Although these coils provide good stability, they do not enclose a large volume of plasma. They can, however, be used to stabilize a large volume of plasma attached to them. A series of large machines called tandem mirrors was built at Livermore with a long region of uniform B-field, which has neutral stability and is stabilized at the ends with yin-yang or baseball coils. One of these, the TMX, is shown in Fig. 10.25. The end coils of these machines became more and more complex as each difficulty was overcome. Intense heating produced enough density in the base­ball coil to stabilize the main plasma in the weaker central region. Thermal barriers used electrostatic potentials to keep the plasma hot in the baseball coils. Sloshing ions were used to shape these potentials. Circularizer coils matched the flattened plasma in the baseball coil to a round one on either side. Anchor coils with higher field were the final plugs at the ends.

The successor to TMX was to be the MFTF-B installation, whose size can be appreciated in Fig. 10.26, which shows one of the yin-yang magnets being moved using the old Roman method of rolling logs. In spite of an earthquake occurring while the coil was being lifted into place, the machine was finished just in time for the entire mirror project to be canceled, much to the dismay of its leader, Keith Thomassen, and, of course, Dick Post.

The 27-m long Gamma 10 machine at Tsukuba, Japan, however, continued to operate and has improved confinement by increasing the potential barrier confining the ions [24]. Instabilities have also been eliminated by producing electric field shear [25]. Results from tandem mirrors, however, pale compared with those from toruses. Densities peak at 4 x 1018 m-3 (4 x 1012 cm-3), ion temperatures at a keV or

two, electron temperatures around 250 eV, and energy confinement times of order 10 ms. In addition, control of electric potentials sometimes requires plasma contact with conducting walls. Though the present state of the art on magnetic mirrors does not suggest their reactor relevance, they may be useful for other tasks that do not require net energy output. These include creating plasmas for transmutation of nuclear wastes or energy production in fission-fusion hybrids. First and foremost, however, is the proposed use of mirror machines as economical neutron sources for materials testing, as described in Chap. 9. Such a machine burning D-T fuel would produce 2 MW/m2 fluxes of 14-MeV neutrons over a sizable area using only 200 g of tritium per year [26].