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John Power, Argonne

Multi-user, High Repetition-Rate, Soft X-ray FEL User Facility (based on a Collinear Dielectric Wakefield Accelerator). Euclid Techlabs LLC: C.Jing , A.Kanareykin, P.Schoessow

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John Power, Argonne

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  1. Multi-user, High Repetition-Rate,Soft X-ray FEL User Facility(based on a Collinear Dielectric Wakefield Accelerator) Euclid Techlabs LLC: C.Jing, A.Kanareykin, P.Schoessow Argonne National Laboratory, HEP: W.Gai, G.Ha, C.Li, J.G.Power Argonne National Laboratory, APS: R.Lindberg, A.Zholents Northern Illinois University: P.Piot John Power, Argonne Assessment of Opportunities High Brightness Beams Workshop, San Juan, Puerto Rico, March 25, 2013

  2. Multi-user, High Rep Rate, Soft X-ray FEL User Facility Capable of serving ~2000 scientists/year • Flexible x-ray beamlines • Tunable pulse length • Seeded • 2 color seeded • SASE Lasers linked with a fiber-optics time distribution network • Low-emittanceinjector: • 1 MHz bunch rep. rate 50 MeV 2 GeV 1 MHz ACCELERATOR (BLACK BOX) experimental end stations • Beam spreader • 100 kHz bunch rep. rate

  3. Multi-user soft x-ray FEL facility based on: SRF linac Capable of serving ~2000 scientists/year • Flexible x-ray beamlines • Tunable pulse length • Seeded • 2 color seeded • SASE Lasers linked with a fiber-optics time distribution network • Low-emittanceinjector: • 1 MHz bunch rep. rate 750m 50 MeV 2 GeV experimental end stations ~ 300 m ~ 50 m ~ 250 m ~100 m ~50 m • CW superconducting linac • ~1MHz bunch rep. rate • ~2 GeV beam energy • ~1 kA peak current • Beam spreader • 100 kHz bunch rep. rate

  4. Multi-user soft x-ray FEL facility based on: DWFA linac Dielectric Wakefield Acceleration (DWFA) linac 750m 350m 200 MeV 2 GeV ~50 m experimental end stations ~50 m • Facility Footprint • 350m x 250m ~50 m ~100 m ~30 m ~25 m ~50 m • Compact DWFA linac • ~1MHz bunch rep. rate • ~2 GeV beam energy • ~1 kA peak current • Compact • Beam • Spreader • Beam • Shaper

  5. Ultra-flexible facility Dielectric Wakefield Acceleration (DWFA) linac • Flexible accelerator beamlines • Flexible x-ray beamlines 1.2 GeV 100 pC 0.5 keV X-rays 2.4 GeV 50 pC End Stations 1 keV X-rays Configurable DWFA Accelerator Configurable FEL Array … … … …

  6. Motivation for DWFA for the High Rep Facility • Compact • Low energy spreader • Accelerating gradient > 100 MV/m • Room temperature quartz fibers • Tunable electron beam energy of a few GeV • Tunable peak current > 1KA • Bunch rep. rate of the order of 1MHz • Inexpensive • Flexible Is it possible to replace some of the SRF linac with a DWFA linac?? • Many hurdles to overcome as you will see…

  7. FUNDAMENTALS: CollinearDielectric Wakefield Acceleration

  8. Cylindrical Dielectric Wakefield Accelerator • Simple geometry • Capable of high gradients • Easy dipole mode damping • Tunable • Inexpensive • Recent results (obtained for Linear Collider development): • 1000MV/m level in the THz domain (UCLA/SLAC group) • 100 MV/m level in the MHz domain (AWA/ANL group)

  9. Wake field in dielectric tube induced by a short Gaussian beam e Q 2a 2b Cu a=240 um; Q=1 nC; bunch length=0.5 ps (FWHM), f=650 GHz

  10. The Wakefield Theorem and the Transformer Ratio Collinear Dielectric Wakefield Acceleration DRIVEWITNESS Wakefield (MV/m/nC) W+ The R< 2 limit has kept interest in collinear wakefield accelerators to a minimum. W- W+ (Maximum energy gain behind the drive bunch) R = = < 2 W- (Maximum energy loss inside the drive bunch)

  11. + r (z) W - W z d d d Road map to a high energy gain acceleration Methods to increase R>2 in a collinear wakefield accelerator Ramped Bunch Reference: Bane et. al., IEEE Trans. Nucl. Sci. NS-32, 3524 (1985) Ramped Bunch Train (demonstrated at ANL) Reference: Schutt et. al., Nor Ambred, Armenia, (1989)

  12. EXAMPLE: A case study of an x-ray FEL user facility based on a 2.4 GeV DWFA 12

  13. Quartz DWFA High rep. rate, X-ray FEL user facilitybased on a 2.4 GeV DWFA ID=400 um freq = 850 GHz FEL10 FEL2 FEL1 TR = 16.5 ~30m P=320 kW, 1 MHz 1.6 nC

  14. Quartz DWFA ID=400 um Key technology:DWFA RF structure design

  15. How can a small DWFA can handle High Rep Rate???? --cooling-- Quartz DWFA ID=400 um RF packet ~333 ps e • Collinear DWFA • Ultra-short RF pulse (~333 ps) • Heating is much less severe than microwave accelerator • Average thermal heating • Average power load 50 W/cm2 @100 kHz rep rate • RF pulsed heating • DT ~ 20 ºC

  16. Key technology: drive bunch shaping enhances transformer ratio Triangular bunch TR~10 Double triangular bunch TR~17

  17. Key technology: witness bunch generation 10 MeV in 10 cm

  18. TM110 TM010 TM010 Deflecting cavity Double EEX technique: a convenient tool for drive and witness bunch shaping Emittance exchange Emittance exchange FODO -I -I -I -I T B B QD QD QF QF B B B B QF QF QD QD QD QD QF QF B B QD QF QD QF x →z emit. exch. z →x emit. exch. mask After mask At EEX exit Before mask 1200 1000 800 600 400 200 0 (c) witness current (A) 2 1 0 -1 -2 time (ps) Drive and Witness from the same source bunch minimal timing jitter

  19. Key technology: How to handle beam loading: • Eacc=115 MV/m • Gaussian Electron bunch • Large energy spread • Strongly chirped in energy Accelerated current Wakefield • ~DE=30 MV/m

  20. Key Technology: Undulator Strongly chirped beams for FEL applications Longitudinal Gradient Tapering the undulator strength or period can counteract large energy chirp and maintain gain Transverse Gradient Varying the undulator strength transversely can counteract large energy chirp and maintain gain N Smaller undulator strength K Larger undulator strength K S BAD: Accelerated beam is strongly chirped (little FEL gain) BAD: Using the chirp to compress the beam does not seem to be useful for radiation GOOD: For short beams (<10 mm rms) the energy chirp is approximately linear in time

  21. Strongly chirped beams for FEL applications: preliminary results witness beam chirp Example: Longitudinal Gradient Tapering the undulator strength K Power evolution of DWFA beam + undulator taper Power profile near saturation z/LG = 20 Chirped SASE spectrum near saturation z/LG = 20 Nonlinear regime Linear gain Some applications favor wide bandwidth 21

  22. Can we reduce energy spread due to beam loading? Gaussian witness bunch • Gaussian • bunch 110 100 90 80 70 • Q=50 pC • Edec=13.6 MV/m • Eacc=81.7 MV/m • sigmaE=5.3% • R=6 Energy (MeV) 15 10 5 0 5 10 15 z (um)

  23. Key idea: Match the curvature of the self-wake to the drive wake • Reverse triangular witness bunch Drive-wake Reverse triangular bunch 110 100 90 80 70 • Q=50 pC • Edec=6.3 MV/m • Eacc=86.3 MV/m Energy (MeV) 20 10 0 10 20 z (um) Witness self-wake • d=0.3% • R=14 ~20x reduction in energy spread

  24. Minimization of the energy spread in a witness bunch Courtesy of E. Simakov, LANL By additionally customizing the shape of the main bunch we designed the configuration which minimizes the wakefield-induced energy spread in the main bunch. The energy spread may be made as low as 0.001%.

  25. General (nonlinear) shapes are possible • Multi-leaf collimator: • Used in medical linacs to shape the x-rays • Each vertical leaf moves independently Multi-leaf collimator Varian's 120-leaf multileaf collimator leaf Varian's high-definition multileaf collimator

  26. Feedback on desired witness and drive shape -I -I Emittance exchange Measured Spectrum B Multi-leaf mask QD QF B B QF QD QD QF 110 100 90 80 70 B QD QF Energy (MeV) http://varian.mediaroom.com/index.php?s=31899&mode=gallery&cat=2473 20 10 0 10 20 z (um) FEEDBACK

  27. BEGINNING EXPERIMENTAL STUDIES 1: Demonstrate EEX based bunch shaping at the Argonne Wakefield Accelerator 27

  28. Demonstrate bunch shaping using a double-dog leg EEX beamline at the AWA Facility The Argonne Wakefield Accelerator Facility  Low Energy (14 MeV) beamline TDC B2 B1 RF Photocathode Gun 8 MeV 14 MeV 20 deg Quads Linac B4 B3 B2 B1 Mask Initial experimental goals: • Demonstrate bunch shaping and compare measured shape to 1st order theory • Measure EEX transfer matrix • Study 2nd order effects in beamline • Study space charge effects in beamline

  29. Demonstrate bunch shaping using a double-dog leg EEX beamline at the AWA Facility The Argonne Wakefield Accelerator Facility  Low Energy (14 MeV) beamline TDC B2 B1 RF Photocathode Gun 8 MeV 14 MeV 20 deg Quads Linac B4 B3 B2 B1 chirp multiple masks on motorized actuator x’ slope x, y beam size Key tunable parameters

  30. Demonstrate bunch shaping using a double-dog leg EEX beamline Example: Experiment I - Shaping capability Multiple masks will be used to study the bunch shaping capability of the double dog-leg EEX beamline

  31. BEGINNING EXPERIMENTAL STUDIES 2: Propagation of drive beam through a 10 meter DWFA linac at APS

  32. Drive bunch through a ID=400 mm fiber !!! ID=400 um • Drive bunch: • Charge = 1.6 nC • Normalized emittance = 2 mm • Beam energy = 50 MeV (close to the accelerator end) • Beam size = 50 mm (Beta function ≈ 10 cm) • Goal: Propagate drive bunch through meter scale DWFA • With no focusing • Beam size will triple in one meter! • External focusing channel around dielectric • ~10-20 cm focal length • Control SBBU with BNS damping

  33. 10 m long structure test in APS LEUTL tunnel • APS will install LCLS type e-gun in 2013 • 0.5 nC, 500 fs, 1 mm bunches • Beam into the LEUTL tunnel in 2014 • Propagate beam through 10 m long DWFA at APS • Single Bunch Beam Break Up (SBBU) • Vacuum pumping • Cooling design • etc. Some equipment exists, new equipment and diagnostics will be needed LEUTL tunnel is ~ 40 m long and is ready to accept the beam

  34. The concept: High Repetition-Rate, Soft X-ray FEL User Facility 10 DWFAs linacs driven by a single SRF linac 10 FEL lines @ 100 kHz rep. rate. Compact, Inexpensive, and Flexible A working group has started feasibility studies Parameter studies of the overall concept Bunch shaping studies at the AWA facility Beam propagation through a 10m DWFA linac at APS Modeling of the large energy spread in the FEL Many more: Drive and witness jitter Dielectric breakdown limitation testing Etc. We welcome collaborators and new ideas! Summary

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