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

Multi-user, High Rep Rate, Soft X-ray FEL User Facility 2 experimental end stations 2 GeV50 MeV Low-emittance injector: 1 MHz bunch rep. rate 1 MHz bunch rep. rate Flexible x-ray beamlines Tunable pulse length Tunable pulse length Seeded Seeded 2 color seeded 2 color seeded SASE SASE Lasers linked with a fiber-optics time distribution network Beam spreader 100 kHz bunch rep. rate 100 kHz bunch rep. rate Capable of serving ~2000 scientists/year

Multi-user soft x-ray FEL facility based on: SRF linac 3 experimental end stations Capable of serving ~2000 scientists/year Low-emittance injector: 1 MHz bunch rep. rate 1 MHz bunch rep. rate Flexible x-ray beamlines Tunable pulse length Tunable pulse length Seeded Seeded 2 color seeded 2 color seeded SASE SASE Lasers linked with a fiber-optics time distribution network Beam spreader 100 kHz bunch rep. rate 100 kHz bunch rep. rate 2 GeV50 MeV ~50 m ~100 m ~ 300 m ~ 250 m ~ 50 m 750m CW superconducting linac ~1MHz bunch rep. rate ~2 GeV beam energy ~1 kA peak current

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

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

Motivation for DWFA for the High Rep Facility  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 6 Compact Inexpensive Flexible Many hurdles to overcome as you will see… Is it possible to replace some of the SRF linac with a DWFA linac??

Collinear Dielectric Wakefield Acceleration FUNDAMENTALS: 7

 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) Cylindrical Dielectric Wakefield Accelerator 8

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

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

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

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

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

ID, OD, Length 400  m,  m, 10 cm , tan  3.75, 0.6x10 -4 Freq. of TM 01, TM 02, TM GHz, 3092 GHz, 5749 GHz Q of TM 01, TM 02, TM , 3173,4401 r/Q of TM 01, TM 02, TM k  /m, 3.2 k  /m, 0.5 k  /m g of TM 01, TM 02, TM c, 0.794c, 0.813c Key technology: DWFA RF structure design 14 Quartz DWFA ID=400 um

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

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

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

Drive and Witness from the same source bunch  minimal timing jitter QFQD QF Emittance exchange T QD QF B QD QF B B B -I QDQF QD QF B QD QF B B B -I QDQF TM 110 TM 010 Deflecting cavity Emittance exchange FODO Double EEX technique: a convenient tool for drive and witness bunch shaping 18 Before mask After mask At EEX exit mask (c) time (ps) witness current (A) z →x emit. exch. x →z emit. exch.

Accelerated current Wakefield Key technology: How to handle beam loading: 19 E acc =115 MV/m  E=30 MV/m Gaussian Electron bunch Large energy spread Large energy spread Strongly chirped in energy Strongly chirped in energy

Key Technology: Undulator 20  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  m rms) the energy chirp is approximately linear in time 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 S Smaller undulator strength K Larger undulator strength K Strongly chirped beams for FEL applications

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

Can we reduce energy spread due to beam loading? 22 Gaussian witness bunch z (um) Energy (MeV) ∙Q=50 pC ∙Edec=13.6 MV/m ∙Eacc=81.7 MV/m ∙sigmaE=5.3% ∙R=6 Gaussian bunch

Key idea: Match the curvature of the self-wake to the drive wake Witness self-wake Drive-wake ~20x reduction in energy spread 23 Reverse triangular bunch ∙Q=50 pC ∙Edec=6.3 MV/m ∙Eacc=86.3 MV/m z (um) Energy (MeV) Reverse triangular witness bunch  =0.3% R=14

Beam pipe OD, 2b1.14 mm Dielectric tube OD, 2a1.24 mm Waveguide cutoff298 GHz Charge of the drive bunch5 nC Length of the drive bunch2.127 ps Charge of the witness bunch250 pC Length of the witness bunch75 fs Time between the bunches9.4 ps Transformer ratio3.16 ΔG/G1.5*10 -5 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%. Minimization of the energy spread in a witness bunch Courtesy of E. Simakov, LANL 24

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

Feedback on desired witness and drive shape 26 ndex.php?s=31899&mode=gal lery&cat=2473 QDQF QD QF B QD QF B B B -I QDQF Emittance exchange z (um) Energy (MeV) Multi-leaf mask Measured Spectrum FEEDBACK

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

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

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

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

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

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

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

Summary 34  The concept: High Repetition-Rate, Soft X-ray FEL User Facility –10 DWFAs linacs driven by a single SRF linac –10 FEL 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!