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