Low Drift Velocity Research Articles

Negative damping of terahertz plasmons in counter-streaming double-layer two-dimensional electron gases

Abstract The plasmon excitation in two-dimensional electron gases (2DEGs) is a significant way of achieving micro-nanoscale terahertz (THz) devices. Here, we establish a kinetic simulation model to study the THz plasmons amplification in a semiconductor double-quantum-well system with counter-streaming electron drift velocities. By comparing the simulation results with theoretical dispersion relations, we confirm two competing mechanisms of negative damping suitable for THz amplification: Cherenkov-type two-stream instability and a new non-Cherenkov mechanism called kinetic relaxation instability. The former is caused by the interlayer coupling of two slow plasmon modes and only exists when the drift velocities are much greater than the fermi velocities. The latter is a statistical effect caused by the momentum relaxation of electron-impurity scattering and predominates at lower drift velocities. We show that an approximate kinetic dispersion relation can accurately predict the wave growth rates of the two mechanisms. The results also indicate that the saturated plasmonic waves undergo strong nonlinearities such as wave distortion, frequency downshift, wave-packet formation, and spectrum broadening. The nonlinear evolution can be interpreted as the merging of bubble structures in the electron phase-space distribution. The present results not only reveal the potential mechanisms of the plasmonic instabilities in double-layer 2DEGs, but also provide a new guideline for the design of on-chip THz amplifiers.

Development and experimental evaluation of visual-acoustic navigation for safe maneuvering of unmanned surface vehicles in harbor and waterway areas

Ship maneuvering in narrow harbor environments requires accurate navigation data to operate safely. Since Global Navigation Satellite Systems (GNSS) can be unreliable and inaccurate in such environments, other aiding techniques should be used for increased redundancy and reliability. To this end, we present a GNSS-free sensor suite consisting of visual, acoustic, and inertial sensors that can be used to navigate small vehicles in a harbor environment. The proposed method is aided by two navigation systems: a doppler velocity log (DVL) for low-drift velocity aiding and a visual fiducial system to estimate the drift-free position and attitude of a camera onboard the vehicle with respect to easily identifiable tags at the dockside. The visual and acoustic measurements are fused with inertial data using an error-state Kalman filter for robust state estimation. To benchmark the performance, we employ an unmanned surface vehicle equipped with a dual-antenna real-time kinematic GNSS receiver for accurate positioning and heading. The benefits of supplementing landmark-based navigation with acoustic measurements are demonstrated by conducting adverse scenarios that include noisy measurements and sensor dropouts in the harbor environment. As a result, the proposed visual-acoustic system can supplement satellite-based navigation systems for safety-critical harbor maneuvering.

On the Response Speed of Narrowband Organic Optical Upconversion Devices

AbstractOrganic upconversion devices (OUCs) consist of an organic infrared photodetector and an organic visible light‐emitting diode (OLED), connected in series. OUCs convert photons from the infrared to the visible and are of use in applications such as process control or imaging. Many applications require a fast OUC response speed, namely the ability to accurately detect in the visible a rapidly changing infrared signal. Here, high image‐contrast, narrowband OUCs are reported that convert near‐infrared (NIR) light at 980 and 976 nm with a full‐width at half maximum of 130 nm into visible light. Transient photocurrent measurements show that the response speed decreases when lowering the NIR light intensity. This is contrary to conventional organic photodetectors that show the opposite speed‐versus‐light trend. It is further found that the response speed increases (when using a phosphorescent OLED) or decreases (for a fluorescent OLED) when increasing the driving voltage. To understand these surprising results, an analysis by numerical simulation is conducted. Results show that the response speed behavior is primarily determined by the electron mobility in the OLED. It is proposed that the low electron drift velocity in the emitter layer sets a fundamental limit to the response speed of OUCs.

The role of noise in PIC and Vlasov simulations of the Buneman instability

The effects of noise in particle-in-cell (PIC) and Vlasov simulations of the Buneman instability in unmagnetized plasmas are studied. It is found that, in the regime of low drift velocity, the linear stage of the instability in PIC simulations differs significantly from the theoretical predictions, whereas in the Vlasov simulations it does not. A series of highly resolved PIC simulations with increasingly large numbers of macroparticles per cell is performed using a number of different PIC codes. All the simulations predict highly similar growth rates that are several times larger than those calculated from the linear theory. As a result, we find that the true convergence of the PIC simulations in the linear regime is elusive to achieve in practice and can easily be misidentified. The discrepancy between the theoretical and the observed growth rates is attributed to the initial noise inherently present in PIC simulations, but not in Vlasov simulations, that causes particle trapping even though the fraction of trapped particles is low. We show analytically that even weak distortions of the electron velocity distribution function (such as flattening due to particle trapping) result in significant modifications of the growth rates. It is also found that the common quiet-start method for PIC simulations leads to more accurate growth rates but only if the maximum growth rate mode is perturbed initially. We demonstrate that the quiet-start method does not completely remedy the noise problem because the simulations generally exhibit inconsistencies with the linear theory.

Statistical results of the numerical model of sea ice drift extremes in the south-western part of the Kara Sea

The aim of the study was to identify the basic characteristics of ice drift in the south-western Kara Sea and to estimate the extreme drift speed of given probability, including its spatial variability and statistical correlation with the main drift-forming factors. In order to obtain the ice drift data, the numerical dynamic-thermodynamic model of ice cover evolution developed in AARI was used. Its basic specific feature is imitation of ice cover with the help of so-called markers (conventional ice floes). Using three variants of the model grid net (25, 12.5 and 5 km), the ice conditions in the Baidara Bay and the adjoining area in January-March 2018 were simulated. The analysis of the simulation results showed that the average ice drift (average ice transport) is directed from the Baidara Bay to the open sea, i.e. northward with slight deviations mostly to the west. A less detailed grid net results in a smoothed ice drift field, while an increase in the spatial resolution of the model increases the spatial contrasts of the ice drift. The maximum values of the extreme ice drift velocity expressed as “once per N years” occur in the northern part of the model area at the directions of the north-western quarter (up to 1.5-1.8 m/s “once per 10 years” - “once per 100 years”, respectively). The frequency of ice drift velocity exceeding 0.3 m/s is about 4-7 %, and that of ice drift velocity exceeding 0.6 m/s is not more than 1 %. At low drift velocity, the role of inertia is very high, but as the drift rate grows, the inertia contribution decreases noticeably. At increasing drift velocity, the statistical correlation between the ice drift (on the one hand) and the wind, current and sea level tilt (on the other hand) becomes evident. This effect is especially evident for the correlation “drift / wind”. The correlation “drift / ice pressure” depends on the drift speed more or less noticeably at low and high drift speeds, when unidirectional changes of the ice drift and ice pressure happen more often than the opposite ones. At the drift velocities within 0.15-0.60 m\s, the correlation between the ice drift and ice pressure is insignificant, i.e. the unidirectional and opposite changes of the ice drift and ice pressure are almost equally likely.

Simulation of the Quantum Hall Effect in Samples with Weak Long-Range Disorder

The fine structure of the density of states is studied numerically in the quantum Hall effect mode during the ballistic transmission of an electron through an area of 1 µm2 of a two-dimensional electron gas with weak long-range disorder. The calculated widths of strict quantum plateaus agree with experimental data. Periodic conductance oscillations corresponding to the addition of two electrons to the simulated area are found in the central part of the lower Landau band. One-dimensional countercurrents are found inside the area and at its edge, which are separated by a magnetic length and explained by the motion of an electron with a low drift velocity.

Interfacial THz generation from graphene/Si mixed-dimensional van der Waals heterostructure.

Even though Si is the most cost efficient and extensively used semiconductor in modern optoelectronics, it is not considered to be an effective THz emitter due to its low carrier drift velocity and small saturated built-in electric field from the inversion layer. Herein, we present an effective way to enhance THz generation using a graphene/Si Schottky junction (GSSJ) excited with a femtosecond laser under electrical gating without rapid saturation and with high carrier drift velocity. This mixed-dimensional van der Waals interface demonstrates large saturation pump fluence with an invalid inversion layer by removing the native oxide on the Si surface. The THz emission amplitude from GSSJ effectively increases with the gate voltage. The THz emission from GSSJ under the same excitation conditions is stronger than that from the surface of InAs (100) and GaAs (100). The results not only show an efficient THz emission from GSSJ but also demonstrate the ability of THz generation for probing the mixed-dimensional van der Waals interface.

Measurements and modeling of charge carrier lifetime in compressed xenon

Gamma-spectrometers based on high-pressure xenon gas (HPXe) are proving themselves as a great potential alternative to the spectrometers based on high-purity germanium crystals and scintillators. The working medium for the high-resolution HPXe detectors, that is, xenon gas compressed up to pressure ∼50 bar and sometimes doped with hydrogen, methane or others gases, needs to be of very high purity. The gas purity level can be determined by direct measurements or, alternatively, its usability in gamma-spectrometers can be evaluated indirectly through the charge carrier (electron) lifetime measurements. Different approaches and specific setups have been used for the lifetime determination, most of those methods involve the measurement and analyses of individual pulses from ionizing particles registered in an ionization chamber filled with Xe. In the present paper, we report on the HPXe electron lifetime study carried out by using measurements in a cylindrical ionization chamber and the respective analytical charge transport model. Our results support the possibility of carrier lifetime determination in the cylindrical configuration. In addition, the voltage regimes for the use of the chamber in the spectroscopic mode were determined. The measurements were conducted in a two-electrode configuration for a range of pressure values (5 to 50 bar) for the Xe+0.25%H2 gas mixture of ∼6N purity. It is shown that in gases with relatively high values of the electron drift velocity and the electron lifetime, for example low-density gases, the charge collection time measurements can give significantly underestimated lifetime assessment. On the other hand, for the low drift velocity gases, they give much more accurate results. With the use of the analytical model, the electron lifetime was determined more precisely.

Evolution of grain structures during directional solidification of silicon wafers

The evolution of grain structures, especially the types of grain boundaries (GBs), during directional solidification is crucial to the electrical properties of multicrystalline silicon used for solar cells. To study this, the electric molten zone crystallization (EMZC) of silicon wafers at different drift speeds from 2 to 6mm/min was considered. It was found that <111> orientation was dominant at the lower drift velocity, while <112> orientation at the higher drift velocity. Most of the non-∑GBs tended to align with the thermal gradient, but some tilted toward the unfavorable grains having higher interfacial energies. On the other hand, the tilted ∑3GBs tended to decrease during grain competition, except at the higher speed, where the twin nucleation became frequent. The competition of grains separated by ∑GBs could be viewed as the interactions of GBs that two coherent ∑3n GBs turned into one ∑3nGB following certain relations as reported before. On the other hand, when ∑ GBs met non-∑ GBs, the non-∑ GBs remained which explained the decrease of ∑ GBs at the lower speed.

Influence of Drift Velocity and Distance Between Jet Particles on the Penetration Depth of Shaped Charges

AbstractThe penetration depth of shaped charge jet into target is strongly affected by the stand‐off. The penetration process terminates even when the jet velocity is still high, and the penetration capability of jet particles degrades after jet breakup at a large stand‐off. This work presents an analytical model to describe the radial drift velocity and distance between jet particles, which leads to decreased penetration depth. The results show that jet particles with low drift velocity impact the crater wall easily. Furthermore, the jet particles cannot reach the crater bottom to increase depth because the crater diameter generated by the jet is quite small. Moreover, the distances between jet particles also play an important role in penetration depth under the influences of strain hardening of target, as well as tumbling and dispersion of jet particles. The radial drift velocity and distance between jet particles are investigated by applying the model to non‐precision charge and precision charge penetrations into target at different stand‐offs. The cutoff jet velocity and cutoff penetration velocity also are determined based on the analytical model. With increased stand‐off, the cutoff jet velocity increases, and the cutoff penetration velocity is almost constant. This result is proven by a number of experiments. The stand‐off curves of two charges are also calculated, and results are in good agreement with experiments. The stand‐off curve can be determined with only two or three experiments using the proposed method. Notably, jet particles should have a slow drift velocity and great penetration capability after breakup for suitable shaped charge.

Wakefield and stopping power of a hydrogen ion beam pulse with low drift velocity in hydrogen plasmas

AbstractA two-dimensional particle-in-cell (PIC) simulation is carried out to study the wakefield and stopping power for a hydrogen ion beam pulse with low drift velocity propagation in hydrogen plasmas. The plasma is assumed to be collisionless, uniform, non-magnetized, and in a steady state. Both the pulse ions and plasma particles are treated by the PIC method. The effects of the beam density on the wakefield and stopping power are then obtained and discussed. It is found that as the beam densities increase, the oscillation wakefield induced by the beam become stronger. Besides, the first oscillation wakefield behind the bunch is particularly stronger than others. Moreover, it is found that the stationary stopping power increases linearly with the increase of the beam density in the linear/semilinear region.

Characteristics of fixational eye movements in people with macular disease.

Fixation stability is known to be poor for people with macular disease and has been suggested as a contributing factor for the poor visual performance of these individuals. In this study, we examined the characteristics of the different components of fixational eye movements and determined the component that plays a major role in limiting fixation stability in people with macular disease. Sixteen observers with macular disease and 14 older adults with normal vision (control observers) monocularly fixated a small cross presented using a Rodenstock scanning laser ophthalmoscope, for trials of 30 seconds. The retinal image and the position of the cross on the retina were recorded digitally. Eye movements were extracted from the recorded videos at a sampling rate of 540 Hz using a cross-correlation technique. A velocity criterion of 8°/s was used to differentiate between slow drifts and microsaccades. Observers with macular disease demonstrated higher fixation instability, larger amplitudes of slow drifts and microsaccades, and lower drift velocities, when compared with older adults with normal vision. The velocity and the rate of microsaccades were comparable between the two groups of observers. Multiple linear regression analysis showed that the amplitude of microsaccades, and to a smaller extent, the amplitude of slow drifts, play a major role in limiting fixation stability. Fixation stability in people with macular disease is primarily limited by the amplitude of microsaccades, implying that rehabilitative strategies targeted at reducing the amplitude of microsaccades should improve fixation stability, and may lead to improved visual functions.

Shear-induced dynamics of polymeric globules at adsorbing homogeneous and inhomogeneous surfaces

The dynamics and adsorption behavior of a single collapsed homopolymer on a surface in shear flow is investigated by means of Brownian hydrodynamics simulations. We study different homogeneous and inhomogeneous surface models and determine dynamic state diagrams as a function of the cohesive strength, the adhesive strength, and the shear rate. We find distinct dynamical adsorbed states that are classified into rolling and slipping states, globular and coil-like states, as well as isotropic and prolate states. We identify two different cyclic processes based on trajectories of the polymer stretching and the polymer separation from the surface. For adsorption on an inhomogeneous surface consisting of discrete binding sites, we observe stick-roll motion for highly corrugated surface potentials. Although the resulting high surface friction leads to low drift velocities and reduced hydrodynamic lift forces on such inhomogeneous surfaces, a shear-induced adsorption is not found in the presence of full hydrodynamic interactions. A hydrodynamically stagnant surface model is introduced for which shear-induced adsorption is observed in the absence of hydrodynamic interactions.

Improvement of Sub-Pixel Position Sensing in Nonuniform Large-Volume Pixelated CdZnTe Crystals

In pixelated 3-D position sensitive CdZnTe detectors, the Compton imaging performance highly depends on the accuracy of the reconstructed 3-D interaction positions. The lateral position resolution is traditionally limited by the anode pixel pitch. The use of transient signals from the pixels that do not collect charge can give a more precise lateral interaction position estimate. Material defects and electric field nonuniformity are present in many large-volume CdZnTe crystals. These detector nonuniformities can severely degrade the accuracy of the sub-pixel position algorithm. It is observed that the rise time of the collecting anode signal tends to be longer for the pixels with nonuniform electron drift velocities. This is likely due to diffusion or trapping and detrapping of the electron cloud as it passes through a region with very low drift velocity. In a nonuniform detector, the sub-pixel position distribution was significantly improved by removing the signals with rise time greater than 500 ns. It is shown that rise time is a good indicator of material nonuniformity and imaging performance. When only events with appropriate rise time of anode signals are used, even poor detector material can yield excellent sub-pixel position information, improving the gamma-ray imaging performance. The sub-pixel position and C/A ratio were calculated based on the digitized preamplifier pulse waveforms from the collecting and non-collecting anode pixels.

Modulation of continuous ion beams with low drift velocity by induced wakefield in background plasmas

AbstractTwo-dimensional particle-in-cell simulations are performed to investigate the propagation of low energy continuous ion beams through background plasmas. It is shown that the continuous ion beam can be modulated into periodic short beam pulses by the induced wakefield, which can be adopted as a method to produce ultrashort ion beam pulses. Furthermore, the transport of the continuous ion beam in plasma with density gradient in the beam propagation direction is proposed and an enhanced longitudinal compression by density gradient is found due to the phase lock of ion pulses in the focusing regions of wakefield and reduced heating of plasma electrons.

Plasma pressure in Mercury's equatorial magnetosphere derived from MESSENGER Magnetometer observations

[1] Since insertion of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft into orbit around Mercury on 18 March 2011, the probe's Magnetometer has routinely observed localized reductions of the magnetic field magnitude below the level predicted by a planetary dipole model corrected for magnetospheric magnetic fields. These magnetic depressions are observed on almost every orbit, and the latitude at which they are observed is local-time dependent. The depression signatures are indicators of the presence of enhanced plasma pressures, which inflate the magnetic field locally to maintain pressure balance, thus lowering the magnetic flux density. Mapping the magnetic depressions in local time and latitude provides insight into the plasma distribution near the planet, which complements that provided by MESSENGER's Fast Imaging Plasma Spectrometer. The spatial distribution shows that magnetic depressions are concentrated in two distinct regions, one near the equator on the nightside and another at high latitudes principally on the dayside. Here we focus on the nightside, equatorial pressure signatures, which we attribute to the magnetotail plasma sheet. The plasma-sheet pressures extend from dusk to dawn and are offset northward from the planetary geographic equator by about 10° in latitude, commensurate with the offset of the planetary dipole. The pressures associated with the plasma-sheet depressions range from 0.1 to 3 nPa and are systematically higher at dawn than at dusk. Proton gradient-curvature and convection drift in Mercury's dipole magnetic field with a dawn-to-dusk electric field result in low drift velocities near dawn, leading to systematically higher densities and pressures at dawn than at dusk, consistent with the observations.

Terahertz instability of optical phonons interacting with plasmons in two-dimensional electron channels

A terahertz (THz) instability can occur when optical phonons interact with drifting solid-state plasmas. We developed a theoretical model for the optical-phonon instability in two-dimensional electron channels. The paper derives the dispersion relation and analyzes the instability using parameters measured in InSb. As the calculations show, strong instability occurs around the longitudinal optical-phonon frequency, and both the growth rate and the unstable frequency band are larger for higher electron densities and lower drift velocities. The results demonstrate the potential of the optical-phonon instability for active THz devices.

Effect of the Drive Parameter on the Differential Fusion Products in Plasma Focus Devices

A moving-boiler model is employed to study the branching ratio of two D-D reaction channels in plasma focus (PF) devices. This model is based on the thermal spreading of the pinched plasma toward the axial direction of the device. In this way, the experimental fusion cross sections of D(d, n)3He and D(d, p)3H reactions are used to calculate the velocity-averaged cross sections due to the penetration of the confined particles into the thermal boiler. The results obtained represent an indispensable effect of the drive parameter on the differential fusion yields. Obviously, an increment of the drive parameter has a high impact on the yields. However, this impact is dominantly in favor of proton yield particularly at lower drift velocity. Furthermore, a saturated condition is obtained at high drift velocity of the boiler. These facts are important for the optimization of fusion emissions in PFs for future applications, particularly in the field of short-lived radioisotope production.

Tide-induced ice cover deformations on the northeastern shelf of Sakhalin Island

The ice drift data obtained with the Furuno radar using a special methodology simultaneously at seven fixed points around the Molikpaq oil-drilling platform on the northeastern shelf of Sakhalin Island in the period from May 14 to 28, 2003 are analyzed. It is shown that at different distances from the shore significant variations are observed both in the alongshore and, especially, transverse components of the tidal seaice drift, which are responsible for the ice cover deformation, whose indices are of a substantial magnitude. The divergence has the maximum positive values (the ice opening is observed) at comparatively low drift velocities of southern bearings soon after the tidal flow direction changed. The ice cover compression was accordingly observed under opposite conditions. The dependence of deformation indices on the tidal cycle phase is well agreed with the results earlier obtained at the coastal Odoptu radar station (RS), where the measurements were carried out using a similar methodology under alike physico-geographic conditions. A steady character of the results obtained allows forecasting the tide-induced ice cover extension and compression, which is of a practical value for providing the ship operations in the region of the Molikpaq platform in the ice season.

Asymmetry reversal of ion collection by mach probes in flowing unmagnetized plasmas

Measurements of ion current in flowing unmagnetized plasmas were performed with planar and spherical Mach probes in two different devices, one a dc multi-dipole plasma device for subsonic flow within a presheath region and the other a double plasma device for supersonic flow. Asymmetry reversal, which is higher ion current to the downstream side of the probe compared with the upstream side current, was observed for high probe bias compared with the electron temperature, relatively low ion drift velocity and Debye length comparable to probe radius. These data are in qualitative agreement with a recent numerical calculation by Hutchinson. As suggested by Hutchinson, it was found that the current ratio depended on the plasma parameters, especially for finite Debye length and high probe bias. Asymmetry reversal emphasizes the lack of validity of using the current ratio except for narrow parameter ranges. This study is the first experiment to demonstrate the non-intuitive phenomenon predicted by Hutchinson's numerical calculation.