1. Pierre-Alexandre Thonet
Design of a Permanent Magnet Dipole For N-Tof Experimental Area 1 to replace the existing M 200 electromagnet
31/10/2017
Pierre-Alexandre Thonet
TE-MSC-MNC

2. Outline A permanent magnet dipole For N-Tof Experimental Area 1
Requirements and constraints
Solution 1: design similar to EAR2 design
Solution 2: design based on a Halbach array
Compactness of the design
Comparison between the 2 solutions
Permanent magnet material
Budget
Planning
Points to be discussed
Pierre-Alexandre THONET

3. A permanent magnet dipole For N-Tof Experimental Area 1
A dipole magnet is required after nTof target. It is used to evacuate all charged particles as protons and electrons from the neutron beam.
Currently a 140 KW M200 dipole magnet is installed in the nTof line to fill this function.
In the frame of the East area renovation a study is being done to replace M200 magnets and DC power supplies by laminated M200 magnets and cycled power supplies for energy savings.
Why a permanent magnet solution?
Taking in consideration the good feedback of the use of a permanent magnet dipole in nTof EAR2, it was decided to study a permanent magnet solution as well for nTof EAR1.
Reliability in radiation area.
Compact design.
Saving opportunities:
Cost of the magnet itself.
No power supply.
No external network such as electrical cabling and demineralized water.
No operational cost.
Pierre-Alexandre THONET

4. Requirements and constraints
Parameter
Value
Unit
Required integrated field
≥ 0.6
T.m
Free aperture
≥ Ø 200 mm
Good field region (GFR) dimension
Ø 200
Pierre-Alexandre THONET

5. 2 solutions are possible
SOLUTION 1: Based on the experience of nTof EAR2, it is possible to make a similar design of this dipole, scaled with a smaller aperture.
SOLUTION 2: In order to reduce the overall weight of the dipole, it is also possible to make a design based on a Halbach array.
Pierre-Alexandre THONET

6. SOLUTION 1: Design similar to EAR2 dipole
Pierre-Alexandre THONET

7. Dipole general view
The magnet design is based on a iron dominated external yoke and poles and Samarium Cobalt Sm2Co17 permanent magnet blocks.
Permanent magnet blocks installed between the external yoke and the poles act as magnetic flux generator.
Iron poles smooth possible deviations of permanent magnet blocks magnetization direction.
Particularity of this design is the addition of permanent magnet blocks on each side of the gap in order to compensate radial stray field losses and improve integrated field homogeneity in good field region.
520 mm
Permanent magnet blocks Sm2Co17, as a flux generator.
Permanent magnet blocks Sm2Co17, compensate radial stray field to improve field quality in GFR.
200 mm
510 mm
Return yoke C10R steel.
Pole tip C10R steel, smooth the possible differences on the easy axis orientation of the permanent magnet blocks.
Pierre-Alexandre THONET

8. 2D magnetic design: field distribution
Parameter
Value
Unit
Central field
0.4 T
Integrated field
0.70
T.m
Good field region
Ø 200 mm
Field homogeneity
+/- 5 %
Free aperture
Magnet length
2 x 880
Magnet weight
2 x 1200 kg
Pictured: Field distribution Bmod (T) in the PM dipole
Pierre-Alexandre THONET

9. SOLUTION 2: Design based on a Halbach array
Pierre-Alexandre THONET

10. Dipole general view
The magnet design is based on a Halbach array. The dipolar field is shaped thanks to the different magnetization directions of the permanent magnet blocks (Samarium Cobalt Sm2Co17). An external magnetic ring (low carbon steel) acts as a rigid frame and magnetic shielding of the system.
Permanent magnet block (Sm2Co17)
Magnetic yoke (low carbon steel)
Non magnetic internal ring (stainless steel)
Non magnetic frame (extruded copper or aluminium)
Non magnetic shim (stainless steel)
Epoxy resin
Pictured: Layout of the permanent magnet dipole
Pierre-Alexandre THONET

11. Magnet assembly
Large size Halbach magnets are normally very difficult to assemble as many permanent magnet blocks are present and strong magnetic forces are induced inside the assembly. Usually it is necessary to glue the permanent magnets blocks together.
The originality of this design is to have a non-magnetic frame made of extruded aluminum or copper which will be used to guide and slide the permanent magnets blocks inside the dipole.
No gluing required.
No assembly tooling required.
Pictured: Magnet frame before blocks insertion
Pierre-Alexandre THONET

12. 2D magnetic design: field distribution
Parameter
Value
Unit
Central field
0.52 T
Integrated field
0.67
T.m
Good field region
Ø 200 mm
Field homogeneity
+/- 2 %
Free aperture
Outside diameter
430
Magnet length
1300
Magnet weight
1200 kg
Pictured: Field distribution Bmod (T) in the PM dipole
Pierre-Alexandre THONET

13. Compactness of the design
In order to produce the required integrated field of 0.6 T.m, 2 sections of a 0.88 meter long permanent magnet dipoles are required for solution 1 but only 1 section of 1.3 meter long for solution 2.
The overall length of the permanent magnet dipole solution will remain similar to the current M 200 electromagnet however the cross section will be much smaller.
M 200 (30 tons) vs Permanent magnet
Solution 1 Solution 2
( 2.4 tons) (1.2 tons)
Pictured: Comparison between M200 electromagnet and permanent magnet (same scale)
Pierre-Alexandre THONET

14. Comparison between the 2 solutions
Advantages of solution 1:
We have the experience of a similar design with EAR2 and we know how to assemble this magnet.
The permanent magnet blocks are all the same with an easy shape for manufacturing.
Advantages of solution 2:
Very efficient design.
Design compact and light.
In theory the assembly would be easier as the dipole frame is used as the assembly tooling.
This design could become a good option for the assembly of large Halbach magnets.
Pierre-Alexandre THONET

15. Permanent magnet material
Samarium Cobalt Sm2Co17
Maximum specific energy product fulfils with the magnet design.
Small temperature coefficient: 0.035%/°C.
Good radiation resistance.
Acceptable corrosion stability without protective coating.
Sm2Co17 grade SmCo30M
High remanent field Br=1.1 T.
Small deviation of the magnetic characteristics (maximum 3% from the nominal values on Br and Hcb).
±5 deg maximum error of easy axis orientation.
Mechanical tolerance of the blocks: +/-0.1 mm
Cobalt content in Sm2Co17 magnets: ≈ 60 %
Magnets weight in the dipole: ≈ 900 kg (for both solutions)
Pictured: demagnetization curve of SmCo30M
Pierre-Alexandre THONET

16. Budget Solution 1: Solution 2: Component Cost (CHF)
Permanent magnet blocks
40000
Machining of magnetic and non-magnetic parts
30000
Assembly of the magnet
10000
Contingency
TOTAL COST (1 section)
90000
TOTAL COST (2 sections)
CHF
Solution 2:
Component
Cost (CHF)
Permanent magnet blocks
100000
Machining of magnetic and non-magnetic parts
30000
Assembly of the magnet
10000
Contingency
TOTAL COST
CHF
Pierre-Alexandre THONET

17. Planning Activity Date Validation of the permanent magnet solution
December 2017
Validation of the design
February 2018
All orders sent out
March 2018
All parts received at CERN
July 2018
Magnet assembled and measured
November 2018
Pierre-Alexandre THONET

18. Points to be discussed
Confirmation that an integrated field of 0.6 T.m is sufficient.
Confirmation that an aperture of 200 mm is sufficient.
Confirmation of the required integrated field homogeneity.
Discussion and choice between solution 1 and solution 2.
Definition of suitable protection covers.
Verification that vacuum chamber insertion procedure is compatible with magnet assembly.
Discussion with transport about the removal of the existing M 200 magnet.
Budget and planning.
Pierre-Alexandre THONET