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Generation of Tunable Microbunch Train

W. D. Kimura

ATF Users MeetingApril 4-6, 2007

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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

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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

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Motivation Ultra-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 electromagneticradiation

Inverse 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-m long microbunches separated by 10.6 m

However, cannot easily change microbunch spacing using IFEL

- Microbunch spacing dictated by laser wavelength

- Also difficult to vary number of microbunches and to provide witnessbunch

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Multibunch PWFA Uses Train of Microbunches

1-D model simulation of wakefields from three microbunches[1]

- Wakefield strength growslinearly with number of bunches

- Resonant process thatrequires:

[1] Courtesy E. Kallos, USC

3-

10

cm

1034.3μmμme

pb n

where b = bunch separation, p = plasma wavelength, ne = plasma density

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Tunable Microbunch Train With Witness Bunch Would Benefit Multibunch PWFA

Present IFEL produces microbunch separation of 10.6 m

- Resonant plasma density is ~1019 cm-3

- Achieving this high density in capillary discharge is difficult

A 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 m

- No convenient 100-m laser source for driving IFEL

Present 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

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Passive, Simple Technique Developed for Generating Tunable Microbunch Train

Basic 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 m)

- Electrons passing through wires create microbunches

- Send microbunches through quadrupolesand dipole to transform sliced electronsinto train of microbunches

Wire-mesh

Reverse transformation also demagnifies microbunch spacing relative to wire spacing

- Demagnifications of 10:1 to 5:1 demonstrated

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x, y, and Dispersion Along Beamline

Note, chicane is not used in this scheme

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Proof-of-Principle (POP) Experiment Performed Using Wire-Mesh

Raw video images of e-beam with approximately 1% energy chirp

Coherent transition radiation (CTR) interferometer measurements confirm microbunch spacing

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Varying High-Energy-Slit Opening Varies Number of Microbunches

Narrow slit opening Medium slit opening Wide slit opening

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Highly Precise Technique – Can Detect Flaw in Wire Spacing

Can detect extra wide space between microbunches caused by two wires touching each other

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Capabilities of Wire-Mesh Technique Depending on wire spacing, can transmit ~50% of beam charge

- Still adequate for many applications including multibunch PWFA

- Does require low emittance beam for “clean” slicing

Diameter of wires affects microbunch length

- Shorter bunch requires thicker wire, which reduces transmitted charge

Spacing 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 mesh

Can 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

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Proposed Program Divided Into Two Phases

Phase 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 characteristics

Phase 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)

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Possible Design for Wire-Mesh Target

Concept strategy is to make multiple wire-mesh cartridges with different wire diameters and spacings

- Use tungsten wire [13 m (0.0005”) diameter and larger available]

Wire

Slotted guide Magnified cross-sectional view

of wire and slotted guide

Wire-mesh holder

Face-onView

TopView

Central axis

Pin for rotationbar

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e-Beam

Vacuumcross

Remote-controlled

insertion and rotation

mechanism

Cartridge Holder Would be Designed to Permit Precision Rotation of Cartridges

Use encoded stepper motor to rotate targets

e-beam

Main support arm

Rotation arm

0-degreeincidence

angle

40-degreeincidence

angle

Main support arm

Rotation arm

e-beam

Wire-meshcartridge

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Wire

Wire-mesh holder

Central axis

Mask

Slit

Can Create Witness Bunch by Placing Mask Over Section of Wire-Mesh

Unblocked wires create microbunch train

Can place witness bunch at any phase relative to microbunches

For multibunch PWFA, witness bunch needs to be at (n + 1/2)p, n = 0, 1, 2…, after train

Maximum accelerationwould occur when n = 0

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Summary of Major Phase I Tasks

Build 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 repeatability

Install 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 spacing

Determine limits of technique

- For example, maximum beam charge may be limited by degradation of emittance

- ATF can deliver 500 – 700 pC with 1 – 2 m emittance

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Model Prediction(1) for Multibunch PWFA Using Wire-Mesh

Assume 6 microbunches, 30 m long, separated by 50 m, corresponding to resonant plasma of 4 × 1017 cm-3

[1] Courtesy E. Kallos, USC

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Model Prediction(1) for “Long” Witness Bunch Assume witness bunch has same length as drive bunches (i.e., 30 m

long) and is at optimum phase for maximum acceleration

[1] Courtesy E. Kallos, USC

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Model Prediction(1) for “Short” Witness Bunch Assume witness bunch length is 1/3 drive bunches (i.e., 10 m long)

and is at optimum phase for maximum acceleration

[1] Courtesy E. Kallos, USC

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Summary of Major Phase II Tasks

Confirm wakefield grows proportional to number of microbunches

- Measure energy gain versus number of microbunches

- Never been verified experimentally

Vary length of witness bunch to sample narrow portion of phase

- Demonstrate narrow energy spread

- Vary position in phase to sample different parts of wakefield

Investigate coherence of wake after bunch train

- Position witness bunch multiple buckets away from bunch train, i.e., n > 0 in (n + 1/2)p

Perform 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

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Proposed Program Schedule and Runtime Needs

Proposing 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

Year 1 Year 2 Year 3

Design improved wire-mesh

Fabricate improved wire-mesh

Testingat STI

Install at ATF

Perform Phase IExperiments

Renewalproposal due

Prepare papersfor publication

Perform Phase IIMultibunch PWFA Experiments

.

.

.

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Role of Collaborators

ATF staff responsible for

- Generating e-beam tune

- Operation of CTR interferometer

- Operation of CTR optical spectrometer

USC responsible for

- Joint operation of multibunch PWFA experiments

- Modeling of multibunch PWFA

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Conclusions

A 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 device

Multibunch 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

Other experiments and applications may benefit from the groundwork laid by this proposed program

- PASER

- As a diagnostic tool,[2] e.g., confirming plasma density

[2] Thanks to Tom Katsouleas and Todd Smith