Are mini fusion power plants possible?
Lockheed Martin’s compact reactor concept / fusion drives for aircraft and trucks?
Building a small, transportable fusion power plant has long been a dream of fusion researchers. In the course of their research, however, it became clear that a functioning power plant has to be of a certain minimum size. Nevertheless, there are occasionally renewed attempts (see “The fusion upstarts”, in Nature, Vol. 511, 14/7/2014, p. 398 ff.). IPP scientists Professor Sibylle Günter and Professor Karl Lackner explain why also the latest version proposed by US technology concern Lockheed Martin might well remain a dream:
The patent applications for the device proposed by Lockheed Martin do not involve a really new concept, but combine the known concepts of a magnetic cusp and a magnetic mirror. Both are impaired by the fact that charged particles can escape along the magnetic field lines out of the confinement region. This leads to an intolerable energy loss, because it is primarily the fast, hot particles that get lost first. Nor does it help here, as proposed, to link several cusps behind one another or combine them with magnetic mirrors.
What is envisaged is incorporating coils in the vessel, i.e. inside the plasma. This needs connections to the outside and fixtures in the plasma vessel. Hot plasma particles from the core of the device would thus come into direct contact with these fixtures. The fundamental idea of magnetic confinement, however, is precisely to keep the high-energy plasma particles in the core moving along the magnetic field lines at always the same volume without impinging on material walls. Otherwise the plasma cools down very fast. One solution here would be superconducting coils levitating in the vessel without support, this leaving, however, the above energy loss problem: The configuration proposed is not suitable for confining hot plasmas.
Furthermore, the coils inside the plasma vessel have to be shielded not only from the surrounding hot plasma, but also from the neutrons produced in the fusion process. With superconducting coils, at least 80 centimetres of shielding around each coil is needed. This does not accord with the power plant size envisaged.
All of these problems have been resolved by the tokamak and stellarator concepts pursued today. Nevertheless, it is not possible to build small, transportable power plants. This is because attaining a positive energy balance, i.e. producing more fusion power than needed for heating the plasma, calls for extremely good thermal insulation of the plasma, viz. about 50 times better than styropor. In a power plant a temperature in the plasma core of 100 to 200 million degrees is needed, while at the walls no more than 1,000 degrees is tolerable. Such large temperature differences in the plasma drive turbulent flows that mix hot and cold regions with one another, i.e. impair the thermally insulating effect of the magnetic field. This has to be compensated with a larger volume. Here it is the size of the temperature gradient that determines the turbulent flows and hence the minimum size of a power plant. How a positive energy balance is to be achieved with the compact version propagated by Lockheed Martin is not even remotely mentioned in the patent applications.