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Results from a series of experiments are described which show that hot, reduced-density channels in the atmosphere usually cool by a process of turbulent convective mixing. Five different types of channels were created: (a) by the interaction of a pulsed CO2 laser with aerosols in the atmosphere, (b) by electric discharges in the atmosphere, (c) by laser-guided electric discharges in the atmosphere, and (d) and (e) by the absorption of CO2 laser radiation in nitrogen doped with sulfur hexafluoride. For channels in which the energy deposition was almost cylindrically symmetric and axially uniform, (e), the rate of cooling, after reaching pressure equilibrium, was within an order of magnitude of thermal conduction. But for channels in which the energy deposition was asymmetric and/or axially nonuniform, the rate of cooling was typically one thousand times faster than thermal conduction (for channels whose radius at pressure equilibrium was ∼1 cm). These channels were seen to be turbulent and to cool by mixing cold surrounding air into the hot channel. Such turbulence has been explained by Picone and Boris [Phys. Fluids 26, 365 (1983)] in terms of a residual vorticity that is caused by the noncylindrical energy deposition. A simple empirical formula is deduced relating the rate of cooling (growth of channel envelope) to the radius of the channel at pressure equilibrium and to the ambient sound speed, which indicates that the effect of vorticity/turbulence saturates for variations in the energy deposition of greater than about 2 to 1.

We report the results of a unique study of the lightninglike phenomena that were seen to accompany the MIKE shot of operation IVY on October 31 1952. MIKE was a thermonuclear surface burst yielding 10.4 MT, which took place on Enewetak Atoll. During the period of approximately 10 ms after detonation, five discrete luminous channels were seen to start from the ground or sea surface at a distance of approximately 1 km from the burst point and to grow up into the clouds. We have reexamined the original photographic records of IVY‐MIKE, obtaining effective brightnesses (optical powers per unit length) for the luminous channels at different altitudes as functions of time. The absolute calibration for the MIKE records was deduced by comparison with the photographic records of other events of that era, laboratory measurements of film sensitivity, and use of atmospheric transmission data taken just prior to the MIKE event. Errors in this analysis lead to an uncertainty of a factor of ∼2 in the brightnesses of the luminous channels. In the laboratory we have used laser‐guided electric discharges to create long (100 cm), arclike plasma channels to simulate the observed luminous channels and to allow determination of an empirical relation between the brightness of the channel and the electric current flowing in the channel. These laboratory discharges had peak currents up to 100 kA and periods of ∼2 ms. Spectroscopic analysis showed that the luminous channels consisted primarily of normal air plasma with typical ground‐level contaminants. Photographic studies showed that these long‐duration discharges are unstable to the m = 1 magnetohydrodynamic (MHD) instability and become severely distorted in less than 1 ms. By direct comparison of the luminous channels seen at MIKE and the laboratory discharges, we deduce: (1) the peak current in the prominent (brightest) channel at MIKE was between 200 and 600 kA. Here the most likely value of the peak current was 250±50 kA, but potential systematic errors in the film calibration and the comparison of MIKE and laboratory data make higher currents possible. (2) The rapid decline in the brightness of the luminous channels seen at MIKE is caused by a combination of the effects of the MHD instability, which eventually leads to a broadening of the current‐carrying channel, and channel cooling by turbulent convective mixing.

A pure, fully ionized, D 2 plasma of 2x 10 19 ions, generated from free-falling D 2 pellets with use of a 100-J Nd-glass laser pulse followed by a 1000-J C0 2 laser pulse, is used to fill ((3 = 1) a cusp-configured magnetic field. The width of the plasma escaping through the ring cusp is determined by laser scattering, chamber wall light and chemical deposits, and plasma lifetime. The ring cusp is an ion gyroradius wide, in contrast to recent experiments that have indicated a considerably narrower cusp width.PACS numbers: 52.55.KeThe cusp geometry was one of the earliest magnetic bottles proposed for plasma containment. It has the virtues of simplicity, axial symmetry, and complete magnetohydrodynamic stability for a /3= 1 plasma. However, its usefulness is limited by the escape of plasma through the ring and point cusps, and the size of these escape holes has been the subject of controversy. Cusp containment research up to 1971 has been reviewed by Spalding 1 and from 1971 to 1977 by Haines. 2 In summary, the cusp hole size is predicted to be of the order of an electron gyroradius, r e9 if electric fields are allowed to exist in the plasmamagnetic field interface, and of the order of an ion gyroradius, r i9 if electric fields are not allowed. In the early experiments of Allen and Spalding, 3 the hole size was estimated to be of the order of an ion gyroradius. This result remained unchallenged until 1974, when Kitsunezaki, Tanimoto, and Sekiguchi, 4 using a laserproduced, /3= 1, D 2 plasma, claimed the cusp width to be of the order of a hybrid gyroradius (r { r e ) 1/2 .