A methodology has been developed to perform electrical characterization of the stripline BPMs for the future Gamma Beam System of ELI Nuclear Physics facility in Romania. Several prototype units are extensively benchmarked and the results are presented in this paper. The BPM sensitivity function is determined using a uniquely designed motorized test bench with a stretched wire to measure the BPM response map. Here, the BPM feedthroughs are connected to Libera Brilliance electronics and the wire is fed by continuous wave signal, while the two software-controlled motors provide horizontal and vertical motion of the BPM around the wire. The electrical offset is obtained using S-parameter measurements with a Network Analyzer (via the “Lambertson” method) and is referenced to the mechanical offse

Cioeta, Fara

Colldelram, C.

Crisol, A.

Falone, A.

Ghigo, A.

Iriso, U.

Mostacci, Andrea

Nosych, A. A.

Olmos, A.

Serio, M.

Stella, A.

English

CIOETA, Fara

MOSTACCI, Andrea

Archivio della ricerca- Università di Roma La Sapienza

MEASUREMENTS AND CALIBRATION OF THE STRIPLINE BPM FORTHE ELI-NP FACILITY WITH THE STRETCHEDWIRE METHODA. A. Nosych, U. Iriso, A. Olmos, A. Crisol, C. Colldelram, ALBA-CELLS, Barcelona, SpainF. Cioeta, A. Falone, A. Ghigo, M. Serio, A. Stella, INFN/LNF, Frascati, ItalyA. Mostacci, Rome University, Rome, ItalyAbstractA methodology has been developed to perform electricalcharacterization of the stripline BPMs for the future GammaBeam System of ELI Nuclear Physics facility in Romania.Several prototype units are extensively benchmarked and theresults are presented in this paper. The BPM sensitivity func-tion is determined using a uniquely designed motorized testbench with a stretched wire to measure the BPM responsemap. Here, the BPM feedthroughs are connected to LiberaBrilliance electronics and the wire is fed by continuous wavesignal, while the two software-controlled motors provide hor-izontal and vertical motion of the BPM around the wire. Theelectrical offset is obtained using S-parameter measurementswith a Network Analyzer (via the “Lambertson” method)and is referenced to the mechanical offset.INTRODUCTIONThe future Extreme Light Infrastructure Nuclear Physics(ELI-NP) facility will be located in Bucharest (Romania),and will be dedicated to the study of secondary light sourcesand attosecond pulses. This will be done by the GammaBeam System (GBS) consisting of a 90 m long Linac produc-ing a 700 MeV electron beam, whose main characteristicsare listed in Table 1.Table 1: Main Characteristics of the ELI-NP LinacNumber of bunches 32Bunch spacing 16 nsCharge/bunch [25–400] pCBunch size, σx 100-200 µmBunch size, σy 100-200 µmBunch length, σz 3–4 psThe stripline BPMs for the GBS of ELI-NP have been orig-inally designed by the Accelerator division of INFN/LNF inFrascati (Rome), and are being manufactured by the com-pany Comeb. After production, the BPM units are shippedto ALBA for electrical characterization and alignment mea-surements. In total there will be 32 BPM units, all of themλ/4 stripline type working at ∼500 MHz, shorted on thedownstream port. Figure 1 shows a schematic drawing and amodel of the stripline, including the port naming conventionused throughout all measurements.This document describes the methodology followed tocharacterize the stripline BPMs. This is done in two ways:first, the so-called electrical offset is obtained (xe, ye) usingthe well-known Lambertson method [1, 2], which is used toanalyze the asymmetries among BPM electrodes.(a) cross-section. (b) 3D BPM Model.Figure 1: Naming convention and 3D BPM Model.Next, we emulate the wire scan by scanning the BPMtransversely around the stretched wire, Fig. 2, to obtain thesensitivity factors kx , ky and the mechanical offset (xw, yw ).These offsets, referenced with respect to BPM’s fiducialpoints, are due to all possible mechanical effects and theywill be taken into account when installing the BPM units inthe GBS Linac oF ELI-NP facility.Figure 2: Photo of the BPM and the test bench in the lab.ELECTRICAL CHARACTERIZATIONThe beam position (xb, yb) in a symmetric BPMmountedin a circular chamber is obtained from the classical differenceover sum (DOS) expression for the electrode signals V1..4with removed offset:xb = kx × V3 − V1V3 + V1 − xoffset (1)yb = ky × V2 − V4V2 + V4 − yoffset (2)Lambertson MethodThe BPM electrical center is defined as the position whereV3 − V1 = V2 − V4 = 0, and it corresponds to the deviation(xe, ye) from the BPM geometrical origin (mechanical cen-ter). Its measurement does not require a BPM preciselyProceedings of IBIC2015, Melbourne, Australia TUPB048BPMs and Beam StabilityISBN 978-3-95450-176-2423 Copyright©2015CC-BY-3.0andbytherespectiveauthorsFigure 3: Sample snapshot of the S-parameter measurementby a Network Analyzer.positioned on a test bench due to an external calibrationmethod developed by D. Lambertson [1, 2].This method uses the coupling between buttons/electrodesto determine the gain factors of each electrode; the ratiosbetween gain factors then provide the difference betweenthe mechanical and electrical centers. It has previously beenapplied to measure electrical offsets of the ALBA BoosterBPMs with 6 µm precision at fixed 500 MHz [3].Each BPM electrode has an associated gain factor g whichcauses the difference between the mechanical and electricalcenter of a BPM. Based on differences between the gainfactors we can obtain the electrical center with respect to themechanical one:xe = kx × g3 − g1g3 + g1, ye = ky × g2 − g4g2 + g4(3)The normalized coupled voltage between two electrodes i(excited) and j (detected) is given by:Vi j = 2 · 50 · Gi jgigj . (4)where Gi j = G ji are the capacitive coupling coefficients.From the asymmetries between the electrodes, the gainfactors gi, j can be obtained from the three alternative com-binations of the measured Vi j , e.g. for g1, this is:2 ∗ 50 ∗ g21 =V21V14V42× G13G12G23==V12V31V32× G23G12G13==V41V31V43× G12G23G13(5)and 3 more similar triplet sets for g2,3,4. Since we are inter-ested in the ratios of gain factors (Eq. (3), the values of Gi,jare not needed. Solving Eq. (5) and using Eq. (3) provides3 different pairs of solutions (xe(a,b,c), ye(a,b,c)) for each off-set, whose good or bad agreement has to do with the qualityof the geometrical symmetry of electrode strips.In practice, the BPM electrical offset is obtained by mea-suring S-parameters of the 4 electrodes with a Network An-alyzer (NA). In this case, we use the 4-port NA (AgilentE5071B, 300 kHz – 8.5 GHz). The NA output signal isinjected through one electrode and the S-parameters of theother electrodes are measured, which correspond to the ele-ments of the 4x4 scattering matrix (or S-matrix). A snapshotof one full measurement is shown in Fig. 3. Ideally the reflec-tion coefficients should be zero and the transmission ones,Sij = Sji, symmetric.The normalized voltage Vi j in Eq. (4) is equal to the trans-mission coefficient Si j . The final calculation formula for thehorizontal electrical offset, includes transformation from dBto linear S-parameter readings:xe = kx√10Sx, ijm/20 −√10Sx,npq/20√10Sx, ijm/20 +√10Sx,npq/20(6)and a similar one for ye, where Sxy,ijm and Sxy,npq are com-binations of S-parameter triplets originating from Eq. (5),which also depend on the solution of gi used.Finally, three variants of the offsets labeled (a), (b) and(c), e.g. (xe(a), ye(a))) are using the corresponding sets ofS-parameter triplets for x and y:(a)Sx,ijm = S32 + S42 − S43Sx,npq = S14 + S42 − S21andSy,ijm = S41 + S31 − S43Sy,npq = S32 + S31 − S21 , (7)(b)Sx,ijm = S43 + S42 − S32Sx,npq = S21 + S42 − S14andSy,ijm = S41 + S31 − S43Sy,npq = S32 + S31 − S21 , (8)(c)Sx,ijm = S43 + S14 − S31Sx,npq = S21 + S32 − S31andSy,ijm = S21 + S14 − S42Sy,npq = S32 + S43 − S42 . (9)ResultsAs an example, Fig. 4 shows the offset for a range offrequencies calculated by Eq. (6) for xe(a) . Similar results arefound for the vertical plane. The periodic notches are due tothe electrode geometry and excitation system (excitation viaone electrode, measuring on the others). This is in agreementwith the CST simulations shown in Fig. 5, which comparesthe case in which the excitation is done via one electrode orthrough the wire. Note that when the excitation is done viaone electrode (red), the notches occur at around 500 MHz,in agreement with Fig. 3. However, in the real case with anelectron beam the notches are displaced to around 1 GHz.Originally the “Lambertson” offset measurement is in-tended to be done at some fixed frequency; however, dueTUPB048 Proceedings of IBIC2015, Melbourne, AustraliaISBN 978-3-95450-176-2424Copyright©2015CC-BY-3.0andbytherespectiveauthorsBPMs and Beam StabilityFigure 4: Horizontal electrical offset type xe(a) .Figure 5: CST simulations exciting one electrode (red) orthe wire (blue).to aforementioned reasons, the measured offsets of the 3striplines do not show a flat behavior at their working fre-quency, but rather a notch (see the zoom in Fig. 6). It there-fore makes sense to take an average value for offsets in bothplanes between [20-200] MHz. This is shown in Table 2 forthe three GBS BPMs measured so far. The electrical offsetsare shown with margins taking into account the differentS-parameter triplets (Eqs. (7), (8) and (9)).The electrical offset measured this way is not affectedby any systematic error due to the test bench or BPM posi-tioning in space. However, this offset can not be compareddirectly with the one obtained by wire scans, because the me-chanical offset measured by the wire scan is a combinationof the offsets due to cable differences, reading electronicsand geometrical imperfections of the BPM.Figure 6: Horizontal electrical offset type xe(a) (zoom).Table 2: Electrical Offsets Measured Using the External“Lambertson” Methodxe, µm ye, µmBPM-01 132 ± 0 −238 ± 8.5BPM-02 25 ± 0.15 −134 ± 10.4BPM-04 165 ± 0.7 −157 ± 10WIRE SCAN CHARACTERIZATIONThemechanical characterization is done via wire scanningthe BPM units. The equipment and connectivity schematicis shown in Fig. 7. The BPM is placed on a test bench and awire is pulled through its theoretical center using fiducialsor other reference points/surfaces. The wire is fed by an RFsignal generator and is terminated by 50 Ω at the other end.The signal, caught by the BPM electrodes, is read by LiberaBrilliance electronics. The wire scan is done by movingthe BPM in the horizontal and vertical directions using themotors in the test stand, controlled by a standard IcePapmotor controller from a remote PC.Figure 7: Schematic of lab setup and equipment connectivityfor wire scanning the BPMs.Test Bench for Wire MappingThe ALBA engineering department has designed an ad-hoc test bench to mechanically hold the stripline BPMsfor stretched wire measurements. The bench is equippedwith two motors (Micos Linear Stage LS-120) with uni-directional repeatability of 0.1 µm for horizontal and verti-cal movement. The test bench ensures reproducibility ofwithin 20 µm between BPM-to-BPM measurements. Figure8 shows a complete 3D model of a BPM mounted on thetest bench; a photo of the setup is shown in Fig. 2.As a compromise between stress and conductivity, thewire material is chosen to be copper of 1 mm in diameter.The wire tension is controlled by the force gauge at nominalforce of 120 N to minimize contribution of the sag effecton measurements. For reference, the sag values in the mid-wire position for various tension, calculated by the catenaryequation, are shown in Table 3.Table 3: Sag Value in the Mid-Wire PositionForce, N 94 100 120Deviation, µm 8.2 7.0 6.4Proceedings of IBIC2015, Melbourne, Australia TUPB048BPMs and Beam StabilityISBN 978-3-95450-176-2425 Copyright©2015CC-BY-3.0andbytherespectiveauthorsFigure 8: A complete 3D model of the test bench for ELIBPM measurements.Fiducial MeasurementsAlthough the test bench ensures acceptable reproducibilityof 20 µm, we have observed several mechanical issues thatlimit our precision to much larger values. Firstly, the BPMfiducials, labeled F1. . . F4 in Fig. 8 are often out of tolerancesin both planes from the manufacturing criteria providedby factory certificates. As an example, Fig. 9 shows thedeviations of fiducial planes as measured by the factory andby the ALBA alignment group for the vertical plane. Whilewe generally measure slightly larger fiducial offsets, in somecases it reaches a 200 µm, and even 500 µm mismatch withrespect to factory measurements.Secondly, the diameters of some fiducial holes are out oftolerances, e.g. the 8 mm slot where the spherical mountedreflector (SMR) of the laser tracker is placed is sometimeslarger by +100–200 µm (up to +400 µm in one case) withrespect to +20 µm specified by the BPM drawings. Thismeans that the horizontal position of the SMR alone canhave a significant displacement.Since its is not straightforward to align a BPM with re-spect to the wire by using BPM’s fiducials, all BPMs arepositioned on the test bench for wire mapping according toan established procedure:a) The stretched wire is considered as the nominal zeroposition, referenced by the three fiducials on the testbench (Z1, Z2 and Z3, as indicated by dashed arrowsin Fig. 8).Figure 9: Fiducial position deviations from nominal in Y.Figure 10: Positioning a BPM on an L-shaped platform bytouching it with 2 reference surfaces.b) A BPM is placed on an L-shaped platform touching itwith its two reference surfaces (bottom and right sidewalls), Fig. 10. The geometrical positions of the planes,manufactured with 20 µm precision, is known fromthe drawings with respect to BPM’s origin. The pitch(20 µm in the horizontal plane) and yaw (10 µm in thevertical plane) errors of the platform are also measured.c) The L-platform, controlled by the motors, is placedsuch that its position with respect to the wire resemblesthe nominal distances from the BPM side walls to itscenter. This platform’s position (motor settings) isdefined as the homing position, meaning the wire hereis at (x, y) = [0, 0] which is same for all BPMs.d) When the BPM is positioned on the platform its fidu-cial positions are measured with the laser tracker withrespect to the stretched wire. Any tilt, yaw or roll imper-fection of the particular BPM is encoded in its fiducialcoordinates with respect to the stretched wire.e) After these considerations and measurements the BPMis mapped.Wire MappingWire mapping is done by exciting the wire to a continuouswave excitation of 499.654 MHz and moving the motors toscan the BPM around the wire. The motor positions are thentranslated into wire movements. The electrode voltages areread by the Libera with averaging over 1024 samples andprocessed by DOS equations with kx = ky = 10:xbpm = kx × V3 − V1V3 + V1 (10)ybpm = ky × V2 − V4V2 + V4 (11)Figure 11 shows the result of wire scanning of one of thefirst BPM units, including the error map defined as abso-lute distance between the measured and actual wire posi-tions. The measured map offset (xw, yw ) is relative to thewire at its homing position. It includes the mechanical andthe “Lambertson” electrical offsets, hence xoffset = xw andyoffset = yw .TUPB048 Proceedings of IBIC2015, Melbourne, AustraliaISBN 978-3-95450-176-2426Copyright©2015CC-BY-3.0andbytherespectiveauthorsBPMs and Beam Stability(a) BPM response map measured by wire scanningwithin ±7 mm. The central point corresponds to thewire homing position (0, 0). Its electrode readingscorrespond to the mechanical offset of the BPM.(b) Error of the response map in mm with respect tomotor read-back positions.Figure 11: Wire mapping the BPM-02 of the ELI-NP.The repeatability of wire scans was also checked by mea-suring the map center with respect to the homing position byrepeating the dismount-mount cycle of same BPM severaltimes. This way the map center was usually measured within50 µm in both X and Y for all BPMs.ResultsTable 4 lists the results of the offsets measured by wirescanning. These values include both the electrical offsets,shown in Tab 2, and mechanical manufacturing imperfec-tions. Besides, while the theoretical value of kx,y = 10, itsTable 4: Offsets Measured Using the Wire Scankx ky xw , µm yw , µmBPM-01 9.57 9.51 146 ± 39 −19 ± 16BPM-02 9.47 9.45 114 ± 14 −210 ± 48BPM-04 9.57 9.54 256 ± 16 −139 ± 34values, measured in 3 × 3 points within ±1 mm, are alsoshown.CONCLUSIONSUsing different techniques, we have measured the elec-trical and the mechanical offsets of several BPM units forthe future Gamma Beam System of the ELI Nuclear Physicsfacility.For the electrical offset an external calibration, called the“Lambertson” method, was applied, estimating the geometri-cal asymmetry of the stripline electrodes. For measuring themechanical offset and the sensitivity factors of the BPMswe have designed and built a motorized test bench for wiremapping the BPM units. The obtained sensitivity factorshave shown to be smaller than their theoretical value. Fidu-cial coordinates of the BPM units were also measured withthe laser tracker and found to be significantly different fromthe factory-provided values.The mechanical offsets, referenced with respect to BPM’sfiducial points, are due to all possible mechanical effectsand they will be taken into account when installing the BPMunits in the GBS Linac oF ELI-NP facility.ACKNOWLEDGEMENTSWe would like to thank the alignment team of ALBA, M.Llonch and J. Landera for their time and extensive alignmentmeasurements.REFERENCES[1] G.R. Lambertson, "Calibration of Position Electrodes usingexternal measurements", LSAP Note-5, LBL, 1987.[2] Y. Chung, G. Decker, "Offset calibration of the Beam PositionMonitor Using External Means", Proc. of 1991 Acc. Instrum.Workshop.[3] A. Olmos, M. Alvarez and F. Perez, "Beam position moni-tors characterization for ALBA", TUPSM075, Proceedings ofBIW10, 2010.Proceedings of IBIC2015, Melbourne, Australia TUPB048BPMs and Beam StabilityISBN 978-3-95450-176-2427 Copyright©2015CC-BY-3.0andbytherespectiveauthors

Measurements and calibration of the stripline BPM for the ELI-NP facility with the stretched wire method

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Measurements and calibration of the stripline BPM for the ELI-NP facility with the stretched wire method

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Archivio della ricerca- Università di Roma La Sapienza