I've been following articles about fusion recently, and with the ITER project mostly. I have noticed that their view point is they need to build a large scale reactor in order to achieve their goals.

I believe their goal is to have the a 10 x power output over consumption. From 50 MW to 500MW.

So what does size of the reactor have to do with an improved output ratio ?

  • $\begingroup$ The best case scenario would be limited by diffusion, which can be made small in a low density collision-less plasma but not at fusion plasma densities. However, diffusion is not even the main problem in magnetic confinement devices, stability is. The impressive list of identified distinct plasma instabilities in en.wikipedia.org/wiki/Plasma_stability should give you an idea what machine designers are up against. Add to that the heat loss problem mentioned by Maxim and then you can see why a real world Tokamak reactor is such a monstrous device. $\endgroup$
    – CuriousOne
    Oct 17, 2014 at 2:25
  • $\begingroup$ see physics.stackexchange.com/questions/28971/… $\endgroup$
    – craq
    Apr 29, 2015 at 15:42
  • $\begingroup$ it's like trying to squeeze jell-O with rubber bands, the more the better. $\endgroup$
    – ntno
    Jul 25, 2015 at 18:02

2 Answers 2


Very high temperature, on the order of 10 keV (100 million degrees Kelvin), is needed for fusion reactions to start to happen at appreciable rates. However, in magnetic fusion devices (tokamak, stellarator) the transport of heat across the plasma (mainly due to plasma turbulence) causes heat losses. Making the system larger allows increasing the heating power (due to fusion reactions, plasma current ohmic heating, external devices launching heat into plasma) while the rate of heat loss decreases with size, so a higher temperature can be achieved for a larger system.


Aside from plasma stability issues : the power generated goes up as the cube of the dimensions, whilst some of the losses (which need to be made up by heating power input) will go up as the square of the dimensions, so the ratio gets better as size grows.


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