David Tzach

Detailed spectra of high power broadband microwave radiation from interactions of relativistic electron beams with weakly magnetized plasmas

Prodigious quantities of microwave energy distributed uniformly across a wide frequency band are observed when a relativistic electron beam (REB) penetrates a plasma. Typical measured values are 20 MW total for Δν≂40 GHz with preliminary observations of bandwidths as large as 100 GHz. An intense annular pulsed REB (I≂128 kA; r≂3 cm; Δr≂1 cm; 50 nsec FWHM; γ≂3) is sent through an unmagnetized or weakly magnetized plasma column (n plasma ∼101 3 cm− 3). Beam‐to‐plasma densities of 0.01≤n beam /n plasma ≤2 are used, the higher values of this range being an unconsidered region for most previous theoretical and experimental efforts. For these higher n b /n p values, the observed emission with ω≫ω p and weak harmonic structure is wholly unanticipated from Langmuir scattering or soliton collapse models. A model of Compton‐like boosting of ambient plasma waves by the beam electrons, with collateral emission of high‐frequency photons, qualitatively explains these spectra. Power emerges largely in an angle ∼1/γ, as required by Compton mechanisms. As n b /n p falls, ω p −2ω p structure and harmonic power ratios consistent with soliton collapse theories appear. With further reduction of n b /n p only the ω p line persists. Thus a transition occurs in spectral behavior from the weak to strong turbulence theories advocated for type‐III solar burst radiation, and further into a regime we characterize as s u p e r s t r o n g REB–plasma interactions, is observed. For frequencies slightly below the broadband region, an ω p line is observed with high power (approximately 1 MW); the line disappears in an external B z ∼400 G. Changing γ b over a range of 2.2–3.7 has little effect on the spectra.

The plasma as a phase conjugate reflector

Plasma is a nonlinear medium and two waves propagating in it interact electromagnetically with each other. If the plasma is pumped by two strong counterstreaming waves of equal frequency, and a third wave enters, the nonlinear interaction generates a fourth wave, phase conjugate to the third wave. This interaction becomes very significant if the frequency and wave vector differences between the third wave and one of the pump waves resonate with the frequency and wave vector of the ion acoustic mode of the plasma. This resonance can be predicted from a fluidlike description of the plasma, but it is shown that the Vlasov description can provide more details of behavior near the resonance. Possible applications of the emergent technology range from improved focusing of radiation in hyperthermia therapy of cancer to the formation of a microwave laser between a phase conjugate plasma reflector and a mirror, for improved radar imaging. Another application is cordless, self‐guiding, power transmission.

MICROWAVE-RADIATION BY A RELATIVISTIC ELECTRON-BEAM PROPAGATION THROUGH LOW-PRESSURE AIR

Intense relativistic electron beams fired into air at varying pressures display a wide range of microwave signatures. These experiments held beam current, energy, and pulse length constant while varying gas pressure. Our observing window is 10 to 40 GHz. At low pressures (<10 mTorr) exponential spectra result, consistent with beam reflexing or virtual cathode oscillations. Above 20 mTorr the spectrum flattens and suggests collective emission at the beam‐generated plasma frequencies. Power falls linearly with pressure above 20 mTorr, until electron‐Neutral collisions damp the emission at a few Torr. However, weak 10 GHz emission appears at full atmospheric pressure.

Small simple hydrogen plasma gun

A small hydrogen plasma gun was constructed using graphite plates and TiH2. The gun, as small as 5 cm in diameter and 7 mm thick, produces ∼1013 cm−3 plasma in a volume of ∼0.1 m3. A 10‐kA current for ∼15 μs is needed to drive the gun. One or more guns can be connected to the same system, an arrangement which allows flexibility in controlling the plasma profile in the experiment. A system employing four similar plasma guns connected in parallel is described.

Coherent curvature radiation from an electron beam rotating in a plasma

Microwave‐radiation bursts at λ∼1 cm are observed when a rotating relativistic electron beam interacts with a plasma. The power level exceeds 1 MW, and the radiation pulse lasts as long as the electron beam pulse for beams up to 1 μsec long. The results are consistent with a model of coherent curvature radiation from electrons bunched due to a two‐stream instability. Other mechanisms are ruled out by observations of harmonic splittings and the power spectrum.

Beam-Plasma Emission near the Plasma Frequency

This paper presents detailed spectra for electromagnetic emission from a strong relativistic beam-plasma instability. Some emission appears early in the beam voltage pulse, apparently from reflexing. This subsides as beam propagation improves, and emission moves to high frequencies. This is compatible with a collective emission process from electrostatic waves built up in the chamber by reflection from the walls. Noise levels suggest about 100 emitting sites switching on and off randomly. The emitters comprise about 1 percent of the plasma volume and survive a time comparable to the lifetime of solitons subject to ion damping.

Polarization of coherent synchrotron radiation from an electron beam rotating in a plasma

After eliminating reflections from the walls of the plasma container, we observed polarization of the coherent synchrotron radiation from a relativistic electron beam rotating in a plasma. Several features of the polarization agree well with calculations based on the single particle synchrotron radiation theory. A particular polarization ratio (Fig. 3) does not, however. We deduce from this direct diffraction of the radiation by the beam electrons. This is strong evidence for beam-particle bunches of size ∼cm. Also, there must be some absorption of the extraordinary wave to account for the observations. We suggest a way to apply these results to measure the pitch angle of the beam.

Laboratory study of coherent curvature radiation as a pulsar emission mechanism

To simulate some of the major physical processes occurring in pulsars, we performed experiments using a relativistic electron beam propatating helically through a magnetized plasma. Microwave radiation with λ≈1 cm emerged when the predicted resonance conditions were satisfied. Power exceeded 1 MW and radiation lasted as long as the electron beam pulse. The spectrum, harmonics, power and scalings were consistent with a model of coherent curvature radiation from electrons which are bunched by a beam-plasma streaming instability. Brightness temperature was ∼1020 degrees. Polarization was that of single-particle emission, but with some evidence for diffraction patterns due to the beam bunches themselves. The Razin effect does not apply to our experiments and was not observed. The fundamental two-step process of electrostatic bunching followed by curvature emission describes well all our results.