Electron beams can be unstable to transverse modes which form small pinch structures, as first pointed out by Weibel. Beams which are not totally self-pinched are liable to this instability, so current neutralized regions near the beam head should display it. If the background response is nonresistive, filamentation is very rapid; if an axial magnetic field is present, interchange instability is competitive and the two may not be distinguishable. Beam motion is usually dominated by the resistive region, where ( omega a/c) 2>>1, which is treated in detail. Using helical particle orbits in a uniform beam of radius a, the dispersion relation and growth rates in a number of differing cylindrical configurations are found. Because all particles have a common betatron frequency a sharp particle resonance appears, which may be broadened by energy spread in the beam. This model is most useful for beams in external axial magnetic fields, which are destabilised by electron drifts.

A broad program of freezing species in threatened ecospheres could preserve biodiversity for eventual use by future generations. Sampling without studying can lower costs dramatically. Local labor can do most of the gathering. Plausible costs of collecting and cryogenically suspending the tropical rain forest species, at a sampling fraction of 10(-6), are about 2 billion dollars for a full century. Much more information than species DNA will be saved, allowing future biotechnology to derive high information content and perhaps even resurrect then-extinct species. Parallel programs of in situ and other ex situ preservation are essential to allow later expression of frozen genomes in members of the same genus. This is a broad proposal that should be debated throughout the entire scientific community.

It is shown that relaxation of electron spins by thermally excited helicons competes with the phonon process at low temperatures in the lighter metals. © 1968.

The plasma current induced by a high-current electron beam generates electron-ion streaming instabilities even if the electron beam itself is stable to electron-electron beam-plasma modes. This paper reviews the temperature- and current-dependence of the electron-ion mode and uses a simple model resistivity to estimate attainable electron temperatures. For beams in the 100 nsec 10 kA/cm2 range, the heating is by ion-acoustic oscillations, and if convective losses are prevented, keV temperatures can be expected in moderately dense (1014 cm-3) plasma.

Observations show that the ubiquitous pulse shortening in high-power microwave (HPM) devices arises from the formation of plasma, electron streaming, high-E-field breakdown, and beam disruption. We review recent experiments in terms of these causes. Linear beam devices exhibit all of these mechanisms; in particular, beam disruption by E × B drifts in the strong microwave fields and diffusion in turbulent electric fields appear common. In relativistic magnetrons, the dominant effect is resonance destruction by cathode plasma motion, possibly from water contamination of the surface. Wall plasma effects shorten pulses in most sources. We call for the introduction of improved surface conditioning, cathodes which do not produce plasmas, and increased effort on the measurements of the high-field and plasma properties of HPM sources. Because of the broad nature of the phenomena in pulse shortening, we appeal for increased participation of the plasma, intense particle beam, and traditional microwave tube communities in pulse-shortening research. © 1997 IEEE.

We present experimental studies of a plasma-filled X-band backward wave oscillator (BWO). Depending on the background gas pressure, microwave frequency upshifts of up to 1 GHz appeared along with an enhancement by a factor of 7 in the total microwave power emission. The bandwidth of the microwave emission increased from -0.5 GHz to 2 GHz when the BWO was working at the rf power enhancement pressure region. The rf power enhancement appeared over a much wider pressure range in a high beam current case (10-100 mT for 3 kA) as compared to a lower beam case (80-115 ml for 1.6 kA). The plasma-filled BWO has higher power output compared to the vacuum BWO over a broader region of magnetic guide field strength. Trivelpiece-Gould modes (T-G modes) are observed with frequencies up to the background plasma frequency in a plasma-filled BWO. Mode competition between the Trivelpiece-Gould modes and the X-band TM01 mode prevailed when the background plasma density was below 6 x 1011 cm-3. At a critical background plasma density of cr 8 x 1011 cm-3 power enhancement appeared in both X-band and the T-G modes. Power enhancement of the S-band in this mode collaboration region reached up to 8 dB. Electric fields measured by the Stark-effect method were as high as 34 kV/cm while the BWO power level was 80 MW. These electric fields lasted throughout the high power microwave pulse. © 1993 IEEE

Radiation at λ∼1 cm and megawatt power level is observed when a rotating, relativistic electron beam interacts with a background plasma. The emission spectrum is peaked at the 20th cyclotron harmonic, and the parametric dependence of the radiation is consistent with a model of interaction between single-particle cyclotron radiation and a beam-plasma streaming instability.

The authors have investigated the possibility of using a plasma as the nonlinear medium for generating phase-conjugate-reflected electromagnetic waves in the microwave region by almost-degenerate four-wave mixing. The reflected beam can be significantly amplified by the plasma. It was found that the phase conjugate reflection is of significant magnitude if the frequency and the wave vector difference of the signal wave, with respect to the pump waves, resonate with the frequency and the wave vector of the ion acoustic mode of the plasma.

We present experimental studies of a plasma-filled X-band backward wave oscillator (BWO). Depending on the background gas pressure, microwave frequency upshifts of up to 1 GHz appeared along with an enhancement by a factor of 7 in the total microwave power emission. The bandwidth of the microwave emission increased from 0.5 GHz to 2 GHz when the BWO was working at the RF power enhancement pressure region. The RF power enhancement appeared over a much wider pressure range in a high beam current case (10-100 mT for 3 kA) as compared to a lower beam case (80-115 mT for 1.6 kA). The plasma-filled BWO has higher power output compared to the vacuum BWO over a broader region of magnetic guide field strength. Trivelpiece-Gould modes (T-G modes) are observed with frequencies up to the background plasma frequency in a plasma-filled BWO. Mode competition between the Trivelpiece-Gould modes and the X-band TM 01 mode prevailed when the background plasma density was below 6×10 11 cm 3. At a critical background plasma density of n cr8×10 11 cm 3 power enhancement appeared in both X-band and the T-G modes, with mode collaboration. Power enhancement of the S-band in this mode collaboration region reached up to 8 dB. Electric fields measured by Stark-effect method were as high as 34 kV/cm while BWO power level was 80 MW. These electric fields lasted throughout the high power microwave pulse. © 1992 National Technical Information Service.