What shape is the JET experimental fusion reactor?

doughnut-shaped chamber called the tokamak. This is where the fusion reactions take place, within hot plasma containing deuterium and tritium atoms.

The shape of JET experimental fusion reactor is the 'D-shaped' vacuum chamber.

Explanation:

JET, "Joint European Torus" is World's biggest magnetically compact plasma physics experiment, in Oxfordshire, UK. The tokamak design paved way to the future nuclear fusion grid energy.

JET was the first tokamaks designed to use this D-shaped chamber. The initial construction of this chamber was purely based on safety factor, but later it was noted that the system was built mechanically to reduce the net force across the chamber which forces the torus towards middle of major axis.

To support the forces, magnets around the chamber should be curved at top and bottom and less on inside to support the acting forces leads to oval shaped chamber. The D shape of the reactor has the dimension of about 2.5 meters in width and 4.2 meters in height with the total of 100 cubic meters of volume for the plasma.