPA, Los Angeles - California)

The intensity of undulator radiation, not amplified by the FEL interaction, can be larger than the maximum FEL signal in the case of an X-ray FEL. In the commissioning of a SASE FEL it is essential to extract an amplified signal early to diagnose eventual misalignment of undulator modules or errors in the undulator field strength. We developed a numerical code to calculate the radiation pattern at any position behind a multi-segmented undulator with arbitrary spacing and field profiles. The output can be run through numerical spatial and frequency filters to model the radiation beam transport and diagnostic. In this presentation we estimate the expected background signal for the FEL diagnostic and at what point along the undulator the FEL signal can be separated from the background. We also discusses how much information on the undulator field and alignment can be obtained from the incoherent radiation signal itself.

Generation of GW-level, sub-Angstrom Radiation in the LCLS using a Second-Stage 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.

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.