The Science of High Magnetic Fields

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The Science of High Magnetic Fields

• Prof. Chris Wiebe
• FSU/NHMFL

The hybrid magnet (45 T!)
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Magnetic fields!
Nikola Tesla

• What is a magnetic field? (symbol B)
• A magnetic field surrounds electric currents. It
  produces a force on moving electric charges
  nearby (such as electrons, protons, etc.)
• F(charge)(velocity of particle)(magnetic
  field)
• Units of magnetic field 1 Tesla (T) 1
  N/(m/sC)

An electron moving in a magnetic field
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Ways of describing magnetic fields

• We usually talk about magnetic fields having
  North and South poles.
• This is a way of describing the direction of the
  magnetic field (it tends to flow out of the North
  pole and into the South pole).
• Iron fillings tend to line up along the magnetic
  field lines.
• Where do magnetic fields come from in bar magnets?

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Where do magnetic fields come from?
N

• Magnetic fields originate from moving charges!
• Everyday objects like lodestone (iron oxide)
  produce their own spontaneous fields due to the
  electrons orbiting the nuclei in atoms.
• The tiny magnetic fields from these atoms add up
  to give a large net magnetic field.
• Why cant we use these to produce large fields?
• The net magnetic field is usually small
• Also, there are other effects which reduce the
  net size of the magnetic field, such as domain
  formation.

S
Orbiting electrons make each atom like a little
magnet.
Domain formation in permanent magnets
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Electromagnets

• What we really want magnetic fields that we can
  turn on and off
• Oersted showed how electric currents can create
  magnetic fields greater than the Earths field
  (can deflect a compass!)
• Electromagnets use electric fields to create
  magnetic fields!
• A current carrying wire produces its own
  magnetic field.
• With the application of an electric field, we can
  create a magnetic field.

No current, no magnetic field
Add a current, compass needle deflects created
a magnetic field!
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Which way is the field pointing?

• Magnetism is a complicated force!
• We can use a right hand rule to determine which
  way the field lines point.
• Thumb in direction of the current, I, fingers
  curl in the B direction.
• How can we create a uniform magnetic field?

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Solenoids and magnetic fields

• Imagine taking these wires and creating loops.
• This is called a solenoid. The magnetic field is
  in the same direction for all of the loops inside
  the solenoid.
• The magnetic field is quite homogeneous too (it
  is the same in the middle of the solenoid
  throughout the whole length).

L
(n no of loops/L)
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Uses of Solenoids

• Automobile starters the magnetic field pulls
  the iron core when current flows. This starts
  the flywheel moving.
• Doorbells Again, when the solenoid is
  energized, the magnetic field generated pulls on
  the plunger, which strikes a bell.
• MRI machines Large magnetic fields are created
  inside the torus shaped areas. These magnetic
  fields align the protons in your body (in water),
  which shows up as contrast depending upon water
  concentrations in certain organs.

A doorbell.
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Getting to high fields

• The net field is to the amount of current you
  can apply (B I)
• So, we can just crank up the current and then get
  to whatever fields we want, right?
• This doesnt really work so well!

(Resistive solenoids by themselves cannot create
large fields)
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Resistive heating!

• The reason why is that there is something called
  resistive heating in conducting wires.
• The electrons that are moving inside the wires
  are bumping into each other and not moving in an
  ideal fashion. They have a resistance.
• Energy is lost as heat (resistive heating).
• The higher the electric field you apply to create
  the current, the more energy that is lost as
  heat.
• Eventually, your wires will melt!

(tiny wires inside the fuse)
(This is how your fuses work at home the wires
melt once a certain current is reached)
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Resistive magnets

• There are some clever ways around this next
  talk.
• Resistive magnets are just circular bunches of
  conductors put together to create a magnetic
  field like a solenoid.
• The holes in the conductors are for cooling,
  either through water or liquid helium ( -270
  degrees C!)
• Even with this cooling, we can only apply
  currents that can get up to 30 T (which is
  still pretty impressive!)

These plates act like solenoids when stacked
together. The holes are for cooling
fluid. (Special size and shape optimized for the
best performance)
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How can we get to higher fields?

• Superconducting magnets!
• (no resistance in superconductors!)

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Superconducting magnets!

• Superconductors conduct electricity with no
  energy loss from heating!
• With a superconducting solenoid, we can apply
  large currents to produce large magnetic fields.
• In fact, these are used all over the place to
  create large fields (eg. MRI machines). Its
  cheaper than using resistive magnets in the long
  run.
• The catch you need to constantly cool down the
  superconductors no room temperature
  superconductors exist!
• This is one of the holy grails of science to
  find a superconductor that works at room
  temperature!

Magnet
Superconductor
That high Tc superconductor is here somewhere!
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High temperature superconductivity
Highest Tc still way below room temperature
298 K

• Our progress towards high temperature
  superconductivity
• This represents a crisis in our understanding of
  superconductors we dont know why many of these
  materials superconduct, so we cant design them
  to have the properties that we want!
• A Nobel prize is waiting for the person that
  discovers a room temperature superconductor.

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There always is a catch
Normal metal
Super- conductor

• So, if we create a superconducting solenoid, we
  should be able to create as a large of a field as
  we need, right?
• There is another catch magnetic fields destroy
  superconductivity!
• Initially, the superconductor will repel magnetic
  fields (something called the Meissner effect).
• Eventually, the magnetic fields will start to
  penetrate the superconductor and we will lose
  superconductivity.
• We call the superconductor normal at this point
  (it has resistive losses like a normal metal)

N
Magnet
S
Superconductor
The Meissner effect is when the magnetic field
lines are pushed outside of the
superconductor. This only happens up to a
certain field before they start to penetrate
and destroy the superconductivity.
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Types of superconductors

• Two types of superconductors
• Type I The magnetic fields are repelled until a
  certain threshold value Hc. After this value is
  reached, superconductivity is destroyed!
• Type II The magnetic fields are repelled until
  a threshold value Hc1 is reached. After this
  value, the material is still superconducting, but
  some of the magnetic field lines penetrate the
  superconductor. These are called vortex lines.
• After Hc2 is reached, the material is no longer
  superconducting.

In type II superconductors, the current can still
flow around the vortex lines, where the magnetic
field penetrates the superconductor.
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Summary

• To sum it up then
• (1) Resistive magnets you can use up to a
  threshold current before they melt.
• (2) Superconducting magnets you can use up to
  a threshold field before they are no longer
  superconducting.
• How can we reach really high fields?
• Combinations of resistive superconducting
  magnets the hybrid magnet!

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The hybrid magnet

• The hybrid magnet at the NHMFL can reach 45 T.
• This works by using an inner resistive magnet and
  an outer superconducting magnet.
• You need to cool down the superconducting magnet
  with liquid helium for the whole experiment!
• Why is the superconductor on the outside?
  Remember, high fields kill superconductivity! We
  cant put the superconductor on the inside.

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Pulsed field experiments

• What about really high fields? (ie. gt 45 T)
• We can reach these, but only for short periods of
  time.
• This is done through pulsed field experiments
  at places such as Los Alamos.
• You can reach these fields by destroying a
  solenoid magnet through an enormous pulse of
  current.

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How to do a pulsed field experiment
RLC circuit

• Pulsed field experiments
• (1) Store up a large amount of current (eg.
  through a capacitor, or two charged plates).
• (2) Release that current to your solenoid
• (3) Do your measurement in less than a
  millisecond!

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Life in Los Alamos

• Video of a pulsed magnetic field experiment on
  YBCO, a high temperature superconductor.
• Typically, the critical fields are very large in
  these materials you need pulsed magnetic fields
  to measure them.

Generator used to store the energy for the 60 T
pulsed fields
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The results!
What did we learn? After all, each data point was
an explosion costing about 10,000! It takes a
huge field to kill the superconducting state in
YBCO, as expected from theoretical
considerations.
So why cant we make a 120 T magnet?
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The road to higher fields

• It is difficult to make YBCO wire that can retain
  this stress.
• The search still continues for ways to create
  better superconducting wires.
• Room temperature Tc values?
• Stable materials that can withstand the forces at
  those fields?
• New discoveries are being made all the time here!
• Stay tuned for Prof. Boebingers talk!