Terahertz Sources
Berkeley plans CIRCE to fill the terahertz gap
A machine has been proposed at Berkeley that would provide radiation at terahertz frequencies, a valuable source for research.
CIRCE: combler le fossé des térahertz
Une équipe à la Source de lumière avancée (ALS) du Laboratoire
national Lawrence de Berkeley propose la construction d'un anneau
totalement optimisé en source de photons produisant un rayonnement
synchrotron cohérent dans la gamme de fréquence des térahertz. Ce
Centre infrarouge cohérent, CIRCE, viserait des performances maximales,
avec un flux de photons dépassant de plus de neuf ordres de grandeur
ceux des sources térahertz "classiques", large bande, actuelles.
A team at the Advanced Light Source (ALS) of the Lawrence Berkeley
National Laboratory (LBNL) has proposed the construction of a
ring-based photon source optimized for generating coherent synchrotron
radiation (CSR) at terahertz frequencies. The Coherent Infrared Center
(CIRCE) will exploit all the CSR production mechanisms currently
available for achieving top-level performance, including a photon flux
exceeding by more than nine orders of magnitude that of existing
conventional broadband terahertz sources.
Interest in the scientific use of radiation at terahertz frequencies
is rapidly increasing: the fields that would benefit range from
solid-state physics (semiconductors, metals, superconductors, strongly
correlated materials, etc) through chemistry and biology to
applications in medical science and security. However, a major problem
is that generating radiation of significant intensity in this frequency
range, which lies between microwaves and infrared, is not
straightforward. Owing to the lack of sources, this region is often
referred to as the "terahertz gap", but storage-ring-based CSR sources
are very promising candidates for addressing this situation.
CSR occurs when the synchrotron emission from the relativistic
electrons in a beam bunch is in phase. This happens when the length of
an electron bunch is comparable to, or shorter than, the wavelength of
the radiation being emitted. At 1 THz, this is about 300 μm.
In the coherent regime, the radiation intensity is proportional to the
square of the number of particles per bunch, in contrast with the
linear dependence of conventional incoherent synchrotron radiation.
Considering that the number of electrons per bunch in a storage ring is
typically very large (106-1011), the potential
intensity gain for a CSR source is huge. However, achievable bunch
lengths and the shielding effect of the conductive vacuum chamber in
storage rings mean CSR can only be generated in the terahertz frequency
range (from about 100 μm to a few millimetres).
Although CSR was predicted to occur in high-energy storage rings
over half a century ago, it has only been observed in the past few
years. Intense bursts of CSR with a stochastic character have been
measured in the terahertz frequency range in storage rings at several
synchrotron light sources. Work carried out by groups at the Stanford
Linear Accelerator Center (SLAC), LBNL and the Berliner
Elektronenspeicherring-Gesellschaft für Synchrotron Strahlung (BESSY)
showed that this bursting emission of CSR is associated with a single
bunch instability (G Shipakov et al. 2002, M Venturini et al. 2002, J M Byrd et al. 2002, M Abo-Bakr et al.
2003a). This "microbunching instability" (MBI) is driven by the fields
of the synchrotron radiation emitted by the bunch itself. Although
interesting in terms of accelerator physics, these bursts of CSR are
not very useful as a terahertz source, because they are intrinsically
unstable and stochastic.
However, CSR emission with remarkably different characteristics was
observed at BESSY when the storage ring was tuned to a special mode for
short bunches (M Abo-Bakr et al. 2002 and 2003b). The emitted
radiation was not the quasi-random bursting previously observed but a
powerful and stable flux of broadband CSR in the terahertz range -
exactly what is required for a source that is useful for scientific
experiments. The LBNL, SLAC and BESSY groups together drew up a model
that reproduces the observations and can be used for designing a
ring-based source optimized for generating stable terahertz CSR (F
Sannibale et al. 2004a and 2004b).
Terahertz CSR in storage rings
An interesting feature of the CSR spectra measured at BESSY is
that they extend to significantly shorter wavelengths than those
expected from a Gaussian longitudinal distribution of the bunch. The
model developed showed that the synchrotron radiation fields can
potentially produce a stable distortion of the bunch distribution from
Gaussian towards a sawtooth-like shape with a sharp leading edge. This
was ultimately responsible for the observed extension of the CSR
spectra towards shorter wavelengths in BESSY. We will refer to this
configuration as the "ultra-stable" mode of operation.
Another development in CSR in storage rings, first demonstrated at
the ALS and more recently at BESSY, was obtained by exploiting
parasitically the "femtoslicing" technique used for producing
femtosecond X-ray pulses. In the femtoslicing scheme, the
co-propagation in a wiggler of a femtosecond optical laser pulse with a
much longer electron bunch generates a modulation of the electron
energy in a femtosecond slice of the bunch (see CERN Courier
July/August 2000 p31). When the bunch propagates in a dispersive
region, the energy-modulated particles are transversely displaced.
Properly masking the synchrotron radiation can remove the part emitted
by the core of the bunch while allowing the transmission of the part
emitted by the displaced electrons. In this way, femtosecond X-ray
pulses are obtained.
At the same time, because of the longitudinal dispersion in the
ring, the modulation in energy induces a density variation in the
longitudinal distribution as the bunch propagates along the ring. The
characteristic length of these longitudinal structures starts from tens
of micrometres (a few tens of femtoseconds duration) immediately after
the laser-beam interaction region in the wiggler. It quickly increases
to the order of a millimetre, before finally disappearing in a few ring
turns. These structures radiate intense CSR in the terahertz range with
appealing characteristics: very short CSR pulses (of the same order as
the laser pulse length), which extend the CSR spectrum towards shorter
wavelengths (to about 10 μm or about 30 THz) than those in
the ultra-stable mode; high energies per terahertz pulse (tens of
micro-joules); and terahertz CSR pulses intrinsically synchronous with
the femtosecond laser and X-ray pulses (allowing for a variety of
pump-probe experiments and/or electro-optic sampling techniques). The
main limitation is the relatively low repetition rate (a few
kilohertz), which is imposed by present laser technology.
Designing CIRCE
In designing the CIRCE ring, the team has provided for optimized
versions of all the techniques for generating terahertz CSR as
described. Figure 1 shows a 3D layout of the ring inside the ALS
facility. The ring, 66 m in circumference and operating at
600 MeV, is designed to be located on top of the ALS Booster Ring
shielding and will share the injector with the ALS Storage Ring.
Figure 2 shows the impressive flux of CIRCE, calculated for three
settings of the ultra-stable mode of operation. The gain of many orders
of magnitude in the terahertz frequency range over the existing
conventional source is clearly visible. Figure 3 shows how the
femtoslicing mode complements the ultra-stable mode of operation in
CIRCE. The calculated spectra for the two modes together cover the
entire terahertz range from wavelengths of about 10 μm
(30 THz) to about 10 mm (0.03 THz). The energy per
terahertz pulse in the example used for the femtoslicing case is about
8.5 μJ, which when focused onto a sample would provide an electric
field of about 106 V/cm. Current laser technology should allow repetition rates as high as 10-100 kHz.
The vacuum chambers in the dipole magnets and the first in-vacuum
mirror have been designed for the efficient collection of terahertz
synchrotron radiation. The design calls for three ports with
100 mrad horizontal by 140 mrad vertical acceptance for each
of the 12 dipole magnets, giving a potential total of 36 dipole beam
lines in CIRCE. The layout of the ring also includes six 3.5 m
straight sections that can be used for insertion devices for possible
future sources (as for the case of the wiggler in the femtoslicing
scheme).
The CIRCE team has completed a detailed feasibility study that
includes electron-beam linear and nonlinear dynamics studies, the
design of all the magnets, the design of the special high-acceptance
dipole vacuum chamber, and evaluating the compatibility of CIRCE with
the ALS facility. Also, the team has experimentally investigated
resonating modes that could be excited by the electron beam in the
high-acceptance dipole vacuum chamber.
These modes, potentially dangerous for the electron-beam stability,
have been measured and characterized by means of radio-frequency
measurements in a prototype dipole chamber. No "show-stoppers" have
been identified and CIRCE is part of the current five-year strategic
plan for the ALS.
• See also http://CIRCE.lbl.gov/. For an historical review of the work on CSR in storage rings see Murphy 2004 below.
Further reading
M Abo-Bakr et al. 2002 Phys. Rev. Lett. 88 254801.
M Abo-Bakr et al. 2003a 2003 IEEE Particle Accelerator Conference 3023.
M Abo-Bakr et al. 2003b Phys. Rev. Lett. 90 094801.
J M Byrd et al. 2002 Phys. Rev. Lett. 89 224801.
J B Murphy 2004 ICFA Beam Dynamics Newsletter 35 20.
F Sannibale et al. 2004a ICFA Beam Dynamics Newsletter 35 27.
F Sannibale et al. 2004b Phys. Rev. Lett. 93 094801.
G Shipakov et al. 2002 Phys. Rev. ST AB 4 5 054402.
M Venturini et al. 2002 Phys. Rev. Lett. 89 224802.
Author: John M Byrd, Michael C Martin and Fernando Sannibale for the CIRCE Team, Lawrence Berkeley National Laboratory.
Article 33 of 35.
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