To some, magnets are just the things that hold family photos and to-do lists to the fridge door
but in reality, magnets and magnetism make the world go around.
Magnets are everywhere, every electric motor, generator, loudspeaker, mechanical hard drive,
microwave oven and of course equipment like MRI machines which I recently found myself
in and which was the inspiration for this video.
Magnets are also the fundamental feature of the new fusion reactors which will
hopefully lead us to a clean energy world which follows on nicely from our last video about
the 3 mile island nuclear incident. So in this video we’ll look at how
magnets are an often overlooked but fundamental part of our modern world.
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Although this video is about magnets it should really be about magnetic fields
because a magnet is just an object or device that creates a magnetic field and it's how we
use these fields that makes them so very useful. Magnetic fields are everywhere too, the biggest
nearest one is generated by the biggest magnet.. the earth itself or to be more
precise the ever-moving molten iron core at the centre of the earth right under our feet.
But even though the magnetic field that this creates extends for tens of thousands of km
into space and protects us from the charged particle radiation given off by the sun,
the power of that field is very weak on a local scale. It only has enough strength to move the
pointer of a compass and can easily be overwhelmed by a tiny magnet nearby.
So what is magnetism and what makes a magnet? Well, Magnetism is defined as is a force
generated in matter by the motion of electrons within its atoms. Magnetism and electricity are
two aspects of electromagnetism which is one of the fundamental forces of the universe.
Magnetism is the force that is created by the orbit of electrons around the nucleus of an atom,
a bit like the planets orbiting the sun and the spin of the electrons themselves
which is like the earth spinning on its axis. In most elements, there are pairs of electrons
spinning in opposite directions which cancels this force out. However, in some so-called transition
metals such as iron, cobalt, and nickel and rare earth elements cerium, neodymium,
samarium, and europium, there are single unpaired electrons and here the forces are not cancelled
out making the electrons like a tiny magnets. When there are large groups of these electrons
all spinning in the same direction the magnetic force is added together creating magnetic domains
and when are a lot of these magnetic domains in a metal it is said to be magnetic.
This creates magnetic field lines in the space around it and the more
lines there are the stronger the magnet is and this is measured in gauss and teslas,
one tesla being 10,000 guass. A typical fridge magnet at its surface is about 100 guass
and the earth’s magnetic field is about 0.5 guass. Because electricity and magnetism are intimately
linked when a current is passed through a conductor like a piece of copper wire,
the movement of electrons in the wire creates a magnetic field around the conductor
but unlike natural permanent magnets this field only exists for as long as a current is flowing
and the greater the amount of current flow the greater the magnetic field
and these are known as electromagnets. The power required for an electromagnet
is the square of the magnetic field, so to go from 1 tesla to 10 teslas requires 100 times the power.
But using just copper wire there is a limit to how much current can flow
and as the wire heats up it’s resistance increases decreasing the current. However,
if superconducting materials are mixed with the copper wire the amount of current can be much
larger the magnetic field generated much stronger. But most superconductors only work when
cryogenically frozen to as low as -269C, limiting where they can be used.
Until relatively recently most permanent magnets were made from alloys of iron, nickel and cobalt
but in 1984 both General Motors and the Japanese company Sumitomo Special Metals,
individually developed Neodymium magnets but using different manufacturing techniques.
These are now the most powerful commercially available permanent magnets. This is because
Neodymium has four unpaired electrons to the three of iron and are capable of storing much more
magnetic energy, typically 18 times that of normal ferrite magnets by volume and 12 time by mass.
This means that they can lift thousands of times their own mass and have become commonplace in
things high power motors of all sizes. Couple that with high energy density Li-ion batteries and you
have devices like battery-powered hand tools that 30 years ago would have needed to be mains
powered, tiny high power motors for things for drones and of course the new
powerful electric motors for electric vehicles. But as powerful as Neodymium magnets are there
are some issues. Firstly when they are heated up to beyond 100C the magnetic energy density drops
dramatically till they reach the Curie point where they permanently lose all their magnetic ability,
though Dysprosium or terbium can be added that will mitigate some of this
to allow higher operating temperatures. The second is the supply of these rare
earth elements is limited to just a few places around the globe and mostly concentrated in China
with dominates the supply chain, which limits the competition and keeps the price artificially high.
When we get the most powerful magnets, Devices like MRI or Magnetic resonance
imaging scanners which I happened to be in one last year have to generate a very
powerful but even magnetic field as possible. This is used to align the spin of the electrons
in hydrogen atoms, and as our bodies are 70% water which is 2 hydrogen atoms to one oxygen,
this works very well to differentiate between fat, water, muscle, and other soft tissue and bone.
Around the core of the MRI scanner other electromagnetic gradient coils then change
that field in a rapid but controlled manner whist radio waves are injected that affect the spin of
the electrons. When the radio waves are turned off the electrons return to their normal spin
and this is picked up by other receiving coils on and using this information the computer
can build up a 3D image of the body in slices. The main electromagnet uses superconductors which
have to be cooled with liquid helium to -269C or 4.2 kelvin to generate a field strength of about
3 tesla which is why you can not enter the room whilst it is on wearing or carrying anything made
from ferrous metals or electronic devices like pacemakers as these could be dislodged
or heated up by the very strong magnetic field. There have been very rare incidents when safety
protocols have not been followed where the M.R.I. magnets have sucked in hospital beds,
screwdrivers, oxygen tanks and other metal objects into the core of the machine.
https://static01.nyt.com/images/2017/06/27/science/ask-mri/ask-mri-jumbo-v2.jpg?quality=75&auto=webp
Now as powerful as MRI magnets are, they far from the largest magnets, that accolade will go to the
experimental ITER fusion reactor currently being built in France by an international consortium.
This is the largest experiment fusion reactor to date and hopefully will be the first to prove
that fusion is viable on a commercial scale even though it won't generate any electrical power,
the heat created will just be vented via a giant heat exchanger.
Now if you're wondering why this is such a big deal,
then this quick explanation will show you why. Existing nuclear power stations use nuclear
fission which takes heavy elements like uranium and bombards them with neutrons,
this splits the uranium atoms and in the process releases more neutrons,
some of which heat’s up cooling water that surrounds the uranium fuel rods
which is used to create steam to power turbines and make electricity.
The problem with fission is that once a fission reaction takes place if things go wrong it can
very quickly escalate out of control with disastrous results, that’s what happened
at 3 Miles Island, Chernobyl and Fukushima. It also creates other highly radioactive elements
which are the waste products and will remain radioactive for hundreds or thousands of years.
Nuclear fusion on the other hand is the same process the sun and all stars use to convert the
lightest element in the universe Hydrogen, into helium. It does this under extreme pressure at
the centre of the sun that fuses the Hydrogen nuclei together to create heavier elements
like helium and in the process, it releases energy which we then capture and use to make electricity.
Fusion reactions release four times the amount of power compared to fission and 4 million times
that of coal. Just 1 gram of deuterium-tritium, isotopes of Hydrogen in a fusion process would
produce 90,000-kilowatt hours of energy or the same as burning 11 tonnes of coal.
Fusion is also inherently safe, unlike fission it is very difficult to start and maintain so
it can not create runaway reactions. If anything goes wrong with the process the reactions stop
automatically and there is no nuclear waste other than the irradiated reaction containment vessels
when the plant is shut down or refurbished. The problem is that, unlike the sun, we can
not recreate the extreme pressure here on earth, at the centre of the sun the density of hydrogen,
the lightest element in the universe is 70x denser than steel.
But fusion can occur on earth at just 10 times atmospheric pressure if the temperature is high
enough, about 10x hotter than the centre of the sun or 150 million degrees Celsius.
Now there is nothing on earth or in the universe that withstand that kind of temperature but
luckily for us matter at that high a temperature is a plasma that reacts to magnetic fields.
So if you have a very strong magnetic field you can contain this super-hot plasma
and that’s exactly what they do in a tokamak fusion reactor but using deuterium & tritium
as fuel instead of normal hydrogen because they react better under less extreme conditions.
ITER or the International Thermonuclear Experimental Reactor is a magnetic confinement
fusion reactor and will be the biggest tokamak reactor built so far and the magnets used in it
are the biggest ever built. The reactor itself is large
torus shape, like a giant doughnut and will use four types of superconducting Magnets,
a central solenoid magnet, poloidal magnets, toroidal-field coils, and correction coils.
The central solenoid alone is the biggest at 18 metres high by 4.3m wide and weighing 1000 tonnes.
Together, these create an intense magnetic field between 11 and 13 tesla that will hold
the plasma in a vacuum in the centre of the reactor vessel away from the walls whilst it
is heated to 150 million degrees by passing huge electrical currents through the plasma.
The central solenoid magnet is made up of six sections stacked on top of each other.
The combined magnets will produce a field strength of 13 Telsa which about 280,000 times
stronger than that of the earths. These will repel each other when running, so they are held together
with massive tie rods which applies 50,000 tons of compression, so great is the magnetic field.
In fact when the central solenoid is active it could pick up the equivalent
of an America class amphibious assault ship weighing in at about 45,000 tons.
The TF coils which surround the ring of the torus generate a force of up to 40,000
tonnes of pressure when active, in fact the whole structure has to withstand the forces greater than
twice that of the space shuttle at take-off. Whilst all this is going on the temperature of
the plasma is 150 million degrees but just a meter or so away the whole vacuum vessel structure and
superconducting coils sits in a giant cryostat tank that cools it to -269 degrees Celius.
ITER will require 300MW of electrical power to get the plasma to absorb 50MW of thermal power in
order to release 500MW of heat in periods of 4-500 seconds when it is finally up and running in 2035.
But if you could make the magnets not only more powerful,
use less power and work at higher temperatures
then the whole fusion process would be much more efficient and make even more power for its size.
So to this end, the most powerful superconducting magnet ever made
has been built and tested by a joint team from MIT and Commonwealth Fusion Systems.
By using a new high-temperature superconducting material it allowed them to make a
record-breaking magnetic field of 20 Tesla's in Sept 2021 in a much smaller space some
40 times smaller than current low-temperature superconducting coils making net power
from fusion possible more quickly. The fusion power produced in a tokamak
is proportional to the strength of the magnetic field to the fourth power, so if you can double
the magnetic field strength you can get a 16x increase in the amount of fusion power.
These new coils will be used in the SPARC fusion project which by
2025 is hoping to produce net positive fusion in smaller more easily manufactured reactors
that can’t come fast enough if we are to move to a net-zero carbon world any time soon.
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