Posted on: 23rd January 2013

Jacob’s Ladder is a demonstration from the former traveling exhibition Fusion Expo that creates a sustained discharge – effectively lightning – between two electrodes. This crackling spark writhing its way up the electrodes is one of the quintessential props from early sci-fi movies.

This Jacob’s Ladder is demonstrated by plasma physicist, Peter de Vries. Dr de Vries also explains how the discharge in JET – thousands of times larger – is formed.


Jacob’s Ladder – Controlling Lightning

Video Teacher resources

Host:                           Phil Dooley – EFDA (predecessor of EUROfusion)

Fusion specialist:        Peter de Vries

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.

https://www.euro-fusion.org/fusion/fusion-technology/magnetic-fields/

https://www.euro-fusion.org/fusion/spot-on-jet-operations/starting-the-plasma/

https://www.euro-fusion.org/fusion/fusion-technology/types-of-fusion-machines/tokamaks/

https://www.euro-fusion.org/fusion/spot-on-jet-operations/maintaining-the-plasma/plasma-edge/

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.

 

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 one hundred 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

 

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.

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