Beam dynamics activities at the Thomas Jefferson National Accelerator Facility (Jefferson Lab) Page: 4 of 6
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compression of the bunch length to picosecond time scales, increasing the
peak current to enhance the FEL interaction. Downstream of the wiggler, a
second quadrupole telescope (with embedded dogleg, to avoid the other end
of the optical cavity) matches the electron beam to the betatron acceptance
of the energy-recovery recirculation arc. The beam is transported through a
linearly achromatic Bates-type end loop, through a FODO backleg, and into
a second end loop to the reinjection point at the cryomodule.
The recirculation transport is nominally is isochronous from wiggler to
reinjection point. Two families of trim quadrupoles allow variation of the
(5,6) matrix element between -0.25 m and +0.25 m while maintaining achro-
maticity; this dynamic range will allow optimization of the RF system behav-
ior and energy compression of the electron beam during FEL operation with
energy recovery. The required acceptance has been achieved through careful
attention to chromatic aberrations. Such aberrations have been controlled
through careful selection of hardware parameters (bend angles, entry/exit
angles, and radii), suppressed by use of transport with rational tunes (the
FODO transport comprises six cells with 90 degrees tune each; the end loops
each are 1.25 wavelengths long horizontally and 0.5 wavelengths long verti-
cally), and corrected using eight sextupoles (in two symmetrically arrayed
families to reduce to zero at the reinjection point the quadratic variations
with momentum of path length, horizontal position, and horizontal angle).
Beam dynamics studies using CEBAF-benchmarked methods predict BBU
thresholds in excess of 60 milliamps . Initial theoretical studies of CSR
effects suggest that phase-space degradation at the wiggler will not preclude
FEL operation; ongoing investigations are considering the impact of CSR
during the energy recovery transport . Modeling of space-charge effects
during transport and acceleration using PARMELA indicate that beam qual-
ity is maintained through the acceleration cycle and the proper phase space
for FEL operation can be produced at the wiggler; space charge does not sig-
nificantly influence the beam performance once the final energy of 42 MeV has
been achieved. Lattice design and analysis studies predict that machine
beam transport parameters are readily achieved and insensitive to typical
errors; beam loss studies suggest the machine will suffer limited losses even
at high currents . Proactive impedance policing has been implemented.
Studies of RF system performance  and the FEL/RF system interaction
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Douglas, D.R. Beam dynamics activities at the Thomas Jefferson National Accelerator Facility (Jefferson Lab), report, December 1, 1997; Newport News, Virginia. (digital.library.unt.edu/ark:/67531/metadc691230/m1/4/: accessed October 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.