Northern Illinois Center for Accelerator
and Detector Development
Ramped-beam generation
Philippe Piot, and Francois Lemery*
Department of Physics and Northern Illinois Center for Accelerator & Detector
Development, Northern Illinois University, DeKalb IL 60115
Accelerator Physics Center, Fermi National Accelerator Laboratory, Batavia IL 60510
*Present address: Center for Free-Electron Lasers (CFEL), Universitat Hamburg,
D-22607 Hamburg, Germany
FACET-II Science Opportunities Workshops
12-16 October, 2015
SLAC National Accelerator Laboratory
Introduction & Motivation
• Possible multipleuser FEL facility
based on wakefield
acceleration [Zholents, FEL14 (2014)]
• Important parameters
for this type of beamdriven acceleration is
the transformer ratio
ebunch
P. Piot, FACET-II workshop, SLAC 10/12/2015
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Transformer-ratio-enhancing shapes
• Various current profiles have been proposed
over the years [Bane et al., SLAC-PUB 3662 (1985)],
• Profiles often challenging to realize in practice
• We seek shapes of the form
tail
• The wake voltage is
where
and
Green’s function of the
e.m. mode excited
P. Piot, FACET-II workshop, SLAC 10/12/2015
wavelength
of mode
3
Quadratic takeoff with linear ramp
• profile with equations
Lemery, PRSTAB 18, 081301 (2015)
ebunch
• decelerating field
• accelerating field
(being
• Transformer ratio
usually take
P. Piot, FACET-II workshop, SLAC 10/12/2015
4
Quadratic takeoff with linear ramp
P. Piot, FACET-II workshop, SLAC 10/12/2015
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Beam shaping via photoemission
• photoemission is a technique of choice
 shaping accomplished via temporal tailoring
of the drive laser.
Simulation of laser shaping (left) and
produced e- beam (right) including
shaper + phoemission response (CsTe)
1 nC
Lemery, PRSTAB 18, 081301 (2015)
P. Piot, FACET-II workshop, SLAC 10/12/2015
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Numerical example
1 nC
• Use LCLS RF gun
• Form “optimum” shape
~1-m downstream of gun
20 cm
60 cm
100 cm
• simulation
-- parabolic +
linear ramp
100 cm from
photocathode
P. Piot, FACET-II workshop, SLAC 10/12/2015
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Production of tailored bunch for beamdriven acceleration
UW SCRF gun
• laser shape +
acceleration &
compression
• close-to-optimal
current distribution
Lemery, PRSTAB 18, 081301 (2015)
5 nC
P. Piot, FACET-II workshop, SLAC 10/12/2015
ebunch
8
• optimum laser shape tuned to
pre-compensate wakefielddistortions
• shape has an inverted
parabolic region
measured nominal
measured w.
optimal laser shape
simulated evolution of
bunch’s current profile
0.5 nC
P. Piot, FACET-II workshop, SLAC 10/12/2015
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Penco, PRL 112 044801 (2014)
Experimental realization (in another
measured UV laser shape
context at Fermi@Elettra)
Multi-frequency linac
• coordinate downstream RF gun
• downstream of 2f linac:
• downstream of compressor:
P. Piot, FACET-II workshop, SLAC 10/12/2015
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Multi-frequency linac (2)
• Example of computed profiles
HEAD
P. Piot, FACET-II workshop, SLAC 10/12/2015
HEAD
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Multi-frequency linac: experiment
• Example of computed profiles
(700 MeV, 0.5 nC)
Standard compression
Ramped bunch 0.5 kA
Head
Head
Ramped bunch 200 A
Ramped bunch 1 kA
Head
P. Piot, FACET-II workshop, SLAC 10/12/2015
Head
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Applicability to FACET-II
• proposed injector + sub-GeV linac has all
necessary ingredients for versatile shaping:
– photocathode RF gun (w. Ti:Sp laser),
– R56 sections, high-harmonic (X-band)
– necessary diagnostics [deflecting structure(s)]
P. Piot, FACET-II workshop, SLAC 10/12/2015
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Summary + outlook
• Practical shapes capable of enhancing
transformer ratio have recently been proposed
– can be produced by conventional photocathode laser
shaping,
– can also be produced
passively (combining
wakefield with R56)
[Lemery, AAC14 (2014)]
– can also be formed
by employing a multifrequency linac
[Piot, PRL 108 (2012)]
P. Piot, FACET-II workshop, SLAC 10/12/2015
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from FACET-II CDR Fig. 3.1
Open questions
• For improved efficiency
need to understand how
to modify these shapes to
achieve high-transformer
ratio in presence of heavy
beam loading,
AFTER BC2
GUN EXIT
witness
drive
drive
witness
• Witness-bunch
formation is not
addressed yet and
setup-specific (here
shown for FLASH).
P. Piot, FACET-II workshop, SLAC 10/12/2015
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