http://accelconf.web.cern.ch/AccelConf/f06/PAPERS/THPPH020.PDFInternational audienceA detailed knowledge of the parameters of a highbrightness electron beam is of major importance for evaluating the performance of a free electron laser using the beam. Therefore, a full characterization of the beam parameters is required during the commissioning of RF photo injectors. As one important contribution to fulfill this task, a multi-purpose high energy dispersive arm (HEDA1) for electron energies up to 40 MeV is presently under construction at the Photo Injector Test Facility at DESY in Zeuthen (PITZ). The dispersive section is designed such that it will combine the functionality of 1) an electron spectrometer, 2) a device for characterization of the longitudinal phase space, and 3) a transverse slice emittance measuring system. The HEDA1 consists of a 180° dipole magnet (C-Bend), followed by removable slit, a quadrupole magnet, and a screen station with a read-out for the streak camera. Design considerations and the detailed lay out of the high-energy dispersive section are presented

Bähr, J.

Garvey, T.

Khodyachykh, S.

Krasilnikov, M.

Lipk, D.

Oppelt, A.

Rönsch, J.

Staykov, L.

Stephan, F.

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DESIGN OF MULTIPURPOSE DISPERSIVE SECTION AT PITZ∗S. Khodyachykh† , J. Ba¨hr, M. Krasilnikov, A. Oppelt, L. Staykov, F. Stephan,DESY, Zeuthen, Germany,T. Garvey, LAL, Orsay, France, D. Lipk, BESSY GmbH, Berlin, Germany,J. Ro¨nsch, University of Hamburg, Hamburg, Germany.AbstractA detailed knowledge of the parameters of a high-brightness electron beam is of major importance for eval-uating the performance of a free electron laser using thebeam. Therefore, a full characterization of the beam pa-rameters is required during the commissioning of RF photoinjectors. As one important contribution to fulfill this task,a multi-purpose high energy dispersive arm (HEDA1) forelectron energies up to 40 MeV is presently under construc-tion at the Photo Injector Test Facility at DESY in Zeuthen(PITZ). The dispersive section is designed such that it willcombine the functionality of 1) an electron spectrometer,2) a device for characterization of the longitudinal phasespace, and 3) a transverse slice emittance measuring sys-tem. The HEDA1 consists of a 180◦ dipole magnet (C-Bend), followed by removable slit, a quadrupole magnet,and a screen station with a read-out for the streak camera.Design considerations and the detailed lay out of the high-energy dispersive section are presented.INTRODUCTIONThe Photo-Injector Test Facility at DESY in Zeuthen(PITZ) is an electron accelerator which was built byDeutsches Elektronen-Synchrotron (DESY) in collabora-tion with international partners with the goal to developand to optimize high brightness electron sources suitablefor SASE FEL operation.The PITZ facility consists of a 1.5 cell L-band RF gun,a photo-cathode laser system, a normal-conducting boostercavity and a diagnostic section. Being an operational ma-chine showing pioneering results in achieving of high-charge low-emittance electron beams for electron bunchtrains [1], PITZ is still under development. The beam linewhich has a present length of about 13 meters will be ex-tended up to about 21 meters within the next two years.Many additional diagnostics components will be added tothe present layout. Together with the deflecting cavity [2],the phase space tomography module [3] and the secondhigh-energy dispersive arm (HEDA2), HEDA1 will extendthe existing diagnostics system of the photo injector. Thescope of the present paper is the multi-purpose high en-ergy dispersive arm (HEDA1). The dispersive section isdesigned to combine the functionality of (i) an electron∗This work has partly been supported by the European Community,contracts RII3-CT-2004-506008 and 011935, and by the ’Impuls- undVernetzungsfonds’ of the Helmholtz Association, contract VH-FZ-005.† sergiy.khodyachykh@desy.despectrometer, (ii) a device for characterization of the lon-gitudinal phase space, and (iii) a transverse slice emittancemeasuring system.SETUPThe layout of the dispersive section HEDA1 is schemat-ically shown in Fig 1.The heart of the dispersive section is a 180◦ dipole mag-net having the bending radius of 300 mm. The bendingradius has been chosen as a compromise between the spacerequirements and momentum resolution on the one handand the ability of the spectrometer to operate within thelarge range of gun and booster parameters. Being deflectedin vertical plane, the electron beam enters the dispersivearm which goes below the main beam line parallel to it.The dispersive arm includes a pumping port combined witha removable slit mask, a quadrupole magnet Q1 for the sliceemittance measurements, a drift space of 600 mm followedby two screen stations (for simplicity only one is shown)and by a beam dump. For measuring of the bunch charge anintegrating current transformer (ICT) is foreseen betweenthe two screen stations (not shown).For the reasons described in the following sections ad-ditional components which contribute to the measurementsat HEDA are located in the main beam line. These are thequadrupole magnet Q2 and the screen station S2 placed up-and downstream to the dipole magnet D, respectively.The main advantage of the 180-degree spectrometeris the simplicity to reconstruct the momentum distribu-tion [4]. One uses the screen S2 in the straight section andmeasures the contribution from the transverse beam sizeand divergence, which can be de-convoluted with the mea-sured spectrum to obtain the pure momentum distribution.This can be illustrated by comparing of the transport ma-trix MD (describing the transport of the electron beam be-tween the position of the quadrupole magnet Q2 and thescreen S1) with the matrix MS which corresponds to thedrift between Q2 and the screen S2. The matrices in thinlens approximation have the formMD =⎛⎝−1 −l1 − l2 2ρ0 −0 0⎞⎠MS =⎛⎝1 l1 + l2 00 1 00 0 1⎞⎠ (1) 1     0  1 Proceedings of FEL 2006, BESSY, Berlin, Germany THPPH020FEL Technology 601Figure 1: Simplified layout of PITZ with the first high energy dispersive section.transverse position, y′i is the divergency and pi/〈p〉 is therelative momentum. Assuming the electron with the co-ordinate y0 is transported through the spectrometer it willget the transverse coordinate on the screen S1 which is de-scribed by the first component of vector y1 = MDy0. Theshift of the particle with respect to the reference trajectoryis thus defined not only by the relative momentum of theparticle, but also by it initial transverse offset and diver-gency. To exclude these two contributions one uses themeasurements on the screen S2 where only initial coordi-nate and initial divergency define the position of the parti-cle (compare M11 and M12 elements of matrices in Eq.1).Moreover, by focusing the beam on the screen S2 with thehelp of the quadrupole magnet Q2 one can control the res-olution of the momentum measurement.Beam momentum and momentum spread dependsstrongly on the machine parameters. It is very sensitiveto the RF phases of the gun and of the booster. Because ofthe large dispersion of the dipole magnet the spot sizes onthe screen S1 are expected to be very large, especially incases of far off-crest RF phases. Figure 2 shows the depen-dence of the vertical beam size on the screen S1 as a twodimensional function of the gun and booster phases. Theaperture of the quadrupole magnet Q1 (80 mm) defines thetransversal acceptance of the dispersive section. Thus, us-ing HEDA1 measurements within the range of −35...35◦of booster phases will be possible in single shot. For largeroff-crest phases a scan of dipole current will be used.LONGITUDINAL PHASE SPACEVery important information for the understanding of thephoto-injector can be gained by studying the electron dis-tribution in the longitudinal phase space, when the electronmomentum is measured as a function of its longitudinal po-sition within the bunch. For this purpose one of the screenstations will be equipped with an aerogel screen used as aCherenkov radiator and a read-out for the streak-camera.Figure 3 (a) shows the results of numerical simulationof the longitudinal phase space using the code ASTRA [7]at the entrance of the dipole magnet. The particles weretracked through the dipole using the matrix formalism,similar to that described in previous section, but using6 × 6 transport matrix. The longitudinal phase space re-gun phase (degree)-20 -10 0 10 20 30booster phase (degree)-40-200204060dx (mm)50100150200250Figure 2: The total beam size in dispersive plane on screenS1 as a function of gun and booster phases.constructed on the screen S1 of the dispersive section afterthe drift having the length of a 1 m is shown in Fig. 3 (b). Acorrection of the influence from the matrix element M56 [6]was applied in Fig. 3 (c). Figure 3 (d) shows the momen-tum distribution as a projection of Fig. 3 (a) in red and 3 (c)in blue.A comparison of the initial and the reconstructed valuesof mean momentum 〈p〉, momentum spread prms and lon-gitudinal emittance l are shownin table 1. The< p > (MeV) prms (keV) l (π keV mm)(a) 18.3111 36.6 75.99(c) 18.3109 49.0 105.08(a − c)/a −0.0009% 33.8% 38.3%results in Fig. 3 and table 1 were done without quadrupole.These results could be improved by applying deconvolu-tion with known beam size at S2. However, this proce-dure unnecessary complicates data analysis comparing tothe method discussed below.The simulations were repeated using a quadrupole triplet4.5 m downstream the photocathode in order to focus thewhere ρ is the bending radius of the dipole magnet andlengths l1 and l2 are defined in Fig 1.The transverse coordinates of the i-th electron can be de-scribed by a vector yi = (yi, y′i, pi/〈p〉)T , where yi is theTableTable 1:Initial and reconstructed values of mean momen-   tum, momentum spread and longitudinal emittance.THPPH020 Proceedings of FEL 2006, BESSY, Berlin, Germany602 FEL TechnologyFigure 3: Longitudinal phase space obtained with thequadrupole magnet Q2 being off: longitudinal phase space(a), reconstructed longitudinal phase space at S1 (b), recon-structed longitudinal phase space at S1 after correction ofM56 (c), momentum distribution (d).beam onto screen S1 (at position 6.5 m). The results areshown in figure 4 and table 2.pmean (MeV) prms (keV) l (π keV mm)(a) 18.3105 36.54 75.72(c) 18.3104 36.59 76.11(a − c)/a −0.0005% 0.16% 0.5%The use of a quadrupole triplet brings a remarkable im-provement to the measurements of the longitudinal phasespace. Both, the longitudinal phase space and the momen-tum distribution are reproduced with very good agreement.The differences between the simulated and reconstructedvalues of the momentum spread and longitudinal emittancereduces by using of the triplet from about 35% down to lessthan one per cent.In order to save some space in the main beamline, westudied the possibility to exchange the quadrupole tripletwith a single quadrupole magnet. Indeed such replacementalmost does not decrease the accuracy of the longitudinalphase-space reconstruction. Figure 5 shows a comparisonof the momentum measurements. The differences betweenthe simulated and reconstructed mean momentum, momen-tum spread and longitudinal phase space are still within1 %.TRANSVERSE SLICE EMITTANCEMEASUREMENTSThe functionality of the HEDA can be enhanced with asetup that allows us to measure the transverse emittance ofFigure 4: Longitudinal phase space obtained with thequadrupole triplet placed on the position of Q2 being on:longitudinal phase space (a), reconstructed longitudinalphase space at S1 (b), reconstructed longitudinal phasespace at S1 after correction of M56 (c). For a compari-son reconstructed longitudinal phase space at S1 [as (c)]but with a single quadrupole instead of a triplet.p (MeV/c)18.15 18.2 18.25 18.3 18.35 18.4Intensity (a. u.)020406080100120140160180Figure 5: Simulated momentum distribution (black) com-pared to its reconstruction in the dispersive arm with thesingle quadrupole Q2 (red), quadrupole triplet (green) andwithout quadrupole (blue).the electron beam at different longitudinal positions alongthe bunch. The so called slice emittance is providing betterunderstanding of the physics of a photoinjector, particularythe emittance compensation and conservation principles. Itis also an important parameter for the SASE FELs sincethe slice emittance and slice peak current define the gainof the SASE FEL process. Using proper phasing of thebooster cavity we can obtain linear correlation between themomentum and longitudinal distribution of the electrons inTable 2: Initial and reconstructed values using a quadru-  pole triplettz (mm)-15 -10 -5 0 5 10p (MeV/c)18.1518.2018.2518.3018.3518.40(a)z (mm)-15 -10 -5 0 5 10p (MeV/c)18.1518.2018.2518.3018.3518.40(b)z (mm)-15 -10 -5 0 5 10p (MeV/c)18.1518.2018.2518.3018.3518.40(c)p (MeV/c)18.15 18.2 18.25 18.3 18.35 18.4Intensity (a. u.)020406080100120140160180 before dipoleafter dipole(d)z (mm)-15 -10 -5 0 5 10p (MeV/c)18.1618.1818.2018.2218.2418.2618.2818.3018.3218.34(a)z (mm)-15 -10 -5 0 5 10p (MeV/c)18.1618.1818.2018.2218.2418.2618.2818.3018.3218.34(b)z (mm)-15 -10 -5 0 5 10p (MeV/c)18.1618.1818.2018.2218.2418.2618.2818.3018.3218.34(c)z (mm)-15 -10 -5 0 5 10p (MeV/c)18.1518.218.2518.318.35(d)Proceedings of FEL 2006, BESSY, Berlin, Germany THPPH020FEL Technology 603the bunch. Afterwards one applies the standard quadrupolescan technique and measures the emittance for differentmomentum/time slices of the bunch.A setup similar to that in [8] will be implemented. A slitat the dipole exit selects the necessary slice from the energychirped beam. This slice is scanned with the quadrupole Q1focusing in a plane orthogonal to the dispersion plane of thedipole, the beam distribution will be observed on screen S1.Due to the short distance between the quadrupole and thescreen the beam size tends to become very small during thescan. For the correction of this the quadrupole magnet Q2with fixed strength placed before the dipole will be used(see Fig.1). The experimental setup was simulated withFigure 6: Slice emittance distribution at the dipole en-trance. The solid line gives the value for the projected emit-tance.ASTRA [7] in attempt to verify the technique and to opti-mize the setup. For the simulations presented here, a gungradient of 60 MV/m and a phase of maximum accelera-tion in the gun, the current in the main solenoid was 385 A,and a booster phase of +70 degrees were chosen. This cor-responds to a projected emittance of 1.6 mm · mrad (seeFig. 6), a mean momentum of 17 MeV/c and an RMS mo-mentum spread of 1.4 MeV/c.The electron bunch was separated in ten longitudinalslices which were scanned with ten different quadrupolestrengths. The distribution is fited to a quadratic equation(2) where the corresponding beam parameters are obtained.〈x2s〉 = M211 ·〈x20〉+2 ·M11M12〈x0 ·x′0〉+M222 ·〈x′20 〉 (2)Here 〈x2s〉 is the beam size coresponding to the particularstrength of the quadrupole magnet, M11, M12 and M22are the elements of the transport matrix, functions of thequadrupole strength, 〈x20〉, 〈x0 ·x′0〉 and 〈x′20 〉 are the initialbeam parameters that need to be found. The correspondingvalue for the emittance then is calculated using Eq. 3.εn = βγ ·√〈x20〉 · 〈x′20 〉 − 〈x0 · x′0〉2. (3)Here 〈x20〉 and 〈x′20 〉 are the rms dimensions of the beam inthe so-called trace phase space and 〈x0 · x′0〉 is the correla-tion between both.Figure 7: Deviation between the result from the simulatedmeasurement and the slice emittance distribution obtainedwith ASTRA.Figure 7 shows the deviation between the result from thesimulated measurement and the slice emittance distributionobtained with ASTRA. The average deviation is 21 % butfor some slices it can reach up to 66 %. Further improve-ment of the measuring accuracy can be reached by takinginto account the space charge during the reconstruction.ACKNOWLEDGEMENTSBesides the funding agencies mentioned on the first pageof the present paper the authors would like to thank S. Rie-mann, H.-J. Grabosch (both DESY) for valuable discus-sions. Without brilliant ideas of our engineers the realiza-tion of this work would not be thinkable.REFERENCES[1] F. Stephan, Status and perspectives of photo injector devel-opments for high brightness beams, proceedings of teh work-shop on the Physics and Applications of High BrightnessElectron Beams. Erice, Italy, 2005.[2] S. Korepanov et al., Design Consideration of the RF Deflectorto Optimize the Photo Injector for PITZ, Proceedings of theFEL2006, Contribution THPPH021.[3] D.J. Holder et al., A phase space tomography diagnostic forPITZ, Proceedings of EPAC 2006.[4] D. Lipka, Spectrometer design for the SRF Gun, BESSYNote SRFgun-BESSY-003-12-2004, 2005.[5] D. Lipka et al, Silica aerogel radiators for bunch length mea-surements, Nucl. Instr. Meth. A 538 (2005) 597.[6] J. Ro¨nsch et al., Measurement of the longitudinal phase spaceat the photo injector test facility at DESY in Zeuthen (PITZ),Proceedings of DIPAC 2005, Lyon.[7] K.Floetmann, ASTRA, http://www.desy.de/ mpyflo.[8] X. Qiu et al., Demonstration of emittance compensationthrough the measurement of the slice emittance of a 10 pi-cosecond electron bunch, Phys. Rev. Let. 76 No. 20 (1996)3723.THPPH020 Proceedings of FEL 2006, BESSY, Berlin, Germany604 FEL Technology

Design of multipurpose dispersive section at Pitz

Design of multipurpose dispersive section at Pitz

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