Magnetic field generation. Sergey L. Bud ko

Transcription

1 Magnetic field generation 590B F18 Sergey L. Bud ko Note: we do not endorse any specific product, they are shown for illustration only

2 Magnetic fields around us

3 Brief history of high field generation

4 Choice of magnets You need to answer the following questions: Do you need in-house magnet? Will 2 trips per year to NHMFL be enough for your research? Can you afford buying and operating magnet? - initial investments, - space, - consumables (LHe), - power, - hands?

5 Choice of magnets Either you need to answer the following questions: What field is needed? How homogeneous the field should be? What is the sample size? What is the sample holder size? Do you need to change the value of the field? Do you need to change the direction of the field? How many axes? What temperature is needed? How fast you can sweep the field (instruments/heating/physics)? What access do you need to the sample (wires, optics, X-rays, neutrons, ) Can you sacrifice the sample? Or you live with whatever you have

6 Units Magnetomotive Force The quantity of magnetic field force, or push. Analogous to electric voltage (electromotive force). Field Flux The quantity of total field effect, or substance of the field. Analogous to electric current. Field Intensity The amount of field force (mmf) distributed over the length of the electromagnet. Sometimes referred to as Magnetizing Force. Flux Density The amount of magnetic field flux concentrated in a given area. Reluctance The opposition to magnetic field flux through a given volume of space or material. Analogous to electrical resistance. Permeability The specific measure of a material s acceptance of magnetic flux, analogous to the specific resistance of a conductive material (ρ), except inverse (greater permeability means easier passage of magnetic flux, whereas greater specific resistance means more difficult passage of electric current). 14/magnetic-units-of-measurement/ Infinite source of confusion for some (many) of us Advise from one of theorist friends: read and understand Ch.4 of v. VIII of Landau & Lifshitz

7 Permanent magnets Alnico Ferrites Neodymium-iron-boride Samarium-cobalt Rarely used as field-generation device in physics labs. Can be used in field instruments for geophysicists, agronomists, < 1 Tesla temperature and history dependent difficult to change field cheap compact can be easily shaped 4.5 koe permanent magnets

8 Permanent magnets Alnico: Al 8-12%, Ni 14-26% Co 5-38 %, Cu 3-6 %, Ti 0-8 %, balance Fe (different grades, identified in 1931 [Japan], Curie temperature o C) Ferrites: Fe 2 O 3 blended with stuff, huge family Neodymium-iron-boride: Nd 2 Fe 14 B and derivatives (Curie temperature ~320 o C) Samarium-cobalt: SmCo 5 and derivatives (Curie temperature o C)

9 Permanent magnets complex field profiles quadrupole magnet (focusing electron beam)

10 Permanent magnets Halbach cylinder Halbach arrays: used in synchrotrons and free electron lasers

11 Electromagnets Biot-Savart law Lab electromagnet

12 Electromagnets Simple and straightforward Field at room temperature Reasonable fields (<3 Tesla) No cryogens Heavy and large Small field volume High field gradients High power consumption Need (water) cooling Stability is determined by power supply Note: ask Prof. Prozorov about effect of magnetic field on chicken s brain

13 Electromagnets

14 Helmholtz coil uniform field in large volume R

15 Maxwell coil uniform field or uniform gradient ni small = 49/69 ni central

16 Solenoids

17 Superconducting solenoids Nb-Ti (T c ~ 10 K, H c2 ~ 15 T) Nb 3 Sn (T c ~ 18 K, H c2 ~ 30 T) Multifilamentary cable Copper (Cu-Ag, ) sheath Serious metallurgical task Epoxy-impregnation

18 First superconducting magnet Phys. Rev. 98 (1955) T at 4.2 K George Yntema

19 Superconducting solenoids Up to 9 T Up to 20 T Current up to ~100 A (current leads are important!) SC persistent switch Quench protection circuit (sometimes proprietary, in old days could use external shunt)

20 Superconducting solenoids Up to 22 T Nb 3 Sn + HTS insert

21 Series Connected Hybrid Magnet (NFMFL - FL) NMR first, then other measurements 11.7M$ to start with CICC = SC cable in conduit conductor cooling!

22 Split pair magnets Standard - up to 9T with 2 split access Angular dependencies, light/x-ray/neutron access (less homogeneous field, more complex magnet design)

23 Vector magnet - expensive - complex - small fields - no moving parts - precise value/direction of the field 1 Tesla 1 Tesla 7 Tesla vertical field

24 Superconducting magnets lambda plate Magnet at 2.2 K, >10% increase in max. field (think of lambda-plate as of VTI with a valve that is cooling the magnet) Can also pump on He-bath but this is too Heconsuming. Superconducting magnets Affordable and compact high magnetic fields. Workhorse in CMP laboratory. Need cryogens Flux jumps Not so trivial if T > 300 K desired

25 cryogen free measurements systems

26 cryogen free measurements systems 3 T Up to 9 T

27 Bitter resistive magnets NHMFL Tallahassee 35 T record 37.5 T Nijmegen, Netherlands less stresses and better cooling

28 NHMFL 45T hybrid magnet Strength Type 45 tesla Hybrid Bore size 32 mm (~1.25 inches) Online since December 1999 Cost Weight $14.4 million 31,752 kg (35 tons) Height 6.7 meters (22 feet) Operating temperature -271 C (-456 F) Water used per minute 15,142 liters (4,000 gallons) Power required 33 MW 33.5 T resistive T superconducting Operation cost (full field) ~ 4,000 $/h

29 NHMFL 45T hybrid magnet

30 Pulsed magnetic fields - history from

31 Pulsed magnetic fields

32 Pulsed magnetic fields Mechanical forces

33 Pulsed magnetic fields Thermal limitations

34 Pulsed field magnets ~ 1 shot every 45 min. Fast data acquisition Heating Fast processes Mechanical strength of the coil is an issue

35 Pulsed field magnets NHMFL Los Alamos Capacitor Bank-Driven Magnets Field Duration Bore 50 T Short Pulse 25 msec 24 mm 50 T Mid-Pulse 400 msec 15 mm 40 T Mid-Pulse 400 msec 24 mm 65 T Short Pulse 25 msec 15 mm 60 T Short Pulse 40 msec 9.8 mm 300 T Single Turn 6 µsec 10 mm Generator-Driven and Multiplex Magnets Field Duration Bore 60 T Controlled Waveform 100 msec 15 mm 100 T Multi-Shot (operational to 90 T) 25 msec 15 mm magneto-optics (IR through UV), magnetization and magneto-transport from 350 mk to 300K; GHz conductivity, MHz conductivity, pulse echo ultra-sound spectroscopy, magnetoconductivity and heat capacity.

36 Pulsed magnetic fields NHMFL Los Alamos

37 100 T pulse magnet NHMFL - LANL

38 Pulsed magnetic fields 100 T pulse magnet NHMFL - LANL

39 Pulsed magnetic fields - give access to very high fields - reasonable space for cryostat and experimental cell - known to be able to accommodate multiple experimental techniques - available in a number of high field labs in the world - often electronics and experimental probe are available (or you work with high field lab scientists to develop something new - need fast electronics - beware of heating (metallic sample/experimental stage) - more noise than in static measurements - suitable only for fast processes - need to go through proposal process and need to travel/send students

40 Pulsed magnetic fields single turn coils Note: semi-destructive magnet destroyed, sample + cryostat survive Herlach et al, 1971

41 Pulsed magnetic fields single turn coils Faraday rotation, T = 5K ZnCr 2 O 4

42 Pulsed magnetic fields single turn coils significantly higher magnetic fields (< 300T) sample and cryostat survive (?) magnet destroyed every experiment (it is inexpensive) limited or no availability for users (ISSP Tokyo? NHMFL-LANL?) limited temperature range limited space fast limited number of experimental techniques safety

43 Pulsed magnetic fields EM flux compression

44 Pulsed magnetic fields EM flux compression magnetic field through Faraday rotation

45 Pulsed magnetic fields EM flux compression significantly higher magnetic fields (up to ~ 1000T) magnet, cryostat and sample are destroyed every experiment no availability for users limited temperature range very limited space fast limited number of experimental techniques safety

46 Destructive pulse field magnets Explosive flux compression - ~ 2800 T Sarov (Russia) Also (past) Ancho Canyon, Los Alamos

47 Destructive pulse field magnets Explosive flux compression - ~ 2800 T Sarov (Russia)

48 Magnetic field measurements (*) Well defined geometry coil can calculate. B θ (**) Faraday s law: E = - db/dt. ns. cosθ several assumptions: constant area S, measurable θ, fast, accurate electronics. Good for pulsed fields. E (***) Hall probes linear in field. 2DEG very sensitive but at low temperatures/high fields QHE and/or SdH are observed. Other (e.g. III V) semiconductors. Semimetals (Bi, ). Either purchase (LakeShore, GMW, ) or DIY if you have even primitive thin film technology. NB: Temperature dependence, angle with magnetic field (but can serve as angle sensor), linearity. Hall arrays, multiaxes sensors. (****) Standards (susceptibility of pure Pd for MPMS)

49 Reading: Fred M. Asner High Field Superconducting Magnets NHMFL web site High Magnetic Fields Conferences and Workshops