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
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