An electric field pushes ions and electrons in opposite directions, and it makes sense that a steady electric field cannot confine a plasma. However, Bob Hirsch, who later headed the AEC’s fusion division, proposed a machine which has become popular with amateur fusioneers because of its simplicity. The device has two spherical grids one inside the other, the outer one grounded and the inner one at a large negative potential [51]. Gas is ionized between the grids, and ions are accelerated toward the center, where they accumulate and create a large posi­tive potential. Subsequent ions are repelled by this “virtual anode” and bounce

away back to the grids. They then oscillate inside the sphere and can collide with one another for an occasional fusion. This suffers from the original reason for a thermal plasma, as explained in Chap. 4. Streaming ions fuse only once in 10,000 collisions. The other collisions degrade their energies so that they no longer can fuse and eventually diffuse out of the system. Grids are OK for small experiments, but they will melt at fusion densities. Furthermore, Debye shield­ing at these densities will prevent the applied voltage from reaching the center of the holes in the mesh.

Migma

Early in the game, colliding accelerator beams were proposed, and several mig — matrons were built. With accelerators, it is easy to get ions up to the energy of the peak of the DT reaction, nearly 80 keV, or even the p-B11 reaction, nearly 300 keV. The beams are of low density, but they can be put into storage rings to circulate past the collision point many times. Elastic scattering, however, gener­ates bremsstrahlung radiation, and there are always instabilities with streaming particles. A comprehensive stability theory has never been worked out for migmas.