Short Microwave Pulses Research Articles

Impulse response measurements with 50-GHz bandwidth

We report on a general technique to characterize the impulse response of a microwave device or network. Very short microwave pulses that approximate an impulse are generated from a mode-locked laser impinging on a fast photodetector. Impulse response measurements have been made with a dynamic range of 40 dB for frequencies up to 50 GHz. A frequency resolution of 50 MHz has been obtained. The accuracy of the measurement is 1 dB in amplitude and 0.1 rad in phase.

Mechanism for microwave pulse shortening in a relativistic BWT

It is shown that reduction in the pulse length in a relativistic BWT, is accompanied by the appearance of explosive-emission plasma on the surface of the ripples of the slow-wave system. Generation stops because electrons emitted by the plasma absorb the electromagnetic wave. This absorption is sharply enhanced by the presence of ions emitted from the plasma.

Experimental plasma relativistic microwave electronics

The presence of plasma affects drastically the operation of microwave devices driven by high-current relativistic electron beams. Loading vacuum microwave oscillators with plasma changes the power and frequency band of the radiation. The Cherenkov plasma maser, that operates only with plasma inside, has both promising advantages and inherent disadvantages comparatively to the vacuum devices. Besides, plasma appears in a relativistic microwave oscillator spontaneously under the influence of high-power microwaves or electron fluxes; this parasitic plasma causes microwave pulse shortening, mainly in devices operating in the microsecond regime. The large body of experimental research carried out in relativistic plasma microwave electronics since 1970 up to now is the subject of this review.

International Workshop on High Power Microwave Generation and Pulse Shortening

International Workshop on High Power Microwave Generation and Pulse Shortening

Optical spectroscopy of plasma in high power microwave pulse shortening experiments driven by a μs e-beam

Microwave pulse shortening experiments have been performed on a rectangular-cross-section (RCS) gyrotron driven by the Michigan Electron Long Beam Accelerator (MELBA) at parameters V=-800 kV, I/sub tube/=0.3 kA and pulselengths of 0.5-1 /spl mu/s. Pulse shortening typically limits the highest (10 MW level) microwave power pulselength to 100-200 ns. Potential explanations of pulse shortening are being investigated, particularly plasma production inside the cavity and at the e-beam-collector. We report the first optical spectroscopy diagnostic measurements inside an operating gyrotron as a means of exploring plasma effects on pulse shortening. Plasma hydrogen H-/spl alpha/ line radiation has been characterized in both time-integrated and temporally-resolved measurements and correlated with microwave power/cutoff. Hydrogen is believed to originate from water absorbed on internal tube surfaces in the gyrotron.

Relativistic Cherenkov plasma maser of microsecond pulse duration

Experiments with a Cherenkov plasma maser (CPM) driven by a high-current relativistic electron beam of microsecond pulse duration are described. The results obtained show that the principle of operation of the CPM makes it possible to avoid the problem of microwave pulse shortening, which is inherent to vacuum devices of relativistic high-current microwave electronics. A 800-ns microwave pulse with an energy of 21 J was obtained at a peak power level of 40 MW (the efficiency being 4%) in a broad (/spl sim/100%) frequency band.

Effect of third-order dispersion on nonlinear magnetostatic spin waves in ferromagnetic films

The effect of high-order dispersion on nonlinear magnetostatic spin waves has been investigated, theoretically, for ferromagnetic films that have metal plates near both surfaces of the film. The conditions under which third-order dispersion has to be considered are thoroughly analyzed and some numerical calculations concerning the propagation of short microwave pulses in ferromagnetic films are presented.

Experimental Observation of Short Microwave Generation via Relativistic Ionization Front Produced by CO2 Laser

A short microwave pulse is generated by the interaction between the relativistic ionization front produced by CO2 laser and a static, zero-frequency electric field. The zero-frequency electric field is produced by an alternately biased capacitor array. Laser light propagation between electrodes produces the ionization front propagating with the velocity equal to the laser group velocity. The maximum frequency of the generated microwave is determined to be in the range of 8.2 to 17.6 GHz by several waveguides with specific cut-off frequency and horn antenna combination.

Bistable Microwave Oscillator Based on Nonlinear Magnetostatic Wave Transmission Line

A microwave oscillator containing a GaAs amplifier, a magnetostatic spin wave (MSW) signal-to-noise enhancer (SNE) used as a nonlinear transmission line, directional couplers, and a variable attenuator has been designed and investigated. It was shown, that nonlinearity of the SNE and amplifier resulted in a bistability in the system: two different levels of oscillation power correspond to one attenuator setting. The oscillator was switched on under action of external short low power microwave pulses.

Demonstration of Microwave Generation from a Static Field by a Relativistic Ionization Front in a Capacitor Array.

We present the results of a proof-of-principle experiment to demonstrate the generation of tunable radiation from a laser-ionize gas-filled capacitor array. This scheme directly converts a static electric field of wave number {ital k}{sub 0} into coherent radiation pulses of frequency {omega}{sup 2}{sub {ital p}}/2{ital k}{sub 0}{ital c}, where {omega}{sub {ital p}} is the plasma frequency. The radiation frequency can be tuned by varying gas pressure and/or capacitor spacing. In this experiment, well-polarized, short (less than 5 ns) microwave pulses have been generated over a frequency range of 6 to 21 GHz. The frequency of the detected signal, as measured with cut-off waveguides, scales linearly with the plasma density, and the relative power of the signal scales quadratically with the dc bias voltage in agreement with the theory. {copyright} {ital 1996 The American Physical Society.}

Bistable microwave oscillator with magnetostaticwave signal-to-noise enhancer in the feedback loop

A microwave feedback oscillator with a saturable amplifier and a nonlinear magnetostatic spin wave signal-to-noise enhancer device in the feedback loop is described. The oscillator has bistable behaviour and can be turned on with short low power microwave pulses.

Mach cone structure generation by the interaction of a short microwave pulse with a plasma

When a non-uniform unmagnetized plasma is irradiated by a short microwave pulse with length of the order of an ion-wave period, the travelling wavepacket of the electron plasma wave, and nonlinear large-amplitude waves in an ion-wave regime have been observed. The observed large-amplitude waves typically have two frequencies. The higher-frequency wave is an ion wakefield excited by the travelling wavepacket of the electron plasma wave, while the lower-frequency mode, also called a density cavity streamer, can be interpreted as a pseudowave induced by the supersonic ion bunch with a typical energy of 25 eV produced at the resonance layer. The experimental results are in reasonable agreement with a theoretical model based on the Mach cone structure.

Short-pulse microwave treatment of disseminated sulfide ores

Theoretical and experimental analyses are used to study the feasibility of using short-pulse microwave energy as a pretreatment step in comminution. Ores of interest contain microwave-absorbing mineral in a low-absorbing gangue. A finite element numerical model is used to predict the thermomechanical response of a single pyrite particle in a calcite host rock during microwave heating. The effects of power density and mineral particle size are presented. Using microwave power densities within those expected in currently available equipment, significant temperature differences are predicted between the pyrite particle and the calcite matrix. Resulting thermomechanical stress exceeds typical strengths of these minerals. Laboratory tests verify rapid, selective heating of crushed sulfide minerals. Tests with mixtures of pure minerals show heating rates directly proportional to the mass of high-loss mineral; thus, shielding of mineral grains by other grains does not appear to be significant. A cursory energy-use analysis suggests that short pulse microwave treatment can be accomplished with relatively small energy input compared with current energy consumption in comminution. Although improved comminution has yet to be experimentally demonstrated, initial results are promising from both a technical and an economic standpoint.

Electron heat transport with non-Maxwellian distributions

Measurements are presented of electron heat transport with non-Maxwellian (flattopped) distributions due to inverse bremsstrahlung absorption of intense microwaves in the University of California at Davis Aurora II device [Rogers et al., Phys. Fluids B 1, 741 (1989)]. The plasma is created by pulsed discharge in a cylindrical vacuum chamber with surface magnets arranged to create a density gradient. The ionization fraction (∼1%) is high enough that charged particle collisions are strongly dominant in the afterglow plasma. A short microwave pulse (∼2 μs) heats a region of the afterglow plasma (ne/ncr≤0.5) creating a steep axial (LT∼1–10λei) temperature gradient. Langmuir probes are used to measure the relaxation of the heat front after the microwave pulse. Time and space resolved measurements show that the isotropic component of the electron velocity distribution is flat topped (∼exp[−(v/vm)m], m≳2) in agreement with Fokker–Planck calculations using the measured density profile. Classical heat transport theory is not valid both because the isotropic component of the electron velocity distribution is flattopped and the temperature gradients are very steep.

Enhanced microwave scattering with time-of-flight resolution

A RADAR-like modification of enhanced microwave-scattering diagnostics is investigated both analytically and numerically. The method uses the fact that backscattering off the density fluctuation is localized to a narrow layer, whose position depends on the fluctuation wavenumber. Therefore, when a plasma is irradiated by a short microwave pulse, the time delay of the backscattered pulse contains the information on the shape of the fluctuation spectrum. This effect is strongly pronounced for scattering near the upper hybrid resonance where both incoming and outgoing waves are very slow. In laboratory and small tokamak plasmas, this technique permits observation of small-scale density fluctuations with a high-spatial and wavenumber resolution.

Observation of temporal plasma-wave echoes in an ion-wave regime.

Temporal echoes in an ion-wave regime are observed in microwave-plasma interaction experiments. When two short microwave pulses (\ensuremath{\tau}\ensuremath{\le}1/${\mathit{f}}_{\mathit{p}\mathit{i}}$, where \ensuremath{\tau} is the pulse width and ${\mathit{f}}_{\mathit{p}\mathit{i}}$ is ion plamsa frequency) are applied successively, one after another, in the nonuniform magnetic-field-free plasma, waves in ion plasma frequency range and plasma density cavity (caviton) are excited at the critical layer of the resonance absorption. After the waves damp out by Landau damping, temporal echoes appear in both the critical layer and underdense region. In the overdense region, however, no echo appears in the present experiments. The results are explained by the theoretical prediction given by Gould, O'Neil, and Malmberg [Phys. Rev. Lett. 19, 219 (1967)].

Excitation of wakefield and electron acceleration by short microwave pulse

Non-linear large amplitude ion wave excitation by short microwave pulse at the resonance absorption region has been observed. The maximum amplitude of the wave is δn/n ≈ 40% in peak-to-peak, when a short microwave pulse of the order of or less than a period of ion oscillation (τi = 2π/ωpi) is used. The excited wave propagates to lower density regions and its velocity is a function of its amplitude, v ∝ (δn/n0)2.2–2.7. At the same time, a strong electron wave is excited after shut-off of the incident microwave pulse and high energy electrons are emitted and accelerated by the excited electron wave wakefield.

Nonlinear propagation of short microwave pulses in magnetostatic volume wave delay lines

The conditions for the excitation of volume wave solitons and self induced transparency in magnetostatic volume wave (MSW) delay lines will be discussed and compared with experimental results. The dispersion and nonlinearity characteristics of compact device structures will be analytically derived in the one and two ground plane configurations, and their influence on the onset of a solitonic behavior will be analyzed in some detail. >

Ramsey interference fringes in single pulse microwave multiphoton transitions.

In microwave multiphoton transitions between Rydberg atoms driven by short microwave pulses we have observed that the rise and fall of the pulse are as important as the peak and together can produce readily visible interference in the transition probability. Furthermore, the transition probability vanishes for some tunings and can be suppressed if the ac Stark shift brings the two states into resonance at the peak of the microwave pulse with a large Rabi frequency.

A numerical solution of the Boltzmann equation for high-powered short pulse microwave breakdown in nitrogen

A theoretical formulation that provides a basis for generating approximate solutions of the Boltzmann equation for the study of high-powered microwave pulses interacting with a background atmosphere is given. A numerical scheme is implemented using realistic cross sections for electron–nitrogen collisions, including excitation and ionization. It is found that the numerical results agree well with previous experimental and theoretical values when α is less than about 0.1 kV/cm Torr, where α is the effective electric field divided by the pressure (α=Ee/P). Results are presented providing a simple fit for the time averaged momentum transfer rate and the ionization rate as a function of the microwave electric field E, wavelength λ, and gas pressure P, for 0.1≤α≤1.0 kV/cm Torr. This fit is compared with recent experimental data for breakdown times.