Virtual bunch length measurement can be carried out by means of ELEGANT code for tracking the bunch particles from RF deflector to the screen. The technique relies on the correlation between the bunch longitudinal coordinate and transverse coordinates induced through a RF deflector. Therefore, the bunch length measurement can be carried out measuring the vertical spot size at the screen, placed after the RF deflector. The deflecting voltage amplitude affects the resolution. Adding a dispersive element, e.g. a magnetic dipole between RF deflector and the screen, the full longitudinal phase space can be measured. In this paper, we discuss some issues relevant for the electron linac of the Compton source at the Extreme Light Infrastructure - Nuclear Physics (ELI-NP)

Alesini D.

Arpaia P.

Franzini G.

Giribono A.

Liccardo A.

Mostacci A.

Palumbo L.

Sabato L.

Vaccarezza C.

Variola A.

English

Archivio della ricerca- Università di Roma La Sapienza

LONGITUDINAL PHASE SPACE MEASUREMENT AT THE ELI-NPCOMPTON GAMMA SOURCEL. Sabato1∗, University of Sannio, Dept. of Engineering, Corso G. Garibaldi, 107, Benevento, ItalyP. Arpaia, A. Liccardo, Federico II University of Naples,Dept. of Electrical Engineering and Information Technology, Via Claudio 21, Naples, ItalyD. Alesini, G. Franzini, C. Vaccarezza, A. Variola, Istituto Nazionale di Fisica Nucleare -Laboratori Nazionali di Frascati (INFN-LNF), via Enrico Fermi, 40, Frascati, ItalyA. Giribono, A. Mostacci, L. Palumbo, Sapienza University of Rome,Dept. Scienze di Base e Applicate per l’Ingegneria (SBAI), Via Antonio Scarpa, 14, Rome, Italy1also at Istituto Nazionale di Fisica Nucleare (INFN) Naples, via Cintia, Naples, ItalyAbstractVirtual bunch length measurement can be carried out bymeans of ELEGANT code for tracking the bunch particlesfrom RF deflector to the screen. The technique relies onthe correlation between the bunch longitudinal coordinateand transverse coordinates induced through a RF deflector.Therefore, the bunch length measurement can be carriedout measuring the vertical spot size at the screen, placedafter the RF deflector. The deflecting voltage amplitudeaffects the resolution. Adding a dispersive element, e.g. amagnetic dipole between RF deflector and the screen, thefull longitudinal phase space can be measured. In this paper,we discuss some issues relevant for the electron linac ofthe Compton source at the Extreme Light Infrastructure -Nuclear Physics (ELI-NP).INTRODUCTIONThe Gamma Beam Source (GBS) at ELI-NP is going tobe an advanced Source of up to 20MeV Gamma Rays basedon Compton back-scattering, i.e. collision of an intense highpower laser beam and a high brightness electron beam withmaximum kinetic energy of about 720MeV. This infrastruc-ture is going to be built in Magurele, near Bucharest (Ro-mania) [1, 2]. The GBS electron linac can run at maximumrepetition rate of 100Hz. Therefore, at room temperaturethe specifications on the requested spectral density can bereached only by multiple bunch collisions. The final opti-mization foresees trains of 32 electron bunches separatedby 16 ns, time needed to recirculate the laser pulse in orderto allow the same laser pulse to collide with all the elec-tron bunches in the RF pulse, distributed along a 0.5 µs RFpulse [1].The properties of the single bunch and the whole trainof bunches have to be measured in order to achieve highbrightness in high repetition rate machine [3, 4]. In particu-lar, bunch length measurement can be done using a RadioFrequency Deflector (RFD) and a screen in an electron linac.This disruptive measurement technique is well-known andwidespread used in high brightness Linacs around the world,e.g. at the SLAC free electron laser [5,6] or at SPARC-LABlinac [7].∗ luca.sabato@unisannio.itIn this paper, the effect of a non-negligible energy chirp onbunch length virtual measurements is treated. Moreover, theenergy chirp affects energy spread measurements. The sim-ulations are carried by means of ELEctron Generation ANdTracking (ELEGANT) code [8]. In section MEASURE-MENT TECHNIQUE, the basic idea, the working prin-ciple and the procedure of the bunch length measurementtechnique using a RFD are explained. In section SIMU-LATION RESULTS, the RFD and bunch parameters ofGBS linac case are reported and the bunch length virtualmeasurements are discussed.MEASUREMENT TECHNIQUEBasic IdeaDifferent types of measurements can be done with a RFD.Bunch length measurements can be done using only a RFDand a screen. Adding a dispersive element, e.g. a magneticdipole between RF deflector and the screen, the full lon-gitudinal phase space can be measured. The basic idea ofthese measurements is based on the property of the RFDtransverse voltage to introduce a correlation between the lon-gitudinal and vertical coordinates of the bunch at the screenposition. Therefore, the bunch length measurement can becarried out measuring the vertical spot size at the screen,placed after the RF deflector [7, 9].Working PrincipleWhen the particles pass through the RFD, they feel adeflecting voltage when they pass through the RFD. Theeffect on every particle is a change in vertical divergence[10]. Considering the bunch length much smaller than RFwavelength (i.e. kz0  1), we can assume the RFD voltageis [6, 11]:V (z0) ≈ Vt [kz0cos(ϕ) + sin(ϕ)] . (1)where z0 is the position of the particles along the beam axiswith the origin in the RFD, k = 2π/λRF , λRF , Vt , and ϕare the deflecting voltage wavelength, amplitude, and phase,respectively.Therefore, RFD gives a vertical divergence change [6]:∆y′0 (z0) = Cr f d [kz0cos(ϕ) + sin(ϕ)] , (2)WEPG44 Proceedings of IBIC2016, Barcelona, SpainISBN 978-3-95450-177-9732Copyright©2016CC-BY-3.0andbytherespectiveauthorsMachine Parameter Measurementswhere Cr f d = qVt/(pc), q is the electron charge, p is theparticle momentum, and c is the speed of light. Assum-ing 〈z0〉 = 〈y0〉 =0m and 〈y′0〉 =0 rad, the vertical bunchcentroid at screen is [6]:Cys = LCr f dsin(ϕ). (3)Assuming also 〈y′0z0〉 =0m, 〈y0z0〉 =0m2, and a negligibleenergy chirp, the rms vertical spot size at screen is [6]:σ2ys = σ2ys,of f+ K2calσ2t0, (4)where σys,of f is the rms vertical spot size at the screen withRFD off [12], σt0 is the rms bunch length (in seconds), andKcal is a calibration factor:Kcal = ωRF LCr f dcos (ϕ) , (5)where ωRF is the deflecting voltage angular frequency. Thecalibration factor is a coefficient that relates the verticalcoordinate at the screen with the bunch longitudinal timecoordinate. Comparing (3) and (5), an important relationcan be noticed:Kcal = ωRF p, (6)where p is the slope of the plot vertical bunch centroid versusRFD phase. Equation (6) means that the coefficient Kcal canbe directly calculated measuring the bunch centroid positionat screen for different values of the RFD phase, i.e. it ispossible to self-calibrate the measurements [13].Measurement ProcedureFigure 1: Bunch length measurement procedure.The bunch length measurement procedure is divided infour steps (Fig. 1) [12, 14]:• first step: carry out the measurement of the rms verticalspot size at the screen with RFD off;• second step: make measurements of vertical bunchcentroid for different values of RFD phase with RFDon and then calculate the calibration factor by meansof a linear fit (6);• third step: make the measurement of the rms verticalspot size at the screen with RFD on;• fourth step: carry out the bunch length measurementfrom (4):σt0,m =√σ2ys − σ2ys,of f|Kcal |.(7)SIMULATION RESULTSFigure 2: Zoom of GBS linac layout between the first andthe second C-band accelerating section [1].A nominal beam represented by 50000 particles has beentracked by means of ELEGANT code from LEL-RF-TDC01RFD to LEL-DIA-SCN08 screen (Fig. 2), placed betweenthe first and second C-band accelerating section of GBSelectron linac [1]. The GBS electron linac bunch at RFD andRFD parameters are reported in Tables 1 and 2, respectively.The correlations between particle longitudinal position andvertical position 〈y0z0〉 and divergence 〈y′0z0〉 at LEL-RF-TDC01 RFD are negligible, on the contrary the energy chirpis non-negligible (see Fig. 3).Table 1: GBS Electron Linac Bunch Parameters (E Is theBunch Energy)σy0 [mm] σy′0 [µrad] 〈y0y′0〉 [m·rad] σt0 [ps] E [MeV]0.3464 57.57 −1.986 · 10−8 0.9117 118Table 2: GBS Electron Linac Bunch Parameters at ScreenVt [MV] fRF [GHz] L [m]1 2.856 1.1380Figure 3: Longitudinal trace space of the Bunch before RFD.Proceedings of IBIC2016, Barcelona, Spain WEPG44Machine Parameter MeasurementsISBN 978-3-95450-177-9733 Copyright©2016CC-BY-3.0andbytherespectiveauthorsIn Fig. 3, the longitudinal trace space of the bunch be-fore RFD is reported and a correlation between the particleenergy expressed in terms of ∆E/Ecentral and the particlelongitudinal coordinate z0 can be noticed. The energy chirpdoesn’t affect the vertical bunch centroid at screen after theRFD, and so the calibration factor (see Figs. 4 and 5). Onthe contrary, the energy chirp affects the vertical spot size atscreen (see Fig. 6) and so the bunch length virtual measure-ment (see Fig. 7).Vertical Bunch Centroid at Screen and CalibrationFactorFigure 4: Theoretical predictions eq. (3) (in red line) andsimulated data (in blue stars) of the vertical bunch centroidat screen versus RFD phase.Figure 4 shows a good agreement between theoreticalpredictions eq. (3) and simulated data of the vertical bunchcentroid at screen versus RFD phase. For every RFD phase,the calibration factor can be calculated by means of a linearfit of five different vertical bunch centroid measurementsvarying the RFD phase in a range of 3° eq. (6). Figure 5shows a good agreement between theoretical eq. (5) andcalculated from vertical centroid calibration factor eq. (6)versus RFD phase.Figure 5: Theoretical eq. (5), in red line, and calculatedfrom vertical centroid eq. (6) calibration factor versus RFDphase.Vertical Spot Size at ScreenFigure 6: Theoretical predictions eq. (6) (in red line) andsimulated data (in blue stars) of the vertical spot size atscreen versus RFD phase.Figure 6 shows a discrepancy between theoretical predic-tions eq. (6) and simulated data of the vertical spot size atscreen versus RFD phase. This discrepancy is due to theenergy chirp of the bunch at the entrance of RFD (see Fig.3). For RFD phase offset of some degrees, correspondingto a vertical centroid offset of about 1mm (see Fig. 4), thevertical spot size at screen relative error due to energy chirpcan be of the order of 1% and this affects the bunch lengthvirtual measurements.Effects of Energy Chirp on Bunch Length VirtualMeasurements at GBS Electron LinacFigure 7: Bunch length virtual measurement relative errorversus RFD phase.The bunch length virtual measurement relative error canbe defined:Er =|σt0 − σt0,m |σt0, (8)where σt0 is the rms bunch length at RFD (see Table 1) andσt0 is the rms bunch length virtual measurement given byeq. (7). In Fig. 7, the bunch length virtual measurementrelative error versus RFD phase is plotted. In the case ofWEPG44 Proceedings of IBIC2016, Barcelona, SpainISBN 978-3-95450-177-9734Copyright©2016CC-BY-3.0andbytherespectiveauthorsMachine Parameter MeasurementsGBS electron linac, for RFD phase offset of some degrees,the bunch length virtual measurement relative error due toenergy chirp can be of the order of 5%. It can be noticedthat the bunch length relative error is minimum when theRFD phase offset is null.Figure 8: Bunch length virtual measurement relative errorversus RFD phase varying the distance between RFD andscreen (blue: ELI-NP case 1.1380 m, red: 1.5 m, green: 2.0m, yellow: 2.5 m, magenta: 3.0 m, black: 3.5 m).The bunch length virtual measurement relative error ver-sus RFD phase varies with the distance between RFD andscreen (see Fig. 8). In Fig. 8, L varies between almost1.14m (GBS linac case) and 3.5m (SPARC-LAB linac case).For a fixed vertical centroid offset, the bunch length virtualmeasurement relative error due to energy chirp is greaterwhen the distance between RFD and screen is smaller. Fora vertical centroid offset of about 1mm and L=3.5m, thebunch length virtual measurement relative decreases to about1%.CONCLUSIONSIn this paper, the importance of the energy chirp for bunchlength measurament is treated. The contribution of the en-ergy chirp in bunch length measurement increases when theRFD offset increases. Therefore, in the case of a bunch witha non-negligible energy chirp, the vertical centroid offsetplays a fundamental role in the bunch length measurementrelative error. In particular, the bunch length virtual mea-surement relative errors varying RFD phase in the case ofGBS electron linac are presented in this paper. The rela-tive errors are due to the non-negligible energy chirp of thebunch at RFD. In particular, the energy chirp doesn’t affectthe vertical bunch centroid at screen after the RFD, and sothe calibration factor. On the contrary, the energy chirp af-fects the vertical spot size at the screen and so the virtualbunch length measurement. When the RFD phase offset isnull, the bunch length relative error is minimum.In the case of GBS electron linac, for RFD phase offsetof some degrees, the bunch length virtual measurement rel-ative error due to energy chirp can be of the order of 5%.For a fixed vertical centroid offset, the bunch length virtualmeasurement relative error due to energy chirp is greaterwhen the distance between RFD and screen is smaller. For avertical centroid offset of about 1mm and L=3.5m (SPARC-LAB linac case), the bunch length virtual measurement rel-ative decreases to about 1%.ACKNOWLEDGEMENTThe authors would like to thank M. Borland for usefulsuggestions and discussion.REFERENCES[1] L. Serafini, et al., “Technical design report eurogammas pro-posal for the eli-np gamma beam system,” arXiv preprintarXiv:1407.3669, 2014.[2] “The white book of eli nuclear physics bucharest-magurele, romania,” avaible on-line at http://www.eli-np.ro/documents/ELI-NP-WhiteBook.pdf.[3] A. Mostacci, et al., “Chromatic effects in quadrupole scanemittance measurements,” Physical Review Special Topics -Accelerators and Beams, vol. 15, no. 8, 2012.[4] A. Cianchi, et al., “Six-dimensional measurements of trains ofhigh brightness electron bunches,” Physical Review SpecialTopics - Accelerators and Beams, vol. 18, no. 8, 2015.[5] G. Loew and O. H. Altenmueller, “Design and applicationsof rf separator structures at slac,” in 5th Int. Conf. on High-Energy Accelerators, Frascati, 1965, pp. 438–442.[6] P. Emma, J. Frisch, and P. Krejcik, “A transverse rf deflectingstructure for bunch length and phase space diagnostics,” LCLSTechnical Note, vol. 12, 2000.[7] D. Alesini, et al., “Rf deflector design and measurements forthe longitudinal and transverse phase space characterizationat sparc,” Nuclear Instruments and Methods in Physics Re-search Section A: Accelerators, Spectrometers, Detectors andAssociated Equipment, vol. 568, no. 2, pp. 488–502, 2006.[8] M. Borland, “Elegant: A flexible sdds-compliant code foraccelerator simulation,” Argonne National Lab., IL (US),Tech. Rep., 2000.[9] D. Alesini and C. Vaccarezza, “Longitudinal and trans-verse phase space characterization,” SPARCRF-03/003,INFN/LNF, Frascati, Tech. Rep., 2003.[10] H. Wiedemann et al., Particle accelerator physics. Springer,2007, vol. 314.[11] R. Akre, L. Bentson, P. Emma, and P. Krejcik, “Bunch lengthmeasurements using a transverse rf deflecting structure in theslac linac,” in Proc. EPAC’02, Paris, France, June, 2002.[12] L. Sabato, et al., “Metrological characterization of the bunchlength system measurement of the eli - np electron linac,”in Proc. 14th IMEKO TC10, Milan, Italy, June 2016, pp.203–208.[13] D. Alesini, et al., “Sliced beam parameter measurements,”in Proc. DIPAC09, Basel, Switzerland, May 2009, paperTUOA01, pp. 146–150.[14] L. Sabato, et al., “Metrological characterization of the bunchlength measurement by means of a rf deflector at the eli-npcompton gamma source,” in Proc. IPAC’16, Busan, Korea,May 2016, paper MOPMB018, pp. 122–125.Proceedings of IBIC2016, Barcelona, Spain WEPG44Machine Parameter MeasurementsISBN 978-3-95450-177-9735 Copyright©2016CC-BY-3.0andbytherespectiveauthors

Longitudinal phase space measurement at the ELI-NP Compton gamma source

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Longitudinal phase space measurement at the ELI-NP Compton gamma source

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