During the mirror hiatus of the last two decades, however, new ideas have emerged that revive the possibility of mirror reactors. The yin-yang and other end coils of tandem mirrors have large magnetic stresses because of their twisted shapes. The new idea is to make mirror machines completely axisymmetric, using only simple circular coils. The feasibility of this was proved by Gas Dynamic Trap experiments in Novosibirsk, Russia [14]. The mirror field can be made extremely strong, creat­ing a large mirror ratio (as large as 2,500), thus reducing the size of the loss cone. A schematic of this is shown in Fig. 10.27. It looks like a large plasma with a pin­hole leak at each end, but the pinhole is not in real space but in velocity space.

The Gas Dynamic Trap produced 10-keV ions with a peak density of 4 x 1019 m-3 (4 x 1013 cm-3) and electron temperatures of 200 eV. The beta value was 60%, com­pared with only a few percent in tokamaks, since only a weak central field is needed to contain large-orbit ions with mainly perpendicular energy.3 In mirrors, neutral beams are used to inject ions, and no energy is wasted in heating the electrons. The machine is pulsed for only 5 ms, and the confinement time is only a millisecond or so. Electric fields are produced by applying voltages to different parts of the walls where the field lines end.

In an axisymmetric tandem mirror, the complexity of the stabilizing coils is gone; the circular coils are easy to make. How, then, can the plasma be stable? It turns out that the outside plasma beyond the mirrors can play an essential role. There the field lines have favorable curvature, bulging inwards toward the plasma. The stability there can overcome the instability driven by the bad curvature at the ends of the central region. It turns out that the density in the outside region does not have to be very high for this to happen as long as the plasma diameter there is large. One can shape the outside field with large coils, of which one is shown, so as to optimize the stabilizing effect [27]. A “kinetically stabilized tandem mirror” machine has been proposed [26, 27] to test this principle. That machine, shown in Fig. 10.28, uses multiple axisymmetric mirrors and injection of ions into the diverging region to improve stability.