Self-similarity and branching of wavelet transforms of microwave signals generated by a vircator

The turbulence regimes of an electron flow with a virtual cathode are studied. The microwave signals generated by a vircator are calculated by the method of large particles. The wavelet transforms of these signals are constructed, and it is shown that they have a complicated self-similar and branching structure. It is concluded that the turbulence of an electron flow is caused by macroscopic instabilities. Pis’ma Zh. Tekh. Fiz. 22, 92–96 (December 26, 1996)     

Generation of superradiance pulses by high-current subnanosecond electron bunches moving in a periodic slow-wave structure

The generation of ultrashort microwave pulses was observed experimentally in the superradiance of high-current electron bunches propagating through a periodic slow-wave structure. The process included particle bunching within the bunch followed by coherent emission from the entire volume of the bunch. Pis’ma Zh. Tekh. Fiz. 24, 7–13 (September 26, 1998)     

Generation of subnanosecond superradiance pulses in the short-wavelength part of the millimeter range

Results of a simulation are presented and experimental observations of the generation of superradiance pulses in the 4 and 2mm wavelength ranges as a result of the rectilinear motion of a subpicosecond high-current electron beam through a periodic retarding system are described. In the 4mm range the microwave pulse power reached 10–15MW with a duration of 150 ps. Pis’ma Zh. Tekh. Fiz. 25, 1–7 (December 12, 1999)     

Spontaneous and stimulated emission in microwave oscillators with a virtual cathode

A one-dimensional model of electron beam emission with a virtual cathode is used to show that spontaneous emission occurs in a reditron while stimulated emission is observed in vircators and negative triodes. However, at a certain stage in the latter, the radiative instability is quenched as a result of the evolution of turbulence in the electron beam. The idea is therefore put forward that this quenching may be eliminated by specially shaping the leading edge of the high-voltage supply pulse to the diode of the microwave oscillator. Pis’ma Zh. Tekh. Fiz. 24, 41–46 (February 26, 1998)     

Determination of the electron density in a discharge with nonmetallic liquid electrodes in atmospheric-pressure air from the absorption of microwave probe radiation

Measurements were made of the absorption of microwave power in a discharge plasma generated using tapwater electrodes in atmospheric-pressure air in order to determine the electron density. The high-voltage discharge burned in a bulk (diffuse) form with a lower current density than an arc discharge. This type of discharge with nonmetallic liquid electrodes is extremely promising for various technical applications. Regimes with I=50–60 mA and voltages U=2.9–3.1 kV were studied. The measurements were made at probe radiation frequencies F=29.6 and 35.2 GHz. A two-conductor transmission line was used to localize the microwave power in the plasma. An estimate was obtained for the average electron density in the central part of the discharge (4×10¹¹)<n _e <,(7×10¹¹)cm⁻³. This result shows good agreement with the results of earlier probe measurements. Pis’ma Zh. Tekh. Fiz. 24, 52–57 (December 12, 1998)     

Evolution of rf instability in a steady-state plasma accelerator

For the first time experimental data are obtained which indicate a stable relation between the microwave radiation intensity of a steady-state plasma accelerator and the rate of bulk erosion of the accelerating channel walls. The frequency range of the electromagnetic radiation corresponds to the excitation of a specific beam instability of the plasma flux at the edge of the accelerating channel, caused, in the opinion of the authors, by electron emission from the eroded part of the dielectric walls. Pis’ma Zh. Tekh. Fiz. 23, 37–41 (May 26, 1997)     

Possibility of using a large orbit regime for operation at bounce-frequency harmonics in a free-electron maser with a guiding magnetic field

It is suggested that a free-electron maser based on the LIU-3000 accelerator (Joint Institute for Nuclear Research, Dubna) can be made to emit in the short-wavelength part of the microwave range by using radiation at harmonics of the bounce frequency of an electron beam propagating along a helical trajectory with a large gyration radius on the transverse inhomogeneity scale of the rf field and selectively exciting a cylindrical waveguide mode whose azimuthal index is equal to the number of the harmonic. Pis’ma Zh. Tekh. Fiz. 25, 30–36 (January 12, 1999)     

Appearance of S-shaped sections on the current-voltage characteristics of p-n junction diodes exposed to microwave radiation

An experimental investigation was made of the influence of microwave irradiation on the low-frequency current-voltage characteristics of p-n junction diodes. It is shown that when the microwave power increases to a certain level, a clearly defined S-shaped section appears on the current-voltage characteristic of the diode. This section becomes broader as the microwave signal increases further and then disappears when the microwave irradiation ceases. Pis’ma Zh. Tekh. Fiz. 25, 42–45 (January 12, 1999)     

Negative differential resistance of a tunnel diode induced by an external microwave signal

An experimentally observed effect is described in which a section of negative differential resistance appears on the current-voltage characteristic of a tunnel diode exposed to an external microwave signal when the diode bias voltage in the absence of the microwave signal is substantially below the peak value. The measurements were made for signal frequencies in the range 25–140 GHz. Pis’ma Zh. Tekh. Fiz. 25, 39–42 (January 26, 1999)     

Microwave phase shifter with planar capacitors using strontium titanate films

A design of a microwave phase shifter based on a loaded microstrip line using planar SrTiO₃ capacitors as nonlinear elements is described. The microwave phase shifter (at T=300 K) demonstrated a continuous phase shift between 0 and ∼55 deg in the frequency range 8.6–9.0 GHz with a phase error not exceeding 5 deg. The quality parameter of the phase shifter in the operating frequency range was 110 deg/dB. Pis’ma Zh. Tekh. Fiz. 25, 78–83 (October 26, 1999)     

Energy-efficient green synthesis of Nd:Y<Subscript>2</Subscript>O<Subscript>3</Subscript> nanopowder by microwave gel combustion

A novel method for the synthesis of nanocrystalline neodymium-doped yttria (Nd:Y₂O₃) by gel combustion in microwave without long period of calcination at high temperature is described. The method leads to rapid formation of phase pure product with saving of time and energy. Gels were prepared by selecting citrate to nitrate (C/N) ratios of 0.5:1 & 1:1 followed by combustion in microwave for drying of gel leading to precursor formation. Thermogravimetric-differential thermal analysis (TG-DTA) of precursor with (C/N) ratios of 1:1 exhibited a total loss in weight of about 31.7% up to 1300 °C and did not show M–O bonds characteristic of yttria by FTIR indicating requirement of calcination at high temperature. Crystallization to phase pure yttria took place only on thermal treatment at 1000 °C for 4 h in oxygen atmosphere confirmed by XRD with particle size 40 nm. While the microwave combusted precursor with C/N ratio of 0.5:1 exhibited a weight loss of only 4.7% up to 1300 °C and showed M–O bonds characteristic of yttria in FTIR spectrum without calcinations at higher temperature. This microwave combustion precursor with C/N ratio of 0.5:1 was found to be crystalline phase pure yttria by X-ray diffraction (XRD) with primary particle size 28 nm by Scherrer’s equation and 30–50 nm of uniform morphology by transmission electron microscopy (TEM). In the present work the ratio of citric acid to nitrates is playing a crucial role in terms of saving time and energy involved in calcinations of microwave combusted precursor for the composition with higher citrate content.     

Preparation of thick YBa2Cu3O7−δ films by dc magnetron sputtering

High-temperature superconducting films up to 3.6 μm thick were obtained and their properties requisite for the development of microwave devices were investigated. It is shown that YBa₂Cu₃O_7−δ films of thickness exceeding 3–5λ _L may be obtained for use in the microwave range. Pis’ma Zh. Tekh. Fiz. 24, 89–94 (October 12, 1998)     

Gas-sensitive properties of copper-containing fullerene membranes

The gas-sensitive properties of copper-doped polycrystalline fullerene members are studied in the temperature range 280–360 K. The presence of isopropanol vapor in the surrounding air is found to increase the microwave absorption of the samples examined. The sensitivity of the samples is temperature-selective and depends on the relative orientation of the crystals, as well as the vector of the electric component of the electromagnetic microwave field. Pis’ma Zh. Tekh. Fiz. 25, 71–75 (April 26, 1999)     

Microwave and rf excitation of a discharge in sulfur vapor with added neon

An experimental investigation was made of the transient emission of a discharge excited by microwave radiation in mixtures of sulfur and rare gases. Several typical spectral types of emission were identified which alternate over time. Pis’ma Zh. Tekh. Fiz. 25, 27–33 (July 12, 1999)     

Interaction between relativistic charged particles and an extended strong rf field

An investigation is made of the interaction between relativistic charged particles and a high-intensity microwave electric field over long interaction lengths. The microwave field has no magnetic component over the entire interaction length. It is shown that the interaction of the relativistic particles has features which depend on the relation between the incoming particle energy and the field strength. In particular, part of a monoenergetic particle beam may be accelerated by the beam’s own energy with some of the energy being transferred from the beam to the field. Pis’ma Zh. Tekh. Fiz. 24, 80–86 (May 26, 1998)     

Indication of the presence or absence of convective motion in water by nuclear magnetic resonance

Nuclear magnetic resonance was used to determine the characteristic parameters for the evolution of an ensemble of gas bubbles in a liquid exposed to microwave irradiation. These parameters can be used to determine whether convective motion takes place in the liquid. It is observed that a liquid may be “strengthened” by low-intensity radio waves. Pis’ma Zh. Tekh. Fiz. 24, 87–90 (September 26, 1998)     

Microwave properties of vanadium borate glasses

A.c. conductivity, dielectric constant and loss, and variation with temperature (302–373 K) for four different compositions of V₂O₅-B₂O₃ glasses were reported at 9.586 GHz microwave frequency. The quality factor (Q) and attenuation factor (α being the important parameters in the microwave range of applications were also studied. The change in the dielectric constant and loss was observed with composition of V₂O₅. The maximum loss was found to be at 15V₂O₅ mol‰ The peak was observed in loss with temperature.     

Features of the microwave breakdown of a high-T c superconducting film at defects

The influence of nonsuperconducting defects on the power density for microwave breakdown of a high-T _c superconducting film is investigated theoretically. It is found that the scenario of the destruction of superconductivity depends substantially on the shape of the defect and on the ratio of the absorption coefficients of the defect and film. It is shown that the breakdown power density at a defect may be higher than or equal to the threshold power density for microwave breakdown of a homogeneous film. Pis’ma Zh. Tekh. Fiz. 24, 74–80 (September 12, 1998)     

Stochastic generation accompanying parametric excitation of spin waves in yttrium iron garnet films

We report the results of experimental investigations of chaotic self-modulation of the envelope of surface spin waves as a result of first-order spin-wave parametric instability. The experimental apparatus consisted of a microwave oscillator with a spin-wave delay line in the feedback circuit. Pis’ma Zh. Tekh. Fiz. 24, 66–72 (April 12, 1998)     

Characteristics of microwave radiation from copper-containing fullerene resonators at liquid-nitrogen temperatures

An investigation is made of the radiative microwave properties (43 GHz frequency) of an open dielectric resonator based on a copper-containing fullerene membrane. It is established that a broad emission peak exists in the range 90–110K caused by an increase in the resonator Q factor in this temperature range. Pis’ma Zh. Tekh. Fiz. 25, 44–47 (October 26, 1999)