Resonant Cavity Beam Position Monitors (RF-BPMs) are diagnostic instruments capable of achieving beam position

resolutions down to the nanometre scale. To date, their nanometric resolution capabilities have been predicted by

simulation and verified through beam-based measurements with particle beams. In the frame of the PACMAN project at

CERN, an innovative methodology has been developed to directly observe signal variations corresponding to nanometric

displacements of the BPM cavity with respect to a conductive stretched wire. The cavity BPM of this R&D study

operates at the TM110 dipole mode frequency of 15GHz.

The concepts and details of the RF stretched wire BPM testbench to achieve the best resolution results are presented,

along with the required control hardware and software

Artoos, K.

Fanucci, Luca

Morel, F. N.

Novotny, P.

Tshilumba, D.

Wendt, M.

Zorzetti, Silvia

English

Resonant Cavity Beam Position Monitors (RF-BPMs) are diagnostic instruments capable of achieving beam position resolutions down to the nanometre scale. To date, their nanometric resolution capabilities have been predicted by simulation and verified through beam-based measurements with particle beams. In the frame of the PACMAN project at CERN, an innovative methodology has been developed to directly observe signal variations corresponding to nanometric displacements of the BPM cavity with respect to a conductive stretched wire. The cavity BPM of this R&D; study operates at the TM110 dipole mode frequency of 15GHz. The concepts and details of the RF stretched wire BPM test-bench to achieve the best resolution results are presented, along with the required control hardware and software

Artoos, Kurt

Morel, Francois

Novotny, Peter

Tshilumba, David

Wendt, Manfred

CERN Document Server

A Wire-Based Methodology to Analyse the Nanometric Resolution of an RF Cavity BPM

Resonant Cavity Beam Position Monitors (RF-BPMs) are diagnostic instruments capable of achieving beam position
resolutions down to the nanometre scale. To date, their nanometric resolution capabilities have been predicted by
simulation and verified through beam-based measurements with particle beams. In the frame of the PACMAN project at
CERN, an innovative methodology has been developed to directly observe signal variations corresponding to nanometric
displacements of the BPM cavity with respect to a conductive stretched wire. The cavity BPM of this R&D study
operates at the TM110 dipole mode frequency of 15GHz.
The concepts and details of the RF stretched wire BPM testbench to achieve the best resolution results are presented,
along with the required control hardware and software

ZORZETTI, SILVIA

FANUCCI, LUCA

Archivio della Ricerca - Università di Pisa

A WIRE-BASED METHODOLOGY TO ANALYSE THE NANOMETRICRESOLUTION OF AN RF CAVITY BPMS. Zorzetti1∗, K. Artoos, F. N. Morel, P. Novotny, D. Tshilumba, M. WendtCERN, Geneva, SwitzerlandL. Fanucci, University of Pisa, Pisa, Italy1also at University of Pisa, Pisa, ItalyAbstractResonant Cavity Beam Position Monitors (RF-BPMs) arediagnostic instruments capable of achieving beam positionresolutions down to the nanometre scale. To date, theirnanometric resolution capabilities have been predicted bysimulation and verified through beam-based measurementswith particle beams. In the frame of the PACMAN project atCERN, an innovative methodology has been developed to di-rectly observe signal variations corresponding to nanometricdisplacements of the BPM cavity with respect to a conduc-tive stretched wire. The cavity BPM of this R&D studyoperates at the TM110 dipole mode frequency of 15GHz.The concepts and details of the RF stretched wire BPM test-bench to achieve the best resolution results are presented,along with the required control hardware and software.INTRODUCTIONThe CLIC experiment at CERN is an international studyof a future Compact LInear Collider. The main purpose isto achieve both high efficiency and luminosity at the Inter-action Point (IP) at a beam energy up to 1.5 TeV. Therefore,the required beam size at the IP is in the range of a fewnanometres. To achieve this, the beam emittance needs tobe maintained over the whole 30 km length of the mainlinac, implying Beam Position Monitor (BPM) technologiescapable of measuring nanometric displacements and a pre-cise alignment between the main accelerator componentsin the micrometric range. In this scenario, the PACMANproject [1], funded by the European Union’s Seventh Frame-work Programme, aims to prove the feasibility of innovative,high precision alignment methods for BPMs, quadrupolesand accelerating structures, based on stretched and vibratingwire technologies.CLIC CAVITY BPM AND TESTSThe CLIC cavity BPM design is based on the combinationof a monopole TM010 mode reference cavity and a dipoleTM110 position cavity, resonating at ∼ 15 GHz (a 3D modelof the position cavity and pickups is shown in Fig. 1). Theinitial design in stainless steel was studied through beammeasurements in the CLIC Test Facility (CTF3) at CERNand it was later improved to achieve an higher Q valuethrough a copper construction. Five cavity BPM prototypesof the new design are now being studied at CERN. ThreeBPMs are installed in CTF3 [2] and two others are used bythe PACMAN project in a laboratory environment to test∗ silvia.zorzetti@cern.chstretched-wire methods.Figure 1: BPM moodel.The BPM resolution depends on many factors. A relevantsignal deterioration is caused by manufacturing imperfec-tions, leading to monopole mode leakage to the positioncavity, asymmetries between the lateral pickup ports, orcross coupling between adjacent ports. To avoid a signif-icant degradation of the performances and accuracy, thefabrication tolerances were set very tight. Taking into ac-count the weakest manufacturing aspects, RF simulations ofthe BPM design anticipated a 50 nm spatial resolution, for50 ns measurement time with beam [3].So far, cavity BPM resolution results have mainly beenachieved through beam-based techniques. The traditionalapproach is to pre-align three BPMs in a straight line on acommon support, and to predict the position of the centralBPM out of the position of the other two. The resolution isextrapolated by the residuals’ standard distribution, returnedby the difference between the measured position and thecalculated one [4].BPM experiments using particle beams are essential testsfor calibrations purposes, e.g. analysing the sensitivity of thecavity as a function of the beam position or the number ofcharges. However, RF tests, without requiring beam, presenta simple way for characterizing first prototypes and pre-align the BPM and its associated quadrupole. The proposedstretched-wire analysis is particularly valuable for findingthe electrical center of the position cavity or analysing itshigh spacial resolution capability.STRETCHED-WIRE SETUPTo study the BPM resolution, a standalone test bench hasbeen assembled. The main bench components, and measure-ment procedure are shown in Fig.2, 3. A Vector NetworkAnalyser (VNA) is added for signal read-out. The scatteringparameters acquired through this setup allow the measureProceedings of IBIC2016, Barcelona, Spain MOPG12BPMs and Beam StabilityISBN 978-3-95450-177-963 Copyright©2016CC-BY-3.0andbytherespectiveauthorsof the relative displacement between the wire and the elec-trical center of the cavity. We found that the most sensitiveinformation is returned by the phase between adjacent ports(e.g. ∠S141), which demonstrates a linear behaviour aroundthe electrical center of the cavity [5].Figure 2: BPM test bench with hexapod and piezo stages.Laboratory EnvironmentEnvironmental tests have been carried out to verify thestability of the laboratory in terms of temperatures and vi-brations.Temperature The resonant frequency of the positioncavity versus temperature has a gradient of −359kHz/◦ C[6]. As a drift in resonant frequency or Q-factor also affectsthe stability of the measurements, it is important that thetemperature of the laboratory is stable. In this laboratory,the recorded temperature drift is less than 0.3◦ C over ∼ 12hours (Fig.4). The signal phase, as measured by the VNA,shows a linear dependence from the temperature Fig.5. It isnecessary to underline that the temperature wasmonitored onthe complete coaxial assembly, composed of both the BPMand the stretched wire, therefore a change of temperaturewas impacting not only on the BPM position cavity but onthe entire installation.Vibrations The test bench is mounted on an active op-tical table to damp vibrations. The chosen model is a VisionIsoStation from Newport, ensuring a reduction of vibrationtransmission to the vertical and horizontal axis of 85% above5 Hz and 95% above 10 Hz. Vibration measurements of theground, the table and the hexapod were performed in orderto estimate their possible effect on the wire position mea-surements. The results are presented in Fig.6 and dependon the period of the day measured: lower vibrations areregistered during the night than during the day. However1 Signal phase acquired exciting from Port4 and reading from Port1 -Reference to cavity depicted in Fig.3the parameter of interest is the relative motion between thetable and the hexapod, as the wire is attached to the tablevia holders and the hexapod moves relatively to this wire.This always remains below 4 nm 2. It is necessary to remarkthat these measurements characterize only the dynamic be-haviour of the BPM measuring system and do not provideany information about possible drift or very low frequencyvibrations.Actuators and SensorsActuators Two types of actuators are used, a Hexapodfor pre-positioning the BPM with respect to the wire (withsub-micrometer minimum incremental motion on 6-DOF 3),and a piezo to move the BPM with respect to the wire witha finer step size.The piezoelectric actuator is a high voltage pre-loaded piezostack P-225 from Physik Instrumente (PI). The actuator hasan embedded strain gauge sensor to measure its elongationduring operation, with 0.3 nm resolution. The actuator ispowered by an E-508 amplifier module and it is driven byan E-509 servo module from the same company. The piezois operated in closed loop to cancel hysteresis and improvelinearity. The input signal to drive the actuator is fed to thecontrol port of the piezo amplifier by a voltage generator,providing DC voltage levels. An input voltage of 10V corre-sponds to 15µm displacement.The actuator is mounted on the hexapod and drives the BPMin a single direction (vertical axis). The BPM center of grav-ity was positioned on the axis of the actuator. Due to thesmall mass (0.7 kg) of the BPM with respect to the high loadactuator (12.5 kN), no external linear guidance was used forthis simple setup. This was considered sufficient as of themeasurement campaign consists of a rather static process.Two lateral parts were used to align around the vertical axis.Sensor A Copper beryllium wire of 0.1 mm diameteris stretched through the BPM cavity to simulate the particlebeam. The choice of the wire is mainly driven by the compat-ibility with magnetic measurements for the Final PACMANBench [7]. In this case it acts as a passive probe, sensing thedipole electric field excited into the cavity. The wire is fixedand mounted to two lateral stretching devices, installed onthe table.Software Automation and VNA SetupThe measurements procedure is completely automatedthrough a controller software developed in LabVIEW (in-terface in Fig.7), with a state machine to synchronize thedifferent steps.1 The hexapod is moved to prepositioned coordinates.This predetermined point is close to the electrical centerand has high sensitivity;2x∆RMS=√x21− x223 three linear axis and three angular onesMOPG12 Proceedings of IBIC2016, Barcelona, SpainISBN 978-3-95450-177-964Copyright©2016CC-BY-3.0andbytherespectiveauthorsBPMs and Beam StabilityFigure 3: Schematic view of the test bench for resolution nanometric measurements.Figure 4: Temperature and cross-section’s phase trends.Figure 5: Temperature vs cross-section’s phase.Figure 6: RMS vibrations of the bench.Figure 7: LabVIEW interface, piezo controller.2 The main controller provides the voltage value to thesignal generator;3 The piezo actuator is controlled in voltage by the signalgenerator’s output;4 After the movement of the piezo, a sensed position issent in closed loop for voltage reading to themultimeter;5 A trigger to a VNA is generated every Dt (a user-defined time constant);6 The VNA, connected to the four BPM ports, performsRF magnitude and phase measurements in frequencysweep. The lower the frequency of the sweep (IFBW),the higher the time constant Dt needs to be. In thepresent experiment IFBW=70 Hz, with a sweep cen-tered around the position cavity’s resonant frequency(∼ 15 GHz). Moreover, before taking data, the VNAwas calibrated and a phase offset has been applied tocenter the BPM signal phase in a 0◦ to ±180◦ range.NANOMETRE RESOLUTION RESULTSTHROUGH STRETCHED-WIREMEASUREMENTSThe signal generator is programmed by the LabVIEWinterface to sweep in a DC voltage range for a number ofsteps. After every movement of the piezo, the VNA acquiresmagnitude and phase data. With this experiment we intendto demonstrate that a few nanometres resolution could beresolved in a static conditions process.Fig.8a shows the linear dependence between piezo elonga-tion (resulting in wire displacement with reference to thecenter of the position cavity) and the adjacent ports phase(∠S14). The position displacement is ∆l = 25nm, from aDC voltage offset driving the piezo of ∆V ≃ 33.33mV . Afiner DC offset voltage sweep (∆V ≃ 2mV ) shows, withless accuracy, the outstanding BPM sensitivity and abilityto detect position displacements of ∼ 3nm (Fig.8b).Impact of Environmental CharacteristicsThe rather high acquisition time (Dt = 240000 ms) resultsin a very low sampling frequency fs = 4 mHz. Possiblesources of noise at frequencies above fs, such as air flowsor vibrations (4 nm at 1 Hz in Fig. 6) could be consideredremoved. Although, practical experiences may still be af-fected by environmental changes over a long time scale (e.g.wire tension or ambient temperature) as they were performedduring different time slots and days. To reduce their impact,tests were repeated several times and averaged.Gradient DistributionPlots in Fig. 8 show the result of the experience. Weobserve a dependence between the phase and the piezo elon-gation expressed in a linear form: y = ax + b, where a is theProceedings of IBIC2016, Barcelona, Spain MOPG12BPMs and Beam StabilityISBN 978-3-95450-177-965 Copyright©2016CC-BY-3.0andbytherespectiveauthors(a) ∠S14 resolution results at 25 nm piezo step size. (b) ∠S14 resolution results at 3.125 nm piezo step size.Figure 8: BPM nanometre resolution results.gradient (angular coefficient) and b the line offset. The tablebelow summarizes those parameters for the mean, upperand lower lines, calculated with the mean squared fit method.Table 1: Gradients and OffsetsParameterUppererrorMeanLowererrorGradient - 25 nm[Degrees/µm]1.7472 1.8881 2.1145Gradient - 3 nm[Degrees/µm]1.5556 1.7842 2.0283Offset - 25 nm[Degrees]-19.9416 -20.1856 -20.4879Offset - 3 nm[Degrees]-23.8501 -24.2595 -24.6557The gradient distribution between every couple of sam-ples (∆∠S41∆l) is shown in Fig. 9. The mean (µ) gradient value,presenting a higher probability of occurrence, is close to thecomputed average value in Table 1. The high variance de-pends mainly on the impact of environmental characteristicsdrifts at very low frequencies (≪ fs), as well as from thelimited number of observations recorded.Figure 9: Histogram of gradient values.CONCLUSIONA test bench to perform static nanometric stretched-wiremeasurements was developed and first results achieved. Thepresented experiences demonstrated the mechanical capabil-ity of the BPM position cavity of acquiring with nanometricsensitivity the position of a floating conductive stretched-wire, out of its coupling with the electrical field excited bythe attached waveguides in the position resonant cavity. Thisresult can be seen as an upper resolution limit for beam-basedtests.REFERENCES[1] H. Mainaud Durand et al., "Status of the PACMAN project",in Proc. IBIC’15, Melbourne, Asutralia, September 2015, pp.483–486.[2] J. R. Towler et al., "Development and Test of High ResolutionCavity BPMs for the CLIC Main Beam", in Proc. IBIC’15,Melbourne, Asutralia, September 2015, pp. 474–478.[3] A. Lunin et al., "Design of a submicron resolution cavity BPMfor the CLIC main linac", Rep. TD-09-028, December 2009.[4] S. Walston et al., "Performace of a nanometer resolution BPMsystem", in Proc. EPAC’06, Edinburgh, United Kingdom, June2006, pp. 1256–1258.[5] S. Zorzetti et al., "Stretched-Wire Techniques and Measure-ments for the Alignment of a 15GHz RF-BPM for CLIC", inProc. IBIC’15, Melbourne, Asutralia, September 2015, pp.487–491.[6] F. Cullinan et al., "A prototype cavity BPM position monitorfor the CLIC main beam", in Proc. IBIC’12, Tsukuba, Japan,October 2012, pp. 1–3.[7] C. Sanz et al., "Characterization and measurement to thesub-micron scale of a reference wire position", in Proc.CIM’15, Paris, France, September 2015, Article Number13005.MOPG12 Proceedings of IBIC2016, Barcelona, SpainISBN 978-3-95450-177-966Copyright©2016CC-BY-3.0andbytherespectiveauthorsBPMs and Beam Stability

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A Wire-Based Methodology to Analyse the Nanometric Resolution of an RF Cavity BPM

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Archivio della Ricerca - Università di Pisa

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