Measurements of Short Bunches at SPPS and E-164X

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1 Measurements of Short Bunches at SPPS and E-164X P. Muggli (USC) M.J. Hogan, C.D. Barnes, D. Walz, R. Siemann P.J. Emma, P. Krejcik (SLAC) H. Schlarb, R. Ischebeck (DESY) P. Muggli, XFEL 24, SLAC 7/29/4 1

2 OUTLINE Motivation CTR Interferometry* Bunch energy spectrum measurements Application to E-164X Conclusions *C. Settakorn, PhD thesis, Stanford (21). P. Muggli, XFEL 24, SLAC 7/29/4 2

3 MOTIVATIONS Length of SLAC ultra-short bunches was never measured! In E-164X plasma wakefield acceleration (PWFA) experiment, the accelerating gradient increases as 1/s z 2 with matching plasma density increasing as 1/s z Bunch incoming energy spectrum and CTR energy varies significantly from bunch to bunch (especially at 1 Hz rep. rate) Outcome of E-164X P. Muggli, XFEL 24, SLAC 7/29/4 3

EXPERIMENTAL SET UP Energy Spectrum X-ray e- IP2: IP: Li Plasma Gas Cell: H2, Xe, NO ne -118 cm-3 L 2.5-2 cm SPPS U. Plasma light N=1.8 11 Coherent sz=2-12µm Transition E=28.

4 EXPERIMENTAL SET UP Energy Spectrum X-ray e- IP2: IP: Li Plasma Gas Cell: H2, Xe, NO ne -118 cm-3 L cm SPPS U. Plasma light N= Coherent sz=2-12µm Transition E=28.5 GeV Radiation and y Optical Transition Radiators Optical Transition Radiation (OTR) Upstream y Downstream y x Imaging Cherenkov Spectrometer Radiator -Energy resolution 6 MeV 25m X-Ray Diagnostic, e-/e+ Production Dump Cherenkov (aerogel) Plasma Light Since E-162: y,e x -1:1 imaging, spatial resolution 9 µm x z Interferometer X-ray Chicane E Cdt l x - Spatial resolution 1 µm - Energy resolution 3 MeV P. Muggli, XFEL 24, SLAC 7/29/4 U C L 4A

5 TRANSITION RADIATION Transition Radiation (TR) becomes Coherent (CTR) for l>s z, with intensity N 2 /s z, N the number of e - /bunch of length s z CTR spectrum extends from for s z <l->, (i.e., broad spectrum in the IR/FIR) CTR spectrum amplitude given by the bunch form factor f(w), i.e, the Fourier transform of the longitudinal charge distribution squared (neglecting transverse variations, in the forward direction of observation). [ ] I total (w) ª NI e (w) 1+ (N -1) f (w) 2 I e (w)= E(w) 2, the TR for a single electron f (w) 2 = e (-ws z /c)2 for E r (z)µ n(z) = (Gaussian bunch) << for w<2πc/s z 1 e -z 2 2 /2s z 2ps z CTR carries longitudinal bunch shape information at long l s P. Muggli, XFEL 24, SLAC 7/29/4 5

6 ITERFEROMETER/ AUTOCORRELATOR Radiation field in the 2 arms of the interferometer with a time of flight difference d=2 z/c: RE TE E E = RTE(t) ref. + E = TRE(t + 2d /c) var. z T, R transmission and reflection coeff. of beam splitter Note: T=T(w), R=R(w)! Intensity I D =(E ref. +E var. ) 2 on autocorrelator detector: I (t;d) = 2 RTE(t) 2 D Ú dt Background +2 Ú RT 2 E(t)E(t + 2d /c) dt Interferogram/autocorrelation P. Muggli, XFEL 24, SLAC 7/29/4 6

7 INTERFEROMETER/ AUTOCORRELATOR For each d or z, measure the energy: S D ( z)= I D (t; z) dt ds Autocorrelation signal characteristics: - Symmetric (even if the bunch shape is not) - Background= 2, peak= 2 + 2, contrast of 2 - Extends to long wavelengths, i.e., to long delays (CTR) - FFT(interferogram) => bunch spectrum - requires multiple (similar) bunches Pros and cons of CTR Interferometry - Simple and inexpensive (<$1k) - No sophisticated timing required - Symmetric trace - Multi-bunch measurement - Requires knowledge of broadband response of the entire system P. Muggli, XFEL 24, SLAC 7/29/4 7

8 CTR MICHELSON INTERFEROMETER 12.5 µm Mylar 1mm HDPE Vacuum Window (3/4 dia) 12.5 µm Mylar Beam Splitters R T.17 Alignment Laser 1 µm Titanium Foil at 45º e - s x =6 µm, s y =17 µm N e - Reference Pyro Detector Variable Position Mirror z Interferometer Pyro Detector Interference signal normalized to the reference signal Motion resolution z min =1 µm or 14 fs (round trip) Mylar: R 22%, T 78%, RT.17 P. Muggli, XFEL 24, SLAC 7/29/4 8

9 CTR INTERFEROGRAM Normalized Interference Signal (a.u) NDR compressor voltage: 41.8 MV/m, 2-6BNS phase -19º, 12.5µm Mylar CombinedCTRInterferograms -5 5 Delay Line Position (µm) Trace is symmetric (even if the bunch shape may not be) Peak/background ratio =2 Large dips on either sides of the peak Modulation far from the peak P. Muggli, XFEL 24, SLAC 7/29/4 9

10 NARROWING/BROADENING SOURCES Interferometer transmission can be affected by: * (amplitude and phase) Water absorption in humid air Vacuum window size cut-off (long l) Interferometer optics aperture (long l) Pyro-electric detector resonances Beam splitter(s)/window Fabry-Perot resonances *C. Settakorn, PhD thesis, Stanford (21). P. Muggli, XFEL 24, SLAC 7/29/4 1

11 BEAMSPLITTER R & T, 45º Thickness d Index of refraction n Angle of incidence 45º 1- eij R(w) = -r 1- r 2 e ij e ij 2 T(w) = (1- r 2 ) 1- r 2 e ij r^(w) = 1-2n n 2-1 r // (w) = n2-2n 2-1 n 2 + 2n 2-1 Mylar: n=3, n=n(w)? Include in a simple autocorellation calculation Interferometer delay z or d => relative phase shift 2k z P. Muggli, XFEL 24, SLAC 7/29/4 11

12 Power Spectrum (a.u.) MYLAR FABRY-PEROT Simple model: Gaussian, s z =2 µm, d=12.7 µm, n=3 Mylar window+splitters Mylar resonances CTRFSpecSigmaz2Mylar12.5_ Wavelength (µm) Filter Amplitue (a.u.) Autocorrelation w/o Filter (a.u.) Æ s z =14 µm CTRautocSigmaz12.7_3.2 Æ s z =9 µm Delay (µm) Fabry-Perot resonance: l=2d/nm, m=1,2,, n=index of refraction Signal attenuated by Mylar beam splitter: (RT) 2 Modulation/dips in the interferogram Smaller measured width: s Autocorrelation < s bunch! P. Muggli, XFEL 24, SLAC 7/29/ Autocorrelation with Filter (a.u.)

13 Normalized Interference Signal (a.u) s z 9 µm CORRECTED GAUSSIAN WIDTH CombinedCTRInterferogramsSm NDR compressor voltage: 41.8 MV/m, 2-6BNS phase -19º CombinedCTRInterferograms -5 5 Bunch s z (µm) Delay Line Position (µm) w Filtering SigmazMylar12.7_3WandBS w/o Filtering Autocorrelation s z (µm) Autocorrelation: s z 9 µm Gaussian Bunch s z 18 µm t 12 fs P. Muggli, XFEL 24, SLAC 7/29/4 13 or

14 FFTB R 56 DEPENDENCY CTRR56=+1.mm2 FFTB R 56 =+1. mm s z =8.4 µm s z 17 µm or 114 fs (corrected) Normlized CTR Int. Sig CTRR56=+1.5mm mm +2. mm 6.9 µm CTRR56=+2.mm Delay Line Position (µm) 7.4 µm s z 13 µm or 86 fs s z 11 µm or 74 fs Measurable, but weak dependency Variations masked by beamsplitter transmission characteristics P. Muggli, XFEL 24, SLAC 7/29/4 14

15 Beam Current (A) BunchField87971M125 MYLAR EFFECT (Example) Beam current profiles for PWFA Phase Ramp "-1º" Phase Ramp "º" Phase Ramp "+1º" s za 18 µm 47 µm 47 µm z (µm) Beamsplitter filtering masks beam profile features Autocorrelation Amplitude (a.u.) Filtered Autocorrelation Amplitude (a.u.) 1 CTRautocorellation87971M µm Mylar CTRautocorellationC87971M125 1 Phase Ramp "-1º" Phase Ramp "º" Phase Ramp "+1º" P. Muggli, XFEL 24, SLAC 7/29/4 Delay (µm) 15

16 CTR AMPLITUDE PEAK COMPRESSION MV, 1. mm MV, 1.5 mm MV, 2. mm MV, 1.5 mm Gaussian Bunch: BPMS DR X E CTR N 2 /s z X(Pyro)R56Comp CTR Pyro Ref. (a.u.) Amplitude variations are clear(er) Amplitude related to bunch current profile P. Muggli, XFEL 24, SLAC 7/29/4 16

17 e - BUNCH MANIPULATION Front LITrack K. Bane, P. Emma Front Back Accelerated electrons Back Accelerated electrons Energy spectrum <-> phase space <-> current profile s z does not fully describe the bunch shape U C L A P. Muggli, XFEL 24, SLAC 7/29/4 17

18 C. Barnes PhD) P. Muggli, XFEL 24, SLAC 7/29/4 18

19 EXPERIMENTAL SET UP Energy Spectrum X-ray e- IP2: IP: Li Plasma Gas Cell: H2, Xe, NO ne -118 cm-3 L cm SPPS U. Plasma light N= Coherent sz=2-12µm Transition E=28.5 GeV Radiation and y Optical Transition Radiators Optical Transition Radiation (OTR) Upstream y Downstream y x Imaging Cherenkov Spectrometer Radiator -Energy resolution 6 MeV 25m X-Ray Diagnostic, e-/e+ Production Dump Cherenkov (aerogel) Plasma Light Since E-162: y,e x -1:1 imaging, spatial resolution 9 µm x z Interferometer X-ray Chicane E Cdt l x - Spatial resolution 1 µm - Energy resolution 3 MeV P. Muggli, XFEL 24, SLAC 7/29/4 U C L19A

20 BPMS DR X X(Pyro)wwoImage BUNCH COMPRESSION & ENERGY SPECTRA #61 I7 Event with Image Event without Image #124 I #348 I CTR Pyro Amplitude (a.u.) Pyro amplitude is ambiguous Energy spectra are not They are complimentary Clear correlation between Energy spectrum and E-164X outcome Amplitude (a.u.) Amplitude (a.u.) XCompressionfrom544X X-ray Over Opitimum Under CCompressionfrom544X Relative Energy (MeV) Cherenkov Over Optimum Under Relative Energy (MeV) P. Muggli, XFEL 24, SLAC 7/29/4 2

21 E164X: A Plasma Wakefield Acceleration Experiment C. Barnes, F.-J. Decker, P. Emma, M. J. Hogan, R. Iverson, P. Krejcik, C. O Connell, H. Schlarb, R.H. Siemann, D. Walz Stanford Linear Accelerator Center C. E. Clayton, C. Huang, C. Joshi, D. Johnson, W. Lu, K. A. Marsh, W. B. Mori University of California, Los Angeles S. Deng, T. Katsouleas, P. Muggli, E. Oz University of Southern California, Los Angeles U C L A P. Muggli, XFEL 24, SLAC 7/29/4 21

22 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 N Linear scaling: E / m) s z /.6mm Plasma Wakefield Accelerator (PWFA) = Transformer Booster for high energy accelerator At n e =2.6x1 17 cm -3 : f rf 4.5 THz accelerator (for s z 2 µm) E acc. 4 GV/m, B q /r 8MT/m ( ) 2 1/s z kpe s z 2 U C L A P. Muggli, XFEL 24, SLAC 7/29/4 22

23 n e cm -3 ENERGY LOSS Relative Mean Bunch Energy (GeV) BPM Measurement L 1 cm, N Max. Loss: 1.5 GeV Plasma OFF Plasma ON MeanEloss(Pyro)2.55e CTR Pyro Amplitude (a.u.) Relative Energy of 95% Contour (GeV) Beam Image Measurement Max. Loss: 3.4 GeV Plasma OFF Plasma ON 95%Eloss(Pyro)2.55e CTR Pyro Amplitude (a.u.) Energy loss correlates with CTR energy (1/s z?) Peak energy gradient 3.4 GeV/1 cm! (or 34 GeV/m!) U C L A P. Muggli, XFEL 24, SLAC 7/29/4 23

8 1 1 Pyro=247 Pyro=299 Pyro=283 Pyro=318 n e

9 GeV Gain Loss -5 +5-5 +5 +5 X (mm) X (mm) X

-5-5 +5 X (mm) 7% of charge or 2 pc with E>E

24 Relative Energy (GeV) n e cm -3, L 1 cm RESULTS N Pyro=247 Pyro=299 Pyro=283 Pyro=318 n e = 3 GeV! 7.9 GeV Gain Loss X (mm) X (mm) X (mm) Energy gain reaches 3+1 GeV or 4 GeV/m X (mm) 7% of charge or 2 pc with E>E Energy gain depends on the details of the incoming beam (x,y,z) U C L A P. Muggli, XFEL 24, SLAC 7/29/4 24

25 CONCLUSIONS First and only(?) measurement of SLAC short bunches CTR interferometry shows bunches as short as 74 fs, but Beam splitter Fabry-Perot alters the measurement and CTR has limitations: multiple bunches, symmetric Short bunch confirmed by ionization of Li, NO, Xe, and H 2 Measure single bunch energy spectrum to retrieve profile/current distribution CTR interferogram and amplitude, and bunch spectrum are key for E-164X and future E- CTR interferometer can be improved: thinner Mylar splitter, vacuum box, Retrieve/incorporate bunch current profiles: in CTR and E-164X, work in progress P. Muggli, XFEL 24, SLAC 7/29/4 25

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