Wire methods for measuring field harmonics, gradients and magnetic axes in accelerator magnets

155 Pages·2012·40.11 MB·English

Wire methods for measuring field
harmonics, gradients and magnetic axes in
accelerator magnets
Candidate:
Ing. Carlo Petrone.
Cern Supervisor:
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11 Prof. Stephan Russenschuck.
3-3-
11
00
22
S-S-
SISI
EE
HH University Supervisor:
TT
N-N-1313
ERER/20/20 Prof. Pasquale Arpaia.
CC//
Department of Engineering,
University of Sannio, Benevento.
(cid:13)c Carlo Petrone

List of publications
[1] Arpaia P, Fiscarelli L, Cardines N, Petrone C, (2010) “Design of an on-line dy-
namic magnetic flux analyzer based on double-buffer sliding DFT”, 7th Sympo-
siumIMEKOTC4,3rd SymposiumIMEKOTC19and15th IWADCWorkshop
Instrumentation for the ICT , Era September 8-10, Kosice, Slovakia.
[2] BuzioM,GalbraithP,GolluccioG,GiloteauxD,GilardoniS,PetroneC,Walck-
iers L, (2010) “Development of upgraded magnetic instrumentation for CERN
real-time reference field measurement system”, IPAC - International Particle
Accelerator Conference, May 23-28, Kyoto, Japan.
[3] Arpaia P, Fiscarelli L, La Commara G, Petrone C, (2011) “A Model-Driven
Domain-Specific Scripting Language for Measurement System Frameworks”,
IEEE - Transactions on Instrumentation and Measurement.
[4] Arpaia P, Buzio M, Perez J G, Golluccio G, Petrone C, Walckiers L, (2011)
“Magnetic field measurements on small magnets by vibrating wire systems”,
Instrumentation and Measurement Technology Conference IEEE, May 10-12.
[5] ArpaiaP,BuzioM,CaspersF,GolluccioG,PetroneC,(2011)“StaticMetrolog-
ical Characterization of a Ferrimagnetic Resonance Transducer for Real-Time
Magnetic Field Markers in Particle Accelerators”, I2MTC - Instrumentation
and Measurement Technology Conference IEEE, May 10-12.
[6] Modena M, Dunkel O, Perez J G, Petrone C, Solodko E, Thonet P, Tommasini
D, Vorozhtsov A, (2012) “Design, assembly and first measurement of a short
model for CLIC final focus hybrid quadrupole QD0”, IPAC - International
Particle Accelerator Conference, New Orleans, Louisiana, USA.
[7] Arpaia P, Buzio M, Perez J G, Petrone C, Russenschuck S, Walckiers L, (2012)
“Measuring field multipoles in accelerator magnets with small-apertures by an
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oscillatingwiremovedonacirculartrajectory”,JINST - Journal of Instrumen-
tation, published by IOP.
[8] Arpaia P, Petrone C, Walckiers L, (2012) “Experimental validation of solenoid
magnetic centre measurement by vibrating wire system”, XX IMEKO World
Congress Metrology for Green Growth, September 914, Busan, Republic of Ko-
rea.1
[9] Arpaia P, Petrone C, Russenschuck S, (2012) “Measuring multipole field errors
in magnet apertures of large aspect ratio by means of an scillating wire on
elliptictrajectories”,XX IMEKO World Congress Metrology for Green Growth,
September 914, Busan, Republic of Korea.2
[10] ArpaiaP,PetroneC,BuzioM,RussenschuckS,andWalckiersL,(2013)“Multi-
pole correction of stretched-wire measurements of field-gradients in quadrupole
accelerator magnets”, JINST - Journal of Instrumentation, published by IOP.
[11] Arpaia P, Petrone C,Russenschuck S, Walckiers L, (2013) “Multipole-field cor-
rectionofstretched-wiremeasurementsofthegradientinacceleratormagnets”,
19th Symposium IMEKO TC 4 Symposium and 17th IWADC Workshop Ad-
vances in Instrumentation and Sensors Interoperability,July18-19,Barcelona,
Spain.
[12] M. Modena, A. Bartalesi, J. Garcia Perez, R. Leuxe, G. Perrin-Bonnet, C.
Petrone, M. Struik, A. Vorozhtsov, (2013) “Performances of the Main Beam
Quadrupole Type1 Prototypes for CLIC”, International Conference on Magnet
Technology, July 14 - 19, Boston, USA.
[13] Arpaia P, Petrone C, Russenschuck S, Walckiers L, (2013) ”Vibrating-Wire
Measurements for Centering and Alignment of Solenoids”, JINST - Journal of
Instrumentation, published by IOP.
1GyorgyStrikerJuniorpaperaward
2GyorgyStrikerJuniorpaperaward

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Introduction
Inparticleaccelerators,thebeamstabilityisaffectedbymagneticfieldin-homogeneity
in the bending and focusing magnets. Therefore, a high field quality [1–4], character-
ized by the harmonic coefficients of the transverse field expansion, is required. High-
accuracy measurements of these coefficients, usually denoted as field multipoles [5],
are mainly based on rotating search coils [6]: a wire loop, of area A and N turns,
is rotated at the a given angular frequency in the magnet field. This method yields
a measurement of the magnetic flux linked with the search coil as a function of its
angular position. The field harmonics are then deduced by exploiting the knowledge
of the search-coil geometry. Uncertainty arises, among other sources, from small lat-
eral movements of the rotation axis due to the coil turning mechanics. Moreover, the
coil’s angular position does not match perfectly the expected angular position for a
particular trigger. This can happen because of torsional vibrations in the coil or tim-
inguncertaintyonthetriggers[7]. Theaccuracyofthehigherharmonicsisimproved
by a compensated (bucked) coil configuration for rejecting effects of geometrical im-
perfections. In the configurations for quadrupole-magnets, a probe contains two or
more off-axis coils, connected such that the voltage signals induced by the main field
are canceled out.
Magnets with small apertures of about 10 mm in diameter are developed for new
linear accelerator studies, such as the Compact Linear Collider (CLIC) [8,9] and the
Linac4 [10] at the European Organization for Nuclear Research (CERN). For high-
precision search coil manufacture, a new printed-circuit board technology is used [11]
forthesmallshafts. Thesingle-stretchedwiremethod[12]iscommonlyusedtomea-
sure the magnetic field strength and axis by moving the wire in radial directions. An
electrical voltage is induced across a moving wire in a magnetic field. The difference
in the integrated voltages measured between two opposite wire displacements yields
the offset of the starting point with respect to the magnetic axis. The multipoles
are determined by moving the single-stretched wire on the generators of a cylindrical
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domain inside the magnet aperture, and by integrating the voltage signal measured
on the wire extremity. However, the wire in the magnet and the cabling to the volt-
meter/integrator create a loop sensitive to environmental electromagnetic noise. The
small apertures of the magnet limit the displacement range, thus reducing the ampli-
tude of the induced voltage and increasing the uncertainty in the axis location and
multipoles measurement. Consequently, small-aperture magnets are challenging for
multipole measurements, both by means of the rotating-coil and the single-stretched
wire methods. In this thesis a new method, based on the oscillating wire particular
suitable for small aperture to measure the multipole is proposed [13].
In particle accelerators, solenoids are used for beam focusing. The emittance
growth is limited by axially symmetric focusing elements with short periods, which
often works better than sequential pairs of quadrupoles that give rise to significant
halo formation [14]. Particles moving exactly along the magnetic axis do not experi-
ence a Lorentz force. Off-axis particles are azimuthally accelerated by the radial field
component in the end regions of the magnet. The resulting helical motion in the lon-
gitudinal field of the magnet center yields a focusing effect due to the radial Lorentz
force. The magnetic axis in a solenoid is defined as the path where the integral over
transverse field components takes its minimum. Measuring the field uniformity and
the magnetic axis position assesses the solenoid design and manufacture. Usually the
field is measured by a Hall probe, mounted on a three-axis coordinate stage. The
magnetic axis position is then measured by means the classical single stretched-wire
(SSW) technique [15]. However, the sensitivity is poor due to small displacement
amplitudes. The vibrating-wire method [16] allows measuring the position of the
magnetic axis in the micrometer range. The method is based on the Lorentz force
displacementofthewire,whichisfedbyanalternatingcurrent. Thewirewillvibrate
in the direction orthogonal to the magnetic field. In a quadrupole, the magnetic axis
isdeterminedbythewirepositioninthemagnetaperture,wherethisoscillationtakes
its minimum [17]. The precision is affected by the relation of the measured axis to
the alignment target of the magnet. In this thesis, a method based on a vibrating
wire is proposed for measuring the position of the magnetic axis of a solenoid. A
copper beryllium wire, fed by an alternating current, oscillates perpendicular to the
transverse magnetic field inside the aperture of the solenoid. The amplitude of this
oscillation is proportional to the current and field magnitudes. When the magnetic
axis coincides with the wire position, the transverse magnetic field components along
the wire cancel out and therefore no motion is induced. Feeding a sinusoidal current
providesmaximumoscillationatthewire’sresonancefrequencyandthusincreasesthe
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measurement sensitivity. Resolution in the micrometre range is easily achieved [18].
Quadrupolemagnetsareinstalledtofocusparticlebeamsinacceleratorsandtheir
focusingstrengthisproportionaltotheradialdistanceofthebeamfromthemagnetic
axis [19]. However, due to manufacturing errors, the magnetic axis (defined as the
locus of points in the magnet aperture with zero magnetic flux density) may not
coincide with the geometrical axis [20].
The magnetic axis is commonly measured in quadrupole magnets, together with
the magnetic field strength, by the single-stretched wire (SSW) method, however,
these measurements are affected by nonlinear field errors in the magnet expressed as
multipole coefficients. Usually these multipoles are referred to as non-allowed mul-
tipoles, because they arise from manufacturing tolerances and deviations from the
quadrupolar symmetry. SSW measurements of the field gradient in quadrupole mag-
nets are affected by this nonlinear field errors, expressed as higher-order field multi-
poles. Inthisthesis, acorrectionoffieldgradientsinquadrupoleacceleratormagnets
measured by stretched-wire methods is described. The gradient is first measured by
means of the single-stretched-wire method. By using the same experimental setup,
therelativemultipole-fielderrorsofthequadrupolearethenmeasuredbymeansofthe
oscillating-wire technique and used for the correction scheme. Results of the experi-
mental validation are presented for a prototype quadrupole for the CLIC accelerator
study at CERN.
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Objectives of this thesis
This thesis proposes a new method based on a forced oscillating wire to measure
magnet field multipoles and a method based on vibrating wire for measuring magnet
axes. These developments involve the following main achievements:
• Design and study of theoretical models. The wire is positioned step-
by-step on different points inside the magnet aperture and the corresponding
oscillation amplitude in each position of the are measured and related to the
magnetic field. A theoretical background, an analytical model, a measurement
procedure,andmeasurementsystemarchitectureoftheoscillating-wiremethod
were defined.
• Design and development measurement bench. A design optimization is
performedforfindingtheoptimalparametersoftheentiremeasurementsystem
andanewbenchwasimplementedtocarryoutameasurementcampaigntoval-
idatethemethods. Todealwiththeforthcomingneedstotesthighperformance
magnets, software components in C++ code and measurement procedures con-
trolling different kinds of hardware sensors and equipment were developed.
• Measurementcampaigntovalidatethemodels. Finally,theexperimental
validation shows satisfying results in comparison to a reference rotating coil
system and, furthermore, the oscillating wire is more powerful than rotating
coils in small-aperture magnets due to the high complexity to manufacture coil
shafts with such small dimensions (few mm).
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