Polarization Analysis for Seeded FELs in a Crossed-Planar Undulator Page: 3 of 3
This article is part of the collection entitled: Office of Scientific & Technical Information Technical Reports and was provided to Digital Library by the UNT Libraries Government Documents Department.
The following text was automatically extracted from the image on this page using optical character recognition software:
for undulator-2. Thus, the phase shifter between the fourth
and the fifth sections can be used for polarization control.
START-TO-END SIMULATIONS AT THE
SECOND HARMONIC WAVELENGTH
For polarization control at the second harmonic wave-
length we use the modified crossed undulator scheme as
proposed in . It consists of three undulators: a first
(main) undulator of length L1 that reaches saturation and
generates microbunching at the fundamental and harmonic
frequencies; two short undulators of equal length L2 L3
with the crossed configuration as shown in Fig.4. The res-
onant wavelength of the two short undulators is chosen to
be the second harmonic of the first undulator (A2 A3
A1/2). The radiator in FEL-3 will again be used as the first
undulator. An important feature of this method is its capa-
bility to generate a high level of circular polarization when
the first undulator operates in saturation. To demonstrate
this feature, we choose the length of undulator-1 to be at
saturation, e.g., Li 17 m.
z$'Z VZ-I- Il n a v.E
e-beam - s
Undulator-1 Undulator-2 sniper Undulator-3
Figure 4: Schematic of the crossed undulator for polariza-
tion control at the 2nd harmonic wavelength.
Similar to the previous section, the S2E beam with re-
alistic distributions are read into Genesis at the beginning
of modulator-1. The same seed laser is used in the simu-
lations. The beam file is dumped at the exit of undulator-1
and then used for the generation of E, and E in undulator-
2 and 3. The radiation field E, from undulator-2 is allowed
to propagate freely to the end of undulator-3. The polariza-
tion analysis will be performed with the on-axis far field
intensity and phase of the two field components.
When we choose L2 L3 1 m with an undulator
period w/2 1.61 cm, the simulation results are shown
in Fig. 5. The upper plot shows the radiation power pro-
files P, and Py at 0.62 nm, and the lower plot shows the
total and the circular degree of polarization at this wave-
length as a function of the phase shift. We see that P and
Py both have spikes at the head/tail part of the pulse due
to the intrinsic evolution of the FEL microbunching after
saturation. The maximum degree of circular polarization is
shown to be 86% with a proper phase shift, again close to
In this paper we have studied the generation of circularly
polarized radiation in a seeded soft x-ray FEL using the
crossed undulator scheme. We have shown that arbitrarily
polarized radiation can be achieved at both the fundamental
-4 -2 0
s ( mj
0 05 1 15 2
phase shtft (cad)
Figure 5: Power profiles from horizontal and vertical un-
dulators (upper plot), and degree of polarization vs. phase
shift (lower plot) at the fundamental wavelength (1.24 nm).
and the 2nd harmonic wavelengths. For the fundamental
radiation, a maximum degree of - 93% is obtainable just
before FEL saturation. For the second harmonic radiation,
the modified scheme can work well in the saturation regime
with over 80% of circular polarization.
Compared to the SASE FEL, the synchrotron oscilla-
tion for the FEL microbunching is more pronounced for
a seeded FEL. Thus, the microbunching decreases quickly
once the part of the bunch reaches saturation. This effect
limits the vertical radiation power and also the uniformity
of the radiation profile, especially for polarization control
at the fundamental wavelength. Therefore, the lengths of
the undulators should be designed properly in order to use
the crossed undulator scheme.
 Fermi@Elettra conceptual design report
http://www.elettra.trieste.it/FERMI, Elettra (2007).
 J. Maraangos et. al., NLS Project: Science Case & Outline
Facility Design, www.newlightsource.org, 2009
 U. Bergmann et al., Science and Technology of Future Light
Sources: A White Paper, SLAC-R-917 (2009).
 K.-J. Kim, Nucl. Instrum. Methods A 445, 329 (2000).
 Y. Ding, Z. Huang, Phys. Rev. ST-AB 11, 030702 (2008).
 H. Geng, Y. Ding, Z. Huang, Proceedings of PAC09, 2009.
 M. Born, E. Wolf, Principles of Optics (1999).
 D. Dunning et. al., these proceedings.
 N.R. Thompson et. al., Proceedings of FEL09, 2009.
 S. Reiche, Nucl. Instr. and Meth. A 429 (1999) 243.
 J. Rowland et. al., these proceedings.
O + total
This article can be searched. Note: Results may vary based on the legibility of text within the document.
Tools / Downloads
Get a copy of this page or view the extracted text.
Citing and Sharing
Basic information for referencing this web page. We also provide extended guidance on usage rights, references, copying or embedding.
Reference the current page of this Article.
Geng, Huiping; /SLAC; Ding, Yuantao; /SLAC; Huang, Zhirong; /SLAC et al. Polarization Analysis for Seeded FELs in a Crossed-Planar Undulator, article, June 25, 2012; United States. (digital.library.unt.edu/ark:/67531/metadc842954/m1/3/: accessed December 9, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.