Modulation compression for short wavelength harmonic generation Page: 2 of 9
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factor of the high harmonic in the echo enabled harmonic generation (EEHG) scheme decreases
only as cubic root of the harmonic number. This enables the EEHG scheme to generate high
harmonic current modulation with a reasonable bunching factor.
Both the HGHG and the EEHG require the use of significant amount of laser power to seed the
electron beam. In this letter, we propose a new scheme for generation short wave length radiation
by modulation compression. Through a modulation compression, the initial energy modulation
amplitude can be amplified by a factor of compression factor while the modulation wavelength
is reduced by a factor of compression factor. This significantly reduces the laser power needed
for generation of short wavelength radiation. Meanwhile, since the modulation wave length has
already been reduced by a factor of the compression factor, a fundamental harmonic mode of such
modulation can be used directly to produce short wave length current modulation. This results
in a large bunching factor for such a short wave length modulation. Furthermore, using a few
cycle laser pulse modulation, such a modulation compressed beam might also be used to generate
A schematic plot of the accelerator beam line element for the proposed scheme is shown in
Figure 1. It consists of a laser modulator, an energy chirper A, a bunch compressor A, another
energy chirper B, and another bunch compressor B. In this scheme, an initial laser modulated
electron beam is transported through an energy chirper to obtain a negative energy chirping across
the beam bunch length. This chirped beam is sent into a standard bunch compressor for bunch
compressing. After the bunch compressor, the beam is transported through another energy chirper
with opposite sign of energy-bunch length correlation compared with that of the first energy chirper.
This chirper undoes the global negative energy chirp across the beam and generates a local positive
energy chirp with an amplified energy modulation amplitude. After the second energy chirper, the
beam is transported through the second bunch compressor that has opposite sign of R56 compared
with the first bunch compressor. This bunch compressor further compresses the positive chirped
beam and results in a compressed energy modulation with increased modulation amplitude and
reduced modulation wave length. Some similar schemes were proposed by Biedron et al. and
Shaftan et al. for tuning HGHG radiation wavelength [6, 7]. A key difference between the scheme
proposed in this Letter and the previous schemes is the usage of the second opposite sign bunch
compressor. Without such a bunch compressor, the initial modulation of the beam will not be
further compressed while the original modulation structure inside the locally chirped beam is
smeared out by the finite energy spread as shown in the latter part of this Letter.
In the following, we will derive the longitudinal phase space distribution of the beam trans-
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Qiang, J. Modulation compression for short wavelength harmonic generation, article, January 11, 2010; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc1014715/m1/2/: accessed November 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.