Enable Recite

Subscribe options

Select your newsletters:

Please enter your email address:

@

Your email address will only be used for the purpose of sending you the ITER Organization publication(s) that you have requested. ITER Organization will not transfer your email address or other personal data to any other party or use it for commercial purposes.

If you change your mind, you can easily unsubscribe by clicking the unsubscribe option at the bottom of an email you've received from ITER Organization.

For more information, see our Privacy policy.

News & Media

Latest ITER Newsline

  • Data | Archiving 20 gigabytes per second—and making it usable

    One of the main deliverables of ITER is the data itself—and there will be a tremendous amount of it to store and analyze. During First Plasma, the highest produ [...]

    Read more

  • Electrical tests | High voltage, high risk

    In the southern part of the construction platform, a one-hectare yard hosts some of the strangest-looking components of the entire ITER installation. Rows of to [...]

    Read more

  • Vacuum vessel | First sector safely docked

    It was 8:00 p.m. on Tuesday 6 April and something quite unusual happened in the ITER Assembly Hall: applause spontaneously erupted from the teams that h [...]

    Read more

  • Remote ITER Business Meeting | Virtual interaction, tangible opportunities

    While the advent of Covid-19 has not stopped the relentless advancement of the ITER Project, it has certainly prompted ingenuity in how ITER conducts its work. [...]

    Read more

  • Manufacturing | Europe completes pre-compression rings

    The French company CNIM (Toulon) has produced a tenth pre-compression ring for the ITER Project on behalf of Fusion for Energy, the European Domestic Agency. Th [...]

    Read more

Of Interest

See archived entries

Lawson's magic formula

In 1955, John D.Lawson (4 April 1923-15 January 2008) demonstrated that the conditions for fusion reactions relied on three vital quantities: temperature (T), density (n) and confinement time (τ). (Click to view larger version...)
In 1955, John D.Lawson (4 April 1923-15 January 2008) demonstrated that the conditions for fusion reactions relied on three vital quantities: temperature (T), density (n) and confinement time (τ).
In 1955 a young engineer working on nuclear fusion decided to work out exactly how enormous the task of achieving fusion is. Although his colleagues were optimistic about their prospects, he wanted to prove it to himself. His name was John Lawson, and his findings—that the conditions for fusion power relied on three vital quantities—became the landmark Lawson Criteria.

The genesis of Lawson's Criteria is simple enough—he calculated the requirements for more energy to be created than is put in, and came up with a dependence on three quantities: temperature (T), density (n) and confinement time (τ)*. With only small evolution thanks to some subtle changes of definition, this is basically the same figure of merit used by today's fusion scientists, the triple product, nτT.

The amount of energy created relies on particles colliding and fusing—the number of collisions is related to the number of particles in a certain region—thus n, the number density (not mass density) is Lawson's first criterion. This would seem encouraging for the prospective experiment, as creating high pressure is relatively easy. However there is a catch. At higher densities a process known as bremsstrahlung rears its ugly head, in which collisions between nuclei and electrons generate radiation. Bremsstrahlung can become so dominant that all the power in the plasma is radiated away; the optimum density conditions are surprisingly low, around a million times less dense than air.

Nonetheless the fusion collisions—between the nuclei—have to be at high speed. This allows the nuclei to overcome their electrostatic repulsion, and get close enough for the strong force that governs fusion to take over and stick the particles together. The speed of a gas or plasma particle is equivalent to its temperature: the second of Lawson's criteria.

Again there is a limit—if the two particles are moving really fast then the time they are in close enough proximity for fusion to occur decreases. The bremsstrahlung also increases at higher temperatures, due to faster moving electrons. The Goldilocks temperature turns out to be in the vicinity of 100—200 million degrees, a seemingly huge task in the fifties that has become a standard condition today.

* "τ" is the Greek letter tau (pronounced like "how").

Read more on the EUROfusion website.


return to the latest published articles