Jacob’s Ladder – Controlling Lightning
Video Teacher resources
Host:
Fusion specialist:
Phil Dooley – European Fusion Development Agreement
Peter de Vries – European Fusion Development Agreement
PART 1 – Jacob’s ladder demonstration
The Jacob’s ladder is an example of a sustained electrical breakdown between two
electrodes – in other words a plasma. The voltage between the two copper electrodes
is high enough to ionize the air (i.e. to separate electrons from their atoms), and this
cloud of ions and electrons – being charged – allows current to flow between the two
electrodes, which air – being neutral – does not. Unlike a spark of static electricity
which might zap between you and a door handle on a dry day, these electrodes have
their charge replenished by the power supply and hence the current continues to flow
through this area of low resistance.
The initial breakdown occurs at the point where the electrodes are closest, which is
designed to be at the bottom of the electrodes. Then the ions, which are heated by this
process, rise, and so the discharge, which is taking the path of low resistance provided
by these ions, travels up the electrodes. At the top, the ions rise so far that the
pathway gets too long. This long pathway means the resistance increases, to a point at
which the resistance of the neutral air at the small gap at the bottom of the electrodes
becomes a preferential path for the current to flow.
Dr de Vries points out that there is an additional mechanism in the breakdown
process, an avalanche process. This occurs when charged species (electrons or ions
which are usually present in small quantities) are accelerated and collide with other
atoms, ionizing them, and creating more electrons which can be accelerated to create
other ions and so on. Hence the path length and density (or mean-free path) are
important quantities as well as the electric field.
PART 2 – Application to JET
Dr de Vries explains that the electric field causing breakdown in JET is not a gap
between two electrodes. Instead it is provided by the transformer effect – the central
solenoid in the middle of the doughnut acts as a primary coil of a transformer, and the
gas in the torus vessel acts as the secondary. By using gas at the optimum pressure
level (about one million times less than atmosphere) the required electric field to
achieve breakdown is only a couple of volts (compared with the 25 000 V/cm
required in air.)
At this pressure the distance an electron would travel before colliding with an atom
(known as the mean-free path) is several metres, which is why it would have time to
pick up enough momentum to ionise an atom.
http://www.efda.org/fusion/fusion-machine/magnetic-fields/
http://www.efda.org/fusion/focus-on/how-to-start-a-fire/
http://www.efda.org/fusion/fusion-machine/types-of-fusion-machines/tokamaks/
http://www.efda.org/fusion/focus-on/limiters-and-divertors/
Jacob’s Ladder Controlling Lightning – Teacher resources
Pre-questions
Does air conduct electricity? Why? No, it doesn’t because it is made up of neutral
atoms (for the most part; there are always trace amounts of free electrons and ions)
If air doesn’t conduct electricity, how does a spark form? If air experiences a
electric field (voltage) large enough to remove an electron from an atom, then you
now have two charged particles, an electron and an ion. These are then affected by the
electric field, i.e. current flows from one electrode to the other.
What makes up the atom, and what charge is each species? Protons (positive),
neutrons (neutral), electrons (negative).
What is the fourth state of matter, and describe it. Plasma – a gas which has been
ionized, and therefore is made up of charged particles (ions) and free electrons.
Because it is charged it conducts electricity, and reacts to electric and magnetic fields.
It can be hot or cold, at high or low pressure. (compare the hot dense sun, with a cold,
low pressure nebula in space.)
What are some examples of plasma: Stars, fluorescent tubes, flames, nebulae,
plasma screen TVs.
How do the particles of a plasma react to a magnetic field? Because they are
charged they experience a force at right angles to both their direction of motion and
the magnetic field. This leads to them following a helical path along the magnetic
field lines, or, if they are initially traveling exactly at right angles to the magnetic
field, to follow a circular path.
Why is plasma required for fusion? Because fusion relies on nuclei colliding, for
this to happen all the electrons around the nucleus must be removed. (Note, not all
plasmas are completely ionized.)
How does a transformer work? A changing current is put through a coil (known as
the primary). A secondary coil, usually wound around the primary, has a current
generated in it by the changing magnetic flux created by the primary.
Jacob’s Ladder Controlling Lightning – Teacher resources
Post-questions (on video content)
What is the voltage between the electrodes in the Jacob’s Ladder ? 10 000 V
What are the two mechanisms for ion formation cited in the video? (1) Electric
field/voltage, which separates electrons from the atoms. (2) Free electrons accelerated
to high speeds by the electric field, which collide with neutral atoms and knock off
other electrons – “Avalanche Process”
What is the shape of the magnetic field in JET? It is toroidal (doughnut shaped) –
although not perfectly so – combined with a poloidal which gives a twist to the
poloidal field.
How big is the voltage inside JET that starts the ionisation? A few volts
What generates the electric field inside the JET torus? The gas inside JET acts as
the secondary coil of a transformer. The primary coil is a solenoid that runs through
the centre of the doughnut – a changing current is run through this solenoid, to
generate the transformer action.
Is this a step-up or step down transformer? Step down, as the plasma has only one
turn. (Note: you are not told the number of turns in the central solenoid, so you have
to estimate that it has more than one turn!)
Why is a voltage so much lower than the Jacob’s Ladder all that is required?
Because the pressure is lower, the electrons travel further between each collision, and
so pick up more energy.
What is the pressure inside JET? A million times less than atmosphere.
How far do the electrons travel on average? Several metres between collisions.
Several kilometres before they hit the wall (due to the imperfections in the toroidal
magnetic field.)
What is the plasma current and temperature at the end of the avalanche
process? 100 000 A, 1 000 000 oC
What are the maximum plasma current and temperature for JET’s fusion
experiments? Several million amps, over a million degrees.
JET’s central solenoid has 710 turns, and the final current is 1 kA, driven by
approximately 5 volts. Calculate the voltage and current in the central solenoid:
The ratio of turns between the primary and secondary coils is 710:1 . Therefore the
voltage is 710 x 5 = 3550, and the current is 1000/710 = 1.41 A
Jacob’s Ladder Controlling Lightning – Teacher resources
Extension Questions
How is transformer action of a tokamak like JET achieved, and how does it limit
the experiment? What methods are addressing this limitation (e.g. stellarators,
or heating systems)? Transformer action requires a changing current in the primary,
so the central solenoid must begin the experiment with a very large current through it.
This is then reduced, through zero, until it reaches a maximum value in the opposite
direction. At this stage the tokamak has to stop. (Sweeping back in the opposite
direction is not practicable as the zero current point confinement would be lost, and
then subsequent reversed direction of the plasma would be problematic).
Newly built tokamaks such as JT60SA in Japan and ITER use superconductors to
increase the current and thereby extend the lifetime of the current-sweep, but it is still
finite. Stellarators use a twisted magnetic field to drive current without the central
solenoid, while other methods use microwave or beam injection systems to drive the
current.
There are two separate magnetic fields at play in JET, toroidal and poloidal.
How are these different, how are they generated and what are they used for? The
central solenoid generates a poloidal magnetic field, similar to the Earth’s magnetic
field. It has a changing current, which has the effect of driving the current in the
plasma. The toroidal magnetic field is ring shaped, and follows the torus vessel. This
is a constant field and is generated by 32 coils encircling the vessel. The combined
vector sum of these two fields gives a helical path, which ensures that particles
rotating around the outside of the vessel are constantly circulated back into the centre,
where the toroidal field is stronger (because the inner parts of the coils are closer
together).
The heating from the transformer effect (known as ohmic heating) is limited to
about one million degrees. Why is this and what other methods are used to heat
the plasma to the required 150 million degrees? Ohmic heating loses efficacy once
the plasma is mostly ionized because the resistance becomes very low, therefore the
power drops because P = I2R. Heating is provided by microwave/RF electromagnetic
radiation for two mechanisms (cyclotron motion, and lower hybrid) or neutral beam
injection.
Jacob’s Ladder Controlling Lightning – Teacher resources
General Fusion Questions
The fusion process turns hydrogen in helium. Compare with fission of uranium.
Fission is splitting atom, to make smaller atoms. Small and very large atoms are both
less stable, the most stable atoms are middle sized – most stable of all is iron.
Compare the process of burning hydrogen to fusing hydrogen. Burning is a
chemical reaction requiring oxygen (oxidation), that produces H2O. Fusion is a
nuclear process that produces helium, releasing more than a million times more
energy per gram than burning releases.
Most Fusion experiments fuse deuterium and tritium (isotopes of hydrogen).
Why is this process used? Is this the same process as occurs in the sun? No the
sun has a complex many step process. DT fusion is much more efficient and easier to
achieve. Fusion reactors put out much more energy per volume than the sun.
What happens to the nucleons (collective term for protons and neutrons) in DT
fusion? They re-arrange themselves to a lower energy state: 3 (T) + 2 (D) rearrange to
become 4(He) + 1(neutron)
Compare the safety considerations for fusion versus a coal or uranium (fission)?
Fusion: use of tritium, activation of vessel
Coal: Carbon dioxide production, other fallout
Uranium: long lived radioactive waste
Jacob’s Ladder Controlling Lightning – Teacher resources
Full Transcript
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Hi, I’m Phil Dooley and today I’m chatting with plasma physicist Peter
de Vries. Thanks for coming along Peter.
Hello Phil.
Thanks for joining us. What have you brought along with you?
Well this is a Jacob’s ladder
Ah, what does it do?
It makes plasma.
It makes plasma! A plasma physicist’s favourite toy! So show me…
Wow it’s pretty easy! Flick of a switch and you have got plasma! OK so
what’s going on here?
Well first of all you can see these two copper plates, we apply a very
high voltage over them, so there’s a difference of about 10 000 Volts
between the two plates, and that ionizes the air in the small gap at the
bottom. And this creates a plasma, an arc, and this arc rises to the top,
where the gap gets too big in order to sustain the arc.
So why does it go up?
Because of course the plasma gets hot and hot air goes up.
Ah, hot air rises, yeah, OK. It’s quite obvious really. So is this a
lightning strike?
Yeah that’s right, only a lightning strike will be of course over a much
larger distance with a much higher voltage; this is a lightning strike over
a very small gap with a lower voltage, but in this case still about 10 000
volts.
Right, so the easiest thing to do obviously would be to get it really close
together, is that right?
That’s what you would think. But if you make the gap too small it will
not work. If you make the gap too big it will also not work.
Ah OK, so you gotta get it just right.
Just right.
Why is that?
Most people probably would think that you would need a voltage in
order to ionise, to create the plasma you have to pull the atoms apart. So
you have to pull the electrons in one direction, and the ions in the other
direction. But that is not the only process that ionizes the plasma. Each
electron is accelerated in the electrical field and collides with the other
atoms, and creates more ion and electron pairs.
Right, so actually end up with a whole lot of them all flowing together, I
mean I guess that’s why it’s such a big… that’s more than one electron
we can see flowing backwards and forwards.
What you get is an avalanche process.
So is this the same thing as goes on inside JET?
Exactly. Not identical, but very similar
Can you explain it to me on the diagram over here?
Yeah, let’s go there
So, where in JET is the gap – where we had a spark on the machine.
Yep, that is what you would think. JET as you can see is a torus, and we
make the plasma in this toroidal shape. The gap should be infinite
because that’s where we want to make the plasma – we have an infinite
Jacob’s Ladder Controlling Lightning – Teacher resources
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gap. But that is of course not the reality. The plasma, or the particles, are
confined by this toroidal magnetic field that we have. But this is not pure
toroidal so the magnetic field lines wobble a little bit. In the end they
will just go around and around, and after several kilometers we just hit
the wall. That is basically the gap we have, so we have a gap of several
kilometers.
So you must need huge voltages for that – like lightning you know –
millions, billions of degrees ( should be “volts”).
You would think that, because like you saw the Jacob’s ladder we
needed 10 000 volts for about half a centimeter, but of course we don’t
break down in air. Here we break down in pressures that are about a
million times less than atmospheric pressure. So a lot less – so a larger
gap, but a lot less pressure, so therefore you can break down at only a
few volts, only a few volts.
So is this to do with the collisions that we were talking about before?
Cos there is less pressure it travels further before it collides.
The electrons will collide in this case, instead of several milli- er micro
metres they will travel several metres before they collide. Of course it
also takes several kilometers before the electrons will meet the anode,
which is the wall here.
You still haven’t told me how we actually… wha, what the gap is…
where… Are there electrodes in here?
You would think like if I stick an anode and a cathode in then I can make
the plasma, and I need that, but of course you only get an arc in between
those two points, and you want to get it to go around toroidally.
..travelling…?
We apply transformer action. And in this central bit here – in the middle
- is a very big coil and on that coil we apply a voltage so that there is a
change in flux, and to compensate for that there will be another voltage
going around in the torus, and that voltage…
So the gas itself becomes the secondary coil
That’s right
So once that’s happened then we’ve got plasma, we’re away, we’re
doing fusion, aren’t we!
Not, not completely yet. I mean like we end at the end of this breakdown
phase, after we have had the avalanche process, with a plasma which is
probably 100 000 amps, and temperatures of something like several tens
of electron volts, er hundreds of electron volts, so a million degrees.
Right
From that point we have to further increase the plasma current in JET to
several million amperes, and also the temperature needs to go even
higher to something like 100 million degrees.
You make it sound easy.
It is!
Thank you for your time today, it’s been very informative
Jacob’s Ladder Controlling Lightning – Teacher resources
28 Countries signed to an agreement to work on an
energy source for the future. EFDA provides the
framework, JET is the shared experiment, Fusion energy
is the goal.
www.efda.org