Generation of attosecond x-ray pulses with a multi-cycle two-color ESASE scheme Page: 2 of 12
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I. INTRODUCTION
Ultrashort soft and hard x-ray sources have the potential to open new regimes in atomic
and electronic processes, benefiting widespread fields in physics, chemistry and biology.
This has motivated the development of laser based ultrashort soft x-ray sources as well
as the construction of fourth generation free electron laser (FEL) sources in the soft and
hard x-ray regime. Due to the duration of the electron bunch, FEL facilities have aimed
for the femtosecond regime, but the formidable challenge of generating attosecond pulses
is attracting much attention within the x-ray FEL user community. Several schemes for
generation of attosecond x-ray pulses have been proposed [1-8], mostly by manipulating of
electron bunches with extremely short laser pulses.
The two-color scheme proposed in Ref. [6] is based on the current-enhanced self-amplified
spontaneous emission (ESASE) technique proposed by Zholents [9]. The electrons interact
with two short laser pulses within two single-period wiggler magnets to generate a time-
dependent energy modulation in the bunch. A dispersive section, such as a four-dipole
chicane, converts the energy modulation of the electron bunch to a density modulation. The
high current spike formed by the overlapping peak intensities of the two lasers dominates the
FEL gain process and produces an attosecond x-ray pulse. Ultra-short lasers with no more
than two optical cycles are required to minimize satellite spikes. In addition, the longitudinal
space charge (LSC) field in the main FEL undulator has to be considered properly [10].
In this paper, we extend the attosecond two-color ESASE scheme to the long optical cycle
regime using a detuned second laser and a tapered undulator by taking advantage of the LSC
field. Our scheme extends to FELs a technique proposed for high-order harmonic generation
(HHG) to manipulate an electron wave packet for the generation of isolated attosecond soft
X-ray pulses [11]. HHG experiments have shown a controlled detuning between the two
colors can control the electron wave packet energy (low energy regime) in the sub-cycle
domain using relatively long infrared laser pulses (up to 15 optical cycles). Here, we apply
this idea to ESASE. Both lasers can be about ten optical cycles long, with the second laser
frequency detuned from the first one to optimize the contrast between the central and side
current spikes. A tapered undulator mitigates the degradation effect of the LSC force in the
undulator [10] and suppresses the FEL gain of all side current spikes.2
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Ding, Y.; Huang, Z.; Ratner, D.; Bucksbaum, P. & Merdji, H. Generation of attosecond x-ray pulses with a multi-cycle two-color ESASE scheme, article, March 4, 2009; United States. (https://digital.library.unt.edu/ark:/67531/metadc934550/m1/2/: accessed March 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.