Colestock, Patrick L

Fitch, M J

Melissinos, A C

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PICOSECOND ELECTRON BUNCH LENGTH MEASUREMENT BYELECTRO-OPTIC DETECTION OF THE WAKEFIELD.M. J. Fitchy , A. C. Melissinosz , University of Rochester, Rochester, NYP. L. Colestockx , FNAL, Batavia, ILAbstractThe longitudinal profile of an 10 nC electron bunch of afew picoseconds duration will be measured by electro-opticdetection of the wakefield. The polarization of a short in-frared probe laser pulse (derived from the photocathodeexcitation laser) is modulated in a LiTaO3crystal by thetransient electric field of the bunch. The bunch profile ismeasured by scanning the delay between the laser and thebunch, and is sensitive to head/tail asymmetries. A single-shot extension of the technique is possible using a longerchirped laser pulse.1 PICOSECOND BUNCH LENGTHMEASUREMENTSThe generation and manipulation of very short electronbunches is important for many applications including fu-ture linear colliders, free electron lasers (FELs), andplasma wakefield acceleration. Consequently, methodsof measuring the bunch length and temporal profiles ofcharged particle beams on picosecond and sub-picosecondtime scales have attracted great interest.Notable recent efforts have used coherent radiation froman electron bunch. Coherent transition radiation (CTR) hasbeen analyzed with far-infrared interferometry [1, 2] withimpressive results. Since the measured signal is an auto-correlation, it is symmetric in time and insensitive to head-tail asymmetries. Cherenkov radiation has been examinedwith a 200-fs streak camera [3]. Below 1 ps, streak camerasare increasingly costly and inefficient. A Hilbert tranformspectrometer employing a Josephson junction detector [4]can also be used to analyze the millimeter and submillime-ter radiation from CTR.Ideally, it is desirable to know the longitudinal phasespace distribution function, not just the rms width (thebunch length) of this distribution. Frequency-domain tech-niques may suffer from the problem of missing phase infor-mation, though recontructing the charge density from theform factor using Kramers-Kronig relations [5] may be animprovement. Time-domain techniques do not suffer fromrequiring a priori assumptions on the longitudinal bunchshape. A recent example of a time-domain study using anrf zero-phasing method is Wang, et al. [6]. The imposedFermilab Beams Division Graduate Fellowy Email: mjfitch@pas.rochester.eduz Email: meliss@urhep.pas.rochester.edux Email: colestock@adcon.fnal.govlongitudinal momentum spread of the zero-phasing cavitiesis transformed into a horizontal position spread in a spec-trometer bend. However, the intrinsic energy spread of thebunch must be small compared with the correlated energyspread from the zero-phasing cavities. This condition iswell satisfied by the DC thermionic beam of Wang, et al.,and they achieve 100 fs resolution. For a high-brightnessphotoinjector beam, this condition seems difficult to sat-isfy.For beams of high average power, such as the TESLAdesign [7], non-interceptive, minimally invasive measure-ment techniques are desired. Our current effort is todevelop a time-domain longitudinal profile measurementbased on electro-optic sampling at Fermilab’s AØ Photo In-jector (AØPI), made possible by the combination of a high-charge (10 nC) high-brightness electron beam and synchro-nized picosecond laser pulses.2 ELECTRO-OPTIC SAMPLINGIn a nonlinear optical crystal, an applied electric field mod-ulates the polarization of light passing through the crystal.This electro-optic, or Pockels effect has wide-ranging ap-plications.The principle of electro-optic sampling (EOS) is to usea short laser pulse as a probe of the fields in the crystalby measuring a polarization change as the relative delay isscanned. Since the first studies of EOS on an electric fieldinjected into a crystal [8, 9], EOS has been demonstrated invarious materials to have multi-THz bandwidths [10, 11].Electro-optic detection of field transients propaging in freespace has been demonstrated in the far-infrared [12] andalso in the mid-infrared [13].The passage of a very short, high-charge, relativisticelectron bunch is accompanied by a strong transient elec-tric field–the wake field. In the lab frame, the electric fieldis relativistically flattened to a radial pancake. A simpleestimation of the magnitude of the field may then be ob-tained using Gauss’ law. Treating the electron bunch as aline charge (Q = 10 nC), the radial field isZS~E ~dA =q0(1)jErj =q20al= 3 MV/m: (2)where we have taken the bunch length to be l = 3 mmfor a 10 ps bunch, and have evaluated the field at a radiusa = 2 cm.0-7803-5573-3/99/$10.00@1999 IEEE. 2181Proceedings of the 1999 Particle Accelerator Conference, New York, 1999Calculating wake fields or the Fourier transform beamimpedances [14] is an important research activity. A moreaccurate calculation including the boundary conditions ofthe beampipe walls can be made with various computercodes. For example, ABCI gives for the iris of the AØPIgun a field magnitude jErj = 1:5 MV/m.2.1 The Photo Injector BeamlineFermilab’s AØPI is a TESLA prototype injector [15, 16]with a Cs2Te photocathode in a 1.625-cell L-band Cu gunwith a solenoid lens. A superconducting Nb 9-cell “cap-ture” cavity [17] accelerates the beam to 18 MeV followedby magnetic compression in a dipole chicane.The laser system is described elsewhere [18, 19]. Briefly,an actively modelocked Nd:YLF oscillator at 81.25 MHzis phase-locked to the rf with  1–2 ps rms jitter. Thesepulses are stretched and chirped in a 2 km single-modefiber. A fast electro-optic pulse picker selects a 1 MHzpulse train for amplification in a chain of Nd:glass am-plifiers. After grating compression to 1–2 ps FWHM,the 1054 nm infrared pulses pass through two BBO crys-tals, generating the second and fourth harmonics to green(532 nm) and UV (263.5 nm). Full charge operation re-quires 3–5 J UV per pulse for quantum efficiencies of or-der 1% , and the laser can provide 200 sec pulse trainsadequate for full charge, and longer 800 sec trains at re-duced charge.After the harmonic generation crystals, a dichroic beam-splitter separates the colors: the unconverted infraredpasses through a delay stage (with a stepper motor) andis expanded and transported to the beamline enclosure asa probe beam; the UV is expanded and transported to thecathode. Since the 1 ps UV pulse is undesirably short, itwill be temporally manipulated to a 10 ps quasi-flattop by apulse stacker. The pulse stacker is a compact arrangementof multiple delay lines arranged around a single UV beam-splitter [20] and this work in progress will be describedseparately. Both the infrared and UV beams share the sameevacuated transport line, and so are combined and sepa-rated at either end with dichroic beamsplitters. From thefirst dichroic to the cathode, the UV transmission is mea-sured to be 78 2%.2.2 Expected SignalWe have chosen a crystal of lithium tantalate (LiTaO3)based on the high electro-optic coefficient and good trans-parency at our infrared laser wavelength and at the mil-limeter waves of the transient field. The crystal is 7 mm 8 mm  1:5 mm, and is cut with the ~c-axis (with ex-traordinary index of refraction ne) parallel to the longestdimension, and the~b-axis the shortest dimension (ordinaryindex nofor this and the ~a-axis). See Fig. 1.To avoid conductors which perturb the electric fields, thecrystal is mounted in a vacuum cross with a machined Ma-cor ceramic holder, and is just recessed into the cross toavoid behind hit by the beam.c-axisb-axisa-axis nenonozyx1.5 mm7 mm8 mmEzExProbe LaserPolarizationEr of e- BunchLithium Tantalate (LiTaO3)Figure 1: Electro-Optic Crystal: Detailed Axis GeometryThe probe laser is linearly polarized at 45 with respectto the ordinary and extraordinary optic axes. The laser andthe transient pulse propagate colinearly in the y-direction,which coincides with the crystal ~b-axis. Due to the staticbirefringence, the laser (}!) becomes elliptically polar-ized:nat=!c(ne  no) y; (3)An additional phase shift is acquired in the presence of thetransient field:tra(T ) =!cn3or22Etra(T ) y: (4)where the appropriate electro-optic coefficient isr22= 1 10 12 m/V and the index is no= 2:154, sothat a 1 MV/m field accumulated over 2 ps (ct = 0:6 mm)gives a phase shift of 36 mrad, or  2.This phase shift is analyzed by a polarizing cube beam-splitter and two photodiodes (see Fig. 2). The static bire-fringence is compensated by a waveplate which balancesthe current in the two photodiodes. By detecting the differ-ence current between the pair of photodiodes, small pho-tomodulation depths can be observed, particularly with thesignal averaging of a lock-in amplifier. While it may bepossible to use a lock-in with our 1 MHz pulse trains, wehave concentrated on single-shot difference current mea-surements. While 10 3 is adequate for the above estimate( 1%), and increased signal-to-noise ratio is greatly de-sired.Group velocity mismatch between the probe laser andthe transient field causes slippage, and degradation of thetime resolution. The common approach is to use a verythin crystal, or a complicated geometrical phase matching.2182Proceedings of the 1999 Particle Accelerator Conference, New York, 1999PD PBSPDQWP10 nC e-3-10 psLiTaO3UV to gun10 psPhotocathodeDrive Laser 1ps IRPBSDelayCompensatorDifferential DetectionFigure 2: The probe laser passes completely through a vac-uum cross with the LiTaO3crystal mounted in a Macorholder (not shown). The polarization change is analyzed bya polarizing beam splitter (PBS), balanced with a quarter-wave plate (QWP) and detected with a balanced pair ofphotodiodes (PD).We have chosen a simple colinear geometry [21, 22] sincefor the large group velocity mismatch of LiTaO3, the tran-sient field is nearly stationary so that the laser sweeps overthe entire portion of the waveform that is inside the crystal.The desired waveform is recovered by numerical differen-tiation.3 EXPERIMENTAL PROGRAMOnce the issues of signal detection, timing, data acquisi-tion, etc. are resolved, there are several interesting studiesthat can be done. A careful study of magnetic bunch com-pression in the dipole chicane would allow comparison ofthe bunch length (and profile) measured with electro-opticsampling to traces of a streak camera. Since the UV pulselength on the cathode has some degree of adjustability, theoptimum can be found.An extension of this scheme to a single-shot measure-ment using a longer chirped laser pulse and a grating spec-trograph [23] is under consideration. In this case, the cor-relation of frequency versus time (the chirp) replaces thescanning delay, although the time resolution is broadeneddue to a convolution effect. As the technology of picosec-ond semiconductor diode lasers improves [24], it may bepossible to greatly reduce the size, cost, and complexity ofthe laser, so that a stand-alone electro-optic bunch lengthmonitor might be realized.4 REFERENCES[1] Pamela Kung, Hung-chi Lihn, Helmut Wiedemann, andDavid Bocek, Phys. Rev. Lett. 73 (1994), p967–970.[2] Y. Shibata et al., Phys. Rev. E 50 (2-B) (1994) p1479.[3] Mitsuru Uesaka, Toru Ueda, Takahiro Kozawa, and ToshiakiKobayashi, Nucl. Instr. Meth. A 406 (1998) p371-279.[4] M. Geitz et al., DESY-TESLA 98-10, April 1998.[5] R. Lai, U. Happek, and A. J. Sievers, Phys. Rev. E 50 (1994),pR4294–R4297.[6] D. X. Wang, G. A. Krafft, and C. K. Sinclair, Phys. Rev. E57 (1998) p2283–2286.[7] D. Edwards, editor, “TESLA Test Facility Linac - DesignReport”, DESY-TESLA 95-01 (1995).[8] J. A. Valdmanis, G. Mourou and C. W. Gabel, AppliedPhysics Letters, 41 (1982), p211–212.[9] J. A. Valdmanis, G. Mourou and C. W. Gabel, IEEE J. ofQuantum Electronics, QE-19, (1983) p664–667.[10] Q. Wu and X.-C. Zhang, Applied Physics Letters, 70, (1997)p1784–1786.[11] Q. Wu and X.-C. Zhang, Applied Physics Letters, 71,(1997), p1285–1286.[12] Peter R. Smith, David H. Auston, and Martin C. Nuss, IEEEJ. of Quantum Electronics, QE-24 (1988), p255–260.[13] Z. G. Lu and P. Campbell and X.-C. Zhang, Applied PhysicsLetters, 71, (1997), p593–595.[14] Alexander W. Chao, “Physics of collective beam instabili-ties in high energy accelerators”, Wiley, 1993.[15] Eric R. Colby, Ph. D. Dissertation, UCLA, 1997.[16] J. P. Carneiro, et al., these proceedings.[17] W. H. Hartung, et al., these proceedings.[18] Alan R. Fry, Ph. D. Dissertation. Univ. of Rochester, 1996.[19] A. R. Fry, M. J. Fitch, A. C. Melissinos, B. D. Taylor,“Laser System for a High Duty Cycle Photoinjector” UR-1559. (1998) Accepted for publication in Nucl. Instr. Meth.in Phys. Res. A.[20] D. Reis and D. Meyerhofer, private communication. 1998.We have adapted their infrared design to the UV.[21] P. Uhd Jepsen et al., Physical Review E 53, (1996) pR3052–R3054.[22] C. Winnewisser et al., Appl. Phys. Lett. 70 (1997) p3069.[23] Z. Jiang and X.-C. Zhang, Appl. Phys. Lett. 72 (1998)p1945.[24] Peter Vasil’ev. Ultrafast Diode Lasers. Artech House,Boston, 1995.2183Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

Picosecond electron bunch length measurement by electro-optic detection of the wakefield

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