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Scientific Report 2007-2009
Particle physics
P34. Results of SPARC Free Electron Laser Experiment
The SPARC project is a Free Electron Laser (FEL)
at the LNF, Frascati (Italy). The main components of
the machine are showed in Fig.1 and is also the test and
training facility for the recently approved VUV/soft X-
ray FEL project named SPARX. The SPARC FEL is
composed by a high brightness photo-injector providing
a high-quality beam at energies up to 150 and 200 MeV
(12 m), a transfer line for beam matching and diagnostic
(6.8 m) and an undulator beam line (13 m) composed
by six undulator sections with variable gap. The gun is
a SLAC/BNL/UCLA 1.6 cell S-band RF photo-injector
and its performances have been studied in a first phase,
with a movable emittance meter. This instrument allowed
the investigation of the beam parameters dynamic
in the first meters after the gun, allowing to optimize
the working point in order to minimize emittance (Ferrario
working point). The final energy is reached with
three SLAC-type linac sections at 2.856 GHz. The first
two sections are surrounded by solenoids providing additional
focusing during acceleration. This solution allows
to work in velocity bunching regime that consists in exploiting
a correlated velocity dispersion for obtaining the
compression of the beam. The magnetic axial field properly
tuned ensures the desired emittance preservation,
i.e. high brightness electron source with short bunch
length without the implementation of a magnetic chicane.
The electron beam injected through the undulator
generates high brilliance and tunable FEL radiation in
the visible region around the fundamental wavelength
(500 nm) and at VUV wavelengths with the harmonics.
The SPARC high brightness electron beam gives the possibility
to develop multidisciplinary activities like coherent
Terahertz radiation with OTR technique and moreover,
in combination with the Terawatt laser of FLAME
experiment at LNF, the Plasma acceleration (PLAS-
MONX project) and coherent X-ray generation by the
Thomson scattering. Experiments are being performed
to generate and manipulate modulated electron bunches
or bunch trains for possible uses in PWFA, pump and
probe FEL experiments, narrow band THz source or enhanced
SASE-FEL. The source’s development of ultrashort
x-ray pulses is both an impressive improvement for
the accelerator and laser physics and it opens a new way
of exploring the ultra-fast dynamics involved in matter
physics (superconductivity, complex and strongly correlated
system) and chemical-biological systems (crystallography,
biomolecular organization, photosynthesis). In
the last few years, the SPARC group of Roma1 has contributed
to develop the high brightness photo-injector
and the choice of machine parameters with some specific
simulation tools for acceleration, transport of the beam
and FEL interaction in the undulator.
Figure 2: First 500 nm Self Amplified Spontaneous Emission
(SASE) at SPARC on February 17 th 2009.
Figure 1: SPARC layout.
The first SASE FEL spectra was obtained on February
17 th (Fig.2) and beam compression via velocity bunching
with emittance compensation was demonstrated in April
2009. In July 2009 a substantial increase of the extracted
radiation from the FEL source was obtained with a longitudinally
flat top e-beam by increasing the bunch charge.
The last stage of the commissioning has established the
characterization of the FEL harmonics (200nm, 133nm)
with SEED laser and the characterization of the spontaneous
and stimulated radiation in the SPARC undulators
with short electron beam (hundreds of fs) in the
so-called single spike regime (full coherent laser pulses).
References
1. T. Watanabe et al., Phys. Rev. Lett. 98, 034802 (2007).
2. M. Ferrario et al., Phys. Rev. Lett. 99, 234801 (2007).
3. A. Cianchi et al., Phys. Rev. ST Accel. Beams 11,
032801 (2008).
4. L. Giannessi et al., NIMA 593, 132 (2008).
Authors
M. Mattioli, P. Musumeci, M. Petrarca, M. Serluca 1
http://www.roma1.infn.it/exp/xfel/
Sapienza Università di Roma 141 Dipartimento di Fisica