Microwave phase shift using ferrite-filled waveguide below cutoff

Neural oscillations play a crucial role in cognition, and several studies emphasize the importance of considering them as traveling waves that propagate through brain regions. In scalp EEG, waves typically travel along the anterior–posterior axis: forward waves (occipital-to-frontal) predominate during sensory stimulation, while backward waves (frontal-to-occipital) emerge during rest and top-down modulation. Within the predictive coding framework, backward waves are proposed to convey predictive signals, whereas forward waves reflect sensory processes and feedforward propagation of prediction errors. In this study, we investigated traveling wave dynamics during a visual entrainment task in neurotypical (N=25) and autistic (Autism Spectrum Disorder - ASD) adults (N=24). Our results replicated previous findings in neurotypical participants, as we observed an increase in backward waves during rhythmic visual stimulation, consistent with enhanced top-down predictions. Notably, we observed the opposite pattern in the ASD group, characterized by a pronounced increase in forward waves at the entrained frequency during visual stimulation. These results align with predictive coding accounts of autistic perception, which hypothesize an imbalance between predictions and sensory evidence. Specifically, an increase in forward wave may reflect a bias toward bottom-up sensory signaling over predictive feedback, due to atypical hierarchical communication across brain regions in ASD. Together, our findings shed new light on the oscillatory dynamics involved in visual entrainment in neurotypical adults and provide novel evidence in favor of predictive coding accounts of autistic perception, as well as a consequent bias toward bottom-up sensory signaling over predictive feedback in a context of visual entrainment. Significance statement Traveling waves reflect the spatiotemporal propagation of neural oscillations, providing a window into hierarchical brain communication. By examining traveling wave dynamics during visual entrainment, we show that neurotypical adults exhibit increased backward (frontal-to-occipital) waves, consistent with enhanced top-down predictive signaling. In contrast, adults with Autism Spectrum Disorder (ASD) display a marked increase in forward (occipital-to-frontal) waves, indicating stronger bottom-up sensory drive. These findings provide electrophysiological evidence for atypical predictive processing in ASD, supporting predictive coding theories that propose an imbalance between sensory evidence and prior expectations. Our results highlight traveling waves as a sensitive neural marker of hierarchical signaling and predictive dynamics across typical and atypical perceptual systems.

A semiclassical model of nonstationary stimulated Raman scattering with account of forward and backward waves of different frequencies is proposed. Equations for the elements of a collective density matrix and electric-field amplitudes are obtained applying the averaging method. It is shown that forward waves are generated by a united polarization component, whereas each of the backward waves is formed by its own component. The description is based on the approximation of small scattering angles and assumes that the quantities considered do not depend on transverse coordinates. The emission dynamics of forward-backward Stokes pulses is investigated both for the linear case and under the strong depletion of pumping and level populations. The domain of applicability of the model of nonstationary forward stimulated Raman scattering is determined.

Pulsatile components of blood pressure may arise from forward (ventricular generated) or backward wave travel in the arterial tree. The objective of this study was to determine the relative contributions of forward and backward waves to pulsatility. We used wave intensity and wave separation analysis to determine pulsatile components of blood pressure during inotropic and vasopressor stimulation by dobutamine and norepinephrine in normotensive subjects and compared pulse pressure components in hypertensive (mean±SD, 48.8±11.3 years; 165±26.6/99±14.2 mm Hg) and normotensive subjects (52.2±12.6 years; 120±14.2/71±8.2 mm Hg). Dobutamine (7.5 μg/kg per minute) increased the forward compression wave generated by the ventricle and increased pulse pressure from 36.8±3.7 to 59.0±3.4 mm Hg (mean±SE) but had no significant effect on mean arterial pressure or the midsystolic backward compression wave. By contrast, norepinephrine (50 ng/kg per minute) had no significant effect on the forward compression wave but increased the midsystolic backward compression wave. Despite this increase in the backward compression wave, and an increase in mean arterial pressure, norepinephrine increased central pulse pressure less than dobutamine (increases of 22.1±3.8 and 7.2±2.8 mm Hg for dobutamine and norepinephrine, respectively; P<0.02). An elevated forward wave component (mean±SE, 50.4±3.4 versus 35.2±1.8 mm Hg, in hypertensive and normotensive subjects, respectively; P<0.001) accounted for approximately two thirds of the total difference in central pulse pressures between hypertensive and normotensive subjects. Increased central pulse pressure during inotropic stimulation and in essential hypertension results primarily from the forward compression wave.

We investigate layered acoustic metamaterials capable of exhibiting a wide variety of wave propagation phenomena, including backward and forward waves with and without negative refraction. The metamaterials are formed by periodically perforated hard plates, which we describe analytically in the limit of small holes compared to both the period and the separation between plates. In particular, we derive expressions for the index of refraction and the transmission and reflection coefficients of finite slabs. We provide illustrative examples of near fields for all four combinations of backward and forward waves with and without negative refraction, along with dispersion diagrams that explain the observed behavior. A comprehensive study of the range of geometrical parameters in which these distinct phenomena are observed is presented as well. These metamaterials hold great promise for achieving full control of sound using simple structures, with potential application to acoustic technologies relying on subwavelength control and imaging of sound.

Summary form only given. The maximum achievable gain in individual sections of helix type traveling wave amplifiers is limited by the requirement that the device be stable with respect to the excitation of backward waves, Generally, there is a critical length above which the backward wave is absolutely unstable. Whether this length is exceeded in a specific device depends on a number of details that must be computed numerically. We will present a model, based on the TWT simulation code CHRISTINE, in which BWO stability is determined including a number of important effects. These are: the placement of severs, their reflection and transmission coefficients, the profile of attenuation along the interaction length, the presence of a driven signal, and the coupling of forward and backward waves due to asymmetries in the helix support structure. Specifically our model calculates the gain for the coupled forward and backward waves in a specified frequency band. Asymmetries result in a stop band near the pi point formed by the coupling of the forward and backward waves. Here the beam interacts with a composite mode formed by the fundamental spatial harmonic of the forward wave and first spatial harmonic of the backward wave. These modes will have differing coupling coefficients with the beam which we calculate using an improved solution of the tape-helix dispersion equations.

A novel nonreciprocal bandstop filter (NBF) using metamaterial Thue-Morse multiferroic superlattices (MTMMS) is presented, where the multiferroic domains with polarization and magnetization simultaneous are arranged along x-axis according to the iterative rule of the Thue-Morse sequence. The dispersion relation is obtained by the transfer matrix method and indicates that the band gaps for the forward wave (FW) and the backward wave (BW) are not symmetrical. Transmission spectrum is utilized to illustrate the band gaps clearly. Two band gaps in the frequency ranges of ω = 20.9 – 23.03 GHz and ω = 63.98 – 64.41GHz can be observed in the MTMMS with the sequence of l=2 for the FW. However, the band gaps for the BW appear in the frequency ranges of ω = 19.49 – 21.92 GHz and ω = 63.12 – 63.98 GHz. The maximum bandwidth of 2.1GHz and an obvious nonreciprocity with respect to the FW and BW can be obtained in this case. The S-parameters can verify this property further. In addition, the NBSF is easy to integrate into practical systems since its size of forming stable band gaps is about one centimeter.

Precursors, or forerunners, are characteristic features of wave propagation in dispersive media. Magnetostatic wave precursors have the feature that their time of arrival does not depend on the center frequency of a propagating microwave pulse but can depend on the dc bias field. In the case of forward volume waves in an infinite, isolated slab, the arrival time depends on neither the frequency nor the bias field. This fact may prove to be of use in the physical characterization of thin films. A phenomenological theory for these precursors based on the approach taken by Sommerfeld and Brillouin is briefly described. In addition, experimental precursors due to forward volume waves, backward volume waves, and surface waves are reported. The qualitative and semiquantitative agreement between theory and experiment is very good.

Spatial inhomogeneity, temporal modulation, and engineered anisotropy of parameters of electromagnetic media offer numerous opportunities for manipulating light–matter interaction over the past decades. Here, we investigate a scenario in which we deal with the temporal interface, hyperbolic anisotropy in the form of layered structures, and frequency dispersion. We theoretically investigate how a monochromatic uniform plane wave – propagating in an unbounded, homogeneous, isotropic dielectric medium – undergoes changes due to the rapid temporal variation of such medium into a hyperbolic dispersive medium formed by the stack of thin metal–dielectric bilayers, in which the metal follows the lossless Drude dispersion and the dielectric is assumed to be dispersionless. We corroborate our analytical results by numerical simulations. We observe several interesting phenomena, such as conversion of the original frequency into three pairs of frequencies, resulting in three sets of forward (FW) and backward (BW) waves. We present the amplitudes and the time-averaged Poynting vectors for such FW and BW waves and discuss some of the salient features of such temporal interface.

Wave separation analysis and wave intensity analysis (WIA) use (aortic) pressure and flow to separate them in their forward and backward (reflected) waves. While wave separation analysis uses measured pressure and flow, WIA uses their derivatives. Because differentiation emphasizes rapid changes, WIA suppresses slow (diastolic) fluctuations of the waves and renders diastole a seemingly wave-free period. However, integration of the WIA-obtained forward and backward waves is equal to the wave separation analysis-obtained waves. Both the methods thus give similar results including backward waves spanning systole and diastole. Nevertheless, this seemingly wave-free period in diastole formed the basis of both the reservoir-wave concept and the Instantaneous wave-Free Ratio of (iFR) pressure and flow. The reservoir-wave concept introduces a reservoir pressure, Pres, (Frank Windkessel) as a wave-less phenomenon. Because this Windkessel model falls short in systole an excess pressure, Pexc, is introduced, which is assumed to have wave properties. The reservoir-wave concept, however, is internally inconsistent. The presumed wave-less Pres equals twice the backward pressure wave and travels, arriving later in the distal aorta. Hence, in contrast, Pexc is minimally affected by wave reflections. Taken together, Pres seems to behave as a wave, rather than Pexc. The iFR is also not without flaws, as easily demonstrated when applied to the aorta. The ratio of diastolic aortic pressure and flow implies division by zero giving nonsensical results. In conclusion, presumptions based on WIA have led to misconceptions that violate physical principles, and reservoir-wave concept and iFR should be abandoned.

In this paper, the numerical solutions of the backward and forward non-homogeneous wave problems are derived to address the nonlocal boundary conditions. When boundary conditions are not set on the boundaries, numerical instability occurs, and the solution may have a significant boundary error. For this reason, it is challenging to solve such nonlinear problems by conventional numerical methods. First, we derive a nonlocal boundary shape function (NLBSF) from incorporating the Pascal triangle as free functions; hence, the new, two-parameter Pascal bases are created to automatically satisfy the specified conditions for the solution. To satisfy the wave equation in the domain by the collocation method, the solution of the forward nonlocal wave problem can be quickly obtained with high precision. For the backward nonlocal wave problem, we construct the corresponding NLBSF and Pascal bases, which exactly implement two final time conditions, a left-boundary condition and a nonlocal boundary condition; in addition, the numerical method for the backward nonlocal wave problem under two-side, nonlocal boundary conditions is also developed. Nine numerical examples, including forward and backward problems, are tested, demonstrating that this scheme is more effective and stable. Even for boundary conditions with a large noise at final time, the solution recovered in the entire domain for the backward nonlocal wave problem is accurate and stable. The accuracy and efficiency of the method are validated by comparing the estimation results with the existing literature.

BackgroundThe age-related increase in pulse pressure (PP) and systolic blood pressure (SBP) is often attributed to alterations in the wave reflection profile and augmented contributions of the reflected waves. However, clinical evidence shows that the stiffening of the proximal aorta with age and the consequent augmentation of the forward pressure wave plays an equally important role. The relative importance of the forward and reflected wave components in essential hypertension has not yet been fully elucidated.ObjectiveThe aim of the current investigation was to simulate the major ageing mechanisms in the arterial system and the heart using a mathematical one-dimensional model of the arterial tree and to assess the evolution of systolic and pulse pressure during normal (non-pathological) ageing.Methods and resultsOur state-of-the-art 1-D model was extended to include turbulence and inertial effects of the flow exiting the left ventricle. Literature data on the age-associated changes in arterial stiffness, peripheral resistance and cardiac contractility were gathered and used as an input for the simulations. The predicted evolution of pressure and augmentation index with age followed accurately the curves obtained in a number of large-scale clinical studies. Analysis of the relative contribution of the forward and backward wave components showed that the forward wave becomes the major determinant of the increase in central and peripheral SBP and PP with advancing age.ConclusionsThe 1-D model of the ageing tree and heart captures faithfully and with great accuracy the central pressure evolution with ageing. The stiffening of the proximal aorta and the resulting augmentation of the forward pressure wave is the major contributor of the systolic pressure augmentation with age.

A study on the transport of charged particles across a magnetic filter field has been carried out in a double plasma device (DPD) and presented in this manuscript. The DPD is virtually divided into two parts viz. source and target regions by a transverse magnetic field (TMF) which is constructed by inserting strontium ferrite magnets into two stainless steel rectangular tubes. Plasma electrons are magnetized but ions are unmagnetized inside the TMF region. Negative voltages are applied to the TMF tubes in order to reduce the loss of electrons towards them. Plasma is produced in the source region by filament discharge method and allowed to flow towards the target region through this negatively biased TMF. It is observed that in the target region, plasma density can be increased and electron temperature decreased with the help of negatively biased TMF. This observation is beneficial for negative ion source development. Plasma diffusion across the negatively biased TMF follows Bohm or anomalous diffusion process when negative bias voltage is very less. At higher negative bias, diffusion coefficient starts deviating from the Bohm diffusion value, associated with enhanced plasma flow in the target region.

SEPARATION OF ARTERIAL PRESSURE INTO SOLITARY WAVES AND WINDKESSEL FLOW

We study the problem of a temporal discontinuity in the permittivity of an unbounded medium with Lorentzian dispersion. More specifically, we tackle the situation in which a monochromatic plane wave forward-traveling in a (generally lossy) Lorentzian-like medium scatters from the temporal interface that results from an instantaneous and homogeneous abrupt temporal change in its plasma frequency (while keeping its resonance frequency constant). In order to achieve momentum preservation across the temporal discontinuity, we show how, unlike in the well-known problem of a nondispersive discontinuity, the second-order nature of the dielectric function now gives rise to two shifted frequencies. As a consequence, whereas in the nondispersive scenario the continuity of the electric displacement D and the magnetic induction B suffices to find the amplitude of the new forward and backward wave, we now need two extra temporal boundary conditions. That is, two forward and two backward plane waves are now instantaneously generated in response to a forward-only plane wave. We also include a transmission-line equivalent with lumped circuit elements that describes the dispersive time-discontinuous scenario under consideration.

A small-signal theory of a resonant relativistic backward-wave oscillator (BWO) operating at a very low-guiding magnetic field is developed. The theoretical approach is based on the successive iteration method of analytical solution of the exact three dimensional (3-D) equations of electrons motion. It was shown that cyclotron resonance effects in relativistic BWO (400 kV) operating at low-focusing magnetic field can lead to three different regimes of combined Cerenkov-cyclotron interaction: 1) cyclotron absorption due to cyclotron resonance between an electron beam and a fast spatial harmonic of forward electromagnetic wave; 2) complete compensation of cyclotron absorption under certain conditions; and 3) efficiency enhancement at low-focusing magnetic field. The theoretical results are compared with measurements of the output microwave power and frequency of the Maryland X-band BWO at very low magnetic field (0.8-4 kG). The measured dependence of the microwave output power on the strength of the guiding magnetic field is consistent with this model. The measured dip in the output power at around 2 kG can be explained as a cyclotron absorption by the forward wave. Design of a BWO operating at low magnetic field has definite advantages compared with a traditional BWO because it provides considerable reduction of volume, weight, and energy consumption of BWO.