cern experience on accelerator magnets based on permanent ... · permanent magnet sextupole for...

CERN experience on accelerator magnets based on permanent magnets

Compact and Low Consumption Magnet Design Workshop for Future Linear and Circular Colliders

Geneva 26th-28th November 2014

Pierre-Alexandre THONET

Which types of magnets are used at CERN?

Pictured: SPS dipole magnet

Pictured: LHC superconducting magnet

Pictured: Linac4 permanent magnet quadrupole

Resistive magnets: 4800 magnets (about 50 000 tons) are installed in the CERN accelerator complex. These magnets are air cooled or water cooled.

Superconducting magnets: 10 000 magnets (about 50 000 tons) are installed mainly in LHC. These magnets are cooled with liquid helium.

Permanent magnets: 150 magnets (about 4 tons) are installed in Linacs and experimental areas.

Compact and Low Consumption Magnet Design Workshop Geneva 26-28 November 2014

Pierre-Alexandre THONET 2



Reasons to use permanent magnets

Reliability: • This solution allows to reduce number and time of interventions in high radiation areas as there

are no risk of electrical failure or water leaks. • No need of failure detection or monitoring system. Cost efficiency: • The production of an accelerator magnet based on permanent magnet is often cheaper than a

resistive magnet solution. • Permanent magnets do not require power convertors and external network such as electrical

cabling and demineralized water supply. • The operation of the magnet does not require any electricity. Flexible designs: • Designs can be very compact. • Magnets can be easily integrated in vacuum vessels assemblies.

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Material used at CERN

Mainly Samarium Cobalt type Sm2Co17 is used at CERN because of the following reasons: • High specific energy product material. • High remanence and coercivity. • High Intrinsic coercivity. • Small temperature coefficient: -0.035%/°C. • Good radiation resistance. • Acceptable corrosion stability even without protective coating. Important requirements asked to magnet suppliers: • Very good homogeneity of magnetic characteristics in a permanent magnet batch (typically within 1%). The

absolute value is less important. • Low deviation of easy axis orientation (typically lower than 2°). • Tight geometrical tolerances.

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Projects based on permanent magnets at CERN

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Pierre-Alexandre THONET 6

Linac4 permanent magnet quadrupole

H - H-

source RFQ DTL

45 keV 3 MeV 50 MeV 100 MeV 160 MeV

Chopper line CCDTL PIMS Transfer line to PSB LEBT

76 m

Pictured: Linac4 layout

This quadrupole was designed to provide beam focusing in the Cell Coupled Drift Tube Linac (CCDTL) of Linac4.

Parameter Value Unit

Number of magnets 14

Nominal gradient 11 to 16 T/m

Nominal integrated gradient 1.1 to 1.6 T

Magnet length 103 mm

Magnet aperture (diameter) 45 mm

Gradient integral error <± 0.5 %

Yaw/pitch/roll < 1 mrad

Pictured: A CCDTL cell

Pictured: PMQ characteristics

Permanent magnet quadrupoles



• Iron free quadrupole based on a Halbach array. • The yoke profile is wire EDM cut in one single piece. This

ensures an accurate positioning of the 8 windows holding the permanent magnet blocks (i.e. directly linked to the magnetic center of the quadrupole) with respect to the referential of the magnet.

• Radial position of the blocks is settled with non-magnetic shims inserted between the austenitic steel yoke and the permanent magnet blocks. A range of radial displacement of 6 mm of the permanent magnet blocks permits to adjust the integrated gradient from 1.1 to 1.6 Tesla.

• All of the 14 quadrupoles installed in the Linac4 have a different gradient.

Permanent magnet block (Sm2Co17) type RECOMA 30S from Arnold Magnetics

Non magnetic yoke (austenitic steel 316LN)

Non magnetic shims (austenitic steel 316LN)

Pictured: Permanent magnet quadrupole 3D model

Pictured: Field lines in the PMQ


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Linac4 quadrupole: design



Linac4 quadrupole: magnet irregularities effects

Pictured: harmonic content for each kind of permanent magnet blocks irregularities

The allowed tolerances on the permanent magnet blocks characteristics and the yoke geometrical tolerances were defined following the studies done on a full model of the quadrupole simulating magnet irregularities.

Case 1: deviation of 2% of remanence (Br) and coercivity (Hcb) on 2 blocks.

Case 2: positioning error of 0.05 mm of 2 blocks.

Case 3: error on magnetization direction of 2° on 2 blocks.

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Linac4 quadrupole: gradient adjustment

After magnet assembly, the integrated gradient of each quadrupole is measured with a stretched wire method. Positioning of the permanent magnet blocks is corrected with the thickness adjustment of the non magnetic shims. This necessary correction is due to small variation of magnetic characteristics of the blocks. In general one iteration is necessary to achieve the gradient tolerance of +/- 0.5%.

Copper-beryllium wire stretched through the magnet

Two translation stages move the wire horizontally then vertically.

Induced voltage V in the wire loop, integrated over the duration of the movement is proportional to the average field across the area spanned by the wire.

Pictured: Stretched wire measurement system

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Permanent magnet sextupole for ASACUSA experiment

This sextupole was designed to be installed just after a hydrogen source used by the ASACUSA collaboration to test their hyperfine spectroscopy beam line. It is used to polarize (~95%) the Hydrogen beam coming from the source.

• These 2 sextupoles are installed in high vacuum (about 10-8 mbar) and an electromagnet solution with coils isolated by resin would generate some degasing. It would also be difficult to evacuate the dissipated heat generated by the coils.

• Permanent magnet design provides a compact solution for this sextupole with a high field requirement.

Permanent magnet sextupoles

Pictured: ASACUSA spectrometry beam line setup

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Parameter Value Unit

Number of magnets 2

Magnet length 65 mm

Magnet weight 2 Kg

Magnet aperture (diameter) 10 mm

Integrated sextupole gradient 7435 T/m

Field at r=5 mm 1.36 T

Harmonic content at 2.5 mm radius: Bn/B3 for n=3,4,...

<0.1

%

Pictured: Permanent magnet sextupole characteristics



ASACUSA sextupole: design

External yoke Titanium T40, non magnetic to hold the poles together and guaranty the geometry.

Vacuum brazing Gapasil filler.

Permanent magnet block Sm2Co17, as a flux generator, type RECOMA 30S from Arnold Magnetics.

Pole Fe-Co, to canalize magnetic flux and assure field quality, type VACOFLUX 50 from Vacuumschmelze.

72

mm

98 mm

Shim 316LN non magnetic austenitic steel but possibility to insert iron shims to adjust the sextupole field.

• The magnet design is based on iron dominated poles and Samarium Cobalt Sm2Co17 permanent magnet blocks. • Permanent magnet blocks installed between each poles act as magnetic flux generator. • Due to high magnetic field, the poles are made of a high saturation Fe-Co alloy. • Fe-Co poles smooth possible deviations of permanent magnet blocks magnetization direction. The field quality is

obtained with an accurate cutting of the pole profile. • An adjustment of the sextupole field is possible by inserting some iron shims behind the permanent magnet blocks. • In order to simplify the manufacture of the permanent magnet blocks, it has been decided to have the same

magnetization direction for all the blocks, parallel to internal and external block faces.

Pictured: Permanent magnet sextupole 3D model

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ASACUSA sextupole: manufacturing

Titanium ring.

Fe-Co core • The external ring was made in Titanium because it has a similar thermal expansion coefficient than Fe-Co.

• During the vacuum brazing operation, a 820°C stage was maintained during 4 hours to perform final annealing of Fe-Co and obtain the highest magnetic characteristics.

Pictured: Vacuum brazing of Fe-Co core and Titanium ring

Pictured: BH curve of Fe-Co VACOFLUX 50

Pictured: Magnet yoke wire cut with EDM Pictured: Insertion of permanent magnet blocks

Pictured: Permanent magnet sextupole assembled

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Permanent magnet dipole for n-Tof* experimental area 7

20

mm

34

0 m

m

800 mm

This dipole was designed to evacuate all charged particles as protons and electrons from the neutron beam after n-Tof spallation target. Parameter Value Unit

Field at the center 0.253 Tesla

Field homogeneity +/- 1.5 %

Magnet gap 340 mm

Magnet length 960 mm

Total mass 2670 Kg

Mass of permanent magnets 714 Kg

Permanent magnet blocks Sm2Co17, as a flux generator.

Permanent magnet blocks Sm2Co17, compensate radial stray field to improve field quality in good field region (GFR).

Return yoke pure iron.

Pole tip pure iron, smooth the possible differences on the easy axis orientation of the permanent magnet blocks.

• Magnet design based on an iron dominated external yoke and poles and Samarium Cobalt Sm2Co17 permanent magnet blocks.

• The dipole is composed of 168 permanent magnet blocks of dimension 80 mm*80 mm*80 mm

*n-Tof: Neutron time of flight. Pictured: Permanent magnet dipole 3D model

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n-Tof dipole: magnetic design

• Because of the dipole symmetries, only 1/8 of the magnet was modeled.

• The integrated field homogeneity inside the good field region (radius of 160 mm) is within +/- 1.5%

0

0.05

0.1

0.15

0.2

0.25

0.3

-1200-1000 -800 -600 -400 -200 0 200 400 600 800 1000 1200Mag

net

ic f

ield

By

(T)

Z (mm)

Field distribution along Z axis at the center of the magnet

Lmag= 1134 mm

Pictured: Field distribution Bmod (T) in the dipole

Pictured: Integrated field homogeneity in GFR (%): 100*(Bydz-Bydz(0))/Bydz(0)

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n-Tof dipole: assembly (1)

• The side blocks were inserted first in the 4 windows. • For the insertion of the pole blocks, due to strong forces tending to

repulse the permanent magnet blocks each other, the magnetic field in the gap was shunted with 1200 steel sheets (600 Kg of steel).

• The 168 magnet blocks were individually inserted in the iron yoke using a suction cup for their manipulation.

Pictured: magnet gap filled with steel sheets Pictured: manipulation and insertion of blocks with suction cup

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Pictured: side blocks inserted on one side of the dipole yoke

Backward field



n-Tof dipole: assembly (2)

• The steel sheets were removed at the end of the assembly. • The protection covers were defined to limit the field outside the

yoke to safely manipulate and work in the vicinity of the magnet.

Pictured: Removal of steel sheets at the end of the assembly

Pictured: Magnetic field measurement

Pictured: magnet installed in the experiment

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n-Tof dipole: cost saving

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Cost estimation for each work unit (CHF) Resistive magnet solution

Permanent magnet solution

Dipole magnet manufacturing 115000 100000

Power supply 30000

Electrical and demineralized water network 15000

Operation (cost/year) 3000

TOTAL (over 15 years of operation) 205000 100000

…and reliability over years with no intervention in a radiation area.



Limits of permanent magnets

• Permanent magnets designs have a fixed field and cannot be proposed for machines where the beam energy is not constant. They can be used mainly for LINACS, transfer lines and experimental areas. We are however exploring a number of solutions for remotely tune the magnetic field (ever mechanically or electrically).

• Permanent magnets have a limited field especially for accelerator magnets with large aperture.

• Field quality and field value of the assembled magnet is directly linked and limited by the quality

and homogeneity of the permanent magnet blocks.

• They need to be installed in an area with a controlled and stable temperature.

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Conclusion

More and more permanent magnet solutions are proposed and used at CERN:

• Cost efficiency

• Reliability

• Flexibility of designs

• Rapidity to implement a permanent magnet solution

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Bibliography

- Maurizio VRETENAR et al., ‘’Linac4 Technical Design Report’’, http://project-spl.web.cern.ch/project-spl/documentation/l4tdr.pdf.

- Maurizio VRETENAR et al., ‘’The Linac4 project at CERN’’, https://accelconf.web.cern.ch/accelconf/IPAC2011/papers/tuoaa03.pdf, presented at the International Particule Accelerator Conference (IPAC 11), September 2011, San Sebastian, Spain.

- Pierre-Alexandre THONET, ‘’Linac4 inter-tank permanent magnet quadrupole’’, https://edms.cern.ch/file/1232370/1.0/L4-MQM-ES-0002-10-00.pdf.

- Davide TOMMASINI, Marco BUZIO, Pierre-Alexandre THONET, Alexey VOROZHTSOV, ‘’Design, manufacture and measurements of permanent quadrupole magnets for Linac4’’, http://cds.cern.ch/record/1425452/files/CERN-ATS-2012-023.pdf?version=1, presented at the 22nd International Conference on Magnet Technology (MT-22) 12-16 September 2011, Marseille, France.

- Antonio BARTALESI, Regis CHRITIN, Michele MODENA, ‘’Experimental test to determine the magnet

reversible temperature coefficient for a permanent magnet quadrupole’’, https://edms.cern.ch/file/1240879/1/PMQ_NOTE.pdf

- Evgeny Solodko, Pierre-Alexandre THONET, ‘’Design of the permanent magnet dipole for n-Tof

experimental area 2’’, http://indico.cern.ch/event/213419/material/slides/1?contribId=4

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http://project-spl.web.cern.ch/project-spl/documentation/l4tdr.pdf


















https://accelconf.web.cern.ch/accelconf/IPAC2011/papers/tuoaa03.pdf















https://edms.cern.ch/file/1232370/1.0/L4-MQM-ES-0002-10-00.pdf


























http://cds.cern.ch/record/1425452/files/CERN-ATS-2012-023.pdf?version=1



















https://edms.cern.ch/file/1240879/1/PMQ_NOTE.pdf














http://indico.cern.ch/event/213419/material/slides/1?contribId=4













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