Velocity Bunching: experiment at Neptune Photoinjector P. Musumeci UCLA Dept. of Physics and Astronomy

Outline 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-pulse high 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: phase space bifurcation at Neptune UndercompressedFully compressed Longitudinal phase space bifurcations and distortions also seen in simulation

Damage from bends 2: coherent synchrotron radiation instability Strong energy modulations observed at 70 MeV in SDL experiments (W. Graves, Berlin CSR Workshop, 1/02). Potentially disastrous for LCLS and TESLA FEL Current (A) Time (ps) No compression Current (A) Time (ps) Mild compression Current (A) Time (ps) Strong compression

Damage from bends 3: Phase space distortions in high gradient UCLA/FNAL PWFA experiments 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: 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: “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: “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

PLEAIDES 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….)

A look to the transverse dynamics 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 ORION facility at SLAC S-band injector S-band PWT buncher X-band linacs (NLCTA) HOMDYN simulation of ORION system

ORION 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

Experiment 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 results Sampling the linear part of the RF-fields results in a very short beam (<0.4 ps)!!!

VB knobs 1: What happen if we change Linac accelerating gradient Cancellation between increasing energy spread and decreasing energy.

VB knobs 2: How to change the longitudinal focal distance from the exit of the Linac To pull the longitudinal focus closer to the exit of the Linac, we need to go further off-crest

Can we measure the 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, R 56 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 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.

Thick lens treatment In M we substitute the thin lens matrix with a thick lens transformation Beam parameters are given by: And we obtain the fitting function

Emittance measurement

Simulations Parmela simulations from the cathode. Mechanism for emittance growth yet to understand. (Parmela, TREDI)

Conclusions 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?