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FEBRUARY 1, I'f61 hydromagnetic velocity-space instability of the "mirror" type. In our experiments, using highenergy electrons, it has been observed to cause an enhanced rate of transport of particles across field lines. It may also cause other effects, such as more rapid thermalization of the plasma, or the emission of plasma radiation. These effects, or those which might arise when the instability is associated with anisotropies in the ionic component of the plasma rather than the electronic component, we have not investigated. Although it is possible to avoid the instability by controlling the plasma parameters, we have shown that it may occur in plasma compression experiments. It also seems likely that any method of creating a hot plasma which results in anomalously large anisotropies (such as the method of injecting highly directed high-energy particles), may stimulate this instability during buildup, even if the final plasma state aimed for is stable. Finally, since this instability feeds on perturbation in energy density, and is not of electrostatic origin, it should persist in the limit of very low particle densities, provided the anisotropy becomes correspondingly large. For this reason, it (or related instabilities) might occur in some astrophysical situations or in particle accelerators, even though the conditions are such that cooperative effects would normally be considered unimportant. to be published).9These conditions (of adiabaticity) are undoubtedly not essential to the growth of instabilities of this type. H. Furth and K. Neil (private communication), wheninvestigating the instabilities of relativistic electron streams, have also been able to show that the mirror instability should persist even in the limit of large orbit sizes.We have observed superconductivity in Nb, Sn at average current densities exceeding 100000 amperes/cm' in magnetic fields as large as 88 kgauss. The nature of the variation of the critical current (the maximum current at a given field for which there is no energy dissipation) with magnetic field shows that superconductivity extends to still higher fields. Existing theory does not account for these observations. In addi-tion to some remarkable implications concerning superconductivity, these observations suggest the feasibility of constructing superconducting solenoid magnets capable of fields approaching 100 kgauss, such as are desired as laboratory facilities and for containing plasmas for nuclear fusion reactions. '~'The highest values of critical magnetic fields previously reported for high current densities

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