All present large toroidal experiments such as JET and designs for ITER are of the conventional tokamak type, i.e. shaped like ring-doughnuts (as illustrated by the outer ring in the figure). A spherical tokamak or ST (sometimes called spherical torus) has a much tighter ring shape, more like a cored apple (illustrated by the inner shape in the figure).


Inside the START machine

 

 

 

 

 

 


Neutral Beam Injector on START


This shape brings a number of advantages. One is that because it does not need to be as large as a conventional machine for the same fusion performance, it will cost less to build. Results so far suggest spherical tokamaks also have significant advantages over the conventional design in terms of efficiency.

Demonstrating the advantages

The START (Small Tight Aspect Ratio Tokamak) experiment at Culham provided the world's first experimental results on hot spherical tokamak plasmas, achieving electron temperatures of more than 10 million °C.

Keeping the plasma stable is vital for the efficiency of a fusion machine. The toroidal magnetic field (supplied by the current flowing in the central column) needed to keep the plasma stable can be a factor of 10 less in a spherical tokamak than that of a conventional tokamak carrying the same plasma current. This means a substantial gain in efficiency, i.e. plasma performance for engineering cost.

Efficiency is measured in values of the parameter ß (beta) - the amount of plasma pressure that can be sustained by the magnetic field provided by the tokamak. This is the factor which is likely to have the most influence on the cost of electricity produced by fusion power. START has achieved world record ß values - three times those achieved in conventional tokamaks.

Experiments on START have also indicated:

  • the Plasma energy confinement time (a measure of thermal insulation) is at least as good in a spherical tokamak as in conventional tokamaks of comparable size
  • START is resilient to the 'major disruptions' that conventional tokamaks can suffer, in which the plasma current terminates suddenly, causing stress on the machine structure
  • the most promising method for supplying auxiliary heating to spherical tokamak plasmas is neutral beam injection. START's world record ß values were achieved using a Neutral Beam Injector loaned by ORNL (the Oak Ridge National Laboratory, US) to heat the plasma.