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

2  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

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-  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 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 requires: [1] Courtesy E. Kallos, USC where b = bunch separation, p = plasma wavelength, n e = plasma density

6 Tunable Microbunch Train With Witness Bunch Would Benefit Multibunch PWFA  Present IFEL produces microbunch separation of 10.6  m -Resonant plasma density is ~10 19 cm -3 -Achieving this high density in capillary discharge is difficult  A resonant plasma density of cm -3 would be better -Capillary discharges work well in this regime -Less problems with wakefield damping at lower densities -But, 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 energy spread -Having true witness bunch will permit demonstrating monoenergetic acceleration

7 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  m) -Electrons passing through wires create microbunches -Send microbunches through quadrupoles and dipole to transform sliced electrons into train of microbunches Wire-mesh  Reverse transformation also demagnifies microbunch spacing relative to wire spacing -Demagnifications of 10:1 to 5:1 demonstrated

8  x,  y, 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 Varying High-Energy-Slit Opening Varies Number of Microbunches Narrow slit openingMedium slit openingWide 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  m (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) p, 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  m emittance

18 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 × 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  m 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  m long) and is at optimum phase for maximum acceleration [1] Courtesy E. Kallos, USC

21 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

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 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  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