CERN experience on accelerator magnets based on permanent magnets

Transcription

1 CERN experience on accelerator magnets based on permanent magnets for Future Linear and Circular Colliders Geneva 26 th -28 th November 2014

2 Which types of magnets are used at CERN? Resistive magnets: 4800 magnets (about tons) are installed in the CERN accelerator complex. These magnets are air cooled or water cooled. Pictured: SPS dipole magnet Superconducting magnets: magnets (about tons) are installed mainly in LHC. These magnets are cooled with liquid helium. Pictured: LHC superconducting magnet Permanent magnets: 150 magnets (about 4 tons) are installed in Linacs and experimental areas. Pictured: Linac4 permanent magnet quadrupole 2

3 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. 3

4 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: %/ 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. 4

5 Projects based on permanent magnets at CERN 5

6 Linac4 permanent magnet quadrupole This quadrupole was designed to provide beam focusing in the Cell Coupled Drift Tube Linac (CCDTL) of Linac4. 45 kev 3 MeV 50 MeV 100 MeV 160 MeV H- source - LEBT H RFQ DTL Chopper line CCDTL PIMS Transfer line to PSB 76 m Pictured: Linac4 layout 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: PMQ characteristics Permanent magnet quadrupoles Pictured: A CCDTL cell 6

7 Linac4 quadrupole: design 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 shims (austenitic steel 316LN) Non magnetic yoke (austenitic steel 316LN) Pictured: Permanent magnet quadrupole 3D model Pictured: Field lines in the PMQ 7

8 Linac4 quadrupole: magnet irregularities effects 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. Pictured: harmonic content for each kind of permanent magnet blocks irregularities Case 3: error on magnetization direction of 2 on 2 blocks. 8

9 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 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. Two translation stages move the wire horizontally then vertically. Pictured: Stretched wire measurement system 9

10 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. Permanent magnet sextupoles 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. Pictured: ASACUSA spectrometry beam line setup Pictured: Permanent magnet sextupole characteristics 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/B 3 for n=3,4,... <0.1 % 10

11 72 mm ASACUSA sextupole: design The magnet design is based on iron dominated poles and Samarium Cobalt Sm 2 Co 17 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. 98 mm Permanent magnet block Sm 2 Co 17, 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. Shim 316LN non magnetic austenitic steel but possibility to insert iron shims to adjust the sextupole field. External yoke Titanium T40, non magnetic to hold the poles together and guaranty the geometry. Vacuum brazing Gapasil filler. Pictured: Permanent magnet sextupole 3D model 11

12 ASACUSA sextupole: manufacturing 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. Titanium ring. 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 12

13 720 mm 340 mm Permanent magnet dipole for n-tof* experimental area 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 Magnet design based on an iron dominated external yoke and poles and Samarium Cobalt Sm 2 Co 17 permanent magnet blocks. The dipole is composed of 168 permanent magnet blocks of dimension 80 mm*80 mm*80 mm Field at the center Tesla Field homogeneity +/- 1.5 % Magnet gap 340 mm Magnet length 960 mm 800 mm Total mass 2670 Kg Mass of permanent magnets 714 Kg Permanent magnet blocks Sm 2 Co 17, as a flux generator. Permanent magnet blocks Sm 2 Co 17, 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. Pictured: Permanent magnet dipole 3D model *n-tof: Neutron time of flight. 13

14 Magnetic field By (T) 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% Pictured: Field distribution Bmod (T) in the dipole Field distribution along Z axis at the center of the magnet L mag = 1134 mm Z (mm) Pictured: Integrated field homogeneity in GFR (%): 100*(Bydz-Bydz(0))/Bydz(0) 14

15 n-tof dipole: assembly (1) Backward field 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: side blocks inserted on one side of the dipole yoke Pictured: magnet gap filled with steel sheets Pictured: manipulation and insertion of blocks with suction cup 15

16 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 16

17 n-tof dipole: cost saving Cost estimation for each work unit (CHF) Resistive magnet solution Permanent magnet solution Dipole magnet manufacturing Power supply Electrical and demineralized water network Operation (cost/year) 3000 TOTAL (over 15 years of operation) and reliability over years with no intervention in a radiation area. 17

18 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. 18

19 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 19

20 Bibliography - Maurizio VRETENAR et al., Linac4 Technical Design Report, - Maurizio VRETENAR et al., The Linac4 project at CERN, presented at the International Particule Accelerator Conference (IPAC 11), September 2011, San Sebastian, Spain. -, Linac4 inter-tank permanent magnet quadrupole, - Davide TOMMASINI, Marco BUZIO,, Alexey VOROZHTSOV, Design, manufacture and measurements of permanent quadrupole magnets for Linac4, presented at the 22nd International Conference on Magnet Technology (MT-22) September 2011, Marseille, France. - Antonio BARTALESI, Regis CHRITIN, Michele MODENA, Experimental test to determine the magnet reversible temperature coefficient for a permanent magnet quadrupole, - Evgeny Solodko,, Design of the permanent magnet dipole for n-tof experimental area 2, 20

21 Thank you for your attention