Proton collimators have been proposed for incorporation into inertial- electrostatic- confinement (IEC) fusion reactors. Such reactors have been envisioned as thrusters and sources of electric power for spacecraft and as sources of energetic protons in commercial ion-beam applications. An artist's concept for an IEC powered spaceship designed for round trip missions to Mars and Jupiter is shown in the figure.

A scale schematic of 100-MWe IEC Fusion-Powered Spacecraft is depicted with dimensions in meters. (Note TWDEC = Traveling-Wave Direct Energy Converter.)
An IEC fusion reactor typically contains a plasma of pure 2H or a 2H-3He mixture. Collisions among the 2H and/or 3He nuclei give rise to fusion reactions, the main energetic products of which are protons with kinetic energy ≈14 MeV and an isotropic velocity distribution. A proton collimator would collect the isotropically emitted protons and form them into a collimated beam.

A proton collimator would include (1) an electromagnet outside the fusion reactor that would impose a substantially uniform magnetic field within the reactor and (2) a pair of electromagnet coils inside the reactor, oriented to generate magnetic fields antiparallel to the one generated by the external electromagnet. The interior electromagnet coils would be positioned so that the fusion reaction would be concentrated in a region between them. The currents in the interior electromagnet coils would be adjusted to minimize the net magnetic field in the fusion-reaction region in order to avoid any adverse effect of the magnetic field on the trajectories of the 2H-3He ions that must collide to cause fusion reactions. The accessible region for the ions and electrons can be completely separated from inner electromagnet coils and support the structure, preventing bombardment damage.

The overall effect of the electromagnets would be to channel the isotropically emitted 14-MeV protons into a beam substantially parallel to the magnetic field. The collimator would also separate the 14-MeV protons from unreacted fuel ions leaking out of the reaction region. The leaking fuel constituents would be collected on plates, condensed to a gas, pumped out, and recycled to the reactor.

The collimated proton beam could be used directly for spacecraft thrust or an industrial ion-beam application. Alternatively, the proton collimator would be used in conjunction with a magnetic expander and an electron/ion separator to generate a net electric current. Another approach under consideration for space propulsion is to focus the beam on a target, e.g., a small plastic pellet, which would be vaporized and exhausted through a magnetic nozzle. Yet another alternative is to introduce the beam into a highly efficient traveling-wave energy-conversion device to extract electric power.

This work was done by George H. Miley and Hiromu Momota of NPL Associates, Inc., for Marshall Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at under the Physical Sciences category. MFS-31734.