Genz, H

Gräf, H D

Hahn, R

Loos, H

Richter, A

Schlott, V

Thomas, M

Wesp, T

Wiencken, M

CERN Document Server

Particle Accelerators, 1996, Vol. 52, pp. 45-53Reprints available directly from the publisherPhotocopying permitted by license only© 1996 OPA (Overseas Publishers Association)Amsterdam B.Y. Published in The Netherlands underlicense by Gordon and Breach Science Publishers SAPrinted in MalaysiaELECTRON BUNCH LENGTH MEASUREMENTSAT THE S-DALINAC FEL FACILITY*V. SCHLOTT, H. LOOS, H. GENZ, H.-D. GRAF, R. HAHN, A. RICHTER,M. THOMAS, T. WESP and M. WIENCKENInstitut fUr Kernphysik, Technische Hochschule Darmstadt,Schlossgartenstr. 9, 64289 Darmstadt, Germany(Received 4 September 1995; infinalform 3 November 1995)We report on the measurement of the longitudinal electron bunch distribution of the free-electron laser (FEL)experiment at the superconducting Darmstadt electron linear accelerator (S-DALINAC) facility. The electronmicropulse length was found to be (4±0.25) ps FWHM. The bunch shape is most likely rectangular. For thefirst time the results were established by applying simultaneously two independent methods, an autocorrelationtechnique using coherent transition radiation in the far infrared and by means of a Hamamatsu C 5680 type streakcamera that determined the pulse structure of the 11th and 13th harmonics of the spontaneous emission of theS-DALINAC FEL.1 INTRODUCTIONThe knowledge of the longitudinal electron bunch distribution in electron accelerators hasbeen of great interest during the past few years. It is an important aspect in many researchareas such as free-electron lasers, linear colliders and the generation of high intensityradiation sources in the far infrared. The observation of the coherent far infrared transitionradiation spectrum1 as well as autocorrelation techniques using transition radiation2 madeit possible to measure short electron bunches in the order of picoseconds3 and even in thefemtosecond region.4As an important step towards laser operation of the free-electron laser experiment atthe superconducting electron accelerator S-DALINAC,5 the longitudinal electron bunchdistribution had to be determined. Therefore, a Michelson interferometer for far infraredtransition radiation was installed and put to use in front of the undulator and in addition, aHamamatsu C 5680 type streak camera6 having a temporal resolution of 1.5 ps FWHM wasapplied to determine the time structure of the spontaneous emission. The comparison of thetwo measurements is particularly important since both methods are known to be prone toerrors and often yield questionable results.* Supported by the BMBF under contract number 05 345 EAI3 and a European Network under contract numberSCI-0471-C(A).4546 v. SCHLOTT et al.A brief introduction to the autocorrelation technique using coherent transition radiationin the first section is followed by the description of both experimental set-ups in the secondsection. The results of transition radiation and streak camera measurements are presentedin the third and fourth section of this paper. A final conclusion compares the results andgives a short outlook on the free-electron laser project.2 AUTOCORRELATION TECHNIQUE USING COHERENT TRANSITIONRADIATIONTransition radiation was first predicted by Frank and Ginzburg7 as a source of radiation,which is emitted when charged particles cross the boundary of two media with differentelectrical properties. In the case ofrelativistic electrons moving from vacuum with dielectricconstant IE I = 1 through a metal foil with IE I > 1 the energy radiated in a frequency rangedw and solid angle dQ for one electron is given by8e2f32 sin2 f}JT:2 C (1 - f32 cos2 f})2 ' (1)where e is the charge of the particle, f3 is the particle velocity expressed in units of c andf) is the angle of emission with respect to the axis of the particle beam. This dipole likeradiation is linearly polarized with the electric field vector lying in the plane spanned bythe normal to the interface and the direction of observation. The radiation is emitted in acone with opening angle f} = 1/y, where y is the Lorentz factor.9 In order to deduce thebackward emitted radiation observed in the experiment, one has to change f3 to - f3 in theformula above. 10 The total energy radiated by a bunch of electrons containing N particles isthe sum over the contribution of each electron, and is therefore proportional to the electronmicropulse charge. Since transition radiation reaches from the x-ray region11 to mm waves,this is only true for wavelengths, which are short compared to the electron bunch length.This part of the radiation spectrum is incoherent and results only from statistical fluctuationsin the particle density. It can be used to examine individual charged particle effects suchas energy, emittance and transverse profile. At wavelengths, which are in the order of theelectron bunch length, it was shown that the energy radiated is coherently enhanced. 12 In thecase of rf accelerators, where the electron bunch length is in the order of a few millimeters,this enhancement occurs in the mm-range. The total energy radiated by a bunch of electronscontaining N particles is then proportional to N 2 and can be expressed as followsd2d:~~' iJ) = (N + N(N - l)f(w))W(w, iJ). (2)Here W (w, f}) is the energy radiated by a single electron. The form factor f (w) is themagnitude squared of the Fourier transform of the mean particle distribution S(r)(3)ELECTRON BUNCH LENGTH MEASUREMENTS 475moFIGURE 1: Experimental set-ups for the transition radiation autocorrelation and for the streak camera measure-ments for the FEL experiment at the S-DALINAC. (1) Undulator, (2) mirror chambers, (3) streak camera in opticslab, (4) Michelson interferometer for transition radiation.One can easily see that f ((J)) describes the coherence ofthe emitted radiation. The incoherentlimit is given for f ((J)) == 0 whereas for f ((J)) == 1 the total energy is coherently enhanced.This part of the radiation spectrum then contains all the information about the collectivelongitudinal properties of the particle beam and can therefore be used by autocorrelation orspectral analysis techniques to measure the bunch length of the particles.3 EXPERIMENTSThe electron bunch length measurements were performed at the 130 MeV superconductingS-band electron accelerator S-DALINAC. The accelerator was operated in the high currentfree-electron laser model3 at a bunch repetition rate of 10 MHz and a beam energy of31.2 MeV. In the present experiment an additional macro structure of 1 ms duration witha repetition rate of 37 Hz was introduced. The Michelson interferometer for the transitionradiation autocorrelation measurements was installed in the PEL bypass in front of theundulator (see Figure 1). Transition radiation was produced by a 25 {tm thick AI-foil,which was introduced into the electron beam at an angle of 45°. The radiation was coupledout of the vacuum beam tube through a polyethylene window, collimated and sent throughthe Michelson interferometer. The interferometer consisted of a 50 {tm thick Mylar beamsplitter, a fixed and a moveable surface gold mirror and a room-temperature pyroelectricbolometer (Moiecron PI-45 with preamplifier) as shown in Figure 2.When removing the AI-foil the electron beam passed the undulator and producedspontaneously emitted radiation on the axis of the FEL resonator. This radiation was coupled48AI-FoilMylarBealllsplitterI I I I Io 10 Clllv. SCHLOTT et al.e--BealllFocusingMirrorInterferoilleterFIGURE 2: Schematic layout of the Michelson interferometer for electron beam autocorrelation measurementsusing coherent transition radiation.out of the optical cavity and sent through a 60 meter long transport system to an opticslaboratory outside the accelerator hall, where the streak camera was set up in order to avoidpossible radiation damage (see Figure 1). The incoming spontaneous emission was focusedonto the entrance slit of the .Hamamatsu C 5680 type streak camera, which had a widthof 15 {tm in order to obtain the highest possible time resolution of 1.5 ps FWHM. It waspossible to switch simultaneously between the two set-ups so that the same electron beamconditions were available for both measurements.4 TRANSITION RADIATION MEASUREMENTSFigure 3 shows a typical interferogram taken by the autocorrelation measurements with theMichelson interferometer. The signal intensity from the pyroelectrical detector is plottedagainst the optical pathlength difference introduced by the moveable mirror in the Michelsoninterferometer. The subharmonic injection with the chopper-prebuncher system as well asELECTRON BUNCH LENGTH MEASUREMENTS20~ .3 .J.4Q)if;0 10P-449o-15 -10 -5 0 5Optical Path Difference (ps)10 15FIGURE 3: Typical interferogram, which was taken with the Michelson interferometer (Figure 2). The solid linerepresents the symmetric portion of the data obtained by applying a Fourier filter technique.the superconducting injector linac has been set up for the shortest possible electron bunchoperation during that run.In order to compensate for the frequency dependence of the Mylar beam splitter due tothin film interference effects and absorption in the far infrared and mm range, where themain intensity of the coherent transition radiation occurs, the efficiency of the beam splitterwas measured from 20 J-Lm up to 1 mm wavelengths. Measurements of the transmissionof the polyethylene outcoupling window as well as the detector response function wereperformed in the same wavelength range. The cut-off frequency of the detector system wasdetermined to be at 300 GHz. In addition water absorption effects due to humidity havebeen considered according to 14. Figure 4 shows the power spectrum of the measured data,which was taken by the technique of Fourier transform spectroscopy, and the efficiency ofall components of the interferometer including water absorption. We found three dominantfrequency components at 14,22 and 30 wavenumbers. Even though the phase information,which is lost in first order autocorrelation measurements, can be recovered,15 in our casethe lack of information below 300 GHz (10 cm-1) makes it impossible to reconstructthe exact electron bunch shape. This is illustrated in Figure 5 which shows the efficiencycorrected spectral intensity of the transition radiation measurement compared with thepower spectrum of a 4 ps long rectangular pulse shape. Nevertheless it was possibleto determine the most likely electron bunch configuration. In our analysis the bunch iscomposed as a histogram containing 21 intervals with a width of 1.25 ps each, according tothe measured spectral range from 10 cm-1 to 45 cm-1 .The measured bunch charge of3.6 pC50ov. SCHLOTT et al.5 10o 20 40Wavenumber k (em-i)60FIGURE 4: Power spectrum of the measured data (upper part) and efficiency of the detector system (lower part).The solid line connects the data points to guide the eye.Frequency CtJ (THz)101Wavenumber k (em-i)FIGURE 5: Power spectra of the efficiency corrected data (dots) and of a 4 ps long rectangular electron bunch(solid line).ELECTRON BUNCH LENGTH MEASUREMENTS 511560,1010...Frequency c.J (THz)520 40Wavenumber k (em-I)-5 0 5Optical Path Difference (ps)20~~MQ) 10~0p..0-15 -100iSC)..........~~~"0..........p....."0 .....00FIGURE 6: The uncorrected measured interferogram is shown in the upper part indicated by dots. The solid linerepresents the fitted interferogram corrected with respect to the response of the detector system. The most likelyelectron bunch configuration is shown in shades. The lower part displays the measured power spectrum (dots) incomparison to the spectrum of the fitted electron bunch configuration (solid line).was taken into account. Power spectra of different bunch shapes were calculated, efficiencycorrected, and compared with the power spectrum of the autocorrelation data. In a fittingroutine this procedure was repeated until the best agreement of the power spectra wasobtained.It turned out that the electron bunch length in the FEL high current mode at the S-DALINAC facility was (4 ± 0.25) ps with a most likely rectangular shape as shown inFigure 6.5 STREAK CAMERA MEASUREMENTSThe streak camera measurements were performed with the spontaneously emitted radiationproduced by the electrons in the undulator. The accelerator was operated in the same wayas for the transition radiation measurements. Since the camera has only been sensitive inthe visible wavelength range, we used the 11th and 13th harmonics of the spontaneousemission at 636 nm and 538 nm, respectively. In order to obtain the lowest possible phasejitter, the streak camera was operated in synchroscan mode at the fastest possible time baseof 20 ms, where only one macropulse of the accelerator was present. Figure 7 shows ameasured streak picture of the spontaneous emission with a total width of 6 ps, which is522o-15FWHM ~ 6 ps-10 -5v. SCHLOTT et al.oPulselength (ps)5 10 15FIGURE 7: Streak camera picture of 11th and 13th hannonics of the spontaneous emission. The measured pulselength was 6 ps FWHM, which results in an electron bunch length of 4 ps FWHM taking the slippage correctioninto account.caused by the micropulse time structure of the electron beam and the lengthening of theoptical pulse due to slippage effects. In the case of the S-DALINAC FEL the slippage is inthe range of 2 ps and thus the corresponding electron bunch length amounts to about 4 psFWHM with a temporal resolution of the streak camera system of 1.5 ps FWHM.6 CONCLUSIONSIt has been possible for the first time to measure the electron bunch length in the free-electron laser experiment at the S-DALINAC facility in Darmstadt. It was determined to be(4 ± 0.25) ps FWHM. This result was obtained by the transition radiation autocorrelationand confirmed by the streak camera measurements of the spontaneous emission whichshows that coherent transition radiation is an appropriate tool to measure the electron bunchlength. It was also for the first time possible to control the bunching process in the injector ofthe 130 MeV superconducting accelerator in order to reach the shortest possible values forthe electron bunch length at that time. Establishing the transition radiation autocorrelationtechnique for determining the electron micropulse length at the S-DALINAC has certainlybeen a major step towards the operation of the FEL. It will be used during the upcomingbeam times to form the electron beam throughout the accelerator so that the design value of2 ps electron bunch length, respectively 2.7 A peak current, necessary for lasing is reached.ELECTRON BUNCH LENGTH MEASUREMENTSACKNOWLEDGEMENTS53We would like to thank K. Berryman, E. Crosson, T.1. Smith and A. Schwettman of theSCA-FEL group at Stanford and D.A. Javoszynski from Bruyeres-Ie-Chatel for manyhelpful informations and fruitful discussions about the transition radiation autocorrelationmeasurements. We would also like to thank Mr U. Denzer from Hamamatsu Photonics Inc.for the short term loan and the installation of the C 5680 type streak camera. Many thanksto W. Grill and A. Eifler from the University of Leipzig for the spectral measurements of thebolometric detector and the Mylar beamsplitter. This work has also been partly supportedthrough a Max-Planck-Forschungspreis.REFERENCES1. Y. Shibata, K. Ishi., T. Takahashi, T. Kanai, M. Ikezawa, K. Takami, T. Matsuyama, K. Kobayashi andY. Fujita, Phys. Rev. A45, (1992) R8340.2. W. Barry, CEBAF PR-91-012 (1991).3. E.R. Crosson, K.W. Berryman, T.I. Smith, R.L. Swent, H.C. Lihn and H. Wiedemann, Nucl. Instrum.Methods A358 (1995) 216.4. P. Kung, H.C. Lihn, H. Wiedemann and D. Bocek, Phys. Rev. Lett. 73 (1994) 967.5. H. Genz, H.-D. Graf, R. Hahn, D.A. Jaroszynski, H. Loos, M. Reichenbach, A. Richter, V. Schlott,M. Thomas, J. Topper, T. Wesp and M. Wiencken, Nucl. Instrum. Methods A358 (1995) ABS 20.6. Guide to Streak Cameras, (Hamamatsu Photonics K.K. 1994).7. LM. Frank and V.L. Ginzburg, J. Phys. (Moscow), 9 (1945) 353.8. L. Wartski, S. Roland, J. Lasalle, M. Bolore and G. Filippi, J. Appl. Phys. 46 (1975) 3644.9. M.L. Ter-Mikaelian, High Energy Electromagnetic Processes in Condensed Media, (Wiley, New York1972).10. V.L. Ginzburg and V.N. Tsytovich, Phys. Rep. 49 (1979) 3.11. M.L. Cherry, G. Hartmann, D. Muller and T.A. Prince, Phys. Rev. DIO (1974) 3594.12. J.S. Nodvick and D.S. Saxon, Phys. Rev. 96 (1954) 180.13. K. Alrutz-Ziemssen, J. Auerhammer, H. Genz, H.-D. Gdif, A. Richter, J. Topper and H. Weise, Nucl.Instrum. Methods A304 (1991) 300.14. D.S. Burch, J. Opt. Soc. Am. 58 (1968) 1383.15. R. Lai and A.J. Sievers, Phys. Rev. E50 (1994) R3342.

Electron Bunch Length Measurements at the S-dalinac Fel facility

Electron Bunch Length Measurements at the S-dalinac Fel facility

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