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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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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No compression

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Mild compression

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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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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

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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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Linac Energy gain (MeV)

Theory Measurement obtained mixing

RF Low Level with Linac Loop

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linacgun

linac

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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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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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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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PWT Linac phase (degrees)

Maximum Faraday cup signal line

Thick lens treatmentThick lens treatment

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cossin

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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

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Measurement on screen 6 Thick lens fit

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BK

And we obtain the fitting function

Emittance measurementEmittance measurement

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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?