  The fusion created by magnetically confined plasma is a promising clean and
essentially unlimited future energy source. However, net energy generation has
not been yet demonstrated in fusion experiments. Some of the main problems
hindering controlled fusion are the imperfect magnetic confinement and the
associated plasma instabilities. Here, we theoretically demonstrate how to
create a fully confined magnetic field with the precise three-dimensional shape
required by fusion theory, using a bulk superconducting toroid with a toroidal
cavity. The combination of the properties of superconductors with the toroidal
topology makes the vacuum field in the cavity volume consisting of nested flux
surfaces, a condition for optimum plasma confinement. The coils creating the
field, embedded in the superconducting bulk, can be chosen with very simple
shapes, in contrast with the cumbersome arrangements in current experiments,
and can be spared from the large magnetic forces between them. Because the
field shape in the cavity is given by the boundary conditions in the
superconductor surface, the system will always tend to maintain the optimum
field distribution in response to instabilities or turbulence in the plasma.
This field shape is preserved even when holes are drilled in the superconductor
to access the plasma region from the exterior. We demonstrate how a
fully-confined magnetic field with the three-dimensional spatial distribution
required in two of the most advanced stellarators, Large Helical Device and
Wendelstein 7-X, can be exactly generated, using simple round coils as magnetic
sources. We argue that state-of-the-art high-temperature superconductors
already have the necessary properties to be employed to construct the bulk
superconducting toroid. The present strategy can lead to optimized robust
magnetic confinement and largely simplified configurations in future fusion
experiments.
