High explosives and flying frogs:
research at high magnetic fields
Stephen Hill, Department of Physics
• General Introduction • Research at high
– History
magnetic fields
– Magneto-optics
• Generation of high
– Diamagnetic levitation
magnetic fields
–
–
–
–
Superconducting
Resistive
Hybrid
Pulsed: Long pulse
Short pulse
– What do we do?
The Magnetic Field
600 BC - Lodestone
The magnetic properties of natural ferric ferrite (Fe3O4) stones
(lodestones) were described by Greek philosophers.
1040 - One of the earliest magnetic compasses
A floating fish-shaped iron leaf, mentioned in the Wu Ching
Tsung Yao which was written around 1040. The book describes
how iron can be heated and quenched to produce thermoremanent magnetisation. The first clear account of suspended
magnetic compasses in any language was written by Shen Kua
in 1088.
1175 - First Reference to a Compass
Alexander Neckem an English monk of St. Albans describes the
workings of a compass.
1269 - First Detailed Description of a Compass
Petrus Peregrinus de Marincourt, a French Crusader, describes
a floating compass and a compass with a pivot point.
The Magnetic Field
1600 - Static Electricity
(De Magnete)
William Gilbert (1544-1603)
studied magnetism and in
1600 wrote "De magnete"
which gave the first rational
explanation to the
mysterious ability of the
compass needle to point
north-south: the Earth itself
was magnetic.
This opened the first era of
modern physics marked by the
great achievements of Galileo,
Kepler, Newton and others.
The Magnetic Field
1820 - Electromagnetism, Current
In 1820, a physicist Hans Christian Oersted, learned that
a current flowing through a wire would move a compass
needle placed beside it. This showed that an electric
current produced a magnetic field.
As you will see
in this talk, this
is the way to
generate high
magnetic fields.
The Magnetic Field
1820 - Electromagnetism, Current
Andre Marie Ampere showed that two parallel wires
carrying current attracted each other if the currents
flowed in the same direction and opposed each other if
the currents flowed in opposite directions. He formulated
in mathematical terms, the laws that govern the
interaction of currents with magnetic fields in a circuit
and as a result of this the unit of electric current, the
amp, was derived from his name.
Birth of 2nd era of modern physics
A sense of scale
1 tesla  10,000 oersted (gauss)
Earth's magnetic field  0.5 gauss
Text books claim strong lab field  1 tesla
Saturation magnetization of Fe  (1 - 2 tesla)/o
108 tesla - surface of a pulsar
104 tesla - white dwarf
3 × 10-1 tesla - in a sun spot
10-2 tesla - surface of sun
The Royal Swedish Academy of Sciences has awarded
The 1985 Nobel Prize in Physics
to Professor Klaus von Klitzing, Max-Planck-Institute, Stuttgart,
Germany
von Klitzing was awarded the Nobel Prize for
discovering the quantized Hall effect. This effect can
only be observed in strong magnetic fields, resulting in
the most precise definition of the unit of resistance –
the ohm.
Citation:
"for his discovery of the quantized Hall effect."
The Royal Swedish Academy of Sciences has awarded
The 1998 Nobel Prize in Physics jointly to
Professor Robert B. Laughlin, Stanford University, California, USA
Professor Horst L. Störmer, Columbia University, New York and
Bell Labs, New Jersey, USA, and
Professor Daniel C. Tsui, Princeton University, Princeton, New
Jersey, USA.
The three researchers are being awarded the Nobel Prize
for discovering that electrons acting together in strong
magnetic fields can form new types of "particles", with
charges that are fractions of electron charges.
Citation:
"for their discovery of a new form of quantum fluid with
fractionally charged excitations."
The Royal Swedish Academy of Sciences has awarded
The 2003 Nobel Prize in medicine jointly to
Professor Paul C Lauterbur, University of Illinois, USA
Professor Peter Mansfield, University of Nottingham, United
Kingdom
Citation:
"for their discoveries concerning magnetic resonance
imaging."
Technology driven
Importance of research at high
magnetic fields
• Up to 60 tesla using
proven technology
– Condensed matter
• Superconductivity
• Magnetism
• Correlated systems
– Magnetic resonance
• EPR/NMR/MRI
– Particle physics
– Zero gravity conditions
• Up to 1000 tesla - currently
under development
– Condensed matter
• Superconductivity
• Magnetism
• Correlated systems
– Spectroscopy of atoms
– New forms of matter
– Astrophysics
Generating high magnetic fields
Biot-Savart law
o J  r 3
B
d r
3

4 r
B  log(Rout/Rin)
where Rout and Rout represent the inner
and outer radii of a solenoidal magnet
B  current density J
• It is only practical to vary
R up to a point
• Key parameter is J
• Major limiting factors
– Power supply
– Joule heating
– Magnetic forces
Method 1: Eliminate Joule heating
Superconducting magnets
• Combination of Nb3Sn and NbTi
• Wires permeated with epoxy
Typical specs
Nb3Sn
wire
Field: 18/20 T @ 4.2/2.2 K
Max current  122 A
Voltage < 10 V
Inductance  240 h
Size Ro  6", Ri  1", h  12"
Wire diameter  1 mm
Filament diameter  75 m
World record: 25 tesla
Pros
and
Magnet is cheap
Low power (< kW)
Can be persistent
Portable
Easy to operate
Large sample space
Extremely reliable*
cons...
Requires liquid Helium
Extremely high inductance
 slow ramp rates
B field kills superconductivity
 2 MJ stored energy @ Bmax
*If operated correctly
Method 2: Resistive magnets
3 coils in series
Inner coil consists of 2 coils in parallel
Cooling water flows axially through holes
Elliptical holes minimize stresses
Conductor: Cu/Ag alloy
Insulation: Kapton
Specifications...
•
NHMFL (Tallahassee, FL)
– Two 20 MW supplies
• Vm = 500 V; Im = 20 kA
– Ripple < 10ppm
– Two 30+ T magnets (32mm
bore)
– 25 T, 52mm
– 20 T, 200mm
– 24.5 T, 32mm bore - high
homogeneity for NMR
Also: Netherlands, France, Japan
Max field: 34 tesla
• World record magnet
–
–
–
–
34 tesla @ 38 kA
500 V max
19 MW!!
Water 140 liters/sec
T=35oC ( 20 MW)
Method 3:
Hybrid magnet
Max field: 50 tesla ?
Superconducting and resistive
•
•
•
•
•
•
Superconducting outer
Resistive inner
V. expensive to run
Not user friendly
Whereabouts:
Days to prepare
Tallahassee, FL –
(world record 45 T)
Dangerous! Energy
Nijmegen, NL –
stored in supercon is
(under construction)
several hundred MJ Tsukuba, Japan
MIT - decomissioned
Method 4: Pulsed magnets
Non-destructive, short pulse
Wire: Cu/Ag and
Cu/Nb Alloy
Kapton insulation
Reinforcement:
Glass/carbon fiber
• Coil consists of few turns
– Keep inductance low
• Magnet cooled to 77 K prior to pulse
– Increases conductivity
• Relatively cheap and easy to build
Max field: 70 tesla
Specifications
Capacitor driven
63 T, 15mm, 7/35ms
50 T, 24mm, 6/30ms
42 T, 24mm, 100/500ms
1 pulse/hour to 63 tesla
1.5 megajoules at 10 kV
(.44 magnum slug 1 kJ)
Imax = 20 kA
500 - 800 pulses, then...
80
70
Magnetic Field (T)
60
50
10 mm SP
40
15 mm SP
30
24 mm SP
20
10
0
0
5
10
15
20
Time (ms)
25
30
35
Non-destructive,
long pulse
260 ton flywheel
1.4 gigawatt
Magnet ~90 MJ
Can deliver 270 MJ
NHMFL/LANL
Amsterdam
60
MAGNETIC FIELD (tesla)
60 T for 0.1 s
or 50 T for 0.5 s
32 mm bore
1 shot/hr
50
40
30
20
10
0
0
1
2
TIME (seconds)
3
Challenges in Producing High Magnetic Fields
Pressure Under Water
4 m Ears
6 psi
1000 m Submarine
1000 psi
4000 m Ocean Floor Submersible 6000 psi
Pressure inside NHMFL Magnets
80T Pulsed Magnet
200,000 psi
(1.3GPascals, 130 kg/mm2)
…exceeds the strength of most materials…
…within a factor of three of theoretical ultimate tensile strength…
Strong
Electromagnets
Generate
HUGE Forces
Flux compression
Single stage
Magnet is destroyed
Sample often
survives
Max field: 200 tesla
3 stage explosive flux compression
Before
During
After
3 Stage 850 tesla generator
Syncronous Detonators
Shockwave
Foam Support
3rd
2nd
Sample
1st Stage
Machined High Explosives
Optical Reflectivity of GaAs sample
60x10
3
Optical Signa ls @ 75 K - Dirac MC1 -D
GaAs QW Refl ectivity @81 0 µm
Quartz Farad ay Rotation @ 632 .8 µm
J. S. Brooks a nd C. H. Mielke
June 18, 1997
60x10
3
55
Refelctivity Signal (arb. units)
50
40
30
45
20
40
2
3
4
5
6
7
8 9
100
Magnetic Field (T)
2
3
4
5
6
7
8
Faraday Rotation (arb. units)
50
Everything is magnetic.....
....Some fun with strong magnetic fields
Diamagnetic levitation
• Molecular diamagnetism
– Common to all matter
– Usually obscured by other
forms of magnetism
• Becomes apparent in
strong magnetic fields
• Study effects of microgravity
– Crystal growth
– Plant growth
Basic principal
m = (/o)VB
Fmag = mB = (/o)VBB
Fmag = (/2o)VB2
Fgrav = Mg = Vg
 levitation requires:
B2
> 2og/
V is volume
 typically -10-5 JT-2kg-1
 103 m-3
requires B2  103 T2m-1
Taking l  0.1 m
We get B  10 tesla
The work was first featured in Physics World, April 1997, p. 28
E.H. Brandt, Science 243, 349 (1989) and Physics World, September 1997
"Of Flying Frogs and Levitrons" by M.V.Berry and A.K. Geim, European
Journal of Physics, v. 18, p. 307-313 (1997).
http://www-hfml.sci.kun.nl/hfml/levitate.html
http://www.nhmfl.gov/movies/levitation/index.html
Very briefly.....
What do we do?
Nano-scale
Single Molecule
Magnets
The Future: writing information
to individual molecules
1 Nanometer
Mn12Ac
Synthesis and characterization:
Christou group, UF chemistry
Advanced characterization:
High frequency EPR, UF physics
Organic superconductors
• One can easily tailor materials with desired properties
• The materials are intrinsically clean, i.e. very pure
• Organic conductors exhibit virtually all known electronic
states of matter, e.g:
Metal, insulator, magnetism, quantum Hall effect,
superconductivity (conventional and exotic forms).
Summary
• History of magnetism and research at high magnetic fields
• Overview of different ways to generate high magnetic
fields
–Superconducting
–Resistive and hybrid
–Long & short pulse (non destructive and destructive)
• Overview of worldwide user facilities
• Fun experiments - diamagnetic levitation
• Examples of what we do