Introduction. LHC High bending field is required for High energy Compact machine.

Transcription

1 Prapaiwan Sunwong

2 Talk Outline General background characteristics of superconductor Material selection and cable structure Multipole magnets Generation of multipole fields Magnet function and coil structure Insertion devices General design requirements Superconducting magnet at SPS Concluding remarks

3 Introduction LHC High bending field is required for High energy Compact machine [ E = 0. 3Bρ]

4 Superconducting Characteristics 1. Zero resistance Discovered by Onnes in 1911 solid mercury exhibits vanishing resistance below 4.2 K. 2. Meissner effect Exclusion of magnetic flux from its interior discovered in 1933 by Meissner and Ochsenfeld.

5 Critical Temperature YBCO

6 Critical Magnetic Fields Type I Nb 3 Al Type II Keys, 2002

7 Critical Current Density Nb 3 Al Keys, 2002

8 Critical Surface Phase Diagram

9 Applications of Superconductivity Superconducting electromagnets (low T c ) Medical uses MRI scanners Scientific research NMR Transportations MAGLEV trains Fusion tokamak ITER Particle accelerators Josephson junction devices SQUID Low-loss power cables (high T c ) Magnet current leads (high T c ) Electric motors, generators, fault current limiters

10 Permanent Resistive Why Superconducting Magnets? Type Advantages Disadvantages Superconducting Compact Low cost ( in small low field magnets) No utilities required No maintenance Simple to operate Can result in very precise fields Variable field No need for complicated cryogenic or vacuum systems Can be built in house or through existing industrial base Relatively low capital cost High and variable field Lower operating costs Reliability Cold beam tubes yield very high vacuums Can be made compact Constant field (mostly) Limited in field Limited in field (up to ~ 2 T) May require large amounts of electrical power and cooling water Possible large operating costs for power & water High capital costs Limited industrial base Requires complicated ancillary systems cryogenics, vacuum, quench protection

11 Material Selection

12 NbTi Wires Alloy of niobium and titanium extremely ductility T c 9 K, B c2 15 T (vary with composition, 46.5% Ti optimum) I c is influenced by microstructure (flux pinning) Copper stabiliser (RRR 100) ATLAS strand - mechanical stability - electrical bypass - heat sink Multifilamentary wire Typical filament diameter 5 50 μm Typical wire diameter mm Twisted filament/wire reduce coupling between filaments for ac field or during field sweep LHC MQY duadrupole strand LHC dipole strand

13 Rutherford-type type Cables Filaments (6 μm each) Wire/strand (6,300 filaments) Rutherford cable (36 strands) Transposed cable: every wire changes places with every other wire along the length of the cable, to decouple the wires with respect to their own self field and promote a uniform current distribution. Rutherford-type cables can be compacted to a high density (88 94 %) and rolled to a good dimensional accuracy.

14 Rutherford Cables Manufacture Martin Wilson s lecture

15 Multipole Magnets Dipole Quadrupole Resistive magnets Superconducting magnets

16 Generation of Multipole Fields In superconducting magnet, field shape is defined by position of each conductor (that carries current) in the coil. Current distribution Magnetic field B B θ r ( r, θ ) ( r, θ ) μ0i = 2a μ0i = 2a 0 0 r a r a I ( φ) = I0 cos( mφ), m = order of multipole m 1 m 1 cos( mθ ) sin( mθ ) y beam axis y r θ P a φ B R x current in z direction r θ B θ B r x

17 Generation of Multipole Fields I ( φ) = I0 cos( mφ) Current distribution can be created by multiple intersecting circles/ellipses carrying constant current densities (J) in different directions. -J y +J x The field inside the current free region is computed by superimposing the field produced by the conductors. Circular conductor: Elliptical conductor: B B x x y = μ 0J, By = μ0j 2 a y J B 1 = μ0, y = μ0 a1 + a2 a2x a + a Difficult to fabricate Use of current shells for practical constant-csa conductors x 2 J 1 2 a 2 a 1

18 Dipole m = 1 Magnet Function and Coil Structure Uniform field for bending Intersecting (overlapping) circles B = B y μ = 0Jd 2 Intersecting (overlapping) ellipses B = B = μ J y 0 a2d a + a 1 2 B

19 Magnet Function and Coil Structure Quadrupole m = 2 Intersecting ellipses Gradient field for focusing B B x y = = μ0j ( a1 a a + a 1 μ0j ( a1 a a + a ) y ) x y Sextupole m = 3 Intersecting ellipses For chromaticity correction B B x y = 2Sxy = S ( x y ) +... a 2 a 1

20 Magnet Function and Coil Structure Some novel designs (for pure multipole fields) Sextupole Octupole Liu, 2011

21 Major Projects USPAS Course on Superconducting Accelerator Magnets, 2003

22 Tevatron Bottura, 2011

23 Major Projects USPAS Course on Superconducting Accelerator Magnets, 2003

24 LHC Bottura, 2011

25 LHC Twin-aperture Dipole cds.cern.ch

26 Insertion Devices Undulator Wiggler K 1, θ 1/γ - many alternating low-field magnetic poles - strong interference effects to increase photon flux Parameter Critical wavelength K λ B λ c u λu 2 E K > 1, θ > 1/ γ Multipole wiggler - several periods to increase photon flux - less important interference effects Wavelength shifter - one period with high field center pole (usually 5-6 T) - very short-wavelength radiation

27 Undulator Helical undulator Planar undulator Superconducting helical undulator for ILC (bifilar helix design) Argonne National Laboratory s planar superconductor undulator YuryIvanyushenkov, ASD Seminar, 2013

28 Undulator Period length switching for hybrid superconducting undulator/wiggler K λub Grau, 2010

29 Multipole wiggler Superconducting wiggler at NSLS The iron yoke and poles of a CESR superconducting wiggler magnet for ILC Superconducting wiggler at DLS

30 Wavelength Shifter Prototype SWLS at NIRS Total power distribution of SWLS at SPS (1.2 GeV, 200 ma)

31 Work on superconducting insertion devices around the world Country Organization Activity Taiwan TLS, TPS SC wigglers, R&D on SCUs Russia Budker Institute SC helical undulator for HEP; SC wavelength shifters; SC wiggler France ACO, Orsay SCU Germany ANKA SCU for Mainz Microtron, R&D on SCUs ACCEL Babcock Noell Two SCUs (for ANKA and for SSLS/NUS, Singapore) New SCU for ANKA UK ASTeC, RAL and DL Helical SCU for ILC Sweden MAX-Lab SC wiggler USA Stanford Helical SCU for FEL demonstration BNL LBNL Cornell NHFML APS R&D on SCUs R&D on SCUs SC wiggler R&D on SCUs R&D on SCUs YuryIvanyushenkov, ASD Seminar, 2013

32 General Design Requirements Keep it superconductive with a comfortable margin Magnet training - well protected (when quench) Reduce heat load - minimise contact resistance - vapour-cooled/hybrid current leads Good cryogenic system to handle all heating Good support structure to handle large Lorentz force Cheap and easy to manufacture Field quality (uniformity) relative field error better than 10-4 is required. Not degraded by exposure to the high radiation levels Well cooling of the chamber and active interlock system

33 Iron Yoke heat exchanger bus-bar saturation control Wilson s and Bottura s lectures

34 Cryostat Radiative heat transfer T 4 Wilson s and Bottura s lectures

35 Thermal Properties Ekin, 2007

36 Quench and Protection Quench = conversion of magnet energy (LI 2 /2) into heat inside the volume of magnet winding which has transited into the resistive state E = J for LHC dipole magnet equivalent to the kinetic energy of 26-tonnes magnet travelling at 88 km/hr Cause of quenching Low specific heat Conductor motion (10μm motion of NbTi raise local temperature to 7.5 K) Resin cracks J c decreases with increasing temperature Wilson s lecture

37 Quench and Protection 3D simulation of quench propagation for a cos theta type magnet

38 Quench and Protection LHC dipole GSI001 Safe hot spot temperature = (300) K Wilson s lecture

39 Quench and Protection 3. Heater activated 1. Normal zone detected 2. Switch opened Bypass diodes for magnets connected in series ten Kate 2013

40 Training of Superconducting Magnets Several thermal and electrical cycles need to be applied to a new coil before the optimal performances are obtained. LHC short prototype dipoles Wilson s lecture

41 Superconducting Magnet at SPS 6.5 T Superconducting Wavelength Shifter (from NSRRC, Taiwan) Current operating field = 4.0 T at 170 A (maximum field = 6.5 T at 308 A ) Critical current of NbTi is 428 A at 8 T inside the coils. Helium consumption = 1.4 L/hr (published value = 1.3 L/hr) Hard x-rays radiation used in macromolecular crystallography - energy range = 12.7 kev -flux = 10 9 photons/s at 100 ma

42 6.5 T Superconducting Wavelength Shifter Liquid nitrogen Liquid helium

43 Cryogenic System Production capacity : 20 L/hr

44 6.4 T Superconducting Wavelength Shifter From MAX-Lab, Sweden Maximum field = 6.4 T at 250 A No liquid nitrogen screening 10 out of 1482 windings in side pole were burnt off and replaced by Cu sheet. Helium consumption < 5 L/hr (???) Wallen, 2002

45 Concluding Remarks Magnet is the most important application of superconductivity. Superconducting magnets provide high magnetic fields, which are required for high-energy and/or compact accelerators. NbTi has been used the most. Magnetic field profiles from superconducting magnets are determined by position of superconducting coils, which can be obtained at high accuracy. Advantages of superconducting insertion devices: - High field increases photon energies - High flexibility - Smaller period for the same peak field - New research possibilities