Insertion Devices Lecture 6 The Application of Superconductors. Jim Clarke ASTeC Daresbury Laboratory

Transcription

1 Insertion Devices Lecture 6 The Application of Superconductors Jim Clarke ASTeC Daresbury Laboratory

2 Introduction For fields greater than ~3T superconductors are the only real option For intermediate fields (~1 to ~3 T) they can have much shorter periods than Permanent Magnets or Normal Conducting Electromagnets The materials used are only superconducting below ~10K Hence the magnets always sit inside a cryostat In the past they would generally have a closed loop liquid Helium refrigerator permanently connected to them The development of closed cycle cryocoolers has enabled a more stand alone approach to be used in some applications liquid helium is not necessarily required and not always used 2

3 Superconducting Basics Superconductivity is an enormous subject there are plenty of good books available (eg Wilson, SC Magnets, OUP, 1983) There are really only two materials that are used at present NbTi and Nb 3 Sn These are both Type II superconductors and they are characterised by a critical surface in temperature, field, and current density space Below the surface they will be superconducting and above they will be normal conducting Type I superconductors are never used as they are only superconducting at low fields (typically < 0.1T) High temperature ceramics have made little impact so far except that they are often now used for the current leads into the cryostat 3

4 Current density (ka.mm -2 ) Niobium-Titanium NbTi is the material of choice 99% of the time It is the only ductile metallic superconductor all of the others are brittle intermetallic compounds Magnets usually operate in boiling liquid helium at 4.2K so a slice through the critical surface at this temperature is often used Martin Wilson, SC Accelerators, CI, 2006 The critical surface for NbTi 4

5 The Critical Line at 4.2K Nb 3 Sn has a much higher performance in terms of critical current, field, and temperature than NbTi But it is brittle and has poor mechanical properties Note that both the field and current density of both superconductors are way above the capability of conventional electromagnets Critical current density A.mm Conventional electromagnets NbTi Nb 3 Sn Magnetic field (Tesla) Martin Wilson, SC Accelerators, CI, 20065

6 Engineering Current Magnet Load Lines 7 6 Engineering Current density (A/mm 2 ) The magnet will go resistive or 'quench' when the peak field load line crosses the critical current line The magnet should be designed to operate at 1000 Martin Wilson, SC Accelerators, CI, Resistive Superconducting Magnetic field on axis * quench operate Peak field at the superconductor Field (T) * density Amm

7 Quench Training Magnets rarely go straight to the expected quench point, as given by the intersection of the load line with the critical current line Instead the magnets go resistive (quench) at much lower currents After a quench, the stored energy of the magnet is dissipated in the magnet, raising its temperature way above critical you must wait for it to cool down and then try again The second try usually quenches at higher current and so on with the third this continued improvement is known as training After a number of training quenches a stable, well constructed magnet gets close to it's expected critical current A poorly constructed magnet will never get there 7

8 Why Do Magnets Quench? The specific heat of all substances falls with temperature At 4.2K, it is ~2,000 times less than at room temperature A given release of energy within the winding thus produces a temperature rise 2,000 times greater than at room temperature The smallest energy release can therefore raise the superconductor above the critical surface Conductors in the magnet are subject to magnetic forces Sometimes they will move suddenly under this force the magnet 'creaks' as the stress comes on This will cause frictional heating Typical values for NbTi suggest that a 10mm motion can raise the local temperature to 7.5K Resin cracking can cause a similar effect 8

9 How to Make Quenches Less Likely Make the winding fit together exactly to reduce the movement of the conductors under the field forces Pre-compress the winding to reduce the movement under field forces Minimize the resin volume and choose a crack resistant type Match the thermal contractions of materials Increase the temperature margin by operating at a lower current Increase the cooling 9

10 Are Quenches Dangerous? If a quench is not handled properly then they can destroy the magnet! The energy stored in a magnet is ½LI 2 For an LHC dipole magnet this is ~10 7 J If this were kinetic energy then the 26 tonne magnet would be travelling at ~25 m/s (~56 mph) Must have a quench protection scheme can be active or passive Often use heaters to warm up the rest of the magnet quickly Also circuit breakers or bypass diodes 10

11 Superconducting Wavelength Shifter Examples The highest field has been achieved by the SPring-8 10T wavelength shifter (from BINP) It uses NbTi and Nb 3 Sn The stored energy is 400kJ The magnet gap is 42 mm 11

12 Superconducting Wavelength Shifter Examples 7T wavelength shifter installed on BESSY-2 SRS 6T wavelength shifter 12

13 Superconducting Multipole Wigglers Superconducting MPWs are popular in the intermediate energy light sources (~3GeV) because the high field enhances the flux in the hard X-ray region A key advantage over permanent magnet hybrid wigglers is not just the higher field but also the reduction in period This lowers the K value and so the radiation is emitted over a narrower horizontal width (±K/g) making it easier to manage the cooling arrangement within the facility Note that the majority of the radiation generated by a MPW will not be used by the beamline and so must be absorbed by cooled surfaces 13

14 Superconducting Multipole Wiggler Examples 3.5T multipole wiggler for Elettra Magnet gap 16.5mm Period 64mm, 45 poles 240kJ of stored energy Note. The highest field hybrids are the SRS 2.4T MPW which has a period of 220mm and magnet gap of 20mm and the ESRF Asymmetric Wiggler that has a field of 3.1T at a gap of 11mm and period of 378mm Iron yoke 14

15 Superconducting Multipole Wiggler Examples Diamond 3.5 T 60 mm period, 45 poles 4.2 T 48 mm period, 45 poles 15

16 Superconducting Multipole Wiggler Examples BESSY-2 7T, 13 pole MPW Magnet gap is 19mm 13 full strength poles 148mm period 16

17 Superconducting Undulators The motivation of using superconductivity is to generate higher fields on axis than are presently available from the best permanent magnet systems They have to have a significantly better performance to make them worthwhile The key region of interest is in short period systems, typically ~15mm The field quality has to be similar to existing undulators An example specification Magnetic Length: 2 m Period : 15 mm Field on Axis: 1.4 T (K =2.0) Electron Beam Aperture: 5 mm (Vert.) x mm (Horiz.) RMS Phase Error: < 3 degrees Trajectory Straightness: +/- 0.5 micron 17

18 Superconducting Undulators Comparison of flux output for a 3GeV, 300mA beam Assuming: K Period (mm) In-Vac CryoPM SC 2.0* 15 * Harmonics 1 and 3 will just overlap 18

19 Undulator Design The standard solution is very simple currents flowing perpendicular to the beam axis The challenge is nearly all engineering Iron yoke Return paths for currents 19

20 Superconducting Undulators A similar scheme has been adopted by several groups S H Kim, APS 20

21 Challenges To achieve the field levels requires the magnets to have a cold beam aperture no room for an insulating vacuum any heat transfer from the beam to the wires could trigger a quench The iron poles need to be accurately machined to minimise field errors The on axis field is also dependent upon the accurate positioning of the wires Correction of the field errors (shimming) is quite tricky several options have been proposed but they all add an extra level of complication not as simple as permanent magnets Ideally the magnet should not need error correction but is this practical? What about transition between iron non-saturation and saturation? 21

22 Wire Position Wires closest to the electron axis must be positioned most accurately but iron yoke accuracy more important 22

23 Error Correction Single turns could be used (blue) or a symmetric pair about a pole (green) Mechanically adjustable shims inside a pole are also possible S Prestemon, LBNL 23

24 Shimming with Coils Correction with dipole coils along the length of the undulator will significantly improve the phase error as demonstrated for the ANKA undulator It is also not necessary to shim every pole an assessment showed that 17 electrical shims for a 100 pole undulator would reduce the phase error of the NUS undulator from 6 to 2 Dolling et al, SRI 06 & D Wollmann, dissertation,

25 Shimming with Coils A more advanced correction method is to have 4 coils per groove on top of the main coil Four power supplies with currents of 250, 100, 50 and 10A are used and each correction coil is either connected to one of the four power supplies or not connected at all This idea gives good phase error correction but would, of course, increase the magnet gap Other variations on this basic idea of a winding on the main coil have been proposed D Wollmann, EPAC

26 Induction Shimming A simple concept A passive superconducting loop one period long will have current induced in it as the magnet is powered to cancel out any field error The enclosed total flux per period should be zero Loops could be multiple periods long D Wollmann, dissertation,

27 Induction Shimming Initial results from a short prototype gave promising results, showing significant field error correction The present HTSC induction loop thickness would imply a magnet gap increase of ~2mm Options are being considered for using thinner substrates, using both sides of the substrate, or using the steel liner as the substrate instead 27

28 Example Superconducting Undulator Installed in ANKA in 2005 The first cold bore undulator ever installed in a storage ring Period is 14mm, 100 periods Magnet gap variable in steps 8 to 16 mm (29 mm for injection) The barrel shape in the field along the axis is due to bending of the two arrays the gap is 0.25mm larger at the entrance and exit of the magnet S Casalbuoni et al, SRI 06 28

29 Example Superconducting Undulator Helical Undulator for the ILC Bifilar helix design 11.5mm period, 2 x 1.75m long 6.35 mm winding bore 29

30 Quench current (A) Example Superconducting Undulator The quench test results show surprising differences between the two identical magnets Both do actually reach the same final quench current which agreed well with expectations magnet 1 magnet 2 nominal current Quench number 30

31 The Application of Nb 3 Sn Nb 3 Sn devices studied intensively at LBNL Quench limit of 8250 A/mm 2 demonstrated But, insulation thickness (packing factor) reduces this to average current density of 1760 A/mm 2 Much of the advantage over NbTi is lost, but not all being actively pursued by RAL/Daresbury team S Prestemon, LBNL 31

32 Sources of Beam Heating The electron beam is able to heat the cold bore of the magnet through a variety of methods! Synchrotron radiation from the upstream magnets or from the undulator itself also indirect SR Wakefield effects resistive wall, geometric, and surface roughness Secondary electron (or ion) bombardment The amount of power that the magnet can cope with depends on the specific heat of the materials and this changes significantly with temperature typically a few W at 4K, 10s of W at 50K, 100 of W at 300K 32

33 Resistive Wall Wakefields The dominant wakefield effect should come from the resistive wall wakefield as the geometric and surface roughness effects can always be reduced to be of lesser significance (in theory!) The resistive wall wakefield relates to the Ohmic heating of the vessel walls due to the image currents flowing in a finite resistance material The size of the effect depends upon the material conductivity (which will be a function of frequency), the electron bunch length (shorter bunches have higher frequency content), the vacuum vessel aperture, the number of bunches, the average electron beam current, and the bunch revolution frequency 33

34 Resistive Wall Wakefields The conductivity of a cold material is often expressed as an RRR value this is the Residual Resistivity Ratio and is the ratio between the resistivity at 300K to the resistivity when cold big numbers are good! The conductivity of a material is a function of frequency The Anomalous Skin Effect (ASE) occurs at very low temperatures and high frequencies at which the thickness of the conducting skin depth is less than the electron mean free path, so that the classical theory of electrical conductivity breaks down this regime needs to be applied for 4K bore magnets 34

35 Example Resistive Wall Power Levels Example curves for Diamond Cylindrical vessel with 2.5mm radius and RRR of 60 The curves assume the ASE regime Multibunch mode 842 bunches Few bunch mode, each bunch is equivalent to 10mA Copper Aluminium (I b ~500mA) D J Scott, Daresbury 35

36 Example Heat Budget This is an example of the heat budget for an undulator These are the anticipated heat loads due to the cryostat rather than the electron beam heat conduction & radiation from 300K surfaces J C Jan, IEEE Trans Appl SC, 17, 2,

37 Example Heat Budget These are the anticipated loads due to the electron beam only assuming a 400mA, 3GeV beam ICPD is Image Current Power Dissipation BMPD is Bending Magnet Power Dissipation ICPD depends on conductivity (RRR) and beam aperture BMPD depends on the absorber position setting and the BM field strength J C Jan, IEEE Trans Appl SC, 17, 2,

38 Thermal Screening Since at 4K the magnet bore can only tolerate a few W intermediate beam screens at ~20K have been used to intercept any beam induced heating These screens are held away from the 4K surface to prevent conduction of heat They have been used in superconducting multipole wigglers, which can tolerate larger magnet gaps (no competing technology!), but are not so practical in undulators Magnet gap Beam Screen at ~20K Vessel at 4.2K 38

39 Thermal Experience The real experience of installed magnets generally seems to be worse than anticipated S Casalbuoni, EPAC 08 39

40 COLDDIAG A collaboration (COLDDIAG) is being led by FZK (Karlsruhe) to systematically measure the beam heating effects inside a cryostat in order to get a better understanding of the individual heating effects S Casalbuoni, EPAC 08 40

41 Magnet Measurement Challenges It is common for superconducting magnets to be tested prior to final assembly into the cryostat by dipping the whole magnet vertically into a liquid helium dewar the probe is also immersed Measuring the finished cold bore magnet in its own cryostat poses some additional problems They must be measured under vacuum to prevent icing up Thermal contraction of the probe arm leads to uncertainty in the exact probe position The Hall probe calibration changes with temperature so it should be calibrated at the magnet bore operating temperature ideally a calibration magnet forms part of the measurement system 41

42 Magnet Measurements A vertical test stand has been developed at NSLS It uses an aluminium guide chamber (3 x 12mm) for the Hall probe array (6 probes, 2 rows of 3) Two needle magnets are installed to help with length calibration A pulsed wire system can be inserted instead of the Hall probes but it is difficult to detect a good signal to noise ratio A calibration magnet and a zero gauss chamber is also incorporated into the facility Harder, PAC 05 42

43 Similar problems for PM cryoundulators as well Joel Chavanne, ESLS XVI,

44 Summary Two materials are dominant, NbTi and Nb 3 Sn, with NbTi being dominant Wavelength shifters (~5 to 10T) are used to generate the highest energy photons There are several recent MPW examples (3.5 to 7T), used to enhance the flux at high photon energies Superconducting undulators have to be pushed close to their limits to gain a significant advantage over permanent magnet alternatives There are several options for shimming out the field errors Cold bore increases the risk of quenching due to beam heating Magnet measurements are more difficult as they have to take account of cold probes, working in a vacuum, and thermal contractions 44