Longitudinal electro-optic electron bunch diagnostics has been successfully applied at several accelerators. The electro-optic effect can be seen as an upconversion of the Coulomb field of the relativistic electron bunch (THz radiation) to the visible spectral range, where a variety of standard diagnostic tools are available. Standard techniques to characterise femtosecond optical laser pulses (auto- and cross-correlators) have led to the schemes that can measure electron bunch profiles with femtosecond resolution. These techniques require, however, well synchronized femtosecond laser pulses, in order to obtain the desired temporal resolution. Currently, we are exploring other electro-optic variants which require less advanced laser systems and will be more amenable to beam based longitudinal feedback applications. The first results of one such new scheme will be presented in this paper

Berden, G.

Gillespie, W. A.

Jamison, S. P.

MacLeod, Allan M.

Phillips, P. J.

van der Meer, A. F. G.

English

Abertay Research Portal

LONGITUDINAL ELECTRON BEAM DIAGNOSTICS VIAUPCONVERSION OF THZ TO VISIBLE RADIATIONG. Berden, A.F.G. van der Meer, FELIX / FOM Institute ‘Rijnhuizen’, Nieuwegein, NLS.P. Jamison, ASTEC, Daresbury Laboratory, STFC, UKA.M. MacLeod, Abertay University, Dundee, UKW.A. Gillespie, P.J. Phillips, Dundee University, Dundee, UKAbstractLongitudinal electro-optic electron bunch diagnosticshas been successfully applied at several accelerators. Theelectro-optic effect can be seen as an upconversion of theCoulomb field of the relativistic electron bunch (THz radi-ation) to the visible spectral range, where a variety of stan-dard diagnostic tools are available. Standard techniquesto characterise femtosecond optical laser pulses (auto- andcross-correlators) have led to the schemes that can measureelectron bunch profiles with femtosecond resolution. Thesetechniques require, however, well synchronized femtosec-ond laser pulses, in order to obtain the desired temporalresolution. Currently, we are exploring other electro-opticvariants which require less advanced laser systems and willbe more amenable to beam based longitudinal feedback ap-plications. The first results of one such new scheme will bepresented in this paper.INTRODUCTIONSingle shot electro-optic detection techniques have beensuccessfully applied in electron-beam diagnostics [1, 2,3, 4]. The longitudinal bunch profile of a single elec-tron bunch can be determined in real-time by detectingits Coulomb field in an electro-optic crystal with a shortoptical laser pulse. The electro-optic method is not onlysingle-shot and real-time, but also non-destructive and non-intrusive. For example, electro-optic measurements at thesoft x-ray free electron laser FLASH show that the SASEprocess is not influenced by the bunch profile measure-ments [4, 5].Several detection techniques exist, but they all have incommon that, ultimately, the temporal resolution is lim-ited by the optical probe laser pulse. Currently the high-est temporal resolution has been demonstrated at FLASHwhere electron bunch profiles with at width of 60 fs rmshave been observed with a laser pulse of 30 fs (FWHM)[4]. Although these techniques directly provide the tempo-ral profile of the electron bunch, they have several ‘disad-vantages’: (i) the laser pulses have to be very well synchro-nized with the electron bunches, (ii) the large bandwidth ofthese ultra-short laser pulses limits the use of many opticalcomponents (such as fibers) because of dispersion effects,and (iii) lasers capable of producing ultra-short laser pulsesare rather expensive.In a proof-of-principle experiment, we demonstrate analternative approach to electro-optic bunch diagnosticswith significantly ‘less demanding’ laser requirements.The schematic of the new technique is shown in figure 1. A(relatively) narrow bandwidth pulsed laser system is usedto measure the spectrum of the Coulomb field of a singleelectron bunch, single-shot and real-time. Here, the resultof the electro-optic effect is the generation of sidebands [6]which directly reveal the spectrum of the Coulomb field.It is important to realize that this new detection technique,which measures the spectrum of the electron bunch electricfield profile, is quite different from the techniques that aimto measure the temporal profile of the electron bunch suchas electro-optic spectral decoding (EOSD) [1] and temporaldecoding (EOTD) [2].EXPERIMENTAL SETUPMeasurements were performed on relativistic electronbunches that are produced in the linear accelerator (250 pC,50 MeV) at the FELIX facility [7]. The spectrum of theCoulomb field of a single electron bunch is measured in-side the accelerator beam pipe at the exit of the undulatorof the FEL (see figure 2). A 0.5 mm thick <110> ZnTecrystal is used as an electro-optic sensor and is placed withCCD gratingps laserP AZnTeFigure 1: (color) Single-shot technique for measuring thespectrum of electric field profiles of single electron bunches(P and A are polarizers).WEPC80 Proceedings of FEL2009, Liverpool, UKStability and Synchronisation688~analyzeracceleratorTi:SapphirelaserpolarizerFEL cavitye-beam  ZnTecrystaldumpundulatorbendingmagnet30 m20 m10 m15 m~~~ ~rf-clockklystron   pulsegeneratorλ/2λ/4rf phaseshifterintensifierCCD spectrometer~ ~30 mFigure 2: (color) Experimental setup at the FELIX facility. The spectrum of the Coulomb field of a single bunch ismeasured inside the FEL cavity at the exit of the undulator. The laser system and the spectrometer are located outside theaccelerator hall. See text for details.its front face perpendicular to the propagation direction ofthe electron beam. The same crystal has been used for mea-surements in the time domain, using the EOTD technique,showing that the FELIX electron bunches have a width of275 fs rms [2].For measurements in the spectral domain, laser pulseswith a narrow bandwidth and ‘long’ pulse duration are re-quired. The laser is a Ti:Sapphire laser with a wavelengthof 790 nm producing pulses of 3 ps (FWHM) which is ac-tively synchronized to the accelerator rf clock. The laseris located in a laser laboratory at a distance of 30 m fromthe accelerator. The laser beam is directed towards electro-optic crystal via mirrors. Before entering the acceleratorbeam pipe, a half wave plate and polarizer are used to re-duce the energy of the laser pulses in order not to damagethe electro-optic crystal [5]. In the beam pipe, the pulsesare passed through the electro-optic crystal. A quarter-wave plate is used to minimize the residual birefringenceof the crystal in the absence of electron bunches. As a re-sult of the electro-optic effect, sidebands are generated bymixing of the frequency components of the bunch electricfield and frequency components of the narrow bandwidthprobe pulse (the carrier). An analyzing polarizer is usedto distinguish the sidebands from the carrier, as they havedifferent polarizations. The spectrum of the sidebands isthen measured with a grating spectrometer and an intensi-fied CCD camera which are located in the laser laboratory.RESULTSFigure 3 shows a typical measurement with and withoutthe electron bunch present. The measurements were takenusing crossed polarizers, which means that the transmit-tance of the narrow bandwidth probe laser pulse is min-imized in the absence of the electron bunch. Neverthe-less, there is still some light at the carrier wavelength leak-ing through the crossed polarizers as can be seen in thefigure. In the presence of an electron bunch, the signalbroadens due to mixing of the frequency components ofthe bunch electric field and the laser pulse. The shorterthe electron bunch, the broader the spectrum of the electricfield of the bunch, and the broader the upconverted spec-trum in figure 3.Figure 3: (color) Single-shot spectral measurement withand without an electron bunch. Proceedings of FEL2009, Liverpool, UK WEPC80Stability and Synchronisation689DISCUSSIONTraditionally, spectral measurements of electronbunches are performed with coherent synchrotron ortransition radiation in the far-infrared region. Thesemeasurements require far-infrared or THz beamlines totransport the radiation from the electron beam pipe to thespectrometer (for example a Michelson Interferometer).The spectral technique discussed in this contributioncircumvents the use of THz beam lines. Here, one needsto deliver relatively narrow bandwidth laser pulses toand from an electro-optic crystal placed adjacent to theelectron beam (inside the accelerator beam pipe). This isstraightforward and rather easy to achieve, and, althoughnot demonstrated here, allows the use of optical fibersto transport the optical pulses to and away from theaccelerator hall.It should be noted that electro-optic spectral measure-ments discussed in this paper are different from the tem-poral measurements using the so-called electro-optic spec-tral decoding (EOSD) technique. In the EOSD technique,which is described in detail in references [1, 5], a long, lin-early chirped probe pulse obtained from a ultra-short broadbandwidth laser pulse is used in a similar setup as shown infigure 1. The electric field profile of the electron bunch iselectro-optically encoded onto this chirped probe pulse. Inthe decoding step, a spectral measurement of the intensitymodulated probe pulse directly reveals the temporal profileof the electron bunch.For the EOSD measurements it is important that thespectral bandwidth of the probe pulse is much larger thanthe spectral bandwidth of the Coulomb field of the bunch.This is exactly the opposite of the conditions required forthe spectral measurements reported in this paper. Here,the spectral bandwidth of the probe pulse should be muchsmaller than the bandwidth of the Coulomb field. Ashas been discussed in several papers [2, 5], EOSD is notcapable of measuring ultra-short electron bunches, sincethe temporal profiles of these ultra-short bunches appearbroader in the EOSD measurements. This limiting res-olution is, in fact, a result of the frequency mixing pro-cess which takes place in the EOSD technique as well, andcan only be circumvented with the EOTD technique whichmeasures the temporal profile of the electro-optically en-coded probe pulse directly [2].The spectral measurements with a narrow bandwidthlaser pulse do not aim to explicitly provide the temporalelectron bunch profile. Although compared to EOTD andEOSD, the loss of phase information may seem a back-ward step, the new measurement technique allows the useof simpler lasers and directly provides a real-time, single-shot spectrum of the electric field. The bandwidth of thistechnique is determined by the electro-optic mixing effect,and depends on the electro-optic crystal. Frequency com-ponents of the bunch electric field up to 4 THz (8 THz) canbe converted to optical frequencies with ZnTe (GaP) [8].The concept can also be extended to high frequency (mid-infrared) spectral components of the bunch profile, throughuse of different non-linear crystals. The bandwidth is notlimited by the beam line for the probe laser, as is the casefor THz beam lines in CTR and CSR spectrometers, whichallows for measurements of the long wavelength compo-nents of the bunch profile.CONCLUSIONWe have demonstrated the measurement of the Coulombfield spectrum of single electron bunches by upconvertingit to optical frequencies. This method allows the use ofrather simple lasers and directly provides the spectrum ofthe electric field in real-time and in a single-shot. This lon-gitudinal bunch diagnostic is quite similar to single shotCTR or CSR spectrometers, but avoids the need for spe-cial THz beam lines, and allows for the measurement oflong wavelength components that cannot be propagated toa remote detector.REFERENCES[1] I. Wilke, A.M. MacLeod, W.A. Gillespie, G. Berden,G.M.H. Knippels, and A.F.G. van der Meer, Single-shotelectron-beam bunch length measurements, Phys. Rev. Lett.88, 124801 (2002).[2] G. Berden, S.P. Jamison, A.M. MacLeod, W.A. Gillespie,B. Redlich, and A.F.G. van der Meer, Electro-optic tech-nique with improved time resolution for real-time, nonde-structive, single-shot measurements of femtosecond electronbunch profiles, Phys. Rev. Lett 93, 114802 (2004).[3] A.L. Cavalieri, et al., Clocking femtosecond X rays, Phys.Rev. Lett 94, 114801 (2005).[4] G. Berden, W.A. Gillespie, S.P. Jamison, E.-A. Knabbe,A.M. MacLeod, A.F.G. van der Meer, P.J. Phillips, H.Schlarb, B. Schmidt, P. Schmüser, B. Steffen, Bench-marking of electro-optic monitors for femtosecond electronbunches, Phys. Rev. Lett. 99, 164801 (2007).[5] B. Steffen, V. Arsov, G. Berden, W.A. Gillespie, S.P. Jami-son, A.M. MacLeod, A.F.G. van der Meer, P.J. Phillips, H.Schlarb, B. Schmidt, and P. Schmüser, Electro-optic timeprofile monitors for femtosecond electron bunches at thesoft x-ray free electron laser FLASH, Phys. Rev. ST Accel.Beams 12, 032802 (2009).[6] S. P. Jamison, A.M. MacLeod, G. Berden, D.A. Jaroszyn-ski, W.A. Gillespie, Temporally resolved electro-optic effect,Opt. Lett. 31, 1753 (2006).[7] Free Electron Laser for Infrared eXperiments (FELIX), seehttp://www.rijnhuizen.nl/felix[8] S. Casalbuoni, H. Schlarb, B. Schmidt, P. Schmüser, B. Stef-fen, and A. Winter, Numerical studies on the electro-opticdetection of femtosecond electron bunches, Phys. Rev. STAccel. Beams 11, 072802 (2008).WEPC80 Proceedings of FEL2009, Liverpool, UKStability and Synchronisation690

Longtitudinal electron beam diagnostics via upconversion of THz to visible radiation

LONGITUDINAL ELECTRON BEAM DIAGNOSTICS VIAUPCONVERSION OF THZ TO VISIBLE RADIATIONG. Berden, A.F.G. van der Meer, FELIX / FOM Institute ‘Rijnhuizen’, Nieuwegein, NLS.P. Jamison, ASTEC, Daresbury Laboratory, STFC, UKA.M. MacLeod, Abertay University, Dundee, UKW.A. Gillespie, P.J. Phillips, Dundee University, Dundee, UKAbstractLongitudinal electro-optic electron bunch diagnosticshas been successfully applied at several accelerators. Theelectro-optic effect can be seen as an upconversion of theCoulomb field of the relativistic electron bunch (THz radi-ation) to the visible spectral range, where a variety of stan-dard diagnostic tools are available. Standard techniquesto characterise femtosecond optical laser pulses (auto- andcross-correlators) have led to the schemes that can measureelectron bunch profiles with femtosecond resolution. Thesetechniques require, however, well synchronized femtosec-ond laser pulses, in order to obtain the desired temporalresolution. Currently, we are exploring other electro-opticvariants which require less advanced laser systems and willbe more amenable to beam based longitudinal feedback ap-plications. The first results of one such new scheme will bepresented in this paper.INTRODUCTIONSingle shot electro-optic detection techniques have beensuccessfully applied in electron-beam diagnostics [1, 2,3, 4]. The longitudinal bunch profile of a single elec-tron bunch can be determined in real-time by detectingits Coulomb field in an electro-optic crystal with a shortoptical laser pulse. The electro-optic method is not onlysingle-shot and real-time, but also non-destructive and non-intrusive. For example, electro-optic measurements at thesoft x-ray free electron laser FLASH show that the SASEprocess is not influenced by the bunch profile measure-ments [4, 5].Several detection techniques exist, but they all have incommon that, ultimately, the temporal resolution is lim-ited by the optical probe laser pulse. Currently the high-est temporal resolution has been demonstrated at FLASHwhere electron bunch profiles with at width of 60 fs rmshave been observed with a laser pulse of 30 fs (FWHM)[4]. Although these techniques directly provide the tempo-ral profile of the electron bunch, they have several ‘disad-vantages’: (i) the laser pulses have to be very well synchro-nized with the electron bunches, (ii) the large bandwidth ofthese ultra-short laser pulses limits the use of many opticalcomponents (such as fibers) because of dispersion effects,and (iii) lasers capable of producing ultra-short laser pulsesare rather expensive.In a proof-of-principle experiment, we demonstrate analternative approach to electro-optic bunch diagnosticswith significantly ‘less demanding’ laser requirements.The schematic of the new technique is shown in figure 1. A(relatively) narrow bandwidth pulsed laser system is usedto measure the spectrum of the Coulomb field of a singleelectron bunch, single-shot and real-time. Here, the resultof the electro-optic effect is the generation of sidebands [6]which directly reveal the spectrum of the Coulomb field.It is important to realize that this new detection technique,which measures the spectrum of the electron bunch electricfield profile, is quite different from the techniques that aimto measure the temporal profile of the electron bunch suchas electro-optic spectral decoding (EOSD) [1] and temporaldecoding (EOTD) [2].EXPERIMENTAL SETUPMeasurements were performed on relativistic electronbunches that are produced in the linear accelerator (250 pC,50 MeV) at the FELIX facility [7]. The spectrum of theCoulomb field of a single electron bunch is measured in-side the accelerator beam pipe at the exit of the undulatorof the FEL (see figure 2). A 0.5 mm thick <110> ZnTecrystal is used as an electro-optic sensor and is placed withCCD gratingps laserP AZnTeFigure 1: (color) Single-shot technique for measuring thespectrum of electric field profiles of single electron bunches(P and A are polarizers).WEPC80 Proceedings of FEL2009, Liverpool, UKStability and Synchronisation688~analyzeracceleratorTi:SapphirelaserpolarizerFEL cavitye-beam  ZnTecrystaldumpundulatorbendingmagnet30 m20 m10 m15 m~~~ ~rf-clockklystron   pulsegeneratorλ/2λ/4rf phaseshifterintensifierCCD spectrometer~ ~30 mFigure 2: (color) Experimental setup at the FELIX facility. The spectrum of the Coulomb field of a single bunch ismeasured inside the FEL cavity at the exit of the undulator. The laser system and the spectrometer are located outside theaccelerator hall. See text for details.its front face perpendicular to the propagation direction ofthe electron beam. The same crystal has been used for mea-surements in the time domain, using the EOTD technique,showing that the FELIX electron bunches have a width of275 fs rms [2].For measurements in the spectral domain, laser pulseswith a narrow bandwidth and ‘long’ pulse duration are re-quired. The laser is a Ti:Sapphire laser with a wavelengthof 790 nm producing pulses of 3 ps (FWHM) which is ac-tively synchronized to the accelerator rf clock. The laseris located in a laser laboratory at a distance of 30 m fromthe accelerator. The laser beam is directed towards electro-optic crystal via mirrors. Before entering the acceleratorbeam pipe, a half wave plate and polarizer are used to re-duce the energy of the laser pulses in order not to damagethe electro-optic crystal [5]. In the beam pipe, the pulsesare passed through the electro-optic crystal. A quarter-wave plate is used to minimize the residual birefringenceof the crystal in the absence of electron bunches. As a re-sult of the electro-optic effect, sidebands are generated bymixing of the frequency components of the bunch electricfield and frequency components of the narrow bandwidthprobe pulse (the carrier). An analyzing polarizer is usedto distinguish the sidebands from the carrier, as they havedifferent polarizations. The spectrum of the sidebands isthen measured with a grating spectrometer and an intensi-fied CCD camera which are located in the laser laboratory.RESULTSFigure 3 shows a typical measurement with and withoutthe electron bunch present. The measurements were takenusing crossed polarizers, which means that the transmit-tance of the narrow bandwidth probe laser pulse is min-imized in the absence of the electron bunch. Neverthe-less, there is still some light at the carrier wavelength leak-ing through the crossed polarizers as can be seen in thefigure. In the presence of an electron bunch, the signalbroadens due to mixing of the frequency components ofthe bunch electric field and the laser pulse. The shorterthe electron bunch, the broader the spectrum of the electricfield of the bunch, and the broader the upconverted spec-trum in figure 3.Figure 3: (color) Single-shot spectral measurement withand without an electron bunch. Proceedings of FEL2009, Liverpool, UK WEPC80Stability and Synchronisation689DISCUSSIONTraditionally, spectral measurements of electronbunches are performed with coherent synchrotron ortransition radiation in the far-infrared region. Thesemeasurements require far-infrared or THz beamlines totransport the radiation from the electron beam pipe to thespectrometer (for example a Michelson Interferometer).The spectral technique discussed in this contributioncircumvents the use of THz beam lines. Here, one needsto deliver relatively narrow bandwidth laser pulses toand from an electro-optic crystal placed adjacent to theelectron beam (inside the accelerator beam pipe). This isstraightforward and rather easy to achieve, and, althoughnot demonstrated here, allows the use of optical fibersto transport the optical pulses to and away from theaccelerator hall.It should be noted that electro-optic spectral measure-ments discussed in this paper are different from the tem-poral measurements using the so-called electro-optic spec-tral decoding (EOSD) technique. In the EOSD technique,which is described in detail in references [1, 5], a long, lin-early chirped probe pulse obtained from a ultra-short broadbandwidth laser pulse is used in a similar setup as shown infigure 1. The electric field profile of the electron bunch iselectro-optically encoded onto this chirped probe pulse. Inthe decoding step, a spectral measurement of the intensitymodulated probe pulse directly reveals the temporal profileof the electron bunch.For the EOSD measurements it is important that thespectral bandwidth of the probe pulse is much larger thanthe spectral bandwidth of the Coulomb field of the bunch.This is exactly the opposite of the conditions required forthe spectral measurements reported in this paper. Here,the spectral bandwidth of the probe pulse should be muchsmaller than the bandwidth of the Coulomb field. Ashas been discussed in several papers [2, 5], EOSD is notcapable of measuring ultra-short electron bunches, sincethe temporal profiles of these ultra-short bunches appearbroader in the EOSD measurements. This limiting res-olution is, in fact, a result of the frequency mixing pro-cess which takes place in the EOSD technique as well, andcan only be circumvented with the EOTD technique whichmeasures the temporal profile of the electro-optically en-coded probe pulse directly [2].The spectral measurements with a narrow bandwidthlaser pulse do not aim to explicitly provide the temporalelectron bunch profile. Although compared to EOTD andEOSD, the loss of phase information may seem a back-ward step, the new measurement technique allows the useof simpler lasers and directly provides a real-time, single-shot spectrum of the electric field. The bandwidth of thistechnique is determined by the electro-optic mixing effect,and depends on the electro-optic crystal. Frequency com-ponents of the bunch electric field up to 4 THz (8 THz) canbe converted to optical frequencies with ZnTe (GaP) [8].The concept can also be extended to high frequency (mid-infrared) spectral components of the bunch profile, throughuse of different non-linear crystals. The bandwidth is notlimited by the beam line for the probe laser, as is the casefor THz beam lines in CTR and CSR spectrometers, whichallows for measurements of the long wavelength compo-nents of the bunch profile.CONCLUSIONWe have demonstrated the measurement of the Coulombfield spectrum of single electron bunches by upconvertingit to optical frequencies. This method allows the use ofrather simple lasers and directly provides the spectrum ofthe electric field in real-time and in a single-shot. This lon-gitudinal bunch diagnostic is quite similar to single shotCTR or CSR spectrometers, but avoids the need for spe-cial THz beam lines, and allows for the measurement oflong wavelength components that cannot be propagated toa remote detector.REFERENCES[1] I. Wilke, A.M. MacLeod, W.A. Gillespie, G. Berden,G.M.H. Knippels, and A.F.G. van der Meer, Single-shotelectron-beam bunch length measurements, Phys. Rev. Lett.88, 124801 (2002).[2] G. Berden, S.P. Jamison, A.M. MacLeod, W.A. Gillespie,B. Redlich, and A.F.G. van der Meer, Electro-optic tech-nique with improved time resolution for real-time, nonde-structive, single-shot measurements of femtosecond electronbunch profiles, Phys. Rev. Lett 93, 114802 (2004).[3] A.L. Cavalieri, et al., Clocking femtosecond X rays, Phys.Rev. Lett 94, 114801 (2005).[4] G. Berden, W.A. Gillespie, S.P. Jamison, E.-A. Knabbe,A.M. MacLeod, A.F.G. van der Meer, P.J. Phillips, H.Schlarb, B. Schmidt, P. Schmu¨ser, B. Steffen, Bench-marking of electro-optic monitors for femtosecond electronbunches, Phys. Rev. Lett. 99, 164801 (2007).[5] B. Steffen, V. Arsov, G. Berden, W.A. Gillespie, S.P. Jami-son, A.M. MacLeod, A.F.G. van der Meer, P.J. Phillips, H.Schlarb, B. Schmidt, and P. Schmu¨ser, Electro-optic timeprofile monitors for femtosecond electron bunches at thesoft x-ray free electron laser FLASH, Phys. Rev. ST Accel.Beams 12, 032802 (2009).[6] S. P. Jamison, A.M. MacLeod, G. Berden, D.A. Jaroszyn-ski, W.A. Gillespie, Temporally resolved electro-optic effect,Opt. Lett. 31, 1753 (2006).[7] Free Electron Laser for Infrared eXperiments (FELIX), seehttp://www.rijnhuizen.nl/felix[8] S. Casalbuoni, H. Schlarb, B. Schmidt, P. Schmu¨ser, B. Stef-fen, and A. Winter, Numerical studies on the electro-opticdetection of femtosecond electron bunches, Phys. Rev. STAccel. Beams 11, 072802 (2008).WEPC80 Proceedings of FEL2009, Liverpool, UKStability and Synchronisation690

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Longtitudinal electron beam diagnostics via upconversion of THz to visible radiation

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