A recirculating linac-based facility for ultrafast X-ray science Page: 2 of 3
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shaped cavities, and operate over 10-100 Hz pulse
repetition rate. The cathode is mounted on the cavity axis
and electrons are rapidly accelerated by the rf electric fields,
minimizing space-charge effects in the low-energy beam.
For high duty cycle operation, thermal limitations prevent
such designs from operating at an electric field sufficiently
high to produce good beam emittance. For the LUX
facility we have produced a conceptual design optimized
for operation at high gradient and high repetition rate, and
producing low-emittance bunches. This design
incorporates features that increase cavity surface area to
reduce deposited power density and enhance the
accelerating electric field at the cathode (82 W/cm2
maximum power density for 64 MV/m at the cathode).
Application of a solenoidal magnetic field on the
cathode of the flat-beam gun, followed by a specially
configured skew-quadrupole channel located after the
injector linac, allows production of a "flat" beam with x/y
emittance ratio 50/ 1 and small vertical normalized
emittance of 0.4 mm-mrad. This technique has been
successfully demonstrated at Fermilab, with properties
close to LUX design parameters .
Following the injector linac, a third-harmonic cavity is
used to linearize the correlated energy spread introduced in
the injector linac, and to manipulate the longitudinal
phase-space in preparation for bunch compression. The
beam is then transported to the entrance of the
recirculating linear accelerator. In the transport line from
injector linac to main linac, the bunches are compressed
from 20 ps to 2 ps, with a final energy spread of 200
keV. This transport line is carefully designed to
compensate for the effects of coherent synchrotron
radiation at the shorter bunch lengths .
In the recirculating linac the maximum energy of 3
GeV is achieved after four passes through the 720 MeV
superconducting rf structure, or 2.5 GeV with the main
linac operating at 600 MeV. Identical cryomodules
containing multiple accelerating cavities are used for the
main linac and the injector linac. The superconducting
linacs have advantages in providing a compact and
efficient accelerator, extremely stable rf fields, and
inherently small perturbative effects on the beam.
Significant advances have been made in superconducting if
technology in recent years, and the parameters of the
proven TESLA superconducting rf systems developed at
DESY have been used in LUX design studies .
Planned upgrades for the CEBAF facility at TJNAF also
meet the requirements for the LUX linacs . Our design
is for an accelerating gradient of up to 20 MV/m in the
main linac. The electron bunch repetition interval in LUX
is less than the superconducting cavity filling time, and
the linacs are more efficiently operated in cw mode.
Engineering modifications to exisiting cryomodule
designs required to allow significantly increased thermal
load in the liquid helium are described in .
The flexibility of the LUX lattice design allows control
and preservation of electron beam transverse and
longitudinal emittances, minimizing the influence of
collective effects . Longitudinal and transverse
dynamics have been modeled from the RF gun through the
injector linac and all passes of the main linac. In the
injector, a harmonic cavity will be used to control the
longitudinal phase-space following the injector linac .
The bunch length and magnet bend angle in the lowest
energy arcs of the machine result in a regime in which
coherent synchrotron radiation emission could be expected,
and the vacuum chamber geometry is designed to
minimize this effect by shielding against lower-frequency
radiation. The recirculating ring arcs are achromatic and
isochronous to preserve beam quality. Our studies
including particle tracking with cavity wakefields,
resistive wall impedance, and magnet errors and
misalignments, show only modest emittance growth, with
negligible impact on machine performance. The lattice is
designed to allow manipulation of the bunch phase space
on each pass if required, and also to accommodate bunch
rates greater than the 10 kHz baseline design.
LUX will have the capacity for energy recovery in the
linacs. However, for the baseline beam power of a few
tens of kW, the beam will be taken directly to a shielded
dump after the x-ray production sections.
At the exit of the final arc the flat-beam electron
bunches receive a time-correlated vertical kick in a dipole-
mode RF cavity. This imparts to the electron bunch a
transverse momentum that is correlated in amplitude to
longitudinal position within the bunch. The electrons then
radiate x-rays in the downstream chain of undulators and
dipole magnets, imprinting this correlation in the
geometrical distribution of the x-ray pulse. The correlated
x-ray pulse is then compressed to 10's fs duration by use
of asymmetrically cut crystal optics. The bunch deflecting
technique is identical to the "crab-cavity" schemes
proposed for several electron-positron colliders. A total
deflecting voltage of 8.5 MV is required, and we have
developed a preliminary design for a 7-cell
superconducting deflecting cavity .
Narrow-gap in-vacuo superconducting undulator designs
provide tunable high-flux sources in the 1-12 keV range.
The flux of 10 keV photons from 1 nC bunches at 10 kHz
is 6x100 photons/ s/0.1%BW for a 4 mm gap, 14 mm
period, 2 T peak magnetic field undulator. Similar
insertion devices are currently being prototyped and
designs are expected to mature in the near future.
A laser-seeded cascaded harmonic-generation scheme
produces high-flux, short-pulse photons over an energy
range of tens of eV to 1 keV. In this process the circular
cross-section high-brightness electron beam is extracted
from the recirculating linac, and passed through an
undulator where a co-propagating seed laser modulates the
charge distribution over a short length of the bunch. The
scheme has been developed and demonstrated at the
Brookhaven DUV FEL facility . The imposed
modulation results in enhanced radiation at specific
wavelengths and a selected wavelength is amplified in a
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Corlett, J.N; Barletta, W.A.; DeSantis, S.; Doolittle, L.; Fawley, W.M.; Green, M.A. et al. A recirculating linac-based facility for ultrafast X-ray science, article, May 6, 2003; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc737871/m1/2/: accessed September 25, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.