Spectral aspect of the onset of subharmonic Shapiro steps in a chain of Josephson junctions connected in parallel

A chain of parallel-connected Josephson elements described by a resistive model is considered as a model for high-temperature Josephson junctions. The calculated spectra of the Josephson generation can account for the experimentally observed subharmonic Sapiro steps on the current-voltage characteristic and for their aperiodic dependence on the external magnetic field. Pis’ma Zh. Tekh. Fiz. 24, 45–52 (September 12, 1998)     

Subharmonic emissions from microbubbles: effect of the driving pulse shape

The aims of this work are to investigate the response of the ultrasonic contrast agents (UCA) insonified by different arbitrary-shaped pulses at different acoustic pressures and concentration of the contrast agent focusing on subharmonic emission. A transmission setup was developed in order to insonify the contrast agent contained in a measurement chamber. The transmitted ultrasonic signals were generated by an arbitrary wave generator connected to a linear power amplifier able to drive a single-element transducer. The transmitted ultrasonic pulses that passed through the contrast agent-filled chamber were received by a second transducer or a hydrophone aligned with the first one. The radio frequency (RF) signals were acquired by fast echographic multiparameters multi-image novel apparatus (FEMMINA), which is an echographic platform able to acquire ultrasonic signals in a real-time modality. Three sets of ultrasonic signals were devised in order to evaluate subharmonic response of the contrast agent respect with sinusoidal burst signals used as reference pulses. A decreasing up to 30 dB in subharmonic response was detected for a Gaussian-shaped pulse; differences in subharmonic emission up to 21 dB were detected for a composite pulse (two-tone burst) for different acoustic pressures and concentrations. Results from this experimentation demonstrated that the transmitted pulse shape strongly affects subharmonic emission in spite of a second harmonic one. In particular, the smoothness of the initial portion of the shaped pulses can inhibit subharmonic generation from the contrast agents respect with a reference sinusoidal burst signal. It also was shown that subharmonic generation is influenced by the amplitude and the concentration of the contrast agent for each set of the shaped pulses. Subharmonic emissions that derive from a nonlinear mechanism involving nonlinear coupling among different oscillation modes are strongly affected by the shape of the ultrasonic driving pulse.     

Noninvasive estimation of dynamic pressures in vitro and in vivo using the subharmonic response from microbubbles

The purpose of this study was to develop and validate a noninvasive pressure estimation technique based on subharmonic emissions from a commercially available ultrasound contrast agent and scanner, unlike other studies that have either adopted a single-element transducer approach and/ or use of in-house contrast agents. Ambient pressures were varied in a closed-loop flow system between 0 and 120 mmHg and were recorded by a solid-state pressure catheter as the reference standard. Simultaneously, the ultrasound scanner was operated in pulse inversion mode transmitting at 2.5 MHz, and the unprocessed RF data were captured at different incident acoustic pressures (from 76 to 897 kPa). The subharmonic data for each pulse were extracted using band-pass filtering with averaging, and subsequently processed to eliminate noise. The incident acoustic pressure most sensitive to ambient pressure fluctuations was determined, and then the ambient pressure was tracked over 20 s. In vivo validation of this technique was performed in the left ventricle (LV) of 2 canines. In vitro, the subharmonic signal could track ambient pressure values with r(2) = 0.922 (p < 0.001), whereas in vivo, the subharmonic signal tracked the LV pressures with r(2) > 0.790 (p < 0.001) showing a maximum error of 2.84 mmHg compared with the reference standard. In conclusion, a subharmonic ultrasound-based pressure estimation technique, which can accurately track left ventricular pressures, has been established.     

Theory of the undulator superradiance of an electron beam pulse in the group synchronism regime

The properties of the superradiance of a short electron beam pulse moving in an undulator field and exciting a waveguide mode field under group synchronism conditions are analyzed. This regime is shown to be characterized by the maximum growth rate of superradiance instability. The development of such instability results in bunching of the particles and the coherent emission of a short electromagnetic pulse by the beam pulse. Pis’ma Zh. Tekh. Fiz. 25, 8–16 (April 26, 1999)     

Influence of the nonideality factor on the formation of a near-surface picosecond laser plasma

It is shown numerically that the influence of the nonideality factor leads to an increase in the breakdown time of a dense near-surface gas by a picosecond laser pulse. Pis’ma Zh. Tekh. Fiz. 25, 37–43 (June 26, 1999)     

Calculating the electromagnetic wave generated by the interaction between an ultrashort laser pulse and matter

A self-consistent solution is obtained for the problem of electron flux generation by interaction between an ultrashort laser pulse and a metal target. Pis’ma Zh. Tekh. Fiz. 24, 84–88 (March 12, 1998)     

Fiber ring laser with locked relaxation oscillations

An investigation is made of the locking of the relaxation oscillations in a fiber ring laser with an active medium of phosphorus aluminosilicate glass doped with erbium and ytterbium ions. It is shown that stable pulse-periodic emission in the 1.5 /gmm range can be achieved at a pulse repetition frequency between 80 and 160 kHz with a pulse length of 2–3 μs. A single-mode model is used to determine the laser parameters (the pumping rate and the photon lifetime in the cavity). Pis’ma Zh. Tekh. Fiz. 25, 24–28 (January 26, 1999)     

Q-switching in single-heterojunction lasers and generation of ultrahigh-power picosecond optical pulses

Q-switching has been obtained for single-heterojunction lasers. The ultrafast saturable absorber in the laser cavity is created by implantation of high-energy heavy ions. The peak power generated by the lasers in a 150 μm stripe width is 380 W with a pulse length of 40 ps. Pis’ma Zh. Tekh. Fiz. 23, 11–16 (February 26, 1997)     

Fiber-optic detector using a mid-infrared diode laser and an acoustooptic modulator

Data are given for an infrared detector based on a semiconductor injection laser, which operates at a wavelength of 3.3 μm and is coupled by a chalcogenide optical fiber to an acoustooptic modulator made from an amorphous Si-Te alloy. The beam modulation coefficient reaches 70% for a pulse duration ≳0.3 μs, making the detector well suited to gas-analysis applications. Pis’ma Zh. Tekh. Fiz. 23, 14–18 (October 26, 1997)     

Tunneling contact under the action of a nanosecond voltage pulse

The initial stage of current flow through a tunneling gap subjected to a short voltage pulse is examined and the dynamics of the contact between a tip and a sample surface under the action of pulses with durations ranging from 1 μs to 1 s is investigated. Pis’ma Zh. Tekh. Fiz. 23, 22–27 (September 12, 1997)     

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. Pis’ma Zh. Tekh. Fiz. 25, 27–36 (March 26, 1999)     

Method of determining dynamic adiabatic compression curves of powders

A method is described for investigating the dynamic adiabatic compression curves of powders by measuring of the pulsed pressure in a powder pressed uniaxially by an elastic instrument. The instantaneous powder density is determined by numerical solution of a pulsed spectral problem using the pressing pressure pulse and the known characteristics of the pressing instrument and the external force. Pis’ma Zh. Tekh. Fiz. 23, 76–80 (July 12, 1997)     

Effect of the rise rate of nanosecond high-voltage pulses on the breakdown of air gaps

The breakdown velocity increases to 10–20 cm/ns when the leading edge of the negative voltage pulse decreases to 0.5–2.5 ns. The sharp increase in the velocity can be explained by the appearance of a short-pulse beam of high-energy electrons which produce in the gap a sufficient number of initial electrons. Pis’ma Zh. Tekh. Fiz. 25, 74–80 (September 26, 1999)     

Influence of tip vibration on the probability of surface modification by a voltage pulse in a scanning tunneling microscope

An investigation was made of the probability of modification of a gold surface by a voltage pulse in a scanning tunneling microscope in the presence of harmonic vibrations of the tip. The dependence of the modification probability on the pulse amplitude revealed a substantial shift of the threshold voltage and a corresponding broadening of the transition section. These results confirm the assumption that the modification probability may undergo a smooth transition from zero to maximum as a result of mechanical vibrations of the tip. Pis’ma Zh. Tekh. Fiz. 24, 42–46 (August 12, 1998)     

Prospects for detection of ultrahigh-energy cosmic rays on the surface of the moon

A theoretical foundation is laid for the development of a radio detector for cosmic rays with energies E≥10²¹ eV. The detection system envisages a radio detector on the Earth or a system of radio antennas on artificial lunar satellites, which would record the radio emission pulse produced by a shower in the lunar soil. Pis’ma Zh. Tekh. Fiz. 23, 57–62 (May 26, 1997)     

Condition for a time-constant current in a high-current planar diode

The conditions required to achieve a constant current from an explosive cathode during the pulse in a high-current planar diode are calculated. Pis’ma Zh. Tekh. Fiz. 24, 63–69 (March 12, 1998)     

Influence of the type of cathode on the dynamic characteristics of ionization waves

The influence of different types of cathodes on the dynamic characteristics of ionization waves is determined experimentally. It is established that when the discharge is initiated by negative pulses, the maximum wave propagation velocity is observed for a cold cathode while the maximum peaking of the output pulse leading edge is observed for a heated cathode. The mechanism for this effect is discussed. Pis’ma Zh. Tekh. Fiz. 24, 39–42 (July 12, 1998)     

Characteristics of a submicrosecond transverse discharge in mixtures of helium with N2 and CO molecules and xenon atoms

An investigation is made of the electrical and optical characteristics of a moderate-pressure transverse discharge in typical active media of infrared CO lasers. The discharge was ignited in a system of unprofiled “ grid-plane” electrodes with automatic ultraviolet preionization by a corona discharge and had a 2×3 cm aperture. The pulse repetition frequency was 1–10 Hz, the current pulse length was ≤0.5 μs, and the pressure of the working mixtures was 10–100 kPa. Studies were made of the transverse discharge current voltage pulses, and also the spectral and temporal characteristics of the plasma radiation in N₂:CO, He:N₂:CO, and He: N₂:CO:Xe mixtures in the ultraviolet and visible ranges. Pis’ma Zh. Tekh. Fiz. 23, 71–76 (October 12, 1997)     

Influence of the parameters of a pulsed electron beam on the removal of nitrogen oxides from flue gases

Results are presented of an experimental investigation of the oxidation of small quantities of nitrogen oxides in air irradiated by a pulsed electron beam. It is shown that the impurity removal process is strongly influenced by the pulse length and current density of the electron beam. It is noted that an adequate model is needed to describe the plasma-chemical oxidation processes of nitrogen oxides involving charged and excited particles. Pis’ma Zh. Tekh. Fiz. 24, 47–50 (August 12, 1998)     

CdS-GaN heterostructure photodetector with switching and memory

A photodetector having two stable states is fabricated. Switching is effected by a light pulse from an external source. The states are stored until a voltage is supplied to the photodetector. The design and properties of the photodetector are presented. The basic device is an M-i-n-GaN light-emitting diode structure. Pis’ma Zh. Tekh. Fiz. 23, 75–78 (October 26, 1997)