Asova G

Floettmann K

Holder DJ

Khodyachykh S

Korepanov S

Krasilnikov M

Muratori Bruno

Rimjaem S

Stephan F

English

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DESIGN OF A TOMOGRAPHY MODULE FOR THE PITZ FACILITY∗G. Asova† , S. Khodyachykh‡ , S. Korepanov, M. Krasilnikov, S. Rimjaem, F. Stephan,DESY, 15738 Zeuthen, GermanyK. Floettmann, DESY, 22761 Hamburg, GermanyD.J. Holder, B. Muratori, ASTeC, STFC Daresbury Laboratory, Warrington, UKAbstractThe goal of the Photo Injector Test Facility at DESY,Zeuthen site (PITZ) is to develop sources of high phase-space density electron beams that are required for the suc-cessful operation of SASE FELs. This requires detailedcharacterisation of the sources and therefore the develop-ment of suitable advanced diagnostics.As part of the ongoing upgrade towards higher beamenergies, new diagnostics components are being installed.An example is a tomography module for transverse phasespace reconstruction which is designed to operate in theenergy range between 15 and 40 MeV. The module con-sists of four observation screens with three FODO cells be-tween them. A number of upstream quadrupoles are usedto match the beam envelope parameters to the optics of theFODO lattice.This contribution presents the final design of the tomog-raphy module. Data from numerical simulations are usedto illustrate the expected performance and to compare it toa simplified setup of two quadrupoles. The quality of thereconstruction is revised with the help of different algo-rithms.INTRODUCTIONSuccessful operation of SASE FELs requires the use ofhigh-quality electron sources and thus a better understand-ing of the processes that contribute to emittance and emit-tance growth in injectors. For these reasons the Photo-Injector Test Facility at DESY, Zeuthen site (PITZ) iscurrently extending its diagnostics instrumentation with amodule for transverse phase-space tomography [1, 2, 3].The PITZ accelerating sections consists of a 1.5 cell RFgun and a booster cavity. Together with the installationof the tomography section, the booster cavity will be ex-changed which will allow the beam momentum to reach30 MeV/c. Low-energy diagnostics components, situatedbetween the gun and the booster, and high-energy ones,downstream of the booster, are used for the characterisationof the electron beam. Some other new additional elementsare described in [1]. In this paper the tomography sectionis presented. The first part reviews the final design layoutwhile the second part is a comparison between such a set∗This work has been partly 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.† galina.asova@desy.de‡ Presently at Siemens AG, Rudolstadt, Germanyup and a simplified version consisting of two quadrupolesand an observation screen. The final part shows the ex-pected performance using numerically simulated distribu-tions. Earlier design iterations can be found in [2, 3].PHASE-SPACE TOMOGRAPHY SETUPThe main components of the module are four observa-tion screens with three FODO cells between them. In [4]it has already been shown that the phase advance that de-livers the smallest emittance measurement error for such asetup is 45◦. This requirement, together with the layout ofthe section and the beam parameters, means that it is neces-sary to have an upstream matching section. The full assem-bly is shown in Fig. 1. As it can be seen there, the beam-line section surrounded by the first two screen stations, alsoforms one of the several identical FODO cells. These fivecells extend over 3.04 m. Four additional quadrupole mag-nets, not shown in Fig. 1, located upstream, are availablefor matching.Quadrupole magnetsAll the quadrupoles in the straight section are identi-cal, produced and measured by DANFYSIK [5]. In oneof the first design iterations a quadrupole effective lengthof 0.06 m was assumed [2] but, due to stringent spacelimitations, this is now the allowed geometrical length ofa magnet. Basic parameters of the quadrupoles used aregiven in Table 1 and the magnetic field necessary for nom-inal gradient of 5.6 T/m, as a function of the longitudinalposition, is shown in Fig 2.Table 1: Main parameters of the quadrupole magnets. Ef-fective length and required current are measured at nominalgradient of 5.6 T/mAverage effective length 0.043 mAverage required current 7.3 APhysical length 0.063 mMaximum gradient 7.5 T/mBore radius 0.04 mWeight 9 kgFrom the hysteresis curve shown in Fig. 3 it can bedemonstrated that the magnetic field stays linear below7 T/m. For that reason the quadrupoles should be operatedbelow this value. From ASTRA [6] numerical simulationsTUPC002 Proceedings of EPAC08, Genoa, Italy06 Instrumentation, Controls, Feedback & Operational Aspects1038T03 Beam Diagnostics and InstrumentationFigure 1: Tomography and matching section top view. The electron beam propagates from right to left.z, mm-100 0 100, Tx, yB-0.100.1Figure 2: Magnetic field mapping as a function of the lon-gitudinal position along the quadrupole for the nominalgradient of 5.6 T/m. The data were taken from quadrupolemeasurements done by DANFYSIK.it can be seen that for some beam energies better matchingis obtained when some of the quadrupoles have gradientsclose to or above 7 T/m. Such a conflict can be resolved ifthe drift space between two of the matching quadrupoles isincreased by 0.08 m.I, A0 2 4 6 8 10 12g, T/m02468Figure 3: Field gradient as a function of the applied current.The data were taken from the DANFYSIK measurements.Each quadrupole is to be powered by a separate powersupply delivering relative accuracy of the magnetic field of10−3.Screen stationsEach of the five screen stations along the section isequipped with a horizontal actuator housing two screens - aYAG and an OTR, planned to be used for different beam en-ergies. The direction of movement of an actuator dependsupon the plane of deflection produced by an RF deflectingcavity to be installed later and the necessary resolution -if the beam is going to be streaked vertically, the actuatorshould be inserted in the beamline in horizontal direction.As well as the main components described above, eachFODO cell is equipped with a BPM and a steering magnet.The misalignment in the components’ mounting shouldbe minimised in order to avoid additional systematic ef-fects. As the error in the emittance is required to besmaller than 10%, the beam size error has to be smallerthan 10% [4]. The calculated misalignment tolerances inrms values are as follows: longitudinal misplacement ofa screen or a quadrupole maximum 100 μm, quadrupolerotation around the z-axis - 10 μrad, quadrupole rotationaround the horizontal and vertical axes - 20 μrad.A SIMPLIFIED SETUP WITH TWOQUADRUPOLESIn order to evaluate the performance of the tomographymodule, the measurement procedure can be compared witha simplified one utilising two quadrupoles and a screen.As a single projection does not uniquely define the ob-ject being imaged, combining information from a numberof viewing angles can result in a more accurate descrip-tion. In medical imaging this number can reach more thanseveral hundred, while in beam physics it is limited by theavailable setup. Using the design described here, only fourprojections are available and the reconstruction is benefi-cial only if the beam is well matched to the optics of thelattice in both transverse planes. If this is not the case,the transfer matrices along the FODO cells do not correctlyrepresent the evolution of the envelope and the phase-spacereconstruction is not correct. Such an example is givenin [3]. Good matching is hard to achieve since the beamis strongly space-charge dominated over the energy rangeProceedings of EPAC08, Genoa, Italy TUPC00206 Instrumentation, Controls, Feedback & Operational Aspects T03 Beam Diagnostics and Instrumentation1039PITZ is operating. It also requires an assumption of a con-stant emittance value in advance. To overcome these is-sues, a quadrupole scan with two magnets has proven tobe useful. A significant drawback is that full π rotation ofthe phase-space ellipse cannot be achieved for both trans-verse planes simultaneously without different drift lengthsbetween the position of reconstruction and the position ofobservation during a single measurement. In PITZ this op-tion gives the possibility of reconstructing the phase-spacedensity distribution at a z-position closer to the gun andto the position where emittance is being measured using aslit-scan technique. Using ASTRA-generated distributions,including space-charge forces in the tracking, it has beenconfirmed that 15 phase-space ellipse rotations are suffi-cient to obtain very good agreement between a generatedand a reconstructed distributions for beam momentum of15 MeV/c. A beam having such a low momentum is hard tomatch due to space-charge effects. The second transverseplane has to be simulated and consequently measured in ad-dition and therefore a detailed quadrupole scan is desired tobe used only as a backup solution and for cross-checking.TRANSVERSE PHASE-SPACERECONSTRUCTION FROM NUMERICALDATAA beam with optimised parameters as required for emit-tance measurement has been generated at the entrance ofthe matching section using ASTRA. Furthermore, it hasbeen matched and tracked, including space charge, alongthe full matching and tomography sections. In Fig. 4, re-constructions of the horizontal trace space using MaximumENTropy [7] and Algebraic Reconstruction Technique [8]are shown. Both algorithms start with the same input dis-tributions and very good agreement can be seen betweenthem, as well as between the reconstructions and the orig-inal distribution, shown at the bottom. The relative errors(a) MENT reconstruction (b) ART reconstruction(c) Original distributionFigure 4: Reconstructed distributions with MENT (top left)and ART (top right) and the original one (bottom).in the beam parameters with respect to the original distri-bution are given in Table 2.Table 2: Relative errors in % of the reconstruction resultswith respect to the original distributionMENT ARTσx 0.21 0.16σx′ 2.75 2.74σxx′ 0.47 0.57εx,N 0.8 0.3MENT was tested on real data from the FLASH FEL. Itshows very good agreement with the results from a mul-tiscreen χ2-fit technique, provided that robust noise re-moval, selection of region of interest and spot-centeringalgorithms are applied in advance.CONCLUSIONSThe tomography setup is currently undergoing techni-cal implementation. Its performance has been verified forthe PITZ working regime using simulations. The require-ments for the precision mounting of the components arechallenging due to the properties of the space-charge dom-inated electron beam and the necessity for strong focusingalong the matching section and in the FODO lattice. Cur-rently, part of the matching section is already installed andfirst experience with the module is expected at the begin-ning of 2009.Two different reconstruction algorithms have been testedon numerical data - MENT and ART. Both of them performvery well if given correct transfer matrices, i.e. there isgood matching of the optical parameters.REFERENCES[1] S. Khodyachykh et al., “New Beam Diagnostic Develop-ments at the Photo-Injector Test Facility PITZ“, proceedingsPAC 2007, Albuquerque, NM, USA.[2] D.J. Holder et al., “A phase space tomography diagnostic forPITZ”, proceedings of EPAC 2006, Edinburgh, UK.[3] G. Asova et al., “Design considerations for phase space to-mography diagnostics at the PITZ facility”, proceedings ofDIPAC 2007, Mestre, Italy.[4] G. Asova et al., “Phase space tomography diagnostics atthe PITZ facility”, proceedings of ICAP 2006, Chamonix,France.[5] http://www.danfysik.com[6] ASTRA: A Space Charge Tracking Algorithm,http://www.desy.de/˜mpyflo/.[7] G. Minerbo, “A Maximum Entropy Algorithm for Recon-structing a Source from Projected Data”, Comp. Graphics Im-age Proc. 10 (1970) 48.[8] R. Gordon, “A tutorial on ART”, IEEE Transactions on Nu-clear Science, Vol. NS-21 (1974).TUPC002 Proceedings of EPAC08, Genoa, Italy06 Instrumentation, Controls, Feedback & Operational Aspects1040T03 Beam Diagnostics and Instrumentation

A Maximum Entropy Algorithm for Reconstructing a Source from Projected Data”, Comp. Graphics Image

A phase space tomography diagnostic for PITZ”, proceedings of EPAC

A tutorial on ART”,

ASTRA: A Space Charge Tracking Algorithm,

Design considerations for phase space tomography diagnostics at the PITZ facility”, proceedings of DIPAC

New Beam Diagnostic Developments at the Photo-Injector Test Facility

Phase space tomography diagnostics at the PITZ facility”, proceedings of ICAP

Design of a Tomography Module for the PITZ Facility

Design of a Tomography Module for the PITZ Facility

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