Both the electrostatic ion acoustic and ion cyclotron waves can be unstable to field-aligned currents comparable to those expected in the auroral zone. However, for a wide range of conditions, the ion cyclotron wave is unstable to the smaller current. In this paper, both instabililies are studied in single and multi-ion plasmas. Profiles of critical current versus height indicate that, as they flow through the topside ionosphere, field-aligned currents of 10 •-xø electrons/tin 2 sec (at 200 km) will destabilize various ion cyclotron waves. part, is the flow of field-aligned currents between the ionosphere and magnetosphere. Clearly, the effective resistance of this current path is an important geophysical parameter. In the neutral and Coulomb collision regions, this resistance can easily be calculated. However, the longest part of the current path still within the ionosphere (i.e., the topside) is collision free. While it is tempting to call the specific resistivity zero in the topside, experience with laboratory experiments indicates this to be seriously misleading, since current densities exceeding ocr-'rain thresholds are unstable to microscopic plasma wave growth.The nonlinear development of such instabilities leads to turbulent or 'anomalous' resistivity.Our eventual goal, elucidation of the anomalous resistance of ionospheric field lines, involves several distinct steps. First, we must ask which of the many wave modes avai!able is generally unstable to the smallest current for a given he•.ght and then, for this wave mode, find the particular frequency and wavelength unstable to the smallest current. Seco.nd, this procedure must be repeated for each height range. This tells us where and when to expect instability. Finally, the nonlinear saturation of the various instabilities must be calculated, and the specific anomalous resistance must be estimated and integrated over height. In this paper, we concentrate upon the first two steps in this chain by surveying certain current-driven instabilities in the ionosphere. Furthermore, we restrict ourselves to the collision-free ionosphere above the F maximum.We consider only instabilities due to currents carried by the cold ionospheric plasma, ignoring any effects due to a hot component of magnetospheric origin. It is easy to show that the hot component will not appreciably affect the cold plasma current instabilities' taken up here, because of the low density and high thermal speeds of the hot component. Instabilities of the hot component alone are a separate topic which will not be pursued. Secondly, we assume the plasma velocity distributions to be drifting isotropic Maxwellians, even though this may be somewhat unrealistic for the collisionless far topside ionosphere. However, in the absence of specifically measured distribution functions, the Maxwellian assumption permits a calculationally convenient exploration of the parametric dependences of the various instabilities. 3O55 KINDEL AND •ENNELKinetic theory contains a variety of instabilities due to field-al...

Analytic theory of the linear and nonlinear behavior of the one-dimensional Brillouin and Raman scattering instabilities is given. Results are presented for the problems of an infinite homogeneous plasma and of a finite inhomogeneous plasma. Nonlinear fluid equations can predict backscatter energies the order of the incident laser energy; however, the size of the interaction region and nonlinear effects on the excited electrostatic wave are very important in determining the amount of backscatter. In many cases of contemporary interest for a high power laser incident on a target plasma, the latter effects can play a crucial role in reducing backscatter to a tolerable level.

A detailed theoretical and simulation study of resonant absorption in a hot plasma is presented which isolates the behavior of the plasma for times short compared to an ion response time. The extent to which an electron fluid model can describe the absorption process in the kinetic regime is discussed. At high intensities the absorbed energy is observed to be deposited in a suprathermal tail of electrons whose energy varies approximately as the square root of the incident power. The density profile modification due to the ion response to the ponderomotive force is also discussed.When an electromagnetic wave is obliquely incident on an inhomogeneous plasma and polarized in the plane of incidence, it is well known that it can be absorbed resonantly by linear mode conversion into an electron plasma wave. ' ' This process, known as resonant absorption, has important implications for laser target experiments and microwave laboratory experiments. ' Most theoretical work has been done for a cold plasma, '" ' while warm-plasma calculations have been either incomplete' or incorrect. 'For gradient lengths long compared to the wavelength of light or koL»1 (where ko is the incident free-space wave number and L is the density scale length), computer simulations in a hot plasma, with fixed ions show that the absorption coefficient is virtually unmodified from the cold-electron case. Theoretical calculations based on a fluid description which agree with these computer simulations indicate that the absorption coefficient is virtually unmodified for temperatures up to 100 keV. At low' intensities these theoretical calculations predict the field structures seen in simulations, while at high intensities a nonlinear dissipation must be added to obtain agreement. This nonlinear dissipation is required at high intensities to account for the acceleration towards the low-density region of the plasma of a small number of electrons to very high energies. To describe resonant absorption in a hot plasma, we combine the linearized electron-momentum equation with Maxwell's equations. An adiabatic pressure law is assumed for the high-frequency electron motion, ion motion is neglected, and the fields are assumed to vary as e' '. V~E = 4~en"V x E = i (~/c) B-, VxB =(4n/c)7 -(i~/c) E, e'in, E ieT, Vn" J= . + yVn -n m(tvvv'v) m(tvviv ) ' ' v, )'where n, is the background plasma density; T, , m, and e are the electron's temperature, mass, and charge, respectively; c is the velocity of light; and y is the usual ratio of specific heats and is chosen equal to 3. In factoring the damping in the electron-momentum equation a different damping rate appears in the electric field term than in the electron pressure term. The significance of this phenomenological damping is discussed below. Combining these equations we obtain the general steady-state wave equation for E:where~~=47)noe'/m. ln particular, we consider the case of a slab of plasma with no =no(x) only and the electromagnetic wave obliquely incident on this slab, with the electric field polarized i...

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The self-focusing of intense electromagnetic waves in a very underdense plasma is studied in computer simulations. Relativistic self-focusing occurs initially. This is followed by "ponderomotive blowout" and filamentation at the edge of the channel walls. The self-focusing is more intense for resonant doublefrequency than for single-frequency illumination. Relativistic self-focusing is illustrated in Fig. 1(a) where the contour plot of the incident field E, is shown at roi, t =330 for fixed ions. The intensity on axis is 3 times larger on the right than the left. By this time the incident beam has already collapsed down to a width of = Sc/rot, in a distance of 30c/rot, . These compare favorably to the predictions of Sprangle, Tang, and Esarey' using cylindrical geometry for an incident power an order of magnitude above the threshold power. Their ana 1298

inated by reducing the source bias voltage by an amount equal to the potential mismatch, i.e., the drift energy 8 eV. The dispersion when F 0 = 18.4 V (a drift is still evident for y 0 = 19 V) is shown by curve b in Fig. 3. I obtain a good fit to the data for u D = 0, T=0.54eV, and n 0 = n B {0.1 kG) (charge collection yields n 0 = 2.0x 10 6 cm" 3 ). In this case, no current is observed on the isolated tubes. The axial drift and the radial transport are reduced by adjusting the source parameters to conform with an allowable plasma equilibrium, in particular, with a plasma density below the Brillouin limit.In summary, we have produced an ion plasma in which ion Langmuir waves are observed. The density constraint imposed by the Brillouin limit is experimentally verified. Near this limit, the plasma waves have dispersion properties similar to those of a weakly magnetized neutral plasma.

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