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

  • Thermal shield repair | Where are we at?

    Fitting the vacuum vessel sectors like a jacket, lining the inner wall of the cryostat, or covering the sides of vertical coil gravity supports, ITER's thermal [...]

    Read more

  • Assembly prep | Reviewing plans for in-vessel installation

    A thorough review of all 'in-vessel' assembly scope was organized by the ITER Machine Assembly Program in early February, with the active participation of senio [...]

    Read more

  • Image of the week | Last measurements before campaign

    In order to precisely identify the bevel regions that need to be rectified, metrologists from the SIMANN (SIMIC-Ansaldo) consortium are performing ultra-precise [...]

    Read more

  • Neutral Beam Test Facility | After upgrades, SPIDER testbed set to restart

    After a two-year shutdown for upgrades, the SPIDER testbed at the ITER Neutral Beam Test Facility in Padua, Italy, is preparing for commissioning and operation. [...]

    Read more

  • ITER Research Plan | Jointly preparing a new blueprint

    As part of work underway to update the ITER Project Baseline, a group of experts nominated by the Members met in February to evaluate the new blueprint for achi [...]

    Read more

Of Interest

See archived entries

Fusion explained

Of tokamaks, hummingbirds and pizza ovens

A common but oversimplistic way of describing fusion is claiming that it's all about reproducing the nuclear reactions that occur in the core of the Sun. In a presentation to non-specialists on 18 January, ITER Director-General Pietro Barabaschi explained why, supposing we could mimic what happens in the Sun, such an undertaking would be totally "useless." The reasons he gave, and the images, parallels and comparisons he used, are what made his presentation particularly illuminating and enjoyable.

For each complex notion of fusion physics ITER Director-General Pietro Barabaschi proposed an analogy. A pile of manure for the Sun's power density; blankets on a cold winter night for energy confinement time (but that's without counting on turbulence); a wood-fired pizza oven to illustrate the issue of size vs power density. (Click to view larger version...)
For each complex notion of fusion physics ITER Director-General Pietro Barabaschi proposed an analogy. A pile of manure for the Sun's power density; blankets on a cold winter night for energy confinement time (but that's without counting on turbulence); a wood-fired pizza oven to illustrate the issue of size vs power density.
In terms of "volumetric power density," a key concept when dealing with energy issues, the Sun and Sun-like stars are very poor, very slow, and very inefficient systems. One cubic metre of the Sun's core generates about as much power as the chemical reactions inside an equivalent volume of manure—something in the range of 250 to 300 watts per cubic metre (W/m3), or much less than a human body (1 kW/m3). Hummingbirds, which hold the record of power density among living organisms, generate 50 kW/m3—that is, 200 times the performance of the proton-proton reactions inside the Sun. However, poor performance as a fusion reactor combined with giant size (333,000 Earth masses) is what makes the Sun what it is: a giant furnace that has been dispensing light and heat to for 4.6 billion years and will continue for another 5 billion years.

Provided one finds pertinent images and analogies, fusion on Earth can be relatively easily explained to non-specialists (with the notable exception of a few quantum phenomena, which according to the ITER Director-General "no one really understands"). Take the probability of deuterium fusing with tritium in a fusion machine, measured against that of hydrogen nuclei in a proton-proton chain. If the probability was a section (and in fact it is, it's called the cross section) deuterium-tritium's section would be comparable to that of the Moon and proton-proton the size of a virus. Energy confinement time, one of the biggest challenges in tokamak physics? Like adding blanket on a cold winter night in order to preserve body heat. (Except that there is no turbulence in bed covers, whereas there is quite a lot in fusion plasmas ... and that's a big issue.)

Then, there is the analogy of the wood-fired pizza oven which, like a tokamak cannot be too small (otherwise confinement is too weak and wood ignition will not occur) or too large (or else the surface power density would be excessive and lead to a carbonized pizza effect). Finding the right balance between size and power density is another considerable challenge that fusion R&D is facing and partly explains, from the ITER Director-General's perspective, why "fusion is taking so long."

Equations, plots and graphs were not absent from the hour-long presentation. Combined with paradoxes and amusing illustrations, it brought home to everyone present, both in the ITER amphitheatre and by remote connection, the most essential notion about fusion: not mimicking the Sun but tackling daunting issues which, helped by the progress of computer modelling in the coming decades, will make it an industrial reality.

Click here to view the recording of the presentation.

 



return to the latest published articles