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Accelerator Seminar


"Few-Femtosecond Synchronism of X-Rays to Visible Light
in an X-Ray Free Electron Laser

Bernard W. Adams,
Argonne National Laboratory



The study of ultrafast elementary processes in solid-state physics, chemistry, and atomic physics will play a major role at the x-ray FEL (XFEL) sources that are soon to come on-line.  The visible/infrared pump, x-ray probe technique will play a prominent role in these studies.

An as-yet unresolved problem is that of synchronizing the pump and probe radiation to the few-femtosecond level, i.e. better than the jitter of hundreds of femtoseconds introduced in the radiofrequency accelerator structure of the FEL.

The scheme proposed here [1] obtains intense femtosecond pulses of x-rays and infrared/visible light from the same electrons in an XFEL undulator, and therefore circumvents the problem of having to synchronize two independent sources.  It combines the recently proposed technique of emittance slicing [2] with the concept of coherence enhancement of transition undulator radiation (TUR) [3], which is most intense in the near-infrared region.

In the emittance-slicing technique, the emittance of an electron bunch is modulated in such a way that only a few-femtosecond slice is of sufficient quality to support self-amplified spontaneous emission (SASE).  In the present proposal, the emittance contrast leads to a contrast in the electron-beam divergence, which, affects the number of electrons contributing the highly collimated TUR visible to a given observer.  Thus, a coherence enhancement (CTUR) of TUR is achieved at wavelengths equal to, or longer than, the length of the slice in the electron bunch, i.e., femtoseconds, corresponding to micron wavelengths.

For the purposes of infrared CTUR generation, the emittance contrast introduced in the original emittance-slicing proposal [2] is not sufficient.  However, using secondary electron-scattering foils, it is possible [1,4] to increase the emittance contrast to the required level.  Originating from the same electrons in the same undulator, the x-rays and infrared light are perfectly synchronized.  With the beam parameters of the LCLS, the pulse energy of the CTUR is about 100 nJ in a single optical cycle of 1-micron light.

In addition to the fundamental concept, the talk will address issues arising in the integration into an XFEL facility, such as placement of secondary scattering foils, direct use of the CTUR for pumping vs. cross-correlation with a short-pulse laser, and scanning delays.  Furthermore, a possible use of CTUR for the purposes of diagnosing the SASE process is discussed [4], and the importance of the velocity term in the Lienard-Wiechert fields is discussed [4].

[1] B.W. Adams, Rev. Sci. Instrum. 76, 063304 (2005)

[2] P. Emma et al., Phys. Rev. Lett. 92, 074801 (2004)

[3] K.-J. Kim, Phys. Rev. Lett. 76, 1244 (1996)

[4] B.W. Adams, Rev. Sci. Instrum. 78, 123302 (2007)

Wednesday, April 1, 2009

3:30 p.m. - 4:30 p.m.
CEBAF Center, Room F113

Talk Slides: (Slides)

For more information, please contact Dr. Alex Bogacz or Anne-Marie Valente.