Generation of Tunable Microbunch TrainW. D. KimuraATF Users MeetingApril 4-6, 2007

Brookhaven National Laboratory (Accelerator Test Facility)-Marcus Babzien -Karl Kusche-Jangho Park -Igor Pavlishin -Igor Pogorelsky -Daniil Stolyarov -Vitaly Yakimenko University of Southern California-Patric Muggli-Thomas Katsouleas-Efthymios (Themos) Kallos

Collaborators

Outline Motivation Description of Approach Review Proof-of-Principle (POP) Experiment Description of Proposed Experimental Apparatus Phase I Demonstrate Improved Wire-Mesh System Phase II Performed Advanced Multi-bunch PWFA Experiments Proposed Schedule and Runtime Needs Conclusions

MotivationUltra-short (subps) microbunches are useful for different applications-Multibunch resonant plasma wakefield acceleration (multibunch PWFA)uses a train of microbunches-Particle Acceleration by Stimulated Emission of Radiation (PASER)also uses a train of microbunches-Microbunches can be used to generate ultrashort electromagneticradiationInverse free electron laser (IFEL) one possible method for generating ultra-short microbunches-STELLA experiment demonstrated utility of IFEL for making microbunches-ATF routinely makes ~1-mm long microbunches separated by 10.6 mmHowever, cannot easily change microbunch spacing using IFEL -Microbunch spacing dictated by laser wavelength-Also difficult to vary number of microbunches and to provide witnessbunch

Multibunch PWFA Uses Train of Microbunches1-D model simulation of wakefields from three microbunches[1]-Wakefield strength growslinearly with number of bunches-Resonant process thatrequires:[1] Courtesy E. Kallos, USCwhere lb = bunch separation, lp = plasma wavelength, ne = plasma density

Tunable Microbunch Train With Witness Bunch Would Benefit Multibunch PWFAPresent IFEL produces microbunch separation of 10.6 mm-Resonant plasma density is ~1019 cm-3-Achieving this high density in capillary discharge is difficultA resonant plasma density of 1017 - 1018 cm-3 would be better-Capillary discharges work well in this regime-Less problems with wakefield damping at lower densities-But, 1017 cm-3 density requires microbunch spacing of order 100 mm-No convenient 100-mm laser source for driving IFELPresent multibunch PWFA experiment also lacks true witness bunch to probe wakefields-Must rely on accelerating background electrons resulting in wide energyspread-Having true witness bunch will permit demonstrating monoenergeticacceleration

Passive, Simple Technique Developed for Generating Tunable Microbunch TrainBasic steps are:-Generate e-beam with correlated energy chirp-Send through quadrupoles and dipole to create spot along beamlinewhere transverse and longitudinal amplitudes are correlated-Place an array of evenly-spaced thin wires (wire-mesh) at spot(typical wire diameter 125 500 mm)-Electrons passing through wires create microbunches-Send microbunches through quadrupolesand dipole to transform sliced electronsinto train of microbunchesReverse transformation also demagnifies microbunch spacing relative to wire spacing-Demagnifications of 10:1 to 5:1 demonstrated

bx, by, and Dispersion Along BeamlineNote, chicane is not used in this scheme

Proof-of-Principle (POP) Experiment Performed Using Wire-MeshRaw video images of e-beam with approximately 1% energy chirpCoherent transition radiation (CTR) interferometer measurements confirm microbunch spacing

Varying High-Energy-Slit Opening Varies Number of Microbunches Narrow slit opening

Highly Precise Technique Can Detect Flaw in Wire SpacingCan detect extra wide space between microbunches caused by two wires touching each other

Capabilities of Wire-Mesh TechniqueDepending on wire spacing, can transmit ~50% of beam charge -Still adequate for many applications including multibunch PWFA-Does require low emittance beam for clean slicingDiameter of wires affects microbunch length -Shorter bunch requires thicker wire, which reduces transmitted chargeSpacing between wires affects microbunch spacing -Can rotate wire-mesh with respect to e-beam to change spacing-Demagnification ratio affected by amount of chirp and dispersion, andangle that beam strikes meshCan create witness bunch by blocking part of the beam except for one slit opening for the witness electrons-Can adjust width of slit opening to vary witness bunch length-Making bunch length less than bunch spacing enables monoenergeticacceleration

Proposed Program Divided Into Two PhasesPhase I: -Design, build, and test at STI improved wire-mesh device suitable forproducing tunable microbunch train and witness bunch-Specifically designed to permit easy adjustments to wire-meshcharacteristics-Install and test wire-mesh at ATF with goal to develop beamtune parameters needed for specific microbunch characteristicsPhase II: -Use improved wire-mesh device to perform advanced multibunch PWFA experiments-Operate at lower plasma densities and use true witness bunch-Experiments would be done in collaboration with USC (Dr. PatricMuggli, Dr. Thomas Katsouleas, and Efthymios Kallos)

Possible Design for Wire-Mesh TargetConcept strategy is to make multiple wire-mesh cartridges with different wire diameters and spacings-Use tungsten wire [13 mm (0.0005) diameter and larger available]

Cartridge Holder Would be Designed to Permit Precision Rotation of CartridgesUse encoded stepper motor to rotate targets

Can Create Witness Bunch by Placing Mask Over Section of Wire-MeshUnblocked wires create microbunch trainCan place witness bunch at any phase relative to microbunchesFor multibunch PWFA, witness bunch needs to be at (n + 1/2)lp, n = 0, 1, 2, after trainMaximum accelerationwould occur when n = 0

Summary of Major Phase I TasksBuild and test improved wire-mesh at STI -Make series of different targets, i.e., with different wire diametersand spacing -Confirm accuracy of angular control and repeatabilityInstall and test wire-mesh at ATF -Use spectrometer to measure energy spectrum-Use CTR interferometer to measure microbunch length and spacing-Use CTR and optical spectrometer to confirm microbunch spacingDetermine limits of technique -For example, maximum beam charge may be limited by degradation of emittance-ATF can deliver 500 700 pC with 1 2 mm emittance

Model Prediction(1) for Multibunch PWFA Using Wire-MeshAssume 6 microbunches, 30 mm long, separated by 50 mm, corresponding to resonant plasma of 4 1017 cm-3[1] Courtesy E. Kallos, USC

Model Prediction(1) for Long Witness BunchAssume witness bunch has same length as drive bunches (i.e., 30 mm long) and is at optimum phase for maximum acceleration[1] Courtesy E. Kallos, USC

Model Prediction(1) for Short Witness BunchAssume witness bunch length is 1/3 drive bunches (i.e., 10 mm long)and is at optimum phase for maximum acceleration[1] Courtesy E. Kallos, USC

Summary of Major Phase II TasksConfirm wakefield grows proportional to number of microbunches -Measure energy gain versus number of microbunches -Never been verified experimentallyVary length of witness bunch to sample narrow portion of phase-Demonstrate narrow energy spread-Vary position in phase to sample different parts of wakefieldInvestigate coherence of wake after bunch train -Position witness bunch multiple buckets away from bunch train, i.e., n > 0 in (n + 1/2)lpPerform extensive study of multibunch PWFA process using true witness bunch Investigate scaling to longer capillary lengths and optimizing for maximum energy gain with narrow energy spread

Proposed Program Schedule and Runtime NeedsProposing 3-year schedule (1 year longer than schedule submitted earlier to ATF Program Advisory Committee)Estimate for runtime requirements-Phase I: 4 weeks-Phase II: 6 weeks

Role of CollaboratorsATF staff responsible for -Generating e-beam tune -Operation of CTR interferometer-Operation of CTR optical spectrometerUSC responsible for-Joint operation of multibunch PWFA experiments-Modeling of multibunch PWFA

ConclusionsA simple, passive technique has been demonstrated for generating a tunable microbunch train with the option of adding a witness bunch-POP experiment at ATF proved concept-This proposed program turns the concept into a workhorse deviceMultibunch PWFA is a promising advanced acceleration technique made even more attractive by the simple wire-mesh technique for generating microbunches-This proposed program provides the means for thoroughly studyingthis process