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Generation of Tunable Microbunch Train

Generation of Tunable Microbunch Train. W. D. Kimura. ATF Users Meeting April 4-6, 2007. Collaborators. Brookhaven National Laboratory (Accelerator Test Facility) - Marcus Babzien - Karl Kusche - Jangho Park - Igor Pavlishin - Igor Pogorelsky - Daniil Stolyarov

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Generation of Tunable Microbunch Train

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  1. Generation of Tunable Microbunch Train W. D. Kimura ATF Users Meeting April 4-6, 2007

  2. Collaborators • 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

  3. 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

  4. 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 electromagnetic radiation • 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-mm long microbunches separated by 10.6 mm • However, cannot easily change microbunch spacing using IFEL - Microbunch spacing dictated by laser wavelength - Also difficult to vary number of microbunches and to provide witness bunch

  5. Multibunch PWFA Uses Train of Microbunches • 1-D model simulation of wakefields from three microbunches[1] - Wakefield strength grows linearly with number of bunches - Resonant process that re quires: where lb = bunch separation, lp = plasma wavelength, ne = plasma density [1] Courtesy E. Kallos, USC

  6. Tunable Microbunch Train With Witness Bunch Would Benefit Multibunch PWFA • Present IFEL produces microbunch separation of 10.6 mm - 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-3density requires microbunch spacing of order 100 mm - No convenient 100-mm 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 energy spread - Having true witness bunch will permit demonstrating monoenergetic acceleration

  7. Wire-mesh 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 beamline where 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 quadrupoles and dipole to transform sliced electrons into train of microbunches • Reverse transformation also demagnifies microbunch spacing relative to wire spacing - Demagnifications of 10:1 to 5:1 demonstrated

  8. bx, by, and Dispersion Along Beamline Note, chicane is not used in this scheme

  9. 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

  10. Medium slit opening Wide slit opening Varying High-Energy-Slit Opening Varies Number of Microbunches Narrow slit opening

  11. Highly Precise Technique – Can Detect Flaw in Wire Spacing • Can detect extra wide space between microbunches caused by two wires touching each other

  12. 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, and angle 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 monoenergetic acceleration

  13. Proposed Program Divided Into Two Phases • Phase I: - Design, build, and test at STI improved wire-mesh device suitable for producing tunable microbunch train and witness bunch - Specifically designed to permit easy adjustments to wire-mesh characteristics - Install and test wire-mesh at ATF with goal to develop beam tune 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. Patric Muggli, Dr. Thomas Katsouleas, and Efthymios Kallos)

  14. 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 mm (0.0005”) diameter and larger available]

  15. Cartridge Holder Would be Designed to Permit Precision Rotation of Cartridges • Use encoded stepper motor to rotate targets

  16. 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)lp, n = 0, 1, 2…, after train  Maximum acceleration would occur when n = 0

  17. Summary of Major Phase I Tasks • Build and test improved wire-mesh at STI - Make series of different targets, i.e., with different wire diameters and 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 mm emittance

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

  19. Model Prediction(1) for “Long” Witness Bunch • Assume 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

  20. Model Prediction(1) for “Short” Witness Bunch • Assume 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

  21. Summary of Major Phase II Tasks • Perform extensive study of multibunch PWFA process using true witness bunch • 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)lp • Investigate scaling to longer capillary lengths and optimizing for maximum energy gain with narrow energy spread

  22. 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

  23. 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

  24. 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 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 studying this process

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