Proceedings of IBIC2012, Tsukuba, Japan

MOPB79DESIGN OF A HIGH-PRECISION FAST WIRE SCANNER FOR THE SPS
AT CERN
R. Veness, N. Chritin, B. Dehning, J. Emery, J. Herranz Alvarez, M. Koujili, S. Samuelsson,
J.L. Sirvent, CERN, Geneva, Switzerland
Abstract
INTEGRATION CONSTRAINTS
Studies are going on of a new wire scanner concept.

INTRODUCTION
Wire scanners are installed in the LHC and all circularmachines in the injector chain as a means to measure thetransverse beam profile and hence emittance. The
motivation for the development of a new scanner design
has been described in a previous article [1], along with theconcept with the rotor of the motor and wire position
measurement system inside the beam vacuum [seeFigure 1]. Development of key components, in particular
the motor and control system, are well advanced [2].
Work is now focussing on the integration of all therequired components with the aim of producing a scannercapable of 20 ms-1 scanning speed combined with 2 µm
position precision.
A number of mechanical components require carefuloptimisation. These include the motor housing, shaft,bearings, fork and wire. In addition, the design concept
includes an in-vacuum optical position encoder in order toreach the required precision. Development of thesecomponents is described in the following sections.

Wire scanners are currently installed in the PS, Booster,
SPS and LHC at CERN. It would greatly simplify
operation and maintenance if the same basic design could
be implemented for all of these machines. To this end, themain constraints in terms of machine physics, operationand environment have been analysed for each machine.
These are summarised in Table 1.
Table 1: Summary of Integration Constraints from theCERN Accelerator Complex
Machine Scan
aperture(mm)PS
146x70
BoosterPS
146x70RF
Bakeout Space
ScreenConstraintNNNNAxial,Transverse
Axial

SPS

152x83YN-LHC

65x65

YY

Transverse

The scan aperture is the horizontal and vertical spacethat must be cleared by the wire. RF screens are requiredin some machines to minimise impedance and RF heating
effects. Integration of new scanners into existingmachines must take into account machine geometries andequipment. Axial space constraints occur in machineswith a tight lattice whereas transverse constraints are seen
with parallel equipment on the beamline (eg, thecryogenic distribution line in the LHC). It can be seenfrom table 1 that each of the machines brings constraints
to the design. A solution has been adopted where the main
components can be integrated into designs for the PS,
SPS and LHC. Each machine will require a different fork
geometry and a different flange interface, but other main
components and principles will be common. The layout ofthe Booster with 4 rings in very close proximity mean thatit has not yet been possible to integrate the design into
this machine.
Combining these constraints leads to a design withaperture range up to 152 by 152, with the option toinclude RF screen and to be bakeable to 200°C in order toactivate a low emission yield getter coating used in theLHC vacuum system.

DESIGN OF COMPONENTS
Motor Housing
Figure 1: Fast Wire Scanner concept.

The motor housing has the function of separating therotor in-vacuum and stator on the atmospheric side of theISBN 978-3-95450-119-9

Transverse Profiles, Screens & Wires Monitors

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○All moving parts are inside the beam vacuum and it isspecified for use in all the machines across the CERN
accelerator complex. Key components have beendeveloped and tested. Work is now focussing on theinstallation of a prototype for test in the Super Proton
Synchrotron (SPS) accelerator.
This article presents the specification of the device andconstraints on the design for integration in the different
accelerators at CERN. The design issues of themechanical components are discussed and optimisation
work shown. Finally, the prototype design, integrating theseveral components into the vacuum tank is presented.

MOPB79

Proceedings of IBIC2012, Tsukuba, Japan

electric motor. The housing needs to be thin to fit in therelatively small air gap (approximately 0.8 mm) between
the rotor and stator. The required wall thickness of thehousing has been determined using finite element method(FEM) analysis. The analysis shows that elastic instability
is the critical failure mode. The stability of the structure
increases strongly with wall thickness. The dependence ofstability on the length of the structure is weaker, where ashorter structure increases the stability. Furthermore, athicker housing (within the limits of this problem) iseasier to manufacture. When all the above mentioned
considerations are put together, the optimum wall
thickness of the motor housing is determined to be 0.4mm.

encoder which the shaft vibrations give rise to. The
calculations show that an outer diameter of 35 mm isneeded to keep the deformation below the tolerated limitof 5 μm.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 3: Graph of relative offset of wire vs. encoder fordifferent shaft diameters and thicknesses.

Figure 2: Graph of motor housing collapse load for
different lengths and wall thicknesses.

Shaft and Bearings
One principal of this design is to support forks andposition measurement system on one rigid structure to
maximise precision. The shaft forms the core of themechanical structure. It is driven by the motor andsupports the rotor, forks and optical disc. The main
constraint on the shaft is its deformation in torsion. The
torsional deformation appears due to the acceleration of
the shaft and the inertias of the components mounted onthe shaft. This deformation must be kept small to ensure
accuracy of the measurements. Although the shaft is not
accelerating when the wire passes through the beam, theshaft will vibrate with a maximum amplitude
corresponding to the torsional deformation due to theacceleration. The shaft needs to be hollow to be able topass cables through it. Analysis of the shaft, using
analytical calculations and FEM simulations, show thatusing a larger outer diameter has a strong effect on thestiffness. It is also shown that the stress in the shaft is low
and the inertia of the shaft is of little importancecompared to the inertias of the components mounted on
the shaft. This means that the strength of the material is
not critical and that a stiff, relatively heavy stainless steel
is a better material choice compared to lighter, more
flexible alternatives such as aluminium or titanium. The
analysis also shows that it is the optical disc and the discholder which give the largest contribution to the shaft
twist. This is because they are mounted on the shaft end
opposite to the motor. Therefore effort should be put intominimising the mass of these components. Figure 3
showsthe offset of the wire position relative to theThe bearings need to assure high precision in terms ofradial runout and the materials used must be UHV
compatible and radiation and bake out resistant. Thismeans that traditional lubricants such as oil and greasecannot be used [3]. Instead one must rely on running thebearings without any lubricant or possibly using solid
lubricants (such as molybdenum disulphide or tungsten
disulphide coatings). It is also recommendable to use
different materials for the races and the rolling elementsin the bearing, to avoid cold welding. One available
alternative for this is hybrid bearings which use steel
races and ceramic rolling elements.

Optical Disc and Support
The principle selected for the high precision
determination of the beam size is an optical system basedon a glass disc with a photo-lithographed μm patternmade of high reflectivity chrome, placed inside thevacuum chamber and fixed on the scanner shaft. This
incremental angle encoder uses single-mode optical fibreand UHV fibre optic feedthrough (9/125μm) to drive1310 nm laser light on a 1:1 lens system in-vacuum that
focuses the light on the disc surface with a 10 μm lightspot size. Using the reflectivity of the chrome pattern, thereflected light is coupled back into the same fibre, andthrough an optical circulator directed to the photodiode.
The laser diode, circulator, photodiode and subsequent
electronics will be located in the surface building andonly one optical fibre will go down to the accelerator
tunnel (250 m). The performance of this single fibre
angular position sensor has been tested and validated on
the bench shown in Fig 4.

ISBN 978-3-95450-119-92

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Proceedings of IBIC2012, Tsukuba, JapanMOPB79

on-g
going to verify
y the use of a 5 μm track too reach betterrmechhanical resollutions (arouund 70 μRaad), and theeincorrporation of otherocalibratioon methods ono the scannerr
workking axis to perform a more reliablle calibrationn
procedure.

Figure 4: Test bench foor the optical position
psensoor.
This fibre encoder provvides a resolu
ution of 157 μRad
with a track of 10 μm sliits, and two position
prefereencesusing only oone channel. An accuracy
y of ±25 μR
Rad isreached by aangular calibraation with a coommercial enccoder
Heidenhain RON225 (FFig. 5). With this calibraation,eccentricity errors, and partly gratinng errors caan beminimized.

Th
he design willl integrate a mmagnetic resttraint in orderr
to preventp
unplaanned movem
ments of the fork duringg
transsportation or installation
iannd also during
g operation inn
case of power or control
csystemm failure.
Unncontrolled movement
mof the wire withhin the beamm
apertture could causecthe mmelting of th
he wire andd
quennching of supeerconducting mmagnets (in th
he case of theeLHCC). This effect is enhanced aas the wire sp
peed due to itss
own unbalanced weightw
wouldd be much slo
ower than theenom
minal speed req
quired for safee operation of the scanner.
Th
he conceptual design of thhe restraint syystem consistss
of a magnetic circcuit in two parrts: One part outside of theevacu
uum has a perrmanent magnnet and electrrical coil; theeotherr part inside thet vacuum cconsists of a ferromagneticfcpiecee fixed to thee shaft. This has two polees that, whennalignned with the external maagnetic circu
uit, define ann
equilibrium positiion corresponnding to the ‘parked’ forkk
posittions. When thet electric ccoil is energizzed it cancelss
the field
fof the permanentpmaagnet allowing
g the shaft too
rotatte freely. This concept can bbe seen in Fig
gure 6.

ure 6: Concep
ptual design for the magn
netic restraintt
Figu
systeem.

ForrksFigure 5: E
Error before calibration (top), calibrration
applied (centter) and error histogram (boottom). Calibrration
made with H
Heidenhein RO
ON225 angular position sennsor.
The final system is UHV comppatible with EMI
immunity, wworks with tem
mperatures up to 200oC, andd it isradiation toleerant due the special fibree used. Studiees areThhe two forks which suppport the wiree need to bee
careffully optimised. They neeed to be stifff enough toomainntain the wiree under tensiion and preveent excessivee
deformation and vibration dduring wire acceleration..
How
wever the fork
ks are also a major compponent of theeinerttial load on the shaft and mmotor, so the mass
m and masss
distrribution neeed to be optimised. From theemanu
ufacturing vieewpoint, theree will be diffferent designss
for each machin
ne due to thhe different apertures, sooquanntities will be small.
Conssidering all of these factorrs, it is logicaal to considerr
makiing these components using meetal additivee
manu
ufacturing [4]] (also referreed to as 3D printing).
pThissmeth
hod will allow
w complex geeometries to be optimiseddusing
g FEM, and produced diirectly in sm
mall quantitiess
fromm 3D CAD moodels allowinng for forms th
hat cannot bee
prod
duced with con
nventional maachining.
ISBN 978-3-95450-119-9

Transverse Profiles, Screens & Wires Monitors

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Maggnetic Restrraint Systemm

MOPB79

Proceedings of IBIC2012, Tsukuba, Japan

Studies of the wire are on-going and will be presented
separately.

STATUS AND NEXT STEPS

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○All major components have been integrated into a 3Dmodel with an envelope which would allow installation
into the PS, SPS or LHC. Figure 7 shows a section
through this model. The scanner is assembled as one selfcontained ‘cartridge’ that will be inserted into theaccelerator vacuum chamber. This will protect the wirefrom damage during insertion – a common problem withexisting designs. A prototype will be constructed andtested in the coming months to verify the operation of thescanner assembly and performance. It is then planned toproduce a first production model for test in the SPS
accelerator. This will be installed in the forthcoming‘Long Shutdown 1’ of all CERN accelerators in 2013-14.
The plan is then to produce a series of scanners for
installation in the second Long Shutdown scheduled for
2018-19.

REFERENCES[1] M.Koujili et al. ‘Fast and High Accuracy Wire Scanner’
DIPAC’09 Conference – 25-27 May 2009 /Basel-CH[2] B.Dehning et al. ‘Vacuum Actuator and Controller Designfor a Fast Wire Scanner’ Proceedings of BIW2012,
Newport News, Virginia, USA[3] D. Ramos ‘Bibliographic search on bearing technology for
ultra-high vacuum applications’ Internal communication(2008).
[4] L.E. Murr et al. ‘Metal Fabrication by Additive
Manufacturing Using Laser and Electron Beam Melting
Technologies’ J. Mater. Sci. Technol., 2012, 28(1), 1-14.

Figure 7: 3-D model section through the scanner.

ISBN 978-3-95450-119-9
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