Velocity Bunching: Velocity Bunching: experiment at Neptune experiment at Neptune
PhotoinjectorPhotoinjector
P. Musumeci
UCLA Dept. of Physics and Astronomy
OutlineOutline
• RF rectilinear compression, an old but trendy idea• Different ways of implementing the velocity
bunching• Two proposed schemes (Pleiades, Orion)• And one experiment (Neptune)• Conclusions
Applications of compressed, ps-Applications of compressed, ps-pulse pulse
high brightness beamshigh brightness beams• Injection into short wavelength
Advanced Accelerators Structures
• Plasma wake-field drivers• SASE FEL (LCLS)• Thomson-scattering sources
(PLEAIDES)
Main issue: can one maintain phase space density(focusability) during compression? Diseases include non-inertial space-charge, CSR,...
Damage from bends 1: Damage from bends 1: phase space bifurcation at Neptunephase space bifurcation at Neptune
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Undercompressed Fully compressed
Longitudinal phase space bifurcations and distortions also seen in simulation
Damage from bends 2: Damage from bends 2: coherent synchrotron radiation coherent synchrotron radiation
instabilityinstability
Strong energy modulations observed at 70 MeV in SDL experiments (W. Graves, Berlin CSR Workshop, 1/02).Potentially disastrous for LCLS and TESLA FEL.
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Cu
rren
t (A
)
Time (ps)
No compression
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rren
t (A
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Mild compression
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rren
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Strong compression
Damage from bends 3: Damage from bends 3: Phase space distortions in high Phase space distortions in high
gradient UCLA/FNAL PWFA gradient UCLA/FNAL PWFA experimentsexperiments
Distribution of highly decelerated beam after plasma. Spectrometer bend is horizontal,chicane bend plane is vertical. Vertical distortions very reproducible; emittance grows from 20 to 50 mm-mrad.
Velocity Bunching: Velocity Bunching: a Cure for “The Bends”?a Cure for “The Bends”?
• Proposed by Serafini, Ferrario (2000) tool for SASE FEL injector, avoids magnetic compression and associated problems
• Compression effectively done at low energy• Inject emittance-compensated beam at 5-7 MeV into
slow-wave linac• Perform one-quarter of synchrotron oscillation to
compress beam • Similar to manipulations in thermionic injector
bunchers, but with high phase space density (emittance preservation???)
Longitudinal phase space schematic for velocity bunching
Options for Velocity Bunching 1: Options for Velocity Bunching 1: “Slow-wave capture”“Slow-wave capture”
• Long slow-wave structure – Choice of phase velocity gives
flexibility in optimizing capture
– Can tune (new source)
– Can tune k (new structure)
• Final bunch length dominated by rf nonlinearities.
• Proposed for INFN FEL injector test facility SPARC (slow-wave integrated system)
• Variation proposed for LLNL PLEIADES
HOMDYN simulation of INFN test facility case
Options for Velocity Bunching 2: Options for Velocity Bunching 2: “Ballistic Bunching”“Ballistic Bunching”
• Do we need the slow wave?• Alternative: use only short bunching section to
split functions of bunching and acceleration• Short section is like a “thin lens” • Good for compact systems
Comparison cartoon
PLEAIDESPLEAIDES
• Sub-ps beam for high flux sub-ps x-rays
• Need very low energy spread and emittance for focusability (<15 microns)
• Four 2.5 m linacs (independent phase, effective slow wave….)
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A look to the transverse A look to the transverse dynamics from the simulationsdynamics from the simulations
• The beam is getting denser, it undergoes a lot of plasma oscillations
• Need to keep it under control with a solenoid field
Simulation by Winthrop Brown (LLNL)
Velocity bunching proposed for Velocity bunching proposed for ORION facility at SLACORION facility at SLAC
• S-band injector
• S-band PWT buncher
• X-band linacs (NLCTA)
HOMDYN simulation of ORION system
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z (m)S-band PWT Buncher
S-band RF gun
X-band travelling wave linacs
ORION velocity bunchingORION velocity bunching
• First PARMELA studies of Velocity Bunching; 1 nC design point (PWFA case)
• Emittance can be preserved • Need ramped magnetic field
profile to match increasing beam plasma frequency
• NLCTA source has magnets!• Need to avoid longitudinal
cross-over
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Experiment at NeptuneExperiment at Neptune• Try velocity bunching idea in the split photoinjector
configuration• Try to investigate the longitudinal and transverse
dynamics as much as possible (no post acceleration, no solenoid for transverse control)
Autocorrelator
CTR foil
PWT Linac
Chicane (used as 45 degrees dispersing dipole)
Vertically focusing
Quadrupole
CCD camera
YaG screen
Neptune 1.6 cell gun+solenoid for
emittance compensation
Transverse diagnostics: emittance
measurement via quad scan
Longitudinal diagnostics : bunch
length
The resultsThe results
Sampling the linear part of the RF-fields results in a very short beam (<0.4 ps)!!!
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Model: ctr autocorrelation Chi 2 = 0.00043R 2 = 0.93021 c 0.34214 ±0.0024A -0.32616 ±0.0125t0 5.06834 ±0.02002rmsz 0.39183 ±0.01868csi 1.93039 ±0.13644
Aut
ocor
rela
tion
func
tion
ps
Q = 210 +- 20 pC
= 700 +- 10
VB knobs 1: VB knobs 1: What happen if we change What happen if we change Linac accelerating gradientLinac accelerating gradient
• Cancellation between increasing energy spread and decreasing energy.
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10
fo
r m
axi
mu
m c
om
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ssio
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Linac Energy gain (MeV)
Theory Measurement obtained mixing
RF Low Level with Linac Loop
zL
EE
zkEL
pp
linacgun
linac
33 sin
cos
VB knobs 2: How to change the VB knobs 2: How to change the longitudinal focal distance longitudinal focal distance
from the exit of the Linacfrom the exit of the Linac
• To pull the longitudinal focus closer to the exit of the Linac, we need to go further off-crest
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5Maximum bunching distance vs. phase
Linac phase
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tanc
e of
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imum
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g
a
Can we measure the Can we measure the emittance of this beam?emittance of this beam?
• Huge energy spread ( 70 degrees off crest) on the beam when the Linac is running at compressing phase. – Only slice emittance (where each slice has a small energy spread) will
be a meaningful measurement.• Measure the emittance around the bend, using the dipole as a
slicer.• Freeze the longitudinal and transversal dynamics at the time the
beam enters the dipole.– In transverse horizontal (x) phase space, the beam blows up because
of the dispersion of the dipole – In longitudinal phase space, R56 is negative and the compression
immediately stops– The transverse vertical (y) phase space will give us information about
the transverse dynamics of the beam before it entered the dipole. The idea is then just to pull the longitudinal focus at the beginning of the dipole by changing linac phase and do vertical quad scans.
• Explore emittance growth using Linac phase to vary position of longitudinal focus
Slicing the beam with the dipoleSlicing the beam with the dipole• Linac + dipole can be used for time resolved
measurements, like slice-emittance• To follow one slice we need to know where it ends up,
when we change the linac phase• Electron beam spectrum measured with the Faraday cup.
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Linac energyLinac phase
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Model: Egun+ Elinac * sin() Chi^2 = 0.08008R^2 = 0.99183 Elinac 7.89671 ±0.56147zerophase 74.33696 ±3.67494period 360 ±0Egun 3.78945 ±0.63804
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erg
y o
f ce
ntr
al s
lice
(M
eV
)
PWT Linac phase (degrees)
Maximum Faraday cup signal line
Thick lens treatmentThick lens treatment
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σ
σ
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cossin
sincoscossin2
sincos
qdq
qdqqdq
qdq
lKlK
lK
lKlKlKlKllKK
lKlKlKK
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σ
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cossin
sincoscossin2
sincos
qdq
qdqqdq
qdq
lKlK
lK
lKlKlKlKllKK
lKlKlKK
MMi
T
f
2221
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LKLKK
LKK
LK
f cossin
sin1
cos1
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In M we substitute the thin lens matrix with a thick lens transformation
Beam parameters are given by:
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Model: thicklens Chi^2 = 0.00007R^2 = 0.99303 P1 0.75404 ±0.03141P2 4.11037 ±0.19404P3 23.65671 ±1.01927
sig
ma2 (
mm2 )
sqrt(K) (m-1)
Measurement on screen 6 Thick lens fit
B
BK
And we obtain the fitting function
Emittance measurementEmittance measurement
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rtic
al e
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Quad scan results
SimulationsSimulations
• Parmela simulations from the cathode.• Mechanism for emittance growth yet to understand.
(Parmela, TREDI)
ConclusionsConclusions
• Velocity bunching is an alternative to magnetic compression
• Neptune experiment as a thin lens “ballistic” bunching experiment: – Compression ratio improved by sampling the linear part of RF
fields– PTW Linac + dipole to study slice emittance– Transverse dynamics: still a lot to understand !!!
• “Slow wave” scheme for rectilinear compression has a more adiabatic, less violent longitudinal dynamics and may compensate better for emittance growth: Pleiades, SPARC.
• How tricky is emittance growth in practice?• How this system integrates into application?
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