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Accelerator Test Facility (ATF) at the Brookhaven National Laboratory
is the first advanced accelerator facility designed and built to
serve the community active in advanced accelerator research. A proposal-driven
user facility, it is dedicated to long-term R&D in the physics
of particle and laser beams. The users, who come from universities,
national laboratories and industry, carry out R&D on advanced
accelerator physics, studying in particular the interactions of
high-power electromagnetic radiation and high-brightness electron
beams, including laser and plasma accelerators of electrons and
free-electron lasers. Other topics include the development of electron
beams with extremely high brightness, photo-injectors, electron
beam and radiation diagnostics and computer controls.
The core of the ATF consists of a high-brightness photoinjector
electron gun, a 75 MeV linac, terawatt picoseconds CO2 laser synchronized
to the electron beam to a picosecond level, four beam lines (most
equipped with energy spectrometers) and a sophisticated computer
control system. The facility, which has been in operation since
1992, provides the best high-brightness electron beams up to an
energy of 75 MeV, with, for example, a normalized rms emittance
of 1 mm at a charge of 0.8 nC. The bunch length is variable from
1 to 8 ps, with a bunch compressor to extend the range down to 100
fs.
The users enjoy extensive support infrastructure, with a few tens
of million dollars of investment, which is embedded in a large and
highly capable national laboratory. The ATF staff provides the users
with close support and expertise in electron-beam dynamics, lasers
and optics, advanced diagnostics, energy spectrometers and computer
control. These supports are free of charge, while the use of other
resources at Brookhaven, as well as the dedicated equipment for
experiments, are the responsibility of the users. The users’
activities are reviewed by the ATF Program Advisory Committee, which
includes members from various universities and national laboratories.
The committee keeps the number of users relatively steady.
The publication rate from experiments at the ATF is high, with an
average of more than three papers in Physical Review per year. The
facility is also an excellent training ground for graduate students
in accelerator physics and the physics of beams with, on-average,
more than two graduations a year. While a large number of students
come from nearby Stony Brook University, the majority come from
universities across the U.S. and throughout the rest of the world.
The ATF staff is proud of its contribution to graduate education
in accelerator and beam physics, through education and support of
the students.
The ATF receives steady support from the U.S. Department of Energy,
which has enabled the facility to evolve not only in terms of hardware
and the expertise of its staff, but also in terms of stability and
in the superb performance of the electron and laser beams. This
environment is beneficial to the rather difficult, cutting-edge
experiments in advanced accelerator and coherent source physics
that are carried out by the users.
From Photocathodes to Plasma Wake Fields
The work of the ATF has pioneered metallic photocathodes such as
copper, magnesium and, most recently, niobium, for robust, good
quantum efficiency operation. These photocathodes are now found
everywhere in the world and are also produced industrially. The
same holds true for the rf guns, with the celebrated Brookhaven
one-and-a-half cell S-band series of guns. The series now stands
at Gun IV, while a new superconducting continuous-wave rf gun is
being developed. Examples of advanced diagnostics undertaken at
the ATF include the first slice-emittance measurement, the first
pulse-length measurement using shot-noise-driven fluctuation in
incoherent radiation, high-resolution phase-space tomography and
more. The ATF is also developing high-performance plasma capillary
channels that channel the carbon-dioxide laser beam and provide
a convenient source of plasma for a variety of experiments. Most
recently, R&D is being carried out on optical stochastic cooling
of hadron beams.
By far the most important aspect of the ATF is the research carried
out by its users. Milestone experiments in laser acceleration include
the work on inverse Cherenkov acceleration and the inverse free-electron
laser (IFEL). The Staged Electron Laser Acceleration experiment,
STELLA, has successfully used two laser accelerators (both IFELs),
demonstrating the steady production of 3 fs electron beam bunches.
With this configuration, STELLA II has demonstrated monoenergetic
laser acceleration for the first time (CERN Courier, March 2004,
p 7).
Experiments on the development of laser-photocathode rf guns include
the “Next Generation Photoinjector,” or Gun III in the
ATF series. Other experiments concern the generation of unique radiation
sources, including the pioneering high-gain harmonic-generation
free-electron laser (FEL) that set a new trend towards coherent,
ultrashort-pulse X-ray FEL. The VISA experiment at the ATF, which
served as a proof-of-principle experiment for the Linac Coherent
Light Sourse project at SLAC (CERN Courier, March 2003, p 5), reached
saturation at visible wavelengths and demonstrated the generation
of harmonics, their growth and saturation properties and the relationship
to microbunching.
The Compton scattering experiment to investigate Compton scattering
between energetic electrons and laser beams produces a record of
about 108 hard X-ray photons per pulse of a few ps. Scattered photons
of the laser light gain kinetic energy from the electrons and become
Doppler-shifted into the X-ray region. This process is called linear
Thomson scattering and normally results in a narrow X-ray beam directed
exactly along the electron path. By elevating laser intensity and
filtering out the linear component in Thomson scattered radiation,
a nonlinear component that splits into two closely separated beams
of twice higher photon energy (Figure 1) was observed for the first
time. The origin of these beams is a theoretically predicted but
never previously demonstrated figure-eight oscillation of a relativistic
electron (Figure 2). An electron acquires such a trajectory in ultra-intense
EM field when its transverse velocity approaches the speed of light.
Recently, a plasma wake-field experiment demonstrated the phase
relationship between the accelerating and focusing component of
the plasma wake. This showed a 90 degree phase difference, thus
allowing plasma wake accelerators to accelerate and focus the beam
at the same phase.
Further reading:
The web site of the ATF is at www.atf.bnl.gov.
fig. 1
fig. 2
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