The THz source produced at the SPARC LAB test facility is able to deliver radiation pulses with time duration of few hundreds of femtoseconds and energy per pulse larger than 10 micro-Joule, corresponding to electric and magnetic

fields of the order of 1MV/cm and 0.5T, respectively. The linac-driven THz radiation is produced as coherent radiation emitted by ultra-short high-brightness electron bunches. Depending on the electron bunch shaping, the THz radiation is characterized by a tunable spectral bandwidth suitable for electron beam longitudinal diagnostics, characterization of novel materials, pump-probe experiments

Chiadroni, E.

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DOI 10.1393/ncc/i2014-11816-1Communications: SIF Congress 2013IL NUOVO CIMENTO Vol. 37 C, N. 4 Luglio-Agosto 2014The SPARC LAB THz source driven by high-brightnesselectron beamsE. Chiadroni(∗)INFN, Laboratori Nazionali di Frascati - Frascati (RM), Italyricevuto il 31 Gennaio 2014; approvato l’1 Aprile 2014Summary. — The THz source produced at the SPARC LAB test facility is ableto deliver radiation pulses with time duration of few hundreds of femtoseconds andenergy per pulse larger than 10 micro-Joule, corresponding to electric and mag-netic fields of the order of 1MV/cm and 0.5T, respectively. The linac-driven THzradiation is produced as coherent radiation emitted by ultra-short high-brightnesselectron bunches. Depending on the electron bunch shaping, the THz radiation ischaracterized by a tunable spectral bandwidth suitable for electron beam longitudi-nal diagnostics, characterization of novel materials, pump-probe experiments.PACS 41.60.Dk – Transition radiation.PACS 41.85.Ja – Particle beam transport.PACS 29.20.Ej – Linear accelerators.PACS 29.27.Bd – Beam dynamics; collective effects and instabilities.1. – IntroductionThe motivation for developing a linac-based THz source stays in the ever growinginterest of filling the so-called THz gap [1] with high peak power radiation sources, i.e.of the order of hundreds of MW.THz radiation is electromagnetic (EM) radiation whose frequency lies between 0.1 e10THz, therefore in the region of spectrum between microwaves and infrared (IR). Beingin between these two bands, THz radiation shares with them several properties, e.g. it isnon-ionizing, it is highly penetrating in a large variety of insulating materials, but not inliquids and metals. In addition, the THz part of the spectrum is energetically equivalentto many important physical, chemical and biological processes.The linac-based THz radiation source developed at the SPARC LAB test facility [2]opens the possibility to perform experiments in different fields of research [3]. Indeed,(∗) E-mail: enrica.chiadroni@lnf.infn.itc© Societa` Italiana di Fisica 349350 E. CHIADRONIsince THz pulses produced at SPARC LAB are intense, peak power of the order of100MW, and as short as 100 fs, they are suitable for investigating ultra-fast and non-linear phenomena and performing THz pump-THz probe spectroscopy.In order to be considered a powerful pump source, THz radiation should have energyper pulse higher than few tens of microjoule, pulse duration of sub-ps scale, since anexcitation starts to exist after half a cycle (T ≈ 100 fs–10 ps), tunable frequency, electricand magnetic fields of the order of MV/cm and T, respectively.Beyond application purposes, coherent THz radiation is also used as powerful longi-tudinal diagnostic [4] of high-brightness electron beams that drive, for example, Free-Electron Lasers (FELs) [5], plasma-based accelerators [6, 7] and advanced radiationsources [8].2. – The SPARC LAB THz radiation sourceThe SPARC photo-injector delivers high current, low emittance electron beams, whichare usually referred as high-brightness beams. Electrons are generated by a UV laser pulsehitting a copper photocathode embedded in the S-band gun. Afterwards, the few MeVelectrons enter in the 3 traveling wave (TW) S-band Linac, which boost their energyup to 170MeV when injected on the crest of the RF field. At the end of the linaca diagnostics and matching section allows to characterize the 6D electron beam phasespace and to match the beam to the four beamlines. The diagnostic section consists oftwo quadrupole triplets, the first one used for emittance measurements, an RF deflectingcavity to measure the longitudinal beam profile and, when combined with the dipole, tomeasure the longitudianl phase space.The high-brightness electron beams so produced and characterized serve as activemedium in the single pass FEL in different schemes, e.g. SASE and SEEDED, serve assource for THz radiation generation, for testing advanced electron beam diagnostics, suchas Electro-Optic Sampling and cavity beam position monitors, as source for gamma rays,and for novel acceleration concepts based on laser and electron beam plasma interactions.The linac-driven THz radiation [9, 10] is produced at the SPARC LAB through ra-diative phenomena based on relativistic electron bunches.Transition (TR) and Diffraction Radiation (DR) can be considered as two aspects ofthe same phenomenon: when a charged particle crosses the interface between two mediawith different refraction indexes, TR is emitted both in forward and backward direction;when the emitted radiation is produced by only a part of the particle field, because ofa slit cut on the screen, it is called DR, in full correspondence with the wave optics inwhich only a part of a wave front is allowed to propagate. Depending on the extensionof the EM field of the relativistic particle, i.e. γλ/2π, with γ the electron Lorentz factorand λ the emitted radiation wavelength, DR is emitted if the slit width is comparablewith γλ/2π, while TR is produced in the limit of zero slit width.Currently, both Coherent Transition Radiation (CTR) and Coherent Diffraction Radi-ation, from both 3 and 5mm slit apertures, are generated from ultra-short (sub-ps, downto 100 fs) electron bunches produced and manipulated at the SPARC high-brightnessphoto-injector.The technique used at SPARC to manipulate the high-brightness electron beams relieson low energy RF compression (the velocity bunching) [11,12] and on the use of properlyshaped trains of UV laser pulses with THz repetition rate hitting the photo-cathode(comb laser beam) [13]. Each laser pulse produces at the photocathode a disk of electronswith the same time spacing. While the longitudinal density modulation is mixed up atTHE SPARC LAB THz SOURCE DRIVEN BY HIGH-BRIGHTNESS ELECTRON BEAMS 351Fig. 1. – Experimental apparatus. The Michelson interferometer is the blue box on the left;the Martin-Puplett interferometer is in the upper center and a Golay cell can be also insertedmoving the flat mirror on the left end of the linear stage.the gun exit due to longitudinal space charge forces, a sawtooth distribution shows upin energy. Therefore, a dispersive system is needed to turn the energy modulation backinto a density modulation, thus restoring the original comb-like longitudinal distribution.In the SPARC photo-injector the dispersive system is represented by the first linearaccelerating section, operated far from the crest of the RF field, in the so-called velocitybunching regime.The coherent radiation spectrum, generated by a longitudinally modulated electronbeam, is a single line at frequencies corresponding to the comb repetition rate and higherharmonics, whose intensity is the same as all the electrons were confined in the singleμ-bunch. The bandwidth narrows with increasing the number of μ-bunches in the train.This technique represents a way to reduce the space-charge force, proportional to Q/γ2,which limits the charge, i.e. the number of electrons, that can be contained in a shortbunch.Taking advantage of advanced electron beam manipulation techniques, well-established at SPARC LAB [14], both broadband and quasi-narrow band THz radiationis produced.Two stations at SPARC LAB are dedicated to THz applications and electron beamlongitudinal diagnostics. Both CTR and CDR sources are currently characterized bymeasuring the radiation spectrum in air both through a set of discrete custom THzfilters, placed in front of a Golay cell detector, and an interferometer, either Martin-Puplett, with wire grids to select polarizations, or Michelson, with a 12 μm Mylar beamsplitter. Pyrodetectors are used in this case.A picture of the complete detecting apparatus is shown in fig. 1.The energy per pulse of the coherent THz radiation as produced by means of anelectron beam with 120MeV energy, 300 pC charge and 160 fs pulse duration is shown infig. 2 for different slit apertures (CTR corresponds to a closed slit).A calibrated Golay cell detector, with diamond window, has been used to estimatethe energy per pulse of the THz radiation reported in fig. 2. Cut-off frequencies areestimated to be 150GHz and 5THz, due to screen finite size, optics acceptance etc. andquartz window transmission, respectively.352 E. CHIADRONIFig. 2. – Energy per pulse of THz radiation as produced by a 300 pC, 160 fs, 120MeV electronbeam. The plot is function of the slit aperture (0mm slit aperture correspond to the CTRemission).From the point of view of narrowband THz emission, the first accelerating section isoperated at a compression phase corresponding to the so-called over-compression regime,where the two bunches are well separated in time, i.e. at a distance of the order of ps.Experimentally a beam composed by two Gaussian bunches of duration smaller than200 fs (FWHM), separated by 4.3 ps, and total charge of 160 pC was extracted fromthe cathode. In order to select the CTR emission at THz frequencies and enhance itsintensity, the pulse inter-distance has been reduced down to 0.8 ps by means of RFcompression in velocity bunching. The longitudinal phase space of this working point isshown in fig. 3.For such a longitudinal distribution, the interferogram is multi-peaked (fig. 4), whosetime distance corresponds to the separation of the bunches in the train.Fig. 3. – Measured longitudinal phase space for a two μ-bunches comb beam, 80 pC/bunch, inthe over-compression regime. The bunch inter-distance is 0.8 ps.THE SPARC LAB THz SOURCE DRIVEN BY HIGH-BRIGHTNESS ELECTRON BEAMS 353Fig. 4. – Autocorrelation function of the CTR spectrum as generated by a two μ-bunches train.The interferogram is measured by means of a Michelson interferometer.The retrieved form factor (fig. 5) is then peaked at the comb repetition frequency,i.e. 1.2THz, whose bandwidth depends on both the modulation of the bunches in thetrain and their length. In the case shown in fig. 5, the large bandwidth depends on themulti-shot nature of the technique.The advanced THz radiation, produced at the SPARC LAB test facility, is char-acterized by 100 fs duration pulses, few hundreds of MW peak power, several tens ofmicro-Joule energy per pulse, broad and narrow bandwidth. Beyond the use of THzradiation for electron beam longitudinal diagnostics, all the features mentioned makethe SPARC LAB THz radiation strongly competitive with respect to conventional THzsources and extremely suitable for investigating non-linear THz spectroscopy and fortesting novel detectors and materials.0 2 4 6 800.20.40.60.81Frequency (THz)Form FactorFig. 5. – Retrieved form factor, peaked at the comb repetition frequency, i.e. 1.2THz.354 E. CHIADRONI3. – ConclusionsCoherent THz radiation is currently produced and optimized at SPARC LAB throughultra-short relativistic electron bunches as both transition and diffraction radiation. Dif-ferent THz emission regimes, both broad band and narrow band, have been achieved byproperly control electron beam shaping, length and charge, therefore by properly set thephoto-injector parameters.∗ ∗ ∗The author would like to thank all the people which contributed to this work, inparticular the whole SPARC team and the TeraLab group (Phys. Dept., University ofRome La Sapienza). This work has been partially supported by the EU Commissionin the Seventh Framework Program, Grant Agreement 312453 - EuCARD-2, Contractn.283745-CRISP and by the Italian Ministry of Research in the framework of FIRB-Fondoper gli Investimenti della Ricerca di Base, Project no.RBFR12NK5K.REFERENCES[1] Williams G. P., Rep. Prog. Phys., 69 (2006) 301.[2] Ferrario M. et al., Nucl. Instrum. Methods Phys. Res. B, 309 (2013) 183.[3] Sherwin M. S. et al., “Opportunities in THz Science”, in Report of a DOE-NSF-NIHWorkshop, edited by Sherwin M. S., Schmuttenmaer C. A. and Bucksbaum P. H.(Arlington, VA) 2004.[4] Chiadroni E., Bunch Length Characterization at the TTF VUV-FEL, in TESLA-FEL2006-09 (2006).[5] Giannessi L. et al., Phys. Rev. Lett., 106 (2011) 144801.[6] Massimo F. et al., Nucl. Instrum. Methods Phys. Res. A, 740 (2014) 242.[7] Rossi A. R. et al., Nucl. Instrum. Methods Phys. Res. A, 740 (2014) 60.[8] Bacci A. et al., J. Appl. Phys., 113 (2013) 194508.[9] Chiadroni E. et al., Appl. Phys. Lett., 102 (2013) 094101.[10] Chiadroni E. et al., Rev. Sci. Instrum., 84 (2013) 022703.[11] Serafini L. and Ferrario M., AIP Conf. Proc., 581 (2001) 87.[12] Ferrario M. et al., Phys. Rev. Lett., 104 (2010) 054801.[13] Ferrario M. et al., Nucl. Instrum. Methods Phys. Res. A, 637 (2011) S43.[14] Petrillo V. et al., Phys. Rev. Lett., 111 (2013) 114802.

The SPARC LAB THz source driven by high-brightness

electron beams

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The SPARC LAB THz source driven by high-brightness electron beams

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