Berden G

Gillespie WA

Jamison SP

Knabbe E-A

MacLeod AM

Schmidt B

Schmuser P

Steffen B

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TIME RESOLVED SINGLE-SHOT MEASUREMENTS OF TRANSITIONRADIATION AT THE THZ BEAMLINE OF FLASH USINGELECTRO-OPTIC SPECTRAL DECODINGG. Berden, A.F.G. van der Meer, FELIX / FOM Institute ‘Rijnhuizen’, Nieuwegein, The NetherlandsS. P. Jamison, ASTeC, Daresbury Laboratory, UKB. Steffen, E.-A. Knabbe, B. Schmidt, P. Schmüser, DESY, Hamburg, GermanyA.M. MacLeod, University of Abertay Dundee, Dundee, UKP. J. Phillips, W.A. Gillespie, University of Dundee, Dundee, UKAbstractSingle-shot electro-optic detection was used to measurethe temporal profile of coherent transition radiation (CTR)pulses at FLASH, the VUV-FEL at DESY. The CTR wasgenerated from single bunches kicked to an off-axis screen,and transported through a 20 m long transfer line from theradiation screen to an experimental station outside the ac-celerator tunnel, where it is imaged. Bipolar pulses with aFWHM less than 1 ps have been measured and are con-sistent with simulations of the propagation of radiationthrough the transfer line.INTRODUCTIONX-ray free electron lasers require short, intense, rela-tivistic electron bunches. Precise measurements of thelongitudinal temporal profile of these compressed electronbunches are essential for a detailed understanding of thelasing and operating principles. At FLASH, the VUV-FELat DESY, three monitors for the longitudinal profile of thecompressed beam are located within a few meters of eachother: the LOLA transverse deflecting RF structure [1], thesingle-shot electro-optic detection monitor which measuresthe Coulomb field of the bunches [2, 3], and the broad-band single shot spectrometer which measures the infraredand far-infrared (or THz) transition or diffraction radiation[4, 5]. For this spectrometer, a 20 meter long THz beamlinehas been constructed which transports the radiation fromthe diffraction and transition radiation screen to the exper-imental hut [6]. The same experimental hut contains thelaser system which is used for electro-optic detection ofthe Coulomb field of the electron bunches [2]. This givesthe opportunity to directly measure the temporal profile ofthe CTR via electro-optic detection as well. Single-shotelectro-optic detection of CSR radiation has been reportedbefore at the FELIX FEL facility [7, 8]. This paper reportsthe first results obtained at FLASH.EXPERIMENTA diagram of the experimental setup is shown in Fig. 1.The CTR was generated from single bunches kicked to anoff-axis screen. A diamond window was used to couple theradiation from the ultra high vacuum environment of theaccelerator into the THz transfer line. All optical compo-nents in the transfer line are reflective, and the line is evac-uated to avoid water absorption. The CTR radiation wascoupled out from the beamline through a crystalline quartzwindow, collected and focussed onto a ZnTe electro-opticcrystal with a 90◦ off-axis parabolic mirror. An indium tinoxide (ITO) beam combiner was used to overlap the THzbeam and the optical probe beam.The probe pulse is the uncompressed optical pulse froma Ti:S amplifier which is synchronised to the accelerator.Alternatively, a stretched pulse from a Ti:S oscillator couldbe used as is shown in Fig. 1 and described in Ref. [3]. Forthe experiments described in this paper, the probe pulse wasa linearly chirped 4 ps FWHM long pulse with a centralwavelength of 792 nm, a bandwidth of 40 nm FWHM, andan energy of less than a microJoule. In a linearly chirpedoptical pulse, the instantaneous wavelength is proportionalto time. Such a probe pulse is needed since we use theZnTeP PCCD grating CTRWBfs laserpulse stretcher CTR screen THz beam line kicker electron beam diamond windowquartz windowFigure 1: (color) Electro optic spectral decoding measure-ments of coherent transition radiation (CTR). See text fordetails; abbreviations: polarizer (P), quarter wave plate(W), ITO beam combiner (B).TUPCH027 Proceedings of EPAC 2006, Edinburgh, Scotland1058 06 Beam Instrumentation and FeedbackT03 Beam Diagnostics and Instrumentation012345 quarter wave plate at +45˚time [ps]spectral intensity [a.u.] -1.0 -0.50.00.51.0background subtracted signaltime [ps]normalised spectral intensity0123456quarter wave plate at  -45˚time [ps]spectral intensity [a.u.] -1.0 -0.50.00.51.0background subtracted signaltime [ps]normalised spectral intensity0 2.5 5 7.50 2.5 5 7.50 2.5 5 7.50 2.5 5 7.5Figure 2: (color) Spectral decoding results for different settings of the quarter wave plate. Upper panels: raw data. Greentraces are without CTR radiation, blue traces are with CTR radiation. Lower panels: normalised data. The leading edgeof the CTR pulse is on the right.technique called spectral decoding [9, 3, 7] for retrievingthe electric field profile of the CTR pulse. The length ofthe probe pulse has to be larger than the length of the CTRpulse in order to be able to measure the whole electric fieldprofile of the CTR pulse. The first polarizer (see Fig. 1)defines the correct polarization. The ITO beam combineroverlaps the THz beam and the probe pulse. In the 500 μmthick ZnTe crystal, the electric field profile of the THz CTRpulse is encoded electro-optically onto the chirped probepulse. In this way, the various wavelength components ofprobe pulse experience a different amount of birefringencedependent of the actual phase of the electric field of theCTR pulse. A quarter waveplate and a second polarizer (theanalyzer) are used to translate the amount of birefringenceinto a modulation depth of the intensity of the probe pulse.The temporal electric field profile of the CTR pulse is nowobtained from a single shot measurement of the wavelengthspectrum of the intensity modulated probe pulse. This caneasily be done with a grating spectrometer and a linear ar-ray detector. In present experiments, the detector is a gatedintensified CCD camera. Raw spectral decoding traces areobtained by proper binning the images.RESULTS AND DISCUSSIONThe upper panels of Fig. 2 show raw spectral decodingresults obtained with two different settings of the quarterwave plate. Green traces are obtained by blocking the CTRat the exit window of the THz beam line, and representmeasurements without CTR radiation present. Blue tracesare obtained with CTR radiation present. The green tracesrepresent a direct measurement of the wavelength spectrumof the probe pulse as well and can be used to convert thehorizontal axes into time, since the probe pulse is linearlychirped and the duration of this pulse is know. The spec-tral decoding signal is obtained by taking the differencebetween spectra recorded with and without CTR presentand normalising by the laser spectrum, and is shown in thelower panels of Fig. 2. The leading edge of the CTR pulseis on the right as confirmed by changing the delay betweenthe CTR pulse and the optical probe pulse.The quarter wave plate at a position of±45◦ correspondsto a so-called balanced detection configuration. This meansthat the probe pulse, in the absence of the CTR pulse, isleft- or right-handed circularly polarized just before the an-alyzing polarizer. Therefore, the positive and negative partsof the electric field of the CTR induce opposite intensitychanges after the analyzing polarizer, allowing the obser-vation of bipolar CTR pulses.In the balanced detection configuration, the signalSBD(ω) is conventionally taken as the difference in inten-sity of two orthogonal polarisations, SBD(ω) ≡ Iy − Ix.In Fig. 2, the signal was defined as the difference in inten-sity of the measured signals with and without CTR present,which is in the small signal limit proportional to the con-ventional balanced detection signal. Jamison et al. [10]showed that the balanced detection signal in the case ofspectral decoding can be described asSBD(ω) ∝{ECTR(τ + t0) ∗ cos(τ24β− π4)}, (1)where τ ≡ 2β(ω − ω0), ω0 is the central frequency of theoptical probe pulse, and β is the chirp parameter of the op-tical probe pulse. The signal depends linearly on the elec-tric field strength, which means that the normalised spec-tral decoding traces in the lower panel directly represent theelectric field profile of the CTR pulses. It is also apparentthat through its convolution with the CTR field ECTR(t)the function cos(τ 2/4β − π/4) plays the role of a time-resolution function. A temporal-limitation τ lim can be de-fined as the separation of the first zeros of cos(τ 2/4β− π4 )Proceedings of EPAC 2006, Edinburgh, Scotland TUPCH02706 Beam Instrumentation and FeedbackT03 Beam Diagnostics and Instrumentation1059centered around τ = 0; τlim can be approximately inter-preted as the minimum THz pulse duration that can be mea-sured without distortion arising from the cosine time reso-lution function. For our experimental conditions we obtainτlim =√12πβ ≈ 2.6√TcT0, (2)where the approximation on the right hand side is obtainedfrom an assumption of a Gaussian probe. Tc is the FWHMof the chirped probe pulse duration and T 0 the FWHM ofthe duration in case the probe pulse was unchirped. For theexperiments described here, τlim is 900 fs, which indicatesthat the CTR measurements shown in Fig. 2 are close tothe limit of distortion-free detection for spectral decodingwith the used chirp parameters. Fully distortion free detec-tion can be achieved using temporal decoding [11, 12] asthe detection scheme. Note that the time resolution for theelectro-optic measurements also depends on the materialand thickness of the electro-optic crystal; for the experi-ment shown here, the resolution limit is about 300 fs (seefor example Ref. [13]).CONCLUSIONThe first electro-optic CTR results at FLASH are verypromising. Work is in progress to perform single-shotelectro-optic spectral decoding and temporal decodingmeasurements of the CTR pulse directly in vacuum (elim-inating the quartz exit window of the THz beam line)and compare the results with spectral measurements of theCTR pulse. Furthermore, the CTR measurements will becompared with measurements obtained from single-shotelectro-optic detection of the Coulomb field of electronbunches and from the LOLA transverse deflecting rf struc-ture.REFERENCES[1] M. Hüning, A. Bolzmann, H. Schlarb, J. Frisch, D.McCormick, M. Ross, T. Smith, J. Rossbach, Obser-vation of femtosecond bunch length using a transversedeflecting structure, Proceedings of the 27th Interna-tional Free Electron Laser Conference (FEL 2005),Stanford, USA 2005, p. 538.[2] G. Berden, A.F.G. van der Meer, S.P. Jamison, B.Steffen, E.-A. Knabbe, B. Schmidt, P. Schmüser,A.M. MacLeod, P.J. Phillips, and W.A. Gillespie. Sin-gle shot longitudinal bunch profile measurements atFLASH using electro-optic techniques, EPAC 2006,TUPCH026.[3] B. Steffen, S. Casalbuoni, E.-A. Knabbe, B. Schmidt,P. Schmüser, and A. Winter. Spectral decodingelectro-optic measurements for longitudinal bunch di-agnostics at the DESY VUV-FEL, Proceedings of the27th International Free Electron Laser Conference(FEL 2005), Stanford, USA 2005, p. 549.[4] H. Delsim-Hashemi, O. Grimm, J. Rossbach, H.Schlarb, B. Schmidt, P. Schmüser, and A.F.G. vander Meer. Broadband single shot spectrometer, Pro-ceedings of the 27th International Free Electron LaserConference (FEL 2005), Stanford, USA 2005, p. 514.[5] H. Delsim-Hashemi, J. Rossbach, P. Schmüser, O.Grimm, H. Schlarb, B. Schmidt, and A.F.G. vander Meer. Bunch compression monitor, EPAC 2006,MOPCH016.[6] S. Casalbuoni, B. Schmidt, P. Schmüser, and B. Stef-fen, TESLA-FEL report 2006-04.[7] G. Berden, B. Redlich, A.F.G. van der Meer, S.P.Jamison, A.M. MacLeod, and W.A. Gillespie. Real-time, single-shot temporal measurements of shortelectron bunches, terahertz CSR and FEL radiation,Proceedings of the 7th European Workshop on BeamDiagnostics and Instrumentation for Particle Acceler-ators (DIPAC 2005), Lyon, France, 2005, p. 69.[8] S.P. Jamison, G. Berden, A.M. MacLeod, D.A.Jaroszynski, B. Redlich, A.F.G. van der Meer, andW.A. Gillespie. Electro-optic techniques for temporalprofile characterisation of relativistic Coulomb fieldsand coherent synchrotron radiation, Nucl. Instr. andMeth. A 557, 305 (2006).[9] I. Wilke, A.M. MacLeod, W.A. Gillespie, G. Berden,G.M.H. Knippels, and A.F.G. van der Meer. Single-shot electron-beam bunch length measurements, Phys.Rev. Lett. 88, 124801 (2002).[10] S.P. Jamison et al., to be published.[11] G. Berden, S.P. Jamison, A.M. MacLeod, W.A. Gille-spie, B. Redlich, and A.F.G. van der Meer, Electro-optic technique with improved time resolution forreal-time, nondestructive, single-shot measurementsof femtosecond electron bunch profiles, Phys. Rev.Lett. 93, 114802 (2004).[12] G. Berden, B. Redlich, A.F.G. van der Meer,S.P. Jamison, A.M. MacLeod, and W.A. Gillespie.High temporal resolution, single-shot electron bunch-length measurements, Proceedings of the ninth Eu-ropean Particle Accelerator Conference (EPAC2004),Lucerne, Switzerland, 2004, p. 2697.[13] S. Casalbuoni, H. Schlarb, B. Schmidt, B. Steffen, P.Schmüser, and A. Winter. Numerical studies on theelectro-optic sampling of relativistic electron bunches,Proceedings of 2005 Particle Accelerator Confer-ence (PAC2005), Knoxville, Tennessee, USA, 2005,p. 3070.TUPCH027 Proceedings of EPAC 2006, Edinburgh, Scotland1060 06 Beam Instrumentation and FeedbackT03 Beam Diagnostics and Instrumentation

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Time resolved single-shot measurments of transition radiation at the THz beamline of FLASH using electro-optic spectral decoding

Time resolved single-shot measurments of transition radiation at the THz beamline of FLASH using electro-optic spectral decoding

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