Dielectric accelerator based 5th generation light source Alexander Zholents, ANL Physics and Applications of High Brightness Beams March 28-April 1, 2016, Havana, Cuba

Single stage acceleration in a hollow dielectric channel* or corrugated wall waveguide 7 ± 0.025 ps 2b 2a Cu Drive bunch Main bunch Dielectric 2a p t d Drive bunch Main bunch Drive and Main from the same source bunch minimal timing jitter Low cost device (likely) Potential for: high field gradients high wall plug power efficiency high bunch repetition rate Quartz tube with 0.2 mm wall Main DWA accelerator components Copper tube Electric field map (300-500 GHz) Cu Dielectric group velocity = (0.7-0.9)c *) G. A. Voss and T. Weiland, DESY M-82-10, 1982; K. L. F. Bane, P. Chen, P. B. Wilson, SLAC-PUB-3662,1985; W. Gai et al. Phys. Rev. Lett. 61, 2756,1988.

A concept of a high repetition rate multi-user FEL facility based on Collinear Wakefield Accelerator Injector Drive accel. Main accel. Undulator x-ray optics ~25 m 20 Compact Inexpensive Flexible

A concept of a high repetition rate multi-user FEL facility based on Collinear Wakefield Accelerator Drive accel. Undulator x-ray optics DWA ~25 m 20 Low energy spreader Up to 100 MV/m High energy efficient Tunable energy ~ a few GeV Tunable Ipk> 1 kA Rep. rate up to 50 kHz/FEL

Beam by design: beam shaper and why we need it Road map to a high energy efficient accelerator: transformer ratio1,2 (Maximum field behind the drive bunch) (Maximum field inside the drive bunch) R = E + E - = Other distributions possible, see also [2] r(z) E - E+ Double triangle distribution R=5 z (mm) R=3 Ez [MV/m/nC] -5 -10 -15 15 10 5 r(z) z (cm) E - E+ Double horn distribution R< 2 E - E+ Wakefield (MV/m) r(z) Gaussian bunch z (mm) 10 -10 5 -5 Goal is to extract maximum energy from drive bunch, up to 80%, and obtain highest energy for the witness bunch DEwitness=0.8R Edrive Bane et. al., IEEE Trans. Nucl. Sci. NS-32, 3524 (1985). F. Lemery, P. Piot, Phys. Rev. Spec. Topics – Acc. and Beams, 18, 081301 (2015).

Mask + Emittance Exchange Beamline Drive and Main Beam Generation Mask + Emittance Exchange Beamline Recent Demonstration at the AWA e- beam profile after mask e- beam profile before mask Talk by Gwanghui Ha Emittance Exchange Beamline Mask Witness Drive Shaping the drive bunch is critical to achieve high efficiency of energy transfer from the drive bunch to the wakefields and eventually to the witness bunch. Only bunches with a very specific shape (double-triangular) exhibit a quasi uniform energy loss for all electrons inside the bunch. This helps obtaining a high transformer ratio of the accelerating field in the wake to the decelerating field within the drive bunch. ~25 kW is deposited on mask Drive Bunch: Double Triangle for High Transformer Ratio Main Beam: produced from same mask to eliminate jitter

Drive bunch shaping using photocathode laser (2) *) Linear ramp with parabolic head r(z) 2 ~ r(z) E+ ~ 1/√a R ~ √a 2a DEwitness=0.8R Edrive High accelerator efficiency favors larger radius *) F. Lemery, P. Piot, Phys. Rev. Spec. Topics – Acc. and Beams, 18, 081301 (2015)

Drive Bunch Beam Breakup Instability (BBU) Examples of longitudinal and transverse wakefields Wz (MV/m), Wx (kV/m/mm) Cumulative collective instability arises from continuous exposure of tail electrons to transverse wake field* tail head Snapshots of a single bunch traversing a SLAC structure *) A.Chao, “Physics of collective beam instabilities in high energy accelerators”, New York: Wiley.

Balakin-Novokhatsky-Smirnov (BNS) damping of BBU Use FODO channel Produce “chirp” in the betatron tune along the electron bunch using the energy “chirp”, and Force tail to oscillate faster than head, thus averaging the impact of transverse wake fields. Illustration for Dielectric Wakefield Accelerator (DWA) Transverse oscillation of particles of a chirped beam p = gb≈ E / mc2 main bunch head After 4 m of propagation tail drive bunch After 8 m Initial energy chirp ~15 % (peak-to-peak) Particles of different energies have different oscillation periods in the FODO lattice

FODO channel (quadrupole wiggler) to control BBU Wakefield accelerator NdFeB 81.3 Tapered quadrupole gradient Dielectric channel imbedded into quadrupole wiggler Wz ~ Q/a2 W ~ Q/a3 Quadrupole wiggler E0 Main E0 Drive dump 45 cm High gradient hybrid quad Bore radius = 1.5 mm. Peak gradient = 0.96 T/mm. Sub-micron precision in the magnetic center position. Length = 40 mm. Weight = 2.5 kg. Magnetic force between top and bottom parts = 30.5 kg.

Slippage effect main, 250 pC drive, 8 nC main, 250 pC Drive bunch and main bunch separation decreases because of their different energies (ps)

Problem mitigation s main Move main bunch to second maximum (can be difficult if done using the mask) Make adaptive frequency channel and always keep main bunch at or near to the maximum (easy) Use drive bunch with higher energy (affects facility cost and energy efficiency) w1 w2 w3 < w2 < w3

Tracking results using 8nC drive and 250pC main bunch After 2 m Ein=400 MeV After 20 m Eout=2.0 GeV 1100 main TM01 freq = 299.7 GHz main 3500 TM01 freq = 336 GHz wake current wake current p=gb p=gb z (mm) z (mm) 700 500 t t 500 50 x (mm) x (mm) -500 -50 t t

Main Bunch Shaping Reduce correlated energy spread in the main bunch Ez (MV/m) Gaussian main bunch 15 10 5 0 5 10 15 z (um) 110 100 90 80 70 Energy (MeV) sE=5.3% Reverse triangular main bunch* Ez (MV/m) 20 10 0 10 20 z (um) 110 100 90 80 70 Energy (MeV) sE=0.3% Self-wake from main bunch matches curvature of drive wake Can be further improved *) T. Katsouleas et al., Particle Accelerators, 1987, Vol. 22, pp. 81-99

Maximum energy gain is defined by the quad strength Ez scales ~ 1/a2 Obtained using a two-particle model, this envelope is defined by the maximum attainable quadrupole gradient at each bore radius *. Ez scales as ~ a1/2 on a boarder line of a stability region 100 nC Ez, (MV/m) 10 nC 1 nC stable region Wz ~ Q/a2 W ~ Q/a3 a, (mm) *) Gaussian peak current distribution of the drive bunch is assumed BBU control favors larger tube radius a *) C. Li, W. Gai, C. Jing, J. G. Power, C. X. Tang, A. Zholents, PRST-AB, 17, 091302 (2014)

Tolerances Misalignment of FODO quadrupoles < 1 mm Random misalignment rms errors 5 mm initial orbit offset No particle losses in the case of correlated variation with amplitude of 2 mm and period > 0.3 m. Random quad roll angles: < 10 mrad Straightness of the dielectric tube: better than 10 mm at 20 m Maximum amplitude is 10 mm and the period is varied

Measurement of quad’s magnetic center with a sub-mm precision* quad, 0.6 T/mm moving wire meas fit r.m.s. error = 0.792 mm 0.75mm a = -1.8857±0.792 mm Scan number Transverse field distribution in 0.75-mm aperture quad Pulsed wire technique will be used for a quadrupole wiggler *) I. Vasserman, J. Xu, APS MD-TN-2016-003

Adjustment of quad’s magnetic center with a sub-mm precision* ±50 mm adjustments of the magnetic poles 1 and 3 shift magnetic center by ±24 mm in x and ±6 mm in y Impact of shifting: Calculate the magnitude of shifting: *) N. Strelnikov, E. Trakhtenberg, I. Vasserman and A. Zholents, APS MD-TN-2016-001

Vacuum chamber RF main mode and HOM coupler every 0.5 m RF coupler removes ~ 2 kW RF coupler BPM Beam power in: 166 kW Beam power out: (32 kW drive +30 kW witness) Vacuum chamber (rotated by 90º ) RF coupler field transmission efficiency DT= 25 °C above cooling water temperature at 20 W/cm power dissipation

Dielectric Charging by Loss Electrons Provide surface conductivity TiN ultrathin film (<10 nm) coating Coating Brazing Drain charge from the bulk Material by design: attain properties of perfect dielectric at THz frequencies and DC conductivity Candidate material: zinc metaniobate (ZnNb2O6), works at (10-150) GHz R&D approach Continue characterization and improvement of ZnNb2O6 or its equivalent Explore CVD diamond with doping Explore conductive material-based structures

Vacuum chamber prototyping 10 cm Copper coated quartz tube Copper coated quartz tube Will compare vacuum in two units, one with and one without the tube

PLANS

The initial goal is to build a 0 The initial goal is to build a 0.5 m long accelerator unit and test it in LEUTL tunnel using APS injector linac 0.5 m Will test: alignment tolerance <0.75 mm rms quad-to-quad and <2 mm rms module-to-module

Linac Extension Unified Test Laboratory (LEUTL) at APS Deflecting cavity 400 MeV LEUTL tunnel Beam parameters in LEUTL tunnel Electron beam energy 300-450 …500 MeV Electron bunch charge 50 – 500 … 1000 pC Beam emittance 0.5 – 1.5 mm-mrad Beam energy spread 250 – 500 keV Bunch length 100 – 1000 fs Bunch rep. rate 6 – 30 Hz PCgun installed at the head of the Linac Laser (Nd:Glass) Wavelength 1053 nm Pulse energy 2 – 5 mJ Pulse length 1 – 4 ps

S-band LCLS-type photocathode gun a the head of the Linac APS 450 MeV injector linac S-band LCLS-type photocathode gun a the head of the Linac S-band deflecting cavity at a test stand during conditioning LEUTL tunnel is ~ 40 m long and is “ready” to accept the beam Go to "View | Header and Footer" to add your organization, sponsor, meeting name here; then, click "Apply to All"

Study cases Case I Case II Fundamental mode Freq. (GHz) 400 300 ID (mm), OD(mm), Length(cm) 1.5, 1.59, 50 2, 2.12, 50 Drive bunch charge (nC) 3.5 8 Double triangular bunch length (mm) 1 Drive/main bunch energy (MeV) Bunch rep. rate (kHz) 100 50 Peak Accelerating Field (MV/m) 42 90 Power dissipation without and with THz field coupler per unit length (W/cm) 19, 5.4 58.5, 18.5 Transformer ratio 5 Main bunch charge (pC), length (mm) 50, 5 300, 10 Total DWA length (m) ~40 ~25 Drive beam use, dump energy (MeV) 80%, ~ 70 80%, 80 Drive beam to main beam efficiency (%) 8.6 15 Main beam energy gain (GeV) 1.5 1.6

FEL simulations (illustration) Undulator period, cm 1.8 parameter, K 1.0 Energy, GeV 1.88 Charge, pC 250 Current, kA 3 Emitt, mm 1 RMS energy spread, % 0.3 Pierce parameter, 0.01 X-ray wave- length, nm 1 Peak power, GW 5 Bandwidth, % 3.8

Summary Multiple stages possible in the next phase Single stage high repetition-rate, soft X-ray FEL user facility 10 CWAs linacs driven by a single 400 MeV SRF linac 10 FEL lines @ 50 kHz bunch repetition rate Compact, inexpensive, and flexible Progress Drive bunch shaping (triangular + quadratic component) Control of beam breakup instability Quadrupole wiggler, adaptive frequency channel Small “main bunch” energy spread Prototype quadrupole design Prototype vacuum chamber design Future development alternative beam shaping techniques– critical space charge effects beam test– critical break sections: BPMs, rf couplers, correctors, etc.- critical Multiple stages possible in the next phase

Acknowledgement S. Doran, W. Gai, J. Power Argonne National Laboratory, HEP R. Lindberg, N. Strelnikov, Y. Sun, E. Trakhtenberg, A. Zholents Argonne National Laboratory, APS S.S. Baturin, C. Jing, A. Kanareykin Euclid Techlabs D. Shchegolkov, E. Simakov Los Alamos National Laboratory P. Piot Northern Illinois University/Fermilab Q. Gao, C. Li Tsinghua University G. Ha Pohang University of Science and Technology

Thank you for your attention

BACKUP

34 m Illustration using 3.5 nC drive and 50 pC main bunches g(t) x(t) p = gb≈ E / mc2 3.5 ps Ip(t) 34 m the drive beam tail decelerates more, develops more lagging, and sees the wake’s accelerating field

Customizing the drive bunch current to reduce the required initial energy chirp* Classical double-triangle bunch current distribution Uniform wake field Wz ωt ωt New bunch current distribution Double-triangle with a parabolic content Linearly growing wake field stimulates energy chirp Wz Reduce initial energy chirp: 15%  9.5% ωt ωt *) D.Y. Shchegolkov, E.I. Simakov, A.A. Zholents, IEEE Trans. Nucl. Sci., vol. 62 (6), pp. 1–8, Dec. 2015.

Planar geometry waveguide vacuum dielectric metal W X Y b a (a=1.5 mm, b = 1.584 mm, W=8 mm, =5) selected to obtain the same Wz as in the circular geometry Drive dipole quadrupole Trailing particles Z Y Even on-axis drive bunch will excite the quadrupole wake Can quadrupole wake produce BNS-like damping?

Planar geometry waveguide vacuum dielectric metal W X Y b a (a=1.5 mm, b = 1.584 mm, W=8 mm, =5) selected to obtain the same Wz as in the circular geometry Drive dipole quadrupole Trailing particles Z Y Even on-axis drive bunch will excite the quadrupole wake Can quadrupole wake produce BNS-like damping?

Is there confinement by quadrupole wake? Quadrupole wakefield and quad focusing are in “phase” sx (m) s (m) Quadrupole wakefield and quad focusing are in opposite “phase” Not much. Although slightly weaker quads can be used to control BBU. sx (m) s (m)

Drive bunch shaping using photocathode laser *) Laser pulse intensity measured parabolic fit space charge I (A) I (A) 0.8 nC head tail head tail Next to the cathode At the end of the injector at 100 MeV Without ramped peak current With ramped peak current … was proposed to remove significant quadratic energy chirp at the end of the FERMI FEL linac *) Cornacchia, Di Mitri, Penco, Zholents, Phys. Rev. ST-AB, 9, 120701(2006); Penco, Danailov, Demidovich, Allaria, et al.,Phys. Rev. Lett, 112, 044801 (2014).

LINAC improvements New Photo-Cathode electron gun Linac structure straightening (ongoing) New slice emittance and slice energy spread diagnostics (ongoing)