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 ?
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.
