Plasma Wakefield Accelerator Experiments and their Diagnostics Patric Muggli University of Southern California

Plasma Wakefield Accelerator Experiments and their Diagnostics Patric Muggli University of Southern California muggli@usc.edu Work supported by US Dept. of Energy

OUTLINE Motivation Plasma Wakefield Accelerator (PWFA) Experimental Setup Beam-driven Plasma-based Accelerator OTR and Beam Transverse Dynamics in PWFA (e - and e + ) CTR and Longitudinal Dynamics in PWFA Trapped Electrons Bunch Trains Energy Doubling Summary and conclusions

PARTICLE ACCELERATORS The 2.4-mile circumference RHIC ring is large enough to be seen from space CERN LHC 8.6 km, 5.3 mi SLAC SLC BNL RHIC 1222 m e - /e + 0-50GeV in 3km Some of the largest and most complex (and most expensive) scientific instruments ever built! All use rf technology to accelerate particles Can we make them smaller (and cheaper) and with a higher energy?

FUTURE LEPTON COLLIDER Linear accelerator to avoid synchrotron radiation limitation (~γ 4 /r 2 ~ E 4 /m 4 r 2 ) Energy frontier: 0.5-3 TeV(?), e - /e + Accelerator length: 1 TeV <50 MeV/m >20 km and $$$$$ Is there a high-gradient alternative to rf technology? Plasmas?

1-D r!" E r Relativistic Plasma Wave: = # $ 0 " E = m 1/ 2 % e $ c2 ' # &! 0 WHY PLASMA-BASED ACCELERATORS? k p E =! pe c E = n e e " 0 $ " pe = n ' e & e2 ) %# 0 m e ( 1/ n 2 e ( 100 n ( e cm )3 ) = 1GV / m @ n e =10 14 cm -3 (f pe 100 GHz) Large E z (accelerating) fields lasting 1 period! λ p E z LARGE Collective response! Plasma: Already (partially) ionized! No damage! H: fully ionized Li: Φ LiI =5.45 ev (easy!), Φ LiII =75 ev (difficult!) High frequency structure without structure No fabrication, no damage, easy to replace, Plasma wave can be driven by: 1/ 2 Intense laser pulse (LWFA) Short particle bunch (PWFA) c

Defocusing PLASMA WAKEFIELD (e - ) Focusing (E r ) Accelerating Decelerating (E z ) --- -- - - - - - - - - --- - -- -------- -- -- - ---- - - - - - - - - - + + + + + + + + + + + + - - + - -- - -- -- - + + + + + + + + + + + + + + + + -- + + + + + + + + + + + + + + + + + + + + + + + + -+ + + + + + + + + - + + + + + + + + + + + + + + + -- - - - - -- ------- - -- --- - - - - - - - -- - - --- -- - -- --- - electron beam Plasma wave/wake excited by a relativistic particle bunch Plasma e - expelled by space charge forces => energy loss + focusing Plasma e - rush back on axis => energy gain Optimize for acceleration and/or focusing (plasma lens) Plasma Wakefield Accelerator (PWFA): high-frequency, high-gradient, strong focusing beam-driven accelerator Large focusing (long propagation distance) + Large accelerating gradient = Large energy gain!

Defocusing PWFA NUMBERS (e - ) Focusing (E r ) Accelerating Decelerating (E z ) --- -- - - - - - - - - --- - -- -------- -- -- - ---- - - - - - - - - - + + + + + + + + + + + + - - + - -- - -- -- - + + + + + + + + + + + + + + + + -- + + + + + + + + + + + + + + + + + + + + + + + + -+ + + + + + + + + - + + + + + + + + + + + + + + + -- - - - - -- ------- - -- --- - - - - - - - -- - - --- -- - -- --- - electron beam Linear theory (n b <<n e ) scaling: Focusing strength: N 2 #10 10 E acc "110(MV / m) N/σ 2 ( $ z /0.6mm) 2 z @ k pe σ z 2 (with k pe σ z <<1) B " r = 1 n e e 2 # 0 c = 3kT / m $ n e (1014 cm %3 ) N=2x10 10 : σ z =600 µm, n e =2x10 14 cm -3, E acc ~100 MV/m, B θ /r=6 kt/m σ z = 20 µm, n e =2x10 17 cm -3, E acc ~100 GV/m, B θ /r=6 MT/m Conventional accelerators: E acc <150 MV/m, B θ /r<2 kt/m (n b >n e )

PWFA EXPERIMENTS @ SLAC 3 km Long-bunch Experiments Short-bunch experiments e - /e + 28.5 GeV N 1.2-1.8x10 10 /bunch e - 28.5, 42 GeV σ z 700 µm σ z 30-20 µm σ r 30 µm k pe σ z 2 σ r 10 µm n e 2x10 14 cm -3 0.1-100 GV/m n e 1-3x10 17 cm -3 L p 1.4 m L p 10, 20, 30, 60, 90, 120 cm Pre-ionized Field-ionized

e - Energy Spectrum X-ray N=1.8 10 10 Coherent σ z =20-12µm Transition E=28.5 GeV Radiation and Interferometer EXPERIMENTAL SET UP IP0: Li Plasma Gas Cell: H 2, Xe, NO n e 0-10 18 cm -3 L 2.5-20 cm Plasma light Optical Transition Radiators Imaging Spectrometer 25m y IP2: x z Cherenkov Radiator Cdt X-Ray Diagnostic, e-/e + Production Dump X-ray Chicane E -Energy resolution 60 MeV Optical Transition Radiation (OTR) y Upstream x y Downstream -1:1 imaging, spatial resolution 9 µm CTR x Peak Current (ka) 25 20 15 10 5 Plasma Light 07142dcpyrovspeakcurrent Cherenkov (aerogel) y,eλ x - Spatial resolution 100 µm - Energy resolution 30 MeV 0 0 200 400 600 CTR Energy (a.u.)

TRANSITION RADIATION Charged particles emit transition radiation (TR) when crossing a dielectric boundary (ε(ω)) [ ] I total (") # NI e (") 1+ (N $1) f (") 2 I e (ω)= E(ω) 2 the TR for a single e - Incoherent Coherent (ω<2πc/σ z, λ>0.6σ z ) TR CTR CTR spectrum amplitude given by the bunch form factor f(ω), i.e, the Fourier transform of the longitudinal charge distribution squared (neglecting transverse variations, in the forward direction of observation). f (") 2 = e (#"$ z / c )2 for E r (z) " n(z) = TR peaks at angle 1/γ, forward and backward TR 1 2#$ z e %z 2 / 2$ z 2 (Gaussian bunch) CTR spectrum extends from λ σ z >, (i.e., broad spectrum in the IR/FIR, THz) Optical TR (OTR) for beam transverse size/shape imaging Integrated CTR energy N 2 /σ z, s-to-s relative σ z measurement (N=cst) CTR carries longitudinal bunch shape information at long λ s CTR interferometry

Defocusing PLASMA FOCUSING OF e - Focusing (E r ) Accelerating Decelerating (E z ) --- -- - - - - - - - - --- - -- -------- -- -- - ---- - - - - - - - - - + + + + + + + + + + + + - - + - -- - -- -- - + + + + + + + + + + + + + + + + -- + + + + + + + + + + + + + + + + + + + + + + + + -+ + + + + + + + + - + + + + + + + + + + + + + + + -- - - - - -- ------- - -- --- - - - - - - - -- - - --- -- - -- --- - electron beam Linear theory (n b <<n e ) scaling: Focusing strength: N 2 #10 10 E acc "110(MV / m) N/σ 2 ( $ z /0.6mm) 2 z @ k pe σ z 2 (with k pe σ z <<1) B " r = 1 n e e 2 # 0 c = 3kT / m $ n e (1014 cm %3 ) N=2x10 10 : σ z =600 µm, n e =2x10 14 cm -3, E acc ~100 MV/m, B θ /r=6 kt/m σ z = 20 µm, n e =2x10 17 cm -3, E acc ~ 10 GV/m, B θ /r=6 MT/m Conventional accelerators: E acc <150 MV/m, B θ /r<2 kt/m (n b >n e )

e - Energy Spectrum X-ray N=1.8 10 10 Coherent σ z =20-12µm Transition E=28.5 GeV Radiation and Interferometer EXPERIMENTAL SET UP IP0: Li Plasma Gas Cell: H 2, Xe, NO n e 0-10 18 cm -3 L 2.5-20 cm Plasma light Optical Transition Radiators Imaging Spectrometer 25m y IP2: x z Cherenkov Radiator Cdt X-Ray Diagnostic, e-/e + Production Dump X-ray Chicane E -Energy resolution 60 MeV Optical Transition Radiation (OTR) y Upstream x y Downstream -1:1 imaging, spatial resolution 9 µm CTR x Peak Current (ka) 25 20 15 10 5 0 Plasma Light 07142dcpyrovspeakcurrent 0 200 400 600 CTR Energy (a.u.) Cherenkov (aerogel) y,eλ x - Spatial resolution 100 µm - Energy resolution 30 MeV

" r (µm) PLASMA FOCUSING OF e - Beam Envelope Model for Plasma Focusing Plasma Focusing Force > Beam Emittance Force (β beam =1/K> β plasma ) 200 150 100 50 0 No Plasma n e =0.03!10 14 cm -3 Plasma Lens n e =1.5!10 14 cm -3 3 rd Betatron Pinch Plasma BeamFocusing(z).graph 0 1 2 3 4 5 λ β z (m) OTR Envelope equation:! 2 "!z 2 + K 2 " = # 2 " 3 In an ion channel: K =! pe 2" c # ( n e ) 1/ 2 with a focusing strength: Multiple foci (betatron oscillation) within the plasma σ x,y (z) at fixed n e => σ x,y (n e ) at fixed z σ x,y (n e ) from OTR images! W = E r rc = B " r = 1 2 n e e # 0 c =6 kt/m @ n e =2 10 14 cm -3

! x (µm) 300 250 200 150 100 L=1.4 m! 0 =50 µm # N =12"10-5 m-rad $ 0 =1.16 m % 0 =-0.5 FOCUSING OF e - OTR Images 1m downstream from plasma Large Beam Size (K 1/β 0 ) Plasma OFF Plasma ON Envelope! x (µm) 600 500 400 300 200 Small Beam Size (K 1/β 0 ) Plasma OFF Plasma ON Envelope Equation Fit!=30 µm # N =44"10-5 m-rad $=0.11 m %=0 50 0 PRL 88(13), 154801 (2002) BetaronFitLongBeta.graph PRL 93, 014802 (2004) 07250cwMatchedBetatron.graph 0 0 0.5 1 1.5 2 0 0.5 1 1.5 2 Plasma Density ("10 14 cm -3 ) Focusing of the beam well described by a simple model (n b >n e ): Plasma = No emittance growth observed as n e is increased Ideal Thick Lens Stable propagation over L=1.4 m up to as n e =1.8 10 14 cm -3 100 n e, matched =1.6-2.5 10 14 cm -3 Plasma Density ("10 14 cm -3 ) Channeling of the beam over 1.4 m or >12β 0 => Matched Propagation over long distance!

e - & e + BEAM NEUTRALIZATION F " = q( E plasma + E b + v b # B b ) e - e + " 1 # $ 0 2 3-D QuickPIC simulations, plasma e - density: σ r =35 µm, σ r =700 µm, Ν=1.8 10 10, d=2 mm e - : n e0 =2 10 14 cm -3, c/ω p =375 µm e + : n e0 =2 10 12 cm -3, c/ω p =3750 µm Back Blow Out Back Front 3σ 0 beam Front 3σ 0 beam Uniform focusing force (r,z) No geometric aberrations Non-uniform focusing force (r,z) Aberrations expected!

Transverse Size (µm) EXPERIMENT/SIMULATIONS: e + BEAM SIZE σ x0 =σ y0 =25µm, ε Nx =390 10-6, ε Ny =80 10-6 m-rad, N=1.9 10 10 e +, L=1.4 m 2000 1500 1000 500 x-size y-size Downstream OTR Experiment P390by80BeamSizes6 Transverse Size (µm) 2000 1500 1000 500 x-size y-size Simulations PE390by80resulysOTR6 0 0 1 2 3 4 5 6 n e (10 14 cm -3 ) 0 0 0.5 1 1.5 2 n e (10 14 cm -3 ) Excellent experimental/simulation results agreement! The beam is round with n e 0

e + : BEAM/SLICE EMITTANCE (SIMULATIONS) σ x0 σ y0 25 µm, ε Nx 390 10-6, ε Ny 80 10-6 m-rad, N=1.9 10 10 e +, L 1.4 m n e =2 10 14 cm -3 10-2 P2.00e14E390by80NE2SlicesNx P2.0e14E390by80NE2SlicesNyL a) x-emittance 5 b) y-emittance All 5 All! N (m-rad) 10-3 10-4 x8 Slices contain 20% of charge 1 4 3 2 x38 4 3 2 1 0 0.5 1 1.50 0.5 1 1.5 z (m) z (m) The e + beam exits the plasma with equal emittances and equal transverse sizes Emittance growth for e +, none for e - (no geometric aberrations) P. Muggli et al., PRL 101, 055001 (2008).

PLASMA WAKEFIELD FIELDS, e -, e + E 0 =28.5 GeV, σ z =0.63 mm (2.1 ps), σ x =σ y =70 µm e -, N=1.8x10 10 e +, N=1.2x10 10 Head P. Muggli et al., PRL 93, 014802 (2004). B.E. Blue et al., PRL 90, 214801 (2003). Energy gain smaller than, hidden by, incoming energy spread Time resolution needed (1ps, streak camera), but shows the physics e - : Peak energy gain: 279 MeV, L=1.4 m, 200 MeV/m e + : Demonstrate acceleration in plasmas

Damping Ring 50 ps RTL Short Bunch Generation In The SLAC Linac Chirping 1 GeV 9 ps 0.4 ps 20-50 GeV Add 12-meter chicane compressor in linac at 1/3-point (9 GeV) SLAC Linac Compression FFTB <100 fs Existing bends compress to <100 fsec 1.5% 30 ka 80 fsec FWHM 28 GeV Courtesy of SPPS ~1 Å Bunch magnetic compression (N=cst) by a factor of 730/25 29! N 2 #10 10 E acc "110(MV /m) ( $ z /0.6mm) 2 100 MV/m => 100 GV/m!

CTR MICHELSON INTERFEROMETER + E ref. = RTE(t) E var. = TRE(t + 2" /c) I D (t;") = 2 RTE(t) 2 # dt Background +2 # RT 2 E(t)E(t + 2" /c) dt Interferogram/autocorrelation Intensity I D =(E ref. +E var. ) 2 on autocorrelator detector Interference signal normalized to the reference signal Motion resolution z min /2=1 µm or 7 fs (round trip) Mylar: R 2 22%, T 2 78%, RT 0.17 ε r =3.2

Amplitude (a.u) CTR INTERFEROGRAM NDR compressor voltage: 41.8 MV/m, 2-6BNS phase -19º, 12.5µm Mylar 1.5 1 0.5 0-1000 -500 0 500 1000 Delay!z (µm) Trace is symmetric (even if the bunch shape may not be) Many bunches for one trace (beam stability) Large dips on either sides of the peak Modulation far from the peak Gaussian fit to central peak: z 18 µm => σ z 13 µm or 43 fs

Simple model: Power Spectrum (a.u.) 10 0.1 0.001 10-5 10-7 10-9 10-11 10-13 10-15 10-17 MYLAR FABRY-PEROT Gaussian, σ z =20 µm, d=12.7 µm, n= 3.2 Mylar window+splitters Mylar resonances CTRFSpecSigmaz20Mylar12.5_3 10 100 1000 Wavelength (µm) 10 0 10-1 10-2 10-3 10-4 Filter Amplitue (a.u.) Autocorrelation w/o Filter (a.u.) 40 30 20 10 σ z =14 µm CTRautocSigmaz12.7_3 1.2 0.8 0.6 0.4 0.2 σ z =9 µm 0 0-300 -200-100 0 100 200 300 Delay (µm) 1 Autocorrelation with Filter (a.u.) Fabry-Perot resonances: λ=2d/nm, m=1,2,, n=index of refraction Signal attenuated by Mylar beam splitter: (RT) 2 Modulation/dips in the interferogram Additional filtering from (pyro) detector Smaller measured width: σ Autocorrelation < σ bunch!

ENERGY SPECTROMETER (NON-INVASIVE, UPSTREAM OF PLASMA) E Plasma Measure incoming bunch spectrum e - C.D. Barnes. PhD. Thesis, Stanford 2004

BUNCH PROFILE RETRIEVAL M. J. Hogan Phys. Rev. Lett. 95, 054802 (2005). I. Blumenfeld for PRST-AB Bunch Peak Current (ka) 20 15 10 5 0 0 100 200 300 CTR Energy (a.u.) LiTrack code (K. Bane, P. Emma) describes longitudinal compression Bunch peak current or length (and profile) can be retrieved Integrated CTR energy N 2 /σ z, s-to-s relative σ z measurement (N=cst)

SIMILAR IN, SIMILAR OUT 5 10 4 4 10 4 Plasma ON Plasma ON Plasma ON 3IdenticalXsprctra IN 10 GeV Amplitude (a.u.) 3 10 4 2 10 4 E x 1 10 4 0-2.5-2 -1.5-1 -0.5 0 0.5 1 Relative Energy (GeV) Identical incoming energy spectra/events Amplitude (a.u.) 1.2 10 5 1 10 5 8 10 4 6 10 4 4 10 4 Plasma ON Plasma ON Plasma ON 3IdenticalCsprctra OUT Identical outgoing energy spectra/events 2 10 4 0-6 -4-2 0 2 Relative Energy (GeV)

n e 2.6 10 17 cm -3 PWFA ENERGY LOSS Relative Mean Bunch Energy (GeV) 0-0.5-1 -1.5 BPM Measurement Max. Loss:!1.5 GeV Plasma OFF Plasma ON MeanEloss(Pyro)2.55e17-2 0 100 200 300 400 500 600 700 CTR Pyro Amplitude (a.u.) L 10 cm, N 1.8 10 10 Relative Energy of 95% Contour (GeV) 0-1 -2-3 -4 Beam Image Measurement Max. Loss:!3.4 GeV Plasma OFF Plasma ON 95%Eloss(Pyro)2.55e17-5 0 100 200 300 400 500 600 700 CTR Pyro Amplitude (a.u.) Energy loss correlates with CTR energy (1/σ z?) Peak energy gradient 3.4 GeV/10 cm! (or 34 GeV/m!)

e - ENERGY DOUBLING E 0 =42 GeV I. Blumenfeld Nature 445, 2007 Two-screen method to remove tail ambiguity Short spectrometer E 0 2E 0 Energy doubling of e - over L p 85 cm, 2.7x10 17 cm -3 plasma Unloaded gradient 52 GV/m ( 150 pc accel.)

e - ENERGY DOUBLING E 0 =42 GeV I. Blumenfeld et al., Nature 445, 2007 Final Focus Test Beam 3 km e - /e + LINAC 90 cm e - E 0 2E 0 Energy doubling of e - over L p 85 cm, 2.7x10 17 cm -3 plasma Unloaded gradient 52 GV/m ( 150 pc accel.)

ORIGIN OF TRAPPED PARTICLES OSIRIS Simulation: Li & He e - in (r-z) Space: n e =1.6 10 17 cm -3 Li at z=11.3 cm Li e - support the wake (not trapped) 3σ z He at z=11.3 cm Multiple trapped e - bunches ~2-3 µ σ z N=0.05x10 10 0.3x10 10 0.25x10 10 He e - born on-axis inside the wake and trapped

EVIDENCE FOR TRAPPED PARTICLES E. Oz et al., Phys. Rev. Lett. 98, 084801 (2007). Incoming: N 1.6 10 10 e - Li I Atomic Lines <18 GV/m >18 GV/m Coherent Cherenkov + CTR Continuum Excess charge/light appear at 18 GV/m or W loss =0.9 GeV Excess charge of the order of incoming charge, 1.6-1.8 10 10 e - Excess visible light >> excess charge Excess visible light<< (excess charge) 2 Partially Coherent

TRAPPED e - CHARCTERISTICS Spectrum of trapped e - TR or Cherenkov visible light: Δz -10 0 10 20 30 40 50 60 Z (µm) λ (nm) x 10 16 6 4 2 Spectral interferences Δλ 0 0.5 1 1.5 2 2.5 λ (nm) Coherent, 10 3-10 5 times more light "z = #2 2"# x 10 4 "# n e ( 1017 cm -3 ) 1.1 2.3 3.2 Trapped particles are emitted in multiple, ultra-short bunches ( µm), spaced by z= λ p fs, multi-gev, mm-mrad trapped bunches! z (µm) 102 84 72! * λ p (µm) calc. * 101 70 59 " p = 2#c $ p = 2#c & n e e 2 ) ( + ' % 0 m * 1 2

PARTICLES TO BUNCH ACCELERATION I. Blumenfeld et al., Nature 445, 2007 Drive Bunch Witness Bunch E/E 0 <1 2E 0 E 0 E 0 2E 0 Energy Doubling & E/E 10% Dream! Fake! Not real! Need a bunch train: drive(s) + withness

MULTIBUNCH PWFA Transformer Ratio: R = E + E " Energy Gain: " RE 0 E 0 : incoming energy Bunch Train E + Ramped Bunch Train * Q=1 3 5 E - Kallos, PAC 07 Proceedings *Tsakanov, NIMA, 1999 R=7.9 => multiply energy by 8 in a single PWFA stage! Can be optimized for beam=>wake efficiency

MULTIBUNCH GENERATION @ ATF Correlated energy chirp from linac To Plasma Emittance selection Choose microbunches spacing and widths with mask and beam parameters: N, z, σ z, Q

BUNCH TRAIN IN TIME Coherent Transition Radiation (CTR) Interferometry PLD Detector e - z=434 µm P. Muggli et al., Phys. Rev. Lett. 101, 054801 (2008) N=4 z=226 µm N=3 Autocorrelation leads spacing z N microbunches => 2N-1 peaks Control N and z (I mean t!) Mask: Period D=1270 µm Wire Dia d=500 µm

BUNCH TRAIN @ BNL-ATF Bunch spacing/plasma density condition: z=λ p (resonance) σ z << λ p z 3λ p /2 Plasma wavelength: λ pe =2πc/ω pe, plasma frequency, density n e, ω pe =(n e e 2 /e 0 m) 1/2 Wake fields add up (linear theory) Select # of drive bunches N=1 to 5 Measure witness bunch energy gain vs N (beyond energy doubling!) Same technique will be used at SLAC-FACET for bunch accel.

SUMMARY/CONCLUSIONS Extensive PWFA results obtained @ SLAC/FFTB Energy doubling of 42 GeV electrons in 85 cm of plasma Energy Doubling Relevant to a future e - /e + plasma-based accelerator/collider Extensive use of TR (OTR, CTR) Very good understanding of the longitudinal and transverse dynamics of e - and e + bunches in the PWFA Much to do: beam acceleration, high gradient acceleration of positrons, beam quality, PWFA for PWFA-LC?

PWFA-LC Concept (an example) TeV CM Energy 10 s MW Beam Power for Luminosity Positron Acceleration Conventional technology for particle generation & focusing Plasma for acceleration A. Seryi, T. Raubenheimer FACET Program will demonstrate most of a single stage July 7, 2008 FACET@ SLAC

E-167 Collaboration: I. Blumenfeld, F.-J. Decker, M. J. Hogan*, N. Kirby, R. Ischebeck, R. H. Iverson, R. H. Siemann and D. Walz Stanford Linear Accelerator Center C. E. Clayton, C. Huang, C. Joshi*,W. Lu, K. A. Marsh, W. B. Mori and M. Zhou University of California, Los Angeles U C L A T. Katsouleas, P. Muggli*, and E. Oz University of Southern California AE31 Collaboration: E. Kallos, P. Muggli University of Southern California Marcus Babzien, Karl Kusche, Vitaly Yakimenko Brookhaven National Laboratory, Upton, Long Island, NY Work Supported by US Dept. of Energy Thank You! Patric Muggli, * Principal AIS, SLAC, Investigators 11/12/08