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.
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.
proposed here  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 
with the concept of coherence enhancement of transition undulator radiation (TUR)
, which is most intense in the near-infrared region.
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.
purposes of infrared CTUR generation, the emittance contrast introduced in the
original emittance-slicing proposal  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 , and the importance of the velocity
term in the Lienard-Wiechert fields is discussed .
 B.W. Adams, Rev. Sci. Instrum. 76, 063304 (2005)
 P. Emma et al., Phys. Rev. Lett. 92, 074801 (2004)
 K.-J. Kim, Phys. Rev. Lett. 76, 1244 (1996)
 B.W. Adams, Rev. Sci. Instrum. 78, 123302 (2007)
Wednesday, April 1, 2009
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.