Distinct Instability Research Articles

Thermalization of a beam by beam-plasma interaction

We present measurements proving the successive excitation of two distinct instabilities by electron beam–plasma interaction along a plasma column. The first, appearing near the gun, grows in space, until the beam is trapped in the wave electric field, and decays. In this process the time-averaged distribution function changes, from a δ-type distribution function, into a plateau. This new beam–plasma distribution is also unstable; and another instability grows until the beam is again trapped. Numerical calculations show that the second instability can be explained, assuming that the beam distribution thermalizes when propagating from the first instability to the second.

Low-Frequency Waves and Instabilities on the Positive Column in a Magnetic Field. IV. Comparison of Various Theories for Helical Modes Illustrated for He at Low Pressures

The relations between three theories for helical modes of a positive column in a magnetic field, namely, those of Kadomtsev and Nedospasov, of Johnson and Jerde, and Ewald, Crawford, and Self are examined, and are shown to be different approximations, valid for different orderings of ω, ωci, νi, to a general theory which includes both ion inertia and the effect of the magnetic field on the ions. A discussion is also given of the relation of these theories to the gradient-driven ion acoustic and drift instabilities of an inhomogeneous magnetoplasma in a simple planar model. An experimental study of the positive column in He at low pressures supports the conclusion that there are two distinct instabilities. The first, setting in for low magnetic field and high frequency, corresponds to the ion acoustic instability and is described by the Ewald-Crawford-Self theory. The second, setting in for high magnetic field and low frequency, corresponds to the drift instability, and is reasonably described by the Johnson-Jerde theory except at very low pressures. Their common asymptote is the Kadomtsev-Nedospasov theory, valid only for high pressures.

The Vertical Mean Wind Profile Over the Ocean for Light to Moderate Winds

Analysis of 299 wind profile observations collected at the Massachusetts Institute of Technology oceano-graphic research platform provides a detailed picture of the behavior of wind profile parameters. A plot of roughness length vs velocity suggests the existence of classical hydrodynamic phenomena such as Jeffreys’ minimum wind speed and the Kelvin-Helmholtz instability. The friction velocity appears to be, in general, a linear function of the wind velocity, except in regions where discontinuities exist. It is suggested that the air flow regime over the ocean in the wind range of 2-10 m sec−1 (measured at a height of 10 m) is best described as a region of constant drag coefficient (CD = 1.6 × 10−3) modified by the superposition of almost singular departures at a number of discrete wind speeds, apparently caused by the onset of distinct instabilities at the water's surface.

Shock-Tube Studies in Exothermic Systems

Three types of behavior are possible: (1) planar incident shocks, with reaction, may be produced and studied; (2) detonation may be initiated by longitudinal nonsteady processes; or (3) distinct transverse instabilities may appear. Case (2) occurs when the incident shock velocity is adjusted to be less than the Chapman-Jouguet velocity. Here, if a reaction occurs, it leads to nonsteady processes which may initiate detonation. Examples are presented to discuss the inherent limitations of this case, and to show that reaction kinetic processes may be studied even though nonsteady flow processes are occurring. Cases (1) and (3) occur when the shock is adjusted to be above the Chapman-Jouguet velocity, usually in diluted mixtures. Case (3) instabilities (which negate the quantitative study of reaction kinetics) are always observed when the reaction zone length becomes of the order of, or less than, the tube diameter. Case (1) behavior is observed for longer reaction zone length, even though there are also recorded cases of unstable flow under these circumstances. An acoustic theory of transverse waves is used to discuss the possible occurrence of case (1) and case (3) flows.