We present the study of the optimization of the optical cavity parameters, in order to maximise the flux of scattered photons in the Compton scattering process. In the optimisation, we compensate the losses of the photon number due to the elliptical shape of the laser pulse in optical cavity with a high focusing electron beam

Adolfo Esposito

Alberto Bacci

Alberto Tagliaferri

Alessandro Gallo

Andrea Rossi

Angelo Bosotti

Angelo Taibi

Antonella Delvecchio

Antonio Sarno

Camilla Curatolo

Cristina Vaccarezza

Daniele Sertore

Dario Giannotti

Dario Giove

Ezio Puppin

Francesco Broggi

Francesco Prelz

Gianfranco Paternò

Gianluca Galzerano

Giorgio Turchetti

Giovanni Mettivier

Illya Drebot

Laura Monaco

Luca Serafini

Luigi Faillace

Marcello Rossetti Conti

Marco Statera

Massimo Placidi

Mauro Gambaccini

Paolo Cardarelli

Paolo Michelato

Paolo Russo

Riccardo Calandrino

Rocco Paparella

Simone Cialdi

Verardo Torri

Vittoria Petrillo

English

Gianfranco Paternò

Archivio istituzionale della ricerca - Politecnico di Milano

OPTIMISATION STUDY OF THE FABRY-PÉROT OPTICAL CAVITY FORTHE MARIX/BRIXS COMPTON XRAY SOURCEI. Drebot∗, A. Bacci, A. Bosotti, F. Broggi, C. Curatolo, L. Faillace, D. Giannotti, D. Giove,P. Michelato, L. Monaco, R. Paparella, F. Prelz, A. R. Rossi, L. Serafini,D. Sertore, M. Statera, V. Torri, INFN, Milano, ItalyS. Cialdi, V. Petrillo, M. Rossetti Conti, Universitá degli Studi di Milano & INFN, Milano, ItalyG. Galzerano, E. Puppin, A. Tagliaferri, Politecnico di Milano, Milano, ItalyA. Esposito, A. Gallo, C. Vaccarezza, INFN/LNF, Frascati Roma, ItalyM. Placidi, G. Turchetti, Universitá di Bologna, Bologna, ItalyG. Mettivier, P. Russo, A. Sarno, Universitá di Napoli & INFN, Napoli, ItalyP. Cardarelli, M. Gambaccini, G. Paternó, A. Taibi, Universitá di Ferrara & INFN, Ferrara, ItalyR. Calandrino, A. Delvecchio, Ospedale San Raffaele, Milan, ItalyAbstractWe present the study of the optimization of the opticalcavity parameters, in order to maximise the flux of scatteredphotons in the Compton scattering process. In the optimisa-tion, we compensate the losses of the photon number due tothe elliptical shape of the laser pulse in optical cavity with ahigh focusing electron beam.INTRODUCTIONMariX (Multidisciplinary Advanced Research Infrastruc-ture with X-rays) is a project of INFN and University ofMilan [1, 2] and has to be constructed at the new scientificcampus at the ex-EXPO site in Milan in the next years. Thefirst component of the X-source MARIX is BriXS (Brightand compact X-ray Source), a double Compton X-ray sourcebased on superconductive cavities technology for the elec-tron beam with energy recirculation and on a laser system inFabry-Pérot cavity at a repetition rate of 100 MHz, produc-ing 20-100 keV radiation. The BriXS accelerator constitutesthen the injector of a superconductive linac which drives aX-ray FEL at 1 MHz, for providing coherent, moderate fluxradiation at 0.3-10 KeV at 1 MHz. The joint presence ofa Compton source and of an hard and soft X-ray FEL willserve a multitude of users, in many fields of science whileits characteristics are described in detail in [2].A main characteristic of this machine is the very highaverage current. The double Compton X-ray sources will op-erate at 100 MHz, with 200 pC electron bunches that means20 mA. These Compton sources are designed to operatewith an electron maximum energy of 100 MeV, which fora 20 mA of current means 2 MW. Such a high beam powercannot be dumped without deceleration, and together withthe CW (Continuous Wave) regime, it justifies an ERL (En-ergy Recovery Linac) machine. Our first analysis is basedon a projects like CBETA ERL crymodule [3].The focus on enabled applications by such an energy rangeand brilliance is on medical oriented research/investigations,mainly in the radio-diagnostics and radio-therapy fields, ex-∗ illya.drebot@mi.infn.itploiting the unique features of monochromatic X-rays, aswell as in micro-biological studies, and, within this main-stream, material studies, crystallography and museology forcultural heritage investigations. In this paper, the layoutand the typical parameter of the BriXS X-ray source will bediscussed.LAYOUTThe BriXS layout, shown in Fig. 1, consists in two sym-metric beam lines, fed from two independent photo injectors,where two symmetric (and coupled) Energy Recovery Linacs(ERL) accelerate the beams. The two ERLs are coupled,accelerating and decelerating (recovery) in a push-and-pullscheme. In this unconventional ERL scheme, beams arecounter-propagating and bunches coming from guns are ac-celerated; those coming from the twins ERLs are deceleratedand brought simultaneously to a single beam-dump. Thispush-and-pull coupled scheme permits to drive two Comp-ton X-ray sources with the same degree of freedom of a linacdriven source, in terms of energy and electron beam qual-ity. Furthermore, the coupled ERL fed by two independentRF system is more stable. Partial beam line modificationsto host additional Compton source Interaction Points (IP)are still under study. CW electron guns capable to produce20 mA average beam current, as BriXS needs, are not yetstate of the art. Some of the most promising photo cathodeguns [4] as the Cornell DC gun [5] and the RF CW Apexgun [6] have been compared by simulations.Figure 1: Scheme of BriXS layout.9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW PublishingISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-THPMF056THPMF0564192ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.02 Photon Sources and Electron AcceleratorsA24 Accelerators and Storage Rings, OtherELECTRON BEAM LINEThe beam lines of BriXS are presented in Fig. 2 andcomprises: 1) a triplet of quadruples at the exit of super-conductive linac matches the beam after the linac to thefirst dog-leg and permits a quadrupole-scan for emittancediagnostic; 2) chicane (dog-leg) with a 20◦ angle to make atranslation of the beam to the IP region; the electron beamparameters at IP presented in Table 1; 3) the IP line is con-stituted by a pair a strong focusing quadruple triplets witha quite long distance in order to host Fabry-Pérot OpticalCavity; 4) Double bend achromat (DBA) section with two90◦ dipole magnets; 5) long chicane (dog-leg) with a 20◦angle to make a translation of the beam to the second super-conductive linac.Figure 2: Principle scheme of magnetic elements of BriXS beam line.The quadruple triplets in the IP region are equal and in-stalled with a mirror symmetry. This structure have identity transport matrix which means that we can use different focus-ing at IP without modification the other magnetic elements after the IP region.Table 1: Table of Electron Beam ParametersElectrons mean energy [MeV] 70-100Bunch charge [pC] 200Bunch length rms [μm] 600Nominal normalized εnx , εny [mm·mrad] 0.99, 0.99Nominal relative energy spread σe % 0.2Focal spot size σx, σy μm 14.9, 14.9Repetition rate [MHz] 100LASER SYSTEMThe optical system of MariX is designed to guide boththe Compton and the FEL sources with the use of a singlelaser oscillator. Concerning the Compton source, the lasersystem delivers the two 100 MHz outputs at 257.5 nm forthe two RF-guns and that at 1030 nm for the high finessecavity. The whole laser is based on Yb-fiber active mediumin order to reach the high mean power due to the 100MHzreprate of the system. This system allow for direct pumpingwith semiconductor lasers at 975 nm, resulting in a highoverall efficiency, relatively low cooling requirements, andcompact footprint. The output dedicated for the FEL isbased on the same technology but the reprate is reduced at1MHz by a commercial amplitude modulator based on aintegrated-optical waveguides Mach-Zender interferometer.In Fig. 3 is showed the scheme of the laser system. Theoscillator is a commercial mode-locked Yb-fiber laser thatdelivers a train of pulses at 100 MHz and 1030 nm. Thanwe have three amplifier systems respectively dedicated tothe high finesse cavity, the two RF-guns for the Compton,and the line for the FEL system that drive one of the two RF-guns. The amplifiers are based on the CPA (chiperd pulseamplification) scheme. The stretcher and the compressorare made by two CVBG (chirped volume Bragg grating)devices and the gain medium is based on a LMA (largemode area) fiber doped with Yb. With this scheme it ispossible to reach the required mean power of 200 W withvery high stability. The fiber technology, beside the thermaladvantages, allows to obtain both optimal spatial qualityand pointing stability, and this is of fundamental importanceto coupling efficiently the high finesse cavity and to drivethe RF-guns. In order to generate the electron bunches byphotoemission the amplified radiation is upconverted up tothe 4th harmonic (257.5 nm) by two nonlinear crystals, oneLBO crystal in non-critical phase matching configuration,and a couple of CLBO crystals. Both the temporal andthe spatial shaping of the laser pulse are implemented inorder to reach the minimum level of the emittance in theelectron bunches. The temporal shaping is obtained by thestacking method [7] with three alpha-BBO crystals and thespatial shaping is based on a commercial pi-shaper deviceand an imaging system. The reprate of the laser oscillatoris stabilized respect to an external reference by a feedbacksystem. In the case of the Compton the external referencecould be an RF reference, but in the case of a pump andprobe experiment with the FEL system, due to the demandof a timing at fs level [8], we need to stabilize the laser bya balanced cross correlator [8] with respect to an opticalmaster oscillator. A second feedback system is needed inorder to stabilize the high finesse cavity with respect to thelaser oscillator. This second feedback is based on the PDH(Pound Drevel Hall) [9] method and a f2f system [10]. Thefirst one is exploited in order to stabilize the reprate of thehigh finesse cavity, an the second one in order to remove thecarrier-envelop-offset from the oscillator spectral comb andin turn to superimpose the oscillator spectral comb with thatof the cavity.HIGH FINESSE CAVITYThe high finesse cavity we are planing to use for the Comp-ton is a 4 mirrors bow-tie cavity in the crossed configuration(see Fig. 4).The mirrors M1, M4 are plane, and the mirrors M2 andM3 are concave with a radius of curvature of 750 mm. Thiskind of cavity allows to obtain a very high mechanical stabil-ity, an also provides high flexibility for the adjustments of thecavity round trip frequency (witch must be matched to that9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW PublishingISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-THPMF05602 Photon Sources and Electron AcceleratorsA24 Accelerators and Storage Rings, OtherTHPMF0564193ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.Figure 3: laser fig 1.Figure 4: laser fig 2.of the electrons, that is fixed at 100 MHz) and the laser beamwaist. Indeed, it turns out that a length variation between thetwo flat mirrors changes the cavity round trip length withoutchanging noticeably the laser beam waist. The laser beamwaist can thus be set by tuning the distance between thetwo spherical mirrors independently of the cavity round-triplength. Moreover when this cavity operates near the edgeof the confocal configuration we obtain at the same time theminimum dimension in the focal point and the maximum di-mension on all the 4 mirrors allowing the working at the veryhigh mean power required for the Compton, staying belowthe damage threshold of the mirrors. We plain to inject thecavity with 200 W in order to obtain an internal mean powerof 500 kW by a gain factor of 2500. In order to minimize thethermal deformation [11], and in turn the variation of themirrors radius of curvature, the substrate of the mirrors M2,M3, M4 is made of ULE (ultra low expansion material) andthat of the M1 mirror of Sapphire to transmit efficiently theinput beam. Due to the mirrors encumbrance the collisionangle will be around 7 degree and this introduces an ellip-ticity in the cavity as function of spatial shift δH betweentwo curved mirrors mode as showed in the Fig. 5. due to theincidence angle on the curved mirrors.Table 2: Table of Laser ParametersLaser pulse energy (mJ) 5Laser pulse length [psec] 2Laser focal spot size w0 RMS [μm] 50Laser papameter α0 0.2Collision angle [deg] 7-10RADIATION SPECTRAThe number of scattered photons (flux) depends on thetransverse size of interacting beams. Due to the fact thelaser spot size in the waist has an elliptical profile, we tryto focus as much as possible the electron beam at IP with around or a similar elliptical profile. Results of simulationsare presented in Fig. 5. One of the main goal of this studyis that electron beam with σx = 25 μm, σy = 15 μm hasalmost the same flux as σx = 15 μm, σy = 15 μm. It meansthat we can have not so strong focusing in the horizontaldimension of electron beam at IP obtaining the same numberof scattered photons.Figure 5: (Blue) Photon flux as function of spatial shift δHbetween two curved mirrors. (Red) Laser waists as functionof δH.Compton back scattering radiation has energy angulardistribution. This quantity make possible to get a monochro-matic radiation by using a collimator. The parameters of thescattered radiadion are presented in Table 3.Table 3: Table of Photon ParametersPeak spectrum [keV] 88Mean spectrum [keV] 85Collimation angle θmax [μrad] 2.9Bandwidth [keV] 4.3Bandwidth % 5# photons per shot Nph_tot 1.5 105# photons per shot after collimation Nph 2.7 104Source rms divergence θRMS [mrad] 1.94 103Spot Size at 10 m [mm] 14.3, 13.1Rad. pulse length σγz [psec] 1.49CONCLUSIONIn this paper conceptual design study are presented. Wehave demonstrated that optimizing a focusing triplet get anelliptical electron spot profile. Using elliptical profile weare in profit.REFERENCES[1] L. Serafini, BriXS: BRIght and compact X-ray Source Expres-sion of Interest 1, DOI:10.13140/RG.2.2.29979.468849th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW PublishingISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-THPMF056THPMF0564194ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.02 Photon Sources and Electron AcceleratorsA24 Accelerators and Storage Rings, Other[2] L. Serafini et al.,“ The MariX source (Multidisciplinary Ad-vanced Research Infrastructure with X-rays)”, presented atIPAC’18, Vancouver, Canada, May 2018, paper THPMF058,this conference.[3] G. Hoffstaetter, et al., “CBETA Design Report”, BNL-114549-3027-IR, CBETA/015b, June 2017.[4] B. Dunham, et al., “Record high-average current from ahigh-brightness photoinjector”, Appl. Phys. Lett., vol. 102,p. 034105 , 2013.[5] C. Gulliford, et al., “Demonstration of cathode emittancedominated high bunch charge beams in a DC gun-based pho-toinjector”, Appl. Phys. Lett., vol. 106, p. 094101, 2015.[6] F. Sannibale, et al., “ Upgrade options towards higher fieldsand beam energies for continuous-wave room-temperatureVHF RF guns”, in Proc. IPAC’17, Copenhagen, Denmark,May 2017, paper MOPIK019.[7] A. K. Sharma, et al., “Theoretical and experimental study ofpassive spatiotemporal shaping of picosecond laser pulses”,Phys. Rev. ST Accel. Beams, vol. 12, p. 033501, 2009.[8] S. Schulz, et al., “Femtosecond all-optical synchronization ofan X-ray free-electron laser”, Nature Comm., vol 6, p. 5938,DOI:10.1038/ncomms6938[9] Eric D. Black, “An introduction to Pound–Drever–Hall laserfrequency stabilization”, Am. J. Phys., vol. 69, p. 79, 2001.[10] H.R. TelleG, et al., “Carrier-envelope offset phase control: Anovel concept for absolute optical frequency measurementand ultrashort pulse generation”, Appl. Phys. B., vol. 69,p. 327, 1999.[11] H. Carstens, et al., “Megawatt-scale average-power ultrashortpulses in an enhancement cavity”, Opt. Lett., vol 39, p. 2595,2014.9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW PublishingISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-THPMF05602 Photon Sources and Electron AcceleratorsA24 Accelerators and Storage Rings, OtherTHPMF0564195ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.

Optimisation Study of the Fabry-Pérot Optical Cavity for the MARIX/BRIXS Compton X-Ray Source

Archivio istituzionale della ricerca - Università di Ferrara

https://re.public.polimi.it/bitstream/11311/1231979/1/I.Drebot%2c...%2cA.Tagliaferri%2c...%2cA.Taibi%2cFabry-P%c3%a9rot_Optical_Cavity_for_the_MARIXBRIXS_Compton_X-Ray_Source%2cIPAC2018-Proc.9thInt.Part.Accel.Conf.%2c0%e2%80%933%282018%29.pdf

Optimisation Study of the Fabry-Pérot Optical Cavity for the MARIX/BRIXS Compton X-Ray Source

Optimisation Study of the Fabry-Pérot Optical Cavity for the MARIX/BRIXS Compton X-Ray Source

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