We propose a versatile THz source named FLUTE (Ferninfrarot Linac- Und Test-Experiment) based on a 30 - 50 MeV S-band linac with bunch compressor, that aimsat not only producing high field THz pulses but also at serving as a test facility to study accelerator physics issues. This source is an important step towards the planned ultra-broadband THz to mid infrared user facility TBONE. Special emphasis is put on studies of bunch compression as a function of bunch charge (0.1-5 nC) and of different generation mechanisms of coherent radiation (CSR, CER, CTR). This paper describes the design and layout of the proposed FLUTE machine and presents results of beam dynamic calculations with the tracking programs ASTRA and CSRtrack. In addition, calculations for the achievable peak electrical field and spectral characteristics for one version of the FLUTE layout are shown

Abela, R.

Baumbach, T.

Braun, H.H.

Ganter, R.

Gies, A.

Hagelstein, M.

Hiller, N.

Huttel, E.

Judin, V.

Kehrer, B.

Kubat, R.

Marsching, S.

Mexner, W.

Müller, A.S.

Naknaimueang, S.

Nasse, M.

Patterson, B.

Plech, A.

Rossmanith, R.

Schmelling, M.

Schuh, M.

Schwarz, M.

English

Müller, A. S.

Braun, H. H.

KITopen

FLUTE, A LINAC BASED THz SOURCES. Naknaimueang∗, T. Baumbach, A. Gies, M. Hagelstein, N. Hiller, E. Huttel, V. Judin, B. Kehrer,R. Kubat, S. Marsching, W. Mexner, A.-S. Müller, M. Nasse, A. Plech, R. Rossmanith, M. Schuh,M. Schwarz, Karlsruhe Institute of Technology, GermanyM. Schmelling, Max-Planck Institute for Nuclear Physics, Heidelberg, GermanyR. Abela, R. Ganter, H. H. Braun, B. Patterson, Paul Scherrer Institute, SwitzerlandAbstractWe propose a versatile THz source named FLUTE (Fer-ninfrarot Linac- Und Test-Experiment) based on a 30 -50 MeV S-band linac with bunch compressor, that aimsat not only producing high field THz pulses but also atserving as a test facility to study accelerator physics is-sues. This source is an important step towards the plannedultra-broadband THz to mid infrared user facility TBONE.Special emphasis is put on studies of bunch compressionas a function of bunch charge (0.1-5 nC) and of differentgeneration mechanisms of coherent radiation (CSR, CER,CTR). This paper describes the design and layout of theproposed FLUTE machine and presents results of beam dy-namic calculations with the tracking programs ASTRA andCSRtrack. In addition, calculations for the achievable peakelectrical field and spectral characteristics for one versionof the FLUTE layout are shown.THE CONCEPT OF FLUTEKIT (Karlsruhe Institute of Technology) has a long tra-dition in generating [1] and using [2] coherent IR and THzradiation. The coherent radiation is produced with thestorage ring ANKA operating in the so-called low alphamode: sub-picosecond electron bunches are produced byreducing the momentum compaction factor α of the storedbeam. The sub-picosecond long bunches emit coherentsynchrotron radiation.Recently, PSI and KIT discussed the possibility tobuild a compact linac-based coherent THz source namedFLUTE. Built at KIT, it could eventually be used later forpump-probe experiments at the SwissFEL [3] while KITdevelops TBONE [4], a THz source based on a supercon-ducting linac.The general parameter list for FLUTE ismax. repetition rate: 100 Hzpulse length: 0.2-1 psmax. pulse charge: 5 nCmax. beam energy: 50 MeVThe general layout of FLUTE is shown in fig. 1. Withan RF laser gun operating at 3 GHz a single bunch witha bunch charge of up to 5 nC is produced. The bunch isaccelerated in the 2 1/2 cell gun to 7 MeV.After a beam diagnostic area the beam enters a 3 GHzlinac and is accelerated to about 45 MeV out of crest. The∗Somprasong.Naknaimueang@kit.eduFigure 1: The concept of FLUTE with a 21/2 cell RF photo-gun and 3 GHz linear accelerator followed by a bunch com-pressor. The total length of FLUTE is about 12 m.bunch, originally several ps long, is afterward compressedin a bunch-compressor to about 300 fs. Different compres-sor layouts, e.g. with a two or a four magnet chicane, willbe investigated. In the following, the four magnet versionwill be discussed.The source of the coherent radiation can be either thesynchrotron radiation at the last bending magnet, edge ra-diation or transition radiation from a foil. The differentsource mechanisms will be studied for the FLUTE beam intheory and experiment.TECHNICAL LAYOUT OF FLUTEThe photo-injector foreseen for FLUTE is the CTF3 gundeveloped for the CLIC test facility [5]. The electron beamis accelerated in the electron gun to 7 MeV.The gun cavity is powered by a 2μs long pulse from aklystron. The estimated 45 MW power from this klystronis split: 22 MW are sent to the gun and 20 MW to the 5.2 mlong S-band linac with 156 cells. The maximum energyof the linac is 50 MeV. For the gun a Ti:Sapphire laser isforeseen with a pulse length of 1 to 4 ps, an energy perpulse of 200μJ, and a repetition rate of up to 100 Hz. Withthese parameters the CTF3 gun reaches gradients of up to100 MV/m. The bunch charge can be as high as 5 nC, usinglarge laser spot size of up to 10 mm full width.BEAM SIMULATIONSThe particles emitted in the 21/2 cell laser gun weretracked with ASTRA [6] through the linac and with CSR-track [7] through the bunch compressor. The bunch is ac-TUPO007 Proceedings of IPAC2011, San Sebastián, Spain1458Copyrightc ○2011byIPAC’11/EPS-AG—ccCreativeCommonsAttribution3.0(CCBY3.0)02 Synchrotron Light Sources and FELsA14 Advanced Concepts0.0280 0.0281 0.0282 0.0283 0.0284 0.028556789Bunch length [um]R_56 [m]3 nCFigure 2: Bunch length (rms) in dependence of R56for a charge of 3 nC. The shortest bunch length is atR56 =0.02825m.celerated in the linac 20 degree off crest. ASTRA calcu-lates the rms bunch length Δsi and the momentum spreadΔp/p at the end of the linac. The following equation givesa rough estimation of the final bunch length Δsf after thechicane:Δsf = Δsi +R56Δpp, (1)R56 =4LB(Θ − tanΘ)sinΘ− 2LD. tan2 ΘcosΘ. (2)Eq. (2) shows the dependence of R56 as a function of thedeflecting angle Θ in the compressor magnet, the lengthof the magnet LB , and the drift space LD for the bunchcompressor. The optimized chicane was found with LB =LD = 0.5m and a total length of 4 m. The optimum choiceof bending radius, derived from Θ and LB, differs for eachbunch current and has to be adapted. Fig. 2 shows the re-sults for a bunch charge of 3 nC and an energy of 41 MeV.Since Eq. (1) does not take into account space charge andCSR effects in the chicane, the selection of bending radiusto achieve maximum compression is followed by a full sim-ulation of the chicane in CSRtrack.Fig. 3 shows the minimum achievable bunch length ob-tained from this procedure as a function of bunch charge.It is clearly visible, that the higher charge has less com-pression than the lower charge. Above 3 nC, the coherentsynchrotron radiation effect has a strong influence on thebunch length.Fig. 4 shows the longitudinal charge profile at the end ofthe optimized bunch compressor for bunch charges 0.1 nCand 3 nC, respectively. The number of particles rises ex-ponentially to a maximum value followed by a sharp drop.The widening due to space charges is clearly visible. Forthe calculation of the peak electric field it is convenient toapproximate the profiles by Gaussians. However, for thecalculation of the spectral power we use the more accurateapproximation by a saw-tooth. The CSR spectrum is pro-portional to the square of the number of particles N e in thebunch and the form factor F (ω), which is the Fourier trans-form of the particle distribution function. Since the formfactor of a saw-tooth like bunch falls off only as ω−4, incontrast to the exponential decrease for a Gaussian bunch,the CSR spectrum extends to higher frequencies.0 1 2 3 4 5150300450600750900Longitudinal Bunch Length [fs]Bunch Charge [nC]Figure 3: Relation between compressed bunch (rms) andbunch charge.1.8 2.0 2.2 2.4 2.6 2.8 3.0t ps 0.20.40.60.81.01.2 part icles 1072 3 4 5 6t ps 123456 part icles 107Figure 4: Longitudinal charge profile for bunches with totalcharge of 0.1 nC (blue) and 3 nC (red). Dashed curves areGaussian approximations with the same total charge andσ = 0.1 ps (blue) and σ = 0.6 ps (red).SPECTRUM AND FIELD STRENGTHFig. 5 shows the calculated CSR power spectra emittedby the 3 nC and 100 pC bunches. As expected for smallfrequencies, the CSR is enhanced by a factor Ne com-pared to the incoherent radiation. The spectral suppres-sion due to the form factor sets in at frequencies above c/lbunch ∼ 0.4THz and 1.5 THz. Because of the above men-tioned slow decrease of the form factor, the coherent radia-tion dominates the spectrum up to frequencies of 100 THz.If the bunches were of a Gaussian shape instead, the in-coherent spectrum would become dominant already at fre-quencies of 10 THz, as shown by the dashed curve in fig. 5.Since the number of particles in the 3 nC bunch is 30times higher than that of the 100 pC bunch, the coherentpower is about 900 times higher for frequencies below0.1 THz, where the form factors are very close to unity.However, the bunch length of the 3 nC bunch is longer andthe drop is not as sharp as that of the 100 pC bunch (seeProceedings of IPAC2011, San Sebastián, Spain TUPO00702 Synchrotron Light Sources and FELsA14 Advanced Concepts 1459 Copyrightc ○2011byIPAC’11/EPS-AG—ccCreativeCommonsAttribution3.0(CCBY3.0)0.01 0.1 1 10 100ΝTHz1013109105peak power WHzFigure 5: Power spectrum of the 3 nC (red) and 100 pC(blue) bunch. The points mark the power at frequenciescorresponding to the bunch lengths (0.4 THz and 1.5T Hz).The dashed curve depicts the spectrum of the Gaussian ap-proximation of the 100 pC bunch with σ = 10THz.fig. 4). This implies that the form factor suppression sets inearlier (at 0.4 THz) and is steeper, and the coherent powersbecome comparable at 1 THz.The time dependence of the electric field, resulting fromcoherent synchrotron radiation, is of interest in pump-probe experiments. Following [8], we calculate the timedependence of the electric field E(t) emitted by a bunchwith charge distribution ρ(x) asE(t) ∼ cosφReA(t) + sinφImA(t) ,A(t) ≡∫ ∞0dω∫ ∞−∞dxs(ω)ρ(x)e−iω(t−x/c) .Here, s(ω) ∝ ω1/6 is the low frequency synchrotron ra-diation spectrum. The tunable phase φ depends on the ma-chine parameters, and is determined by the local accelera-tion process of the emitting electrons. To perform the in-tegrations analytically, we approximated the saw-tooth likebunch profile by three line segments. The resulting pulseshape for 100 pC is shown in fig. 6. To obtain the asym-metric pulse we chose φ = 325 deg. The marked points onthe t-axis correspond to the t−values of the points we usedin the approximation of ρ. The cusps at these points are ar-tifacts of this approximation. Notice that the field is sizableonly during times corresponding to the bunch length.To obtain the peak electric field E0, we use the followingequation, valid for a Gaussian bunch shape,E20ε0cS ≈Wc3.8σ, (3)where W is the total pulse energy radiated into the areaS. Thus, we determine the peak field strength to be3.012 × 108 V/m and 7.495 × 107 V/m for the 3 nC and100 pC bunch, respectively.It is remarkable, that even though the power ratio is900, the electric fields for the two situations differ onlyby a factor of 4. Fig. 7 shows the dependence of thepeak electric field on bunch length and charge for Gaussianbunch shapes. The regions of constant field for differentcharge/length combinations are clearly visible.  1.5 2.0 2.5 3.0 3.5 4.0t ps5.0  10171.0  10181.5  10182.0  1018E a .u Figure 6: Pulse shape of the electric fields emitted by the100 pc bunch with φ = 325 deg. For the cusps see text.The dashed curve shows the field for the Gaussian approx-imation.-310-210-1101-110 1-110bunch charge / nC bunch length / psFigure 7: Strength of the electric field in GV/m (color) forGaussian bunches according to Eq. (3). The markers denotethe fields for the Gauss equivalent bunches of Fig. 4.SUMMARYFLUTE is a planed compact linear accelerator for theproduction of broadband coherent THz radiation with highpeak fields. We presented the basic machine setup andperformed simulations, showing that FLUTE can providebroadband THz radiation with peak field strengths of theorder of 3× 108 V/m.REFERENCES[1] A.-S. Müller et al., Proc. EPAC 2006, Edinburgh, p. 2868.[2] Y.-L. Mathis et al., UVSOR workshop on coherent syn-chrotron radiation, Myodaiji, Japan, 2007.[3] R. Ganter, SwissFEL Conceptional Design Report, PSIBericht 10-04, 2010.[4] A.-S. Müller, Proc. PAC09, Vancouver, p. 1153.[5] R. Bossard, M. Dehler, Proc. EPAC 96, Sitges, Spain.[6] K. Flöttman, ASTRA, http://www.desy.de/∼mpyflo/[7] M. Dohlus, T. Limberg, CSRtrack, DESY[8] A.-S. Müller, RAST, 3, (2011).TUPO007 Proceedings of IPAC2011, San Sebastián, Spain1460Copyrightc ○2011byIPAC’11/EPS-AG—ccCreativeCommonsAttribution3.0(CCBY3.0)02 Synchrotron Light Sources and FELsA14 Advanced Concepts

FLUTE, a linac based THz source

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FLUTE, a linac based THz source

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