For several years, numerical simulation of intense microwave and laser beam propagation in the atmosphere has been conducted at Lawrence Livermore National Laboratory. For very short pulses of 20 ns or less, full-wave electron fluid computer codes have investigated atmospheric propagation, as well as propagation in low-pressure, air-filled waveguides. These one- and two-dimensional codes solved time-dependent equations for electron number, momentum, and energy conservation self-consistently with Maxwell`s curl equations. Because of machine limitations, these codes, which resolve variations within a wave cycle, have been impractical for pulses longer than several hundred cycles. A one-dimensional, time-harmonic, envelope electron fluid code has been developed for calculation of long-pulse, cascade ionization microwave and laser beam effects in the atmosphere. In this investigation, the authors consider envelope code calculations for incident pulses from 0.1--100 ns in the laser wavelength regime for propagation in the lower atmosphere. Both CO{sub 2} and neodymium glass laser wavelengths are addressed. Square pulse breakdown electric field thresholds are calculated and compared with analytic predictions from the literature. Gaussian envelope thresholds are also calculated. Propagated and tail-eroded electric field waveshapes and electric density and energy profiles for several incident amplitudes, waveshapes, and pulse lengths will be presented.

Summary form only given. Two one-dimensional computational methods have been used for the investigation of the ionospheric compression of plane wave pulses. One method is a finite difference computer code which spatially integrates the dispersive effect of the electron density on the pulse over a specified path. The second method is a computationally efficient numerical convolution algorithm which is especially applicable to long initial pulses. The finite difference code is used to investigate the compression of relatively short pulses (< 50 ns) with different waveshapes and chirp profiles over various atmospheric paths. An 18-ns-long square pulse with an inverse square root chirp at 90 km produces an amplitude enhancement of 3.5 at 450 km for a perfectly vertical trajectory along an extended earth radius. In contrast, the convolution algorithm is used for the investigation of much longer chirped initial pulses. A properly chirped pulse with a bandwidth of one decade can be compressed temporally by a factor of about 40 into a nearly monopolar pulse.

In propagating significant distances in the ionosphere, a short microwave pulse will undergo strong dispersion from the ambient electrons it encounters. The pulse will be extended in time with dramatic reductions in peak amplitude and power. This undesirable dispersive reduction may be counteracted by transmission of a frequency-chirped pulse. In principle, with the proper initial pulse and a sufficient knowledge of the ionospheric electron density distribution, compressed pulses are possible at many points in the ionosphere. We use two computational methods to examine ionospheric compression of plane-wave pulses with several waveforms.

After traveling a sufficient distance through a Lorentz medium, the frequency components of a short pulse become spatially separated along the direction of propagation, with each component traveling at its own energy velocity. For the case of radio and microwave pulses propagating in the earth's ionosphere, the dispersion process is nearly time-reversible, since collisional losses are very small. Thus, any desired final waveform can be obtained, in principle, by appropriate frequency and amplitude modulation of the initial waveform. In practice, obtainable degrees of pulse compression will be limited by source bandwidths and modulation control, distortion of broadband signals by antennas, and uncertainties and fluctuations in the propagation medium. We have investigated computationally the possibilities and practical limitations of compressing radio and microwave pulses in the ionosphere. The calculations were made using a time-domain formulation for plane waves, which is essentially a Green function method based on the existence of an asymptotic expression for the dispersion of a short pulse. We have also made some calculations of pulse compression for oblique trajectories, where diffractive effects must be included, and for the case of a focusing antenna.

Summary form only given. A pulse envelope formulation for computationally modeling electron layer generation in laboratory chamber experiments in low-pressure (0.1-10-torr) air irradiated with long-pulse (100-600-ns), 2.856-GHz crossed microwave beams has been previously described (see D.J. Mayhall et al., 1989). A second calculation has been done for a linearly rising incident pulse, which reaches a plateau of 0.159 MV/m at 80 ns. In this case, five layers also occur, but their character and timing are much more like those of the experimental layers than are those in the first calculation. These layers form from a number of coalescing blobs. Flutes form on both sides of the first layer. Blobs in the first and second layers coalesce. The second and third layers consist of isolated blobs. The calculated appearance times vary from the experimental by 6% to 26%. When the plateau value of the incident pulse is reduced by 30% to 0.111 MV/m at 80 ns, the appearance times vary from 70.6 to 84.9 ns and differ from the experimental by 5-20%. This calculation thus gives the best agreement with experimentally observed appearance times

Detailed laboratory measurements and theoretical modeling relevant to the production, geometrical description and decay of microwave-induced air ionization for an upper atmospheric RF reflecting layer are reported. It is found that breakdown thresholds are adequately predicted by fluid models and simplified scaling models with refinement by kinetic models being important at lower pressures. Repetitive pulse sustainment has been demonstrated to be straightforward with a commensurate reduction in sustainment power levels. However, establishment of a convenient breakdown geometry for specular RF reflections, other than a single layer in a crossed beam geometry, was not obtained. Detailed density decay measurements qualitatively support estimates of decay times and indicate ionization dwell times of tens of milliseconds. Chemistry studies indicate three N{sub x}O{sub x} species will be produced. Further study of these collateral reactions is required to establish whether adverse atmospheric consequences can result. However, large N{sub x}O{sub x} production does not appear as a concern for relatively small, low repetition rate, proof of concept atmospheric experiments. A realizable proof of concept experiment is found with simple optimization criteria which is corroborated by laboratory measurements and theoretical simulations. Tail-erosion appears as a potentially severe limitation in atmospheric experiments beyond the proof of concept level, more » suggesting use of multiple-beam systems. 20 refs., 18 figs. « less

High power microwave radiation is under consideration for use as an ionizing agent to form a radio reflecting region in the stratospheric/mesospheric altitude range. The ionization region is referred to as an Artificial Ionization Layer (AIL) or Artificial Ionization Mirror (AIM). An AIL could conceivably avoid constraints of reflection from the ionosphere: (1) limitation to {approximately}30 MHz reflection frequencies, (2) erratic variations due to it's natural origin, and (3) reduction of a blind skip'' area for high frequency reflections. Work in the United States in this area began with the introduction of the concept in the Air Force Forecast II review of new technologies during 1986. Soviet work began earlier in the 1970's and continued into the 1980's with a benchmark paper by Gurevich appearing in 1980. The experiment goals of AIM-II were to (1) measure ionization thresholds for single and multiple layer formation, (2) characterize the geometry of layer formation, and (3) determine density and effective collision rates in the ionization layer. The AIM-II research campaign, which was performed in the same experimental geometry, was dedicated to repetitive pulse operation to explore the issues surrounding prolonged or sustained'' AIL production that would be necessary in many application scenarios. Themore » primary issues to be examined included: (1) reproducibility of breakdown geometry, (2) utility of increased ionization background from previous pulses in reducing the power threshold for breakdown by subsequent pulses, and (3) ionization persistence or decay.« less

Summary Form only given, as follows. Laboratory chamber experiments in low-pressure (0.1-10 torr) air with long-pulse (100-600-ns), 2.856-GHz crossed microwave beams formed by reflection of a single beam from an inclined metal plate have shown that multiple, luminous electron density layers form sequentially in time toward the source of the incident beam. To model this experiment, a two-dimensional, long-pulse electron fluid computer code has been developed. Initially, the electron evolution in the rectangular, x-y space transverse to the driving E/sub z/ microwave field is described by a convective continuity equation. The convection is approximated by electric drift and thermal diffusion terms in the x and y directions. After the peak electron density has increased to a few percent of the critical density (10/sup 17/ m/sup -3/ at 2.856 GHz), the initial electrostatic equations are replaced by a new set. This features a nonconvective continuity equation, in which drift and diffusion are ignored but ionization is retained, and a harmonic, slowly varying envelope approximation to the wave equation for the driving field. This wave equation appears as two coupled diffusion equations for the real and imaginary parts of the driving field. Calculations have been carried out at 1-torr pressure with a linearly ramped pulse, which reaches a plateau value of 0.159 MV/m at 80 ns.<>

At very high power levels pulsed microwave beams can generate air-breakdown plasmas which may limit the fluence that the beam can transport through the atmosphere. Conventional air breakdown is an avalanche process wherein free electrons, driven by the microwave fields, produce ionization through collisions with air molecules. Propagation of a beam is affected when the plasma electron density approaches the critical density for the particular microwave frequency. The rate of growth of the plasma depends on the competition between the ionization probability and electron loss processes such as attachment and diffusion. The physics of the avalanche process is reasonably well understood, and fluence limits can be fairly accurately predicted, so long as there are free seed electrons to initiate the breakdown. At sea level and low altitudes, seed electrons are, in fact, expected to be fairly rare, and air breakdown, and the consequences for beam propagation, must be treated as a statistical problem; the effective fluence limit may be much greater than would be predicted on the basis of conventional breakdown thresholds. The statistical effects are currently being investigated. 13 refs., 2 figs.