Traveling-wave Regime Research Articles

An Efficient Tidal Dissipation Mechanism via Stellar Magnetic Fields

Recent work suggests that inwardly propagating internal gravity waves (IGWs) within a star can be fully converted to outward magnetic waves if they encounter a sufficiently strong magnetic field. The resulting magnetic waves dissipate as they propagate outward to regions with lower Alfvén velocity. While tidal forcing is known to excite IGWs, this conversion and subsequent damping of magnetic waves have not been explored as a tidal dissipation mechanism. In particular, stars with sufficiently strong magnetic fields could fully dissipate tidally excited waves, yielding the same tidal evolution as the previously studied “traveling wave regime.” Here, we evaluate the viability of this mechanism using stellar models of stars with convective cores (F-type stars in the mass range of 1.2–1.6 M ⊙), which were previously thought to be weakly tidally dissipative (due to the absence of nonlinear gravity-wave breaking). The criterion for wave conversion to operate is evaluated for each stellar mass using the properties of each star’s interior along with estimates of the magnetic field produced by a convective core dynamo under the assumption of equipartition between kinetic (convective) and magnetic energies. Our main result is that this previously unexplored source of efficient tidal dissipation can operate in stars within this mass range for significant fractions of their lifetimes. This tidal dissipation mechanism appears to be consistent with the observed inspiral of WASP-12b and more generally could play an important role in the orbital evolution of hot Jupiters—and to lower-mass ultra-short-period planets—orbiting F-type stars.

Dynamics of immune memory and learning in bacterial communities.

From bacteria to humans, adaptive immune systems provide learned memories of past infections. Despite their vast biological differences, adaptive immunity shares features from microbes to vertebrates such as emergent immune diversity, long-term coexistence of hosts and pathogens, and fitness pressures from evolving pathogens and adapting hosts, yet there is no conceptual model that addresses all of these together. To this end, we propose and solve a simple phenomenological model of CRISPR-based adaptive immunity in microbes. We show that in coexisting phage and bacteria populations, immune diversity in both populations is coupled and emerges spontaneously, that bacteria track phage evolution with a context-dependent lag, and that high levels of diversity are paradoxically linked to low overall CRISPR immunity. We define average immunity, an important summary parameter predicted by our model, and use it to perform synthetic time-shift analyses on available experimental data to reveal different modalities of coevolution. Finally, immune cross-reactivity in our model leads to qualitatively different states of evolutionary dynamics, including an influenza-like traveling wave regime that resembles a similar state in models of vertebrate adaptive immunity. Our results show that CRISPR immunity provides a tractable model, both theoretically and experimentally, to understand general features of adaptive immunity.

Development of a Millimeter-Long Travelling Wave THz Photomixer

THz sources based on the optical-heterodyne (photo)mixing in an ultrafast photodetector are very promising since they operate at room temperature, are potentially compact, cost-efficient and, above all, are widely frequency-tunable. However, their widespread use is currently hampered by available power levels in the μW range at THz frequencies. We present here a travelling wave structure, with millimeter level coherence length at THz frequencies opening the way to large active area (~4000 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) photomixing devices capable of handling optical pump power beyond 1 W well beyond the capabilities of standard lumped-element devices using small active areas (<; 50 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) needed to maintain a capacitance level (<; 10 fF) compatible with THz operation. It is based on a silicon nitride waveguide coupled to a membrane-supported low-temperature-grown GaAs photoconductor embedded in a coplanar waveguide. Milliwatt power levels up to 1 THz and still above 1 μW up to 4 THz are expected according to the optoelectronics model of this device elaborated in this study. Experimentally, the frequency response of a 1-mm-long structure, measured up to 100 GHz by using the beatnote produced by two 780-nm-DFB lasers, shows clearly the expected travelling wave signature consisting in a 6-dB-decrease ending at ~50 GHz when the contribution of the backward travelling wave is fully cancelled, following by a constant level up to ~100 GHz. The experimental demonstration of operation in the travelling wave regime is a first step towards the fulfillment of the original promises of this concept in terms of power level and frequency bandwidth.

The evolutionary origin of the universal distribution of mutation fitness effect.

An intriguing fact long defying explanation is the observation of a universal exponential distribution of beneficial mutations in fitness effect for different microorganisms. To explain this effect, we use a population model including mutation, directional selection, linkage, and genetic drift. The multiple-mutation regime of adaptation at large population sizes (traveling wave regime) is considered. We demonstrate analytically and by simulation that, regardless of the inherent distribution of mutation fitness effect across genomic sites, an exponential distribution of fitness effects emerges in the long term. This result follows from the exponential statistics of the frequency of the less-fit alleles, f, that we predict to evolve, in the long term, for both polymorphic and monomorphic sites. We map the logarithmic slope of the distribution onto the previously derived fixation probability and demonstrate that it increases linearly in time. Our results demonstrate a striking difference between the distribution of fitness effects observed experimentally for naturally occurring mutations, and the "inherent" distribution obtained in a directed-mutagenesis experiment, which can have any shape depending on the organism. Based on these results, we develop a new method to measure the fitness effect of mutations for each variable residue using DNA sequences sampled from adapting populations. This new method is not sensitive to linkage effects and does not require the one-site model assumptions.

Computation of the Linear and Nonlinear Stability of a Two-Layer Couette Flow

Abstract—This work considers the linear and nonlinear stability of a two-layer Couette flow in a horizontal channel. At the first stage, the Navier–Stokes equations are linearized in both phases. We then solve the spectral problem and investigate the dynamics of periodic disturbances in a wide range of changes in the volumetric content of liquids and the velocity of the top plate. Both the neutral and the fastest growing disturbances of the unstable interface mode are calculated. At the second stage, nonlinear stationary traveling wave regimes were calculated for the Couette flow in a horizontal channel using the complete Navier–Stokes equations for both liquids. The results are compared to the experimental data available in the literature.

Response of a Cold-Electron Bolometer on THz Radiation from a Long YBa2Cu3O7−δ Bicrystal Josephson Junction

The response of the Cold-Electron Bolometers (CEBs), integrated into a 2-D array of dipole antennas, has been measured by irradiation from YBa2Cu3O7−δ (YBCO) 50 μm long Josephson junction into the THz region at frequencies from 0.1 to 0.8 THz. The possibility of controlling the amplitude-frequency characteristic is demonstrated by the external magnetic field in the traveling wave regime of a long Josephson junction. The YBCO junction has been formed on the bicrystal Zr1−xYxO2 (YSZ) substrate by magnetron sputtering and etching of the film. CEBs have been fabricated using an Al multilayer structure by a self-aligned shadow evaporation technique on Si substrate. Both receiver and oscillator have been located inside the same cryostat at 0.3 K and 2.7 K plates, respectively.

Wave Regimes of Electroconvection of a Low Conducting Liquid under Unipolar Injection of a Charge in a Steady Electric Field

We have analyzed nonlinear regimes for a nonuniformly heated low conducting liquid in a horizontal capacitor in the gravity field and in a steady electric field under unipolar injection of a charge from the cathode and heating from above. We have considered a model in which the density of charges injected from the cathode is proportional to the electric field strength in the capacitor. For thermal Rayleigh number Ra = –1000, we have constructed the bifurcation diagram of regimes. We have analyzed various electroconvection regimes (stationary regime as well as the regime of standing and traveling waves). Analyzing the effect of increasing heating from above, we have detected transitions between the regimes: the traveling wave regime changes to the standing wave regime, which is transformed into the regime of a modulated standing wave.

Oscillatory Regimes in a 1D Josephson Junction Array with a Nonlocal Delayed Coupling

We have numerically investigated a series array of electromagnetically coupled Josephson junctions considering the coupling delay. In the general case of a nonzero delay, we have derived equations for the slow and fast phases in the low-frequency approximation. We have studied the regimes of oscillations of a Josephson junction array for different positions of the bias point on the current–voltage characteristics (including its reverse branch). Similar analysis has been performed for systems of equations without coupling delay and for an arbitrary bias current. Several regimes of steady-state oscillations have been detected, i.e. synchronous oscillations, traveling wave regime, regime of partial switching-off of junctions, and chimera states.

On the Convection of a Binary Mixture in a Horizontal Layer Under High-frequency Vibrations

The convective instability and non-linear flows are considered in a horizontal, binary-mixture layer with negative Soret coupling, subjected to the high-frequency vibration whose axis is directed at an arbitrary angle to the layer boundaries. The limiting case of long-wave disturbances is studied using the perturbation method. The influence of the intensity and direction of vibration on the spatially-periodic traveling wave solution is analyzed. It is shown that the shift in the Rayleigh number range, in which the traveling wave regime exists, toward higher values is a response to a horizontal-to-vertical transition in the vibration axis orientation. The characteristics of amplitude- and phase-modulated traveling waves are obtained and discussed.

Josephson Traveling-Wave Antennas

We propose a new approach to the problem of obtaining coherent radiation from systems with a great number of Josephson junctions, which is based on the concept of traveling-wave antennas. The traveling wave in a line ensures identity of the electrodynamic conditions, under which the junctions operate, whereas the energy leakage to radiation in the lateral direction prevents saturation of the nonlinearity of the individual junctions having a small dynamic range. Simple analytical models, which demonstrate feasibility of the traveling-wave regime, are considered. A code for direct numerical simulation of Josephson microchips including microantennas, lumped elements, and power supply circuits have been developed. Using the direct numerical simulation, a version of the Josephson antenna, which is similar to the simplest single-wire antenna, is studied and the possibility to realize the traveling-wave regime is demonstrated.

Stationary and non-stationary chimeras in an ensemble of chaotic self-sustained oscillators with inertial nonlinearity

We study spatial structures arising in an ensemble of Anishchenko–Astakhov chaotic self-sustained oscillators with non-local coupling. Diagrams of the regimes realizing in this system are constructed numerically. The peculiarities of formation of chimera states appearing with decreasing the coupling strength are analyzed. A new type of the chimera state which is born from the traveling wave regime is demonstrated.

Investigation of the hybrid electron linac with negative group velocity

Hybrid accelerator, incorporating travelling wave (TW) and standing wave (SW) structures is proposed and discussed. Accelerator can sum up an RF focusing in the SW buncher and lower losses in the TW accelerating structure walls. Moreover, the structure without dumping load is proposed. Input power and beam loading are chosen to minimize power reflection from buncher maintain travelling wave regime in the accelerating structure while beam loading is on. In this case SW buncher operates as a dumping load, so all input power either goes to beam or dissipates in the wall losses, it increases structure efficiency. RF characteristics and beam dynamics simulations have been performed.

Flow regimes for fluid injection into a confined porous medium

AbstractWe report theoretical and numerical studies of the flow behaviour when a fluid is injected into a confined porous medium saturated with another fluid of different density and viscosity. For a two-dimensional configuration with point source injection, a nonlinear convection–diffusion equation is derived to describe the time evolution of the fluid–fluid interface. In the early time period, the fluid motion is mainly driven by the buoyancy force and the governing equation is reduced to a nonlinear diffusion equation with a well-known self-similar solution. In the late time period, the fluid flow is mainly driven by the injection, and the governing equation is approximated by a nonlinear hyperbolic equation that determines the global spreading rate; a shock solution is obtained when the injected fluid is more viscous than the displaced fluid, whereas a rarefaction wave solution is found when the injected fluid is less viscous. In the late time period, we also obtain analytical solutions including the diffusive term associated with the buoyancy effects (for an injected fluid with a viscosity higher than or equal to that of the displaced fluid), which provide the structure of the moving front. Numerical simulations of the convection–diffusion equation are performed; the various analytical solutions are verified as appropriate asymptotic limits, and the transition processes between the individual limits are demonstrated. The flow behaviour is summarized in a diagram with five distinct dynamical regimes: a nonlinear diffusion regime, a transition regime, a travelling wave regime, an equal-viscosity regime, and a rarefaction regime.

Convection in a colloidal suspension in a closed horizontal cell

The experimentally detected [1] oscillatory regimes of convection in a colloidal suspension of nanoparticles with a large anomalous thermal diffusivity in a closed horizontal cell heated from below have been simulated numerically. The concentration inhomogeneity near the vertical cavity boundaries arising from the interaction of thermal-diffusion separation and convective mixing has been proven to serve as a source of oscillatory regimes (traveling waves). The dependence of the Rayleigh number at the boundary of existence of the traveling-wave regime on the aspect ratio of the closed cavity has been established. The spatial characteristics of the emerging traveling waves have been determined.

It takes a village to raise a tide: non-linear multiple-mode coupling and mode identification in KOI-54

We explore the tidal excitation of stellar modes in binary systems using Kepler observations of the remarkable eccentric binary KOI-54 (HD 187091; KIC 8112039), which displays strong ellipsoidal variation as well as a variety of linear and non-linear pulsations. We report the amplitude and phase of over 120 harmonic and anharmonic pulsations in the system. We use pulsation phases to determine that the two largest amplitude pulsations, the 90th and 91st harmonics, most likely correspond to axisymmetric m = 0 modes in both stars, and thus cannot be responsible for resonance locks as had been recently proposed. We find evidence that the amplitude of at least one of these two pulsations is decreasing with a characteristic time-scale of ∼100 yr. We also use the pulsations’ phases to confirm the onset of the travelling wave regime for harmonic pulsations with frequencies ≲50 Ωorb, in agreement with theoretical expectations. We present evidence that many pulsations that are not harmonics of the orbital frequency correspond to modes undergoing simultaneous non-linear coupling to multiple linearly driven parent modes. Since coupling among multiple modes can lower the threshold for non-linear interactions, non-linear phenomena may be easier to observe in highly eccentric systems, where broader arrays of driving frequencies are available. This may help to explain why the observed amplitudes of the linear pulsations are much smaller than the theoretical threshold for decay via three-mode coupling.

Quasioptical Theory of Relativistic Čerenkov Generators and Amplifiers

We study propagation of electromagnetic waves above periodically corrugated surfaces and excitation of such waves by rectilinear relativistic electron flows within the framework of the quasioptical approach. Near a periodic structure, the electromagnetic field can be expanded into a sum of spatial harmonics, which in the case of a low corrugation depth are paraxial wave beams whose relationship is described within the framework of the method of equivalent surface magnetic currents. Under these assumptions, we obtain a dispersion relation for normal waves, and two extreme cases are singled out on its basis. In the first case, the radiation frequency is far from the Bragg resonance frequency, and wave propagation can be described within the framework of the impedance approximation, where the field is represented in the form of a fundamental slow wave and its spatial harmonics. In the case of interaction with the electron flow, this case corresponds to a convective instability, where the particles are synchronous with the fundamental (zeroth) or higher spatial harmonics (the traveling-wave tube regime), and an absolute instability, where the particles are synchronous with the spatial harmonic of the backward waves (the backward-wave oscillator regime). In the second extreme case, which is realized at frequencies close to the Bragg resonance, the field is represented as two counter-propagating quasioptical wave beams. This determines the absolute character of the instability, which is used in generators of the surface wave at the π-type modes. Basing on the developed theory, we determined the main characteristics of the amplifier and generator schemes including growth rates, energy exchange efficiency, and the formation of a spatial self-consistent structure of the radiated field. Good prospects for application of relativistic amplifiers and generators of surface waves in the submillimeter-wave band is demonstrated.

Traveling-wave convection in colloids stratified by gravity

Thresholds of convection excitation and nonlinear convective flow patterns in a horizontal colloidal-mixture layer heated from below are investigated. We take into consideration the fact that, provided the barometric stratification has been reached prior to imposing the temperature gradient, only oscillatory disturbances develop. The influence of separation ratio, sedimentation length, and Prandtl number on the thresholds of oscillatory convection is studied. To examine the complex nonlinear dynamics of the system, numerical simulations with realistic boundary conditions have been carried out using a finite difference method. Long-wave and cellular instability modes as well as transitions between the conductive state and the traveling-wave regime are discussed. It is shown that the traveling-wave regime is stable within a specified range of heating intensity (the Rayleigh number interval). Complex bifurcation and spatiotemporal properties of dissipative structures caused by the interaction of the gravity-induced concentration gradient, nonlinear advection, and mixing of the fluid with nanoparticles are considered.

Artificial Faraday rotation using a ring metamaterial structure without static magnetic field

A metamaterial structure composed of a periodic array of conductive rings including each a semiconductor-based isolator is experimentally shown to produce Faraday rotation. Due to the presence of the isolators, a unidirectional traveling-wave regime is established along the rings, generating rotating magnetic moments and hence emulating the phenomenon of electron spin precession. The metamaterial exhibits the same response as a magnetically biased ferrite or plasma, but without the need of any static magnetic field bias, and therefore, it is easily integrated in printed circuit technology.

Design of hybrid electron linac with standing wave buncher and traveling wave structure

A disk-loaded waveguide (DLW) is the most common structure for compact linear accelerators working in a traveling wave (TW) regime. Among its advantages are high shunt impedance and manufacturing simplicity. The other popular structure is an on-axis coupled bi-periodical accelerating structure (BAS) that works in standing wave (SW) regime. Both the standing and the traveling wave regimes have their own advantages and disadvantages. The design of the hybrid accelerator with SW buncher and TW accelerating section presented in this paper unites the advantages of both regimes. For example, the buncher in the hybrid accelerator is shorter than in a pure TW accelerator, and it requires no solenoid; this structure is more technologically convenient as it does not require a circulator.The other way to combine the advantages of DLW and BAS is to design a magnetic coupled disk-loaded waveguide (DLW-M). This paper also presents the results of a survey study that analyzed the electrodynamical parameters of such a structure and compared them with those of DLW. The experimental data is also presented. Higher order modes, multipacting discharge and thermal simulations show that DLW-M structure is more preferable to classical DLW.

The properties of a dynamo wave in the case of intense meridional circulation

The solution of a Parker dynamo was constructed for the case of intensive meridional circulation using a method similar to WKB. We show how to build a solution in the transition from the traveling-wave regime to the regime of stationary magnetic field configuration. A solution of the Hamilton-Jacobi equations was discovered for the problem of a dynamo containing a triple point in the complex plane wave vector.