ASSESMENT OF OPPORTUNITY FOR A COLLINEAR WAKEFIELD ACCELERATOR FOR A MULTI BEAMLINE SOFT X-RAY FEL FACILITY W. Gai, C. Jing, A. Kanareikin, C. Li, R. Lindberg, J. Power, D. Shchegolkov, E. Simakov, Y. Sun, C.X. Tang, A. Zholents Many hurdles to overcome as you will see
Collinear acceleration in a dielectric-lined or corrugated wall waveguide* 7 ± 0.025 ps E z ~ Q/a 2, f 0 ~ 300 GHz 2b 2a Main bunch Drive bunch Dielectric Cu 2a Main bunch t d p Drive bunch Drive and Main from the same source bunch minimal timing jitter Electric field map Cu Dielectric Phase velocity ~ c, group velocity = (0.7-0.9)c * W. Gai et al. Phys. Rev. Lett. 61, 2756,1988. Low cost device (likely) Potential for: high field gradients high wall plug power efficiency high bunch repetition rate
A concept of a multi-user FEL facility E-gun SRF: 2.5 GeV ~1 MHz ~300 m Spreader ~200 m Undulators ~50 m ~50 m experimental end stations Based on: High repetition rate SRF linac (NGLS-like) Collinear Wakefield Accelerator (CWA) Low E spreader Up to 100 MV/m CWA imbedded in quadrupole wiggler Tunable E ~ a few GeV Tunable I pk > 1KA Rep. rate ~50 khz/fel Compact Inexpensive Flexible 3
Beam shaper and why we need it Drive Bunch Shaping Increase Transformer Ratio (Double triangle peak current distribution) Reduce Beam Break Up (Parabolic current distribution) Main Bunch Shaping Reduce Energy Spread (Trapezoidal current distribution) Bunch Shaping Method AWA Bunch Shaping Experiment
Road map to a high energy gain acceleration: Transformer Ratio 1-4 Wakefield (MV/m) 10 5-5 -10 (Maximum field behind the drive bunch) R = E+ = E - (Maximum field inside the drive bunch) Gaussian bunch Double triangle bunch r(z) E - E + z (mm) R< 2 r(z) E - z (mm) E + R>2 Goal is to extract maximum energy from drive bunch, up to 80% 1) Bane et. al., IEEE Trans. Nucl. Sci. NS-32, 3524 (1985). 2) Schutt et. al., Nor Ambred, Armenia, (1989). 3) C. Jing, A. Kanareykin, J. Power, M. Conde, Z. Yusof, P. Shoessow, and W. Gai. Phys. Rev. Lett., v.98, pp. 144801-1,-4 2007. 4) C. Jing, J. G. Power, M. Conde, W. Liu, Z. Yusof, A. Kanareykin, and W. Gai. Phys. Rev ST- Accelerator Beams, v 14, pp. 021302-6, 2011. 5
Drive bunch shaping using emittance exchange EEX quads TDC dipole FMC dipole quads sextupoles dipole dipole mask ~ 1.5% of part. Transverse particle distribution after mask After EEX y x y (m) 45% transmission efficiency x (m) ~25 kw is deposited on mask at low energy 5 MeV, i.e. below threshold energy for isotope production M. Cornacchia and P. Emma, Phys. Rev. ST Accel. Beams 5, 084001 (2002) P. Emma, et al, Phys. Rev. ST Accel. Beams 9, 100702 (2006) D. Xiang and A. Chao, Phys. Rev. ST Accel. Beams 14, 114001 (2011) Y. E. Sun, et al, Phys. Rev. Lett. 105, 234801 (2010) B. Jiang, et al, Phys. Rev. Lett. 106, 114801 (2011) B. E. Carlsten, et al, Phys. Rev. ST Accel. Beams 14, 084403 (2011) P. Piot, et al, Phys. Rev. ST Accel. Beams 14, 022801 (2011) Shchegolkov and Simakov, Phys. Rev. ST Accel. Beams 17, 041301 y (m) current t (s) t (s)
Main Bunch Shaping E z (MV/m) Reduce correlated energy spread in the main bunch Gaussian main bunch Energy (MeV) 110 100 90 80 70 s E =5.3% 15 10 5 0 5 10 15 z (um) Reverse triangular main bunch* 110 E z (MV/m) Self-wake from main bunch matches curvature of drive wake Energy (MeV) 100 90 80 70 s E =0.3% 20 10 0 10 20 z (um) Can be further improved *) T. Katsouleas et al., Particle Accelerators, 1987, Vol. 22, pp. 81-99 7
Drive bunch shaping using self-wakefields* g(z) g(z) g(z) ~1% r(z) r(z) r(z) Bunch shaping with photocathode laser is also an option *) G. Andonian, Advanced Accelerator Workshop - AAC 2014, San Jose, (2014)
Self-wakefield shaping can be made more precise using Double EEX technique (a) B Emittance exchange QF QD At entrance -I -I B B QD QF TM 010 TM 110 TM 010 Deflecting cavity z x emit. exch. B QD QF QD QF FODO T mask QD QF B QF QD Emittance exchange -I -I B B QD QF Before mask After mask At exit (c) x z emit. exch. better transmission efficiency witness B QD QF (d) 2 1 0-1 -2 time (ps) 1200 1000 800 600 400 200 0 current (A) 9
AWA experiment is focused on bunch shaping demonstration Beamline installation: Fall 2014 First beam: Winter 2015 The Argonne Wakefield Accelerator Facility (AWA) 14 MeV beamline RF Photocathode Gun 8 MeV chirp 14 MeV Dog-leg EEX at the AWA Facility Linac Quads 20 Multiple masks on motorized actuator will be used to study the bunch shaping capability of the dog-leg type EEX beamline B1 B1 B2 B2 TDC B3 B4 deg 10
Drive Bunch Beam Break Up Instability Examples of longitudinal and transverse wakefield functions W z (MV/m), W x (kv/m/mm) W z ~ Q/a 2 W ~ Q/a 3 Cumulative collective instability arises from continuous exposure of tail electrons to transverse wake field* *) A.Chao, Physics of collective beam instabilities in high energy accelerators, New York: Wiley.
Balakin-Novokhatsky-Smirnov (BNS) damping of BBU 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. After 4 m of DWA p = gb E / mc 2 main Transverse oscillation of particles of a chirped beam Initial energy chirp ~15 % (peakto-peak) Particles of different energies have different oscillation periods in the FODO lattice
Customizing the drive bunch current to reduce the required initial energy chirp Classical doubletriangle bunch current distribution Uniform wake field W z ωt New bunch current distribution ωt Double-triangle with a parabolic content Linearly growing wake field stimulates energy chirp W z Reduce initial energy chirp: 15% 9.5% ωt ωt
Estimates using two particle BBU model* W z ~ Q/a 2 W ~ Q/a 3 Main Drive E 0 Tapered quadrupole gradient Wakefield accelerator High gradient permanent magnet quad Quadrupole wiggler E 0 dump Copper Dielectric tube with copper cladding imbedded into quadrupole wiggler Soft iron NdFeB 8 cm Cooling channel *) C Li et al., to be published Dielectric or corrugation
Maximum attainable energy gain* Field scales ~ 1/a 2 when BBU is ignored Field scales as ~ a 1/2 (BBU). E z, (mm) 1 nc 10 nc 100 nc stable region a, (mm) *) Gaussian peak current distribution of the drive bunch is assumed Quadrupole gradient, (T/mm) This envelope is defined by the attainable quadrupole gradient at a given bore radius. Permanent magnet quad** a, (mm) **) Abliz, Vasserman, Zholents, to be published
Slippage effect main Drive-to-main bunch separation decreases because of their different energies drive s
p = gb E / mc 2 g(t) 3.5 ps Illustration main x(t) Note an off-center shift I p (t) 0 m Space charge effects are not included
17 m
34 m the drive beam tail decelerates, gets a lag, and sees the wake s accelerating field
Problem mitigation Move main bunch to second maximum (can be difficult if done using the mask) s main 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) w 1 w 2 w 3 w 1 < w 2 < w 3
Study cases Case I Case II Fundamental mode Freq. (GHz) 400 300 ID (mm), OD(mm), Length(cm) 1.5, 1.59, 10 2, 2.12, 10 Drive bunch charge (nc) 3.5 8 Double triangular bunch length (mm) 1 1 Drive/main bunch energy (MeV) 300 400 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 54, 10.8 Transformer ratio 8 5 Main bunch charge (pc), length (mm) 50, 5 250, 10 Total DWA length (m) ~40 ~20 Drive beam use, dump energy (MeV) 80%, ~ 70 80%, 80 Drive beam to main beam efficiency (%) 8.6 15.5 Main beam energy gain (GeV) 1.5 1.6
Result of tracking for 8nC drive and 250 pc main bunch 1100 After 2 m E in =400 MeV After 18 m E out =2.0 GeV main TM 01 freq = 299.7 GHz current wake 3500 main TM 01 freq = 336 GHz current wake p=gb 700 z (mm) p=gb 500 z (mm) t t x (mm) 50 0 x (mm) 500 0-50 -500 t t
Undulator period, cm 1.8 Undulator 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 wavelength, nm 1 Peak power, GW 5 Bandwidth, % 3.8 FEL simulations (illustration)
Summary 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 Future development improving transmission efficiency through the mask important accounting for space charge effects maintaining trajectory straightness (~ 1 mm) - vital modular design: quadrupole wiggler, vacuum chamber, heat load/ cooling, BPMs, rf couplers, etc. - critical