Backward-traveling Waves Research Articles

High-power performance studies of an S-band high-gradient accelerating cavity for medical applications

High-Gradient accelerating cavities are one of the main research lines in the development of compact linear accelerators. However, the operation of such accelerating cavities is currently limited by non-linear electromagnetic effects that are intensified at high electric fields, such as RF breakdowns, dark currents and radiation. A novel normal-conducting High Gradient S-band Backward Travelling Wave accelerating cavity for medical application (v = 0.38c) has been designed and constructed at CERN with a design gradient of 50 MV/m. In this paper, the high-power performance studies of this novel design carried out at the IFIC high-power laboratory are presented, as well as the analysis of the conditioning parameters in combination with numerical simulations.

Investigating Pressure Patterns in Transformer Tanks after an Interturn Short Circuit: A Finite Element Approach

When an inter-turn short-circuit fault occurs during the operation of a transformer, the arc generates energy that causes the temperature in the tank to rise. This in turn increases the temperature of the insulating oil, vaporizing it, and the rising pressure in the tank acts on the tank such that it can easily explode. The arc energy is related to the initial pressure of the gas and its production in the tank. The pressure wave propagates in insulating oil, and the transient pressure at any point in the path of the pressure wave is the superposition of vectors of forward- and backward-traveling waves. The authors of this study applied a finite element simulation software to establish a model of the transformer tank and used it to analyze the changes in pressure in the tank and on the wall as well as the factors influencing this phenomenon. The results show that the wall pressure of the transformer increases with time after the failure of the interturn short circuit. The pressure wave travels from the initial position of the arc to the periphery and decreases with diffusion effects. The influence of pressure on the transformer tank can be reduced by selecting an appropriate location for the pressure-release valve and can in turn prevent the tank from rupturing due to the impact of rising pressure.

Single-Ended Protection of VSC-HVdc Outgoing Line Based on Backward Traveling Wave Time-Domain Weighted Optimization

Single-Ended Protection of VSC-HVdc Outgoing Line Based on Backward Traveling Wave Time-Domain Weighted Optimization

Protection of Line Faults in HVDC Grids Through Convexity Detection in Backward Traveling Wave Voltages

Short-circuit faults in high-voltage DC (HVDC) grids must be timely isolated. Conventional fault isolation schemes for HVDC grids typically rely on numerical simulations or analytical calculations for threshold setting, which complicate engineering implementation. To solve this issue, this paper first introduces the convex point of backward traveling wave (BTW) voltage as the reference for the faulty line isolation in HVDC grids, which is interpreted as the singular feature in the second derivative (SD) of BTW voltage. The singularity in the SD of BTW voltage is independent of system or fault parameters, having good generality in HVDC grids. Through recognizing the unique pattern in the wavelet transform modulus maxima of the singular feature, the proposed protection method can identify faulty lines in HVDC grids in the initial phase of faults. Numerical studies carried out with PSCAD/EMTDC and MATLAB/Simulink demonstrate the effectiveness, robustness and applicability of the proposed method in detecting and isolating various single- and double-pole faults in HVDC grids.

Mathematical Derivation of Wave Propagation Properties in Hierarchical Neural Networks with Predictive Coding Feedback Dynamics

Sensory perception (e.g., vision) relies on a hierarchy of cortical areas, in which neural activity propagates in both directions, to convey information not only about sensory inputs but also about cognitive states, expectations and predictions. At the macroscopic scale, neurophysiological experiments have described the corresponding neural signals as both forward and backward-travelling waves, sometimes with characteristic oscillatory signatures. It remains unclear, however, how such activity patterns relate to specific functional properties of the perceptual apparatus. Here, we present a mathematical framework, inspired by neural network models of predictive coding, to systematically investigate neural dynamics in a hierarchical perceptual system. We show that stability of the system can be systematically derived from the values of hyper-parameters controlling the different signals (related to bottom-up inputs, top-down prediction and error correction). Similarly, it is possible to determine in which direction, and at what speed neural activity propagates in the system. Different neural assemblies (reflecting distinct eigenvectors of the connectivity matrices) can simultaneously and independently display different properties in terms of stability, propagation speed or direction. We also derive continuous-limit versions of the system, both in time and in neural space. Finally, we analyze the possible influence of transmission delays between layers, and reveal the emergence of oscillations.

Non-invasive Assessment by B-Mode Ultrasound of Arterial Pulse Wave Intensity and Its Reduction During Ventricular Dysfunction.

Arterial pulse waves contain clinically useful information about cardiac performance, arterial stiffness and vessel tone. Here we describe a novel method for non-invasively assessing wave properties, based on measuring changes in blood flow velocity and arterial wall diameter during the cardiac cycle. Velocity and diameter were determined by tracking speckles in successive B-mode images acquired with an ultrafast scanner and plane-wave transmission. Blood speckle was separated from tissue by singular value decomposition and processed to correct biases in ultrasound imaging velocimetry. Results obtained in the rabbit aorta were compared with a conventional analysis based on blood velocity and pressure, employing measurements obtained with a clinical intra-arterial catheter system. This system had a poorer frequency response and greater lags but the pattern of net forward-traveling and backward-traveling waves was consistent between the two methods. Errors in wave speed were also similar in magnitude, and comparable reductions in wave intensity and delays in wave arrival were detected during ventricular dysfunction. The non-invasive method was applied to the carotid artery of a healthy human participant and gave a wave speed and patterns of wave intensity consistent with earlier measurements. The new system may have clinical utility in screening for heart failure.

Development of a compact linear accelerator to generate ultrahigh dose rate high-energy X-rays for FLASH radiotherapy applications.

In recent years, the FLASH effect, in which ultrahigh dose rate (UHDR) radiotherapy (RT) can significantly reduce toxicity to normal tissue while maintaining antitumor efficacy, has been verified in many studies and even applied in human clinical cases. This work evaluates whether a room-temperature radio-frequency (RF) linear accelerator (linac) system can produce UHDR high-energy X-rays exceeding a dose rate of 40Gy/s at a clinical source-surface distance (SSD), exploring the possibility of a compact and economical clinical FLASH RT machine suitable for most hospital treatmentrooms. A 1.65m long S-band backward-traveling-wave (BTW) electron linac was developed to generate high-current electron beams, supplied by a commercial klystron-based power source. A tungsten-copper electron-to-photon conversion target for UHDR X-rays was designed and optimized with Monte Carlo (MC) simulations using Geant4 and thermal finite element analysis (FEA) simulations using ANSYS. EBT3 and EBT-XD radiochromic films, which were calibrated with a clinical machine Varian VitalBeam, were used for absolute dose measurements. A PTW ionization chamber detector was used to measure the relative total dose and a plane-parallel ionization chamber detector was used to measure the relative normalized dose of each pulse. The BTW linac generated 300-mA-pulse-current 11 MeV electron beams with 29kW mean beam power, and the conversion target could sustain this high beam power within a maximum irradiation duration of 0.75 s. The mean energy of the produced X-rays was 1.66 MeV in the MC simulation. The measured flat-filter-free (FFF) maximum mean dose rate of the room-temperature linac exceeded 80Gy/s at an SSD of 50cm and 45Gy/s at an SSD of 67.9cm, both at a 2.1cm depth of the water phantom. The FFF radiation fields at 50cm and 67.9cm SSD at a 2.1cm depth of the water phantom showed Gaussian-like distributions with 14.3 and 20cm full-width at half-maximum (FWHM) values, respectively. This work demonstrated the feasibility of UHDR X-rays produced by a room-temperature RF linac, and explored the further optimization of system stability. It shows that a simple and compact UHDR X-ray solution can be facilitated for both FLASH-RT scientific research and clinical applications.

Non-synchronous blade vibration analysis of a transonic fan

This study numerically investigates the aeromechanic behavior of a transonic fan model with a flat tip-leading-edge on the NASA rotor 67 test case. Single-passage unsteady calculations at a near stall operating point of 82% design speed show that the dominant frequencies of mass flow were not the harmonics of the rotor rotational frequency. A full-annulus fluid–structure interaction analysis was subsequently carried out to examine the unsteady flows and their interactions with blade vibrations. The results show that the modal displacement of the backward traveling seventh nodal diameter of the second torsion mode grew exponentially, which reveals that the blade vibration was non-synchronous. The vibration pattern indicates that the aerodynamic mode was resonant with the structural vibration mode. Around the rotor tip, the circumferential vortical propagation induced by interactions among the main flow, tip leakage flow, and tip clearance vortex was the source of aerodynamic excitation. To clarify the mechanism of the non-synchronous vibration, the coupling between aerodynamic disturbance and structural response, i.e., aliasing, was summarized. The frequency spectra of the fluctuating pressure show that an aerodynamic Backward Traveling Wave (BTW) was co-aliased to a structural BTW due to the propagation of the circumferential vortex. The correlation between the frequency and free convective speed of the aerodynamic disturbance determined the directions of aliasing.

Spatial and spectral filtering of tapered lasers by using tapered distributed Bragg reflector grating

A 1040 nm tapered laser with tapered distributed Bragg reflector (DBR) grating is designed and fabricated. By designing the grating with tapered layout, the tapered DBR grating exhibits the scattering effect on side backward-traveling waves, thus achieving additional suppression of parasitic oscillation. Under the suppression of parasitic oscillation, the spatial and spectral characteristics of the tapered laser are improved. The experimental results show that a near-Gaussian far-field distribution and a kink-free P–I characteristics are achieved, and a single peak emission with a wavelength of 1046.84 nm and a linewidth of 56 pm is obtained.

Factorized One-Way Wave Equations

The method used to factorize the longitudinal wave equation has been known for many decades. Using this knowledge, the classical 2nd-order partial differential Equation (PDE) established by Cauchy has been split into two 1st-order PDEs, in alignment with D’Alemberts’s theory, to create forward- and backward-traveling wave results. Therefore, the Cauchy equation has to be regarded as a two-way wave equation, whose inherent directional ambiguity leads to irregular phantom effects in the numerical finite element (FE) and finite difference (FD) calculations. For seismic applications, a huge number of methods have been developed to reduce these disturbances, but none of these attempts have prevailed to date. However, a priori factorization of the longitudinal wave equation for inhomogeneous media eliminates the above-mentioned ambiguity, and the resulting one-way equations provide the definition of the wave propagation direction by the geometric position of the transmitter and receiver.

Dispersion diagrams of linear slow-wave structures. Identification of electromagnetic waves, all electromagnetic waves: forward-traveling, backward-traveling and standing electromagnetic waves

The analytically calculated dispersion diagram of an overmoded 9-period V-band slow-wave structure (SWS) designed to operate in a high-order transverse-magnetic TM03 waveguide mode of a backward-traveling electromagnetic wave [Ye et al., Phys. Plasmas, 22, 063104 (2015)] is re-examined using the two-dimensional (2D) Poisson SuperFish code allowing to precisely calculate spatial patterns of electric field vectors, wavenumbers and characteristic frequencies of all azimuthally-symmetric TM0nl cavity modes that exist in a closed SWS, where n and l are the radial and axial indices of TM0nl cavity modes, respectively [Main et al., IEEE Trans. Plasma Sci., Vol. 22, No. 5, Oct. 1994, p. 566]. It is shown that the analytically calculated higher-order TM0n waveguide modes of the open SWS are actually not pure TM0n modes, but fascinating combinations of selected portions of the modes with selected portions of all possible lower-order TM0(n-j) waveguide modes, where j=1 … (n-1). An analytically calculated TM03 waveguide mode, therefore, turns out to be in fact a combination of three different TM0n waveguide modes sequentially identified by numerically calculated TM0nl cavity modes: (i) TM03l mode with l=0 … 4, (ii) TM01l mode with l=13 … 11, and (iii) TM02l mode with l=8 … 9.

Experiment study on traveling wave resonance of fatigue fracture of high-speed bevel gear in aero-engne based on acoustic measurement method

High-speed, light-weight and high-load gearing is widely used in the transmission systems of modern aero-engines. Some dynamic phenomena, such as the traveling wave resonance, can be more easily encountered. It generally occurs quite suddenly and causes catastrophic accidents. This study presents an experimental investigation of the signal characteristics of the gear fatigue fracture process caused by the traveling wave resonance. The acoustic waveguide system and the dynamic calibration system are developed. The traveling wave resonance monitoring test and the fatigue performance test of the central drive bevel gear in an aero-engine are carried out. The results show the traveling wave resonant frequency and rotational speed of the driven bevel gear are identified and monitored effectively by the acoustic waveguide system. The dangerous rotational speeds range and the sound pressure energy radiated of the vibration are discussed. The results reveal the gear resonant phenomena are narrow rotational speed band sensitivity and more dangerous at high speed. The fracture mode of the driven bevel gear of aero-engine is reproduced. When the gear has initial defects and works within the dangerous rotational speed of the traveling wave resonance, the gear fatigue fracture will occur quickly. The harmonic frequency, the frequency of forward traveling wave (FTW) and backward traveling wave (BTW), and their combination frequency will appear alternately in the noise spectrum of the gear before fatigue fracture.

A new method of single terminal traveling wave location based on characteristic of superposition of forward traveling wave and backward traveling wave

This paper proposed a novel one-terminal traveling -wave-based fault-location method for transmission lines. Traveling waves (TWs) are induced in transmission lines by the occurrence of a fault. Different are formed by the superposition of forward and backward TWs when a fault occurs. The proposed fault-location method used a fault-location function to determine the location of faults from corresponding TW change-points. The proposed fault-location method was evaluated through modeling and simulating a three-phase 500 kV/50 Hz transmission system using PSCAD/EMTDC. The performance of the proposed fault-location method was evaluated for using different angles of fault inception, fault impedances, sampling rates, and noise. The fault-location method proposed in this paper was verified using real-world testing, which involved taking measurements from a TW-based fault-location device.

Mechanisms of gradual pressure drop in angiographically normal left anterior descending and right coronary artery: Insights from wave intensity analysis

BackgroundThis study evaluated the mechanism of decline in coronary pressure from the proximal to the distal part of the coronary arteries in the left anterior descending (LAD) versus the right coronary artery (RCA) from the insight of coronary hemodynamics using wave intensity analysis (WIA). MethodsTwelve patients with angiographically normal LAD and RCA were prospectively enrolled. Distal coronary pressure, mean aortic pressure, and average peak velocity were measured at 4 different positions: 9, 6, 3, and 0 cm distal from each coronary ostium. ResultsThe distal-to-proximal coronary pressure ratio during maximum hyperemia gradually decreased in proportion to the distance from the ostium (0.92±0.03 and 0.98±0.03 at 9 cm distal to the LAD and RCA ostium). WIA showed the dominant forward-traveling compression wave gradually decreased and the backward-traveling suction wave gradually decreased in proportion to the decrease in coronary pressure through the length of the non-diseased LAD but not the RCA. ConclusionsThe pushing wave and suction wave intensities on WIA were diminished in proportion to the distance from the ostium of the LAD despite the wave intensity not changing across the length of the RCA, which may lead to gradual intracoronary pressure drop in the angiographically normal LAD.

Mechanism of pulsus bisferiens in thoracoabdominal thoracic aneurysms: Insights from wave intensity analysis.

Aortic pulsatile hemodynamics are important in various clinical conditions. Whereas the importance of wave reflections of the closed type in pulsatile hemodynamics has been extensively studied, less is known about the impact of reflections of the open type, in which reflected waves changes both direction and type (compression vs suction) compared to the incident wave. In this report, we present careful pulsatile hemodynamic analyses of a case in which prominent reflections of the open type occur in a patient with a thoracoabdominal aortic aneurysm, causing a highly abnormal proximal aortic and peripheral arterial hemodynamic pattern, known as pulsus bisferiens. Wave intensity analysis of central pressure‐flow data demonstrated an early systolic forward‐traveling compression wave followed by a prominent late systolic forward‐traveling expansion wave, along with an abnormal prominent late systolic/early diastolic backward‐traveling compression wave which produced a sharp rise in diastolic pressure, and was responsible for the pulsus bisferiens pattern.

An Improved DC Fault Protection Scheme Independent of Boundary Components for MMC Based HVDC Grids

For modular multilevel converter (MMC) based DC grids, current-limiting reactors (CLRs) are mainly employed to suppress the fault current and provide boundary effects to detect internal faults. Thus, most existing protection schemes are highly dependent on the larger CLRs to guarantee high selectivity. However, in existing MMC based HVDC projects, the size of CLRs is restrained by the cost, weight, and system stability under normal state. Thus, boundary protections may fail to detect high-resistance faults and pole-to-ground faults under weak boundary conditions. To overcome these shortcomings, this paper proposes a fast and selective DC fault detection scheme independent of boundary components. The propagation characteristics of line-mode backward traveling-waves (TW) are analyzed to identify external and internal faults. The polarities of zero-mode backward TWs are employed to select faulted poles. The directional overcurrent based pilot protection is adopted as a complementary criterion to detect remote faults. The proposed method can be applied in MMC-HVDC systems with small CLRs that cannot provide strong boundary conditions. Besides, the detection speed is fast (less than 1.5ms). Moreover, it is robust to fault resistance and immune to noise. Various simulation results in PSCAD/EMTDC verifies the effectiveness of the proposed scheme.

High-gradient testing of an S -band, normal-conducting low phase velocity accelerating structure

A novel high-gradient accelerating structure with low phase velocity, $v/c=0.38$, has been designed, manufactured and high-power tested. The structure was designed and built using the methodology and technology developed for CLIC $100\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ high-gradient accelerating structures, which have speed of light phase velocity, but adapts them to a structure for nonrelativistic particles. The parameters of the structure were optimized for the compact proton therapy linac project, and specifically to 76 MeV energy protons, but the type of structure opens more generally the possibility of compact low phase velocity linacs. The structure operates in S-band, is backward traveling wave (BTW) with a phase advance of 150 degrees and has an active length of 19 cm. The main objective for designing and testing this structure was to demonstrate that low velocity particles, in particular protons, can be accelerated with high gradients. In addition, the performance of this structure compared to other type of structures provides insights into the factors that limit high gradient operation. The structure was conditioned successfully to high gradient using the same protocol as for CLIC X-band structures. However, after the high power test, data analysis realized that the structure had been installed backwards, that is, the input power had been fed into what is nominally the output end of the structure. This resulted in higher peak fields at the power feed end and a steeply decreasing field profile along the structure, rather than the intended near constant field and gradient profile. A local accelerating gradient of $81\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ near the input end was achieved at a pulse length of $1.2\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$ and with a breakdown rate (BDR) of $7.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}\text{ }\text{ }1/\mathrm{pulse}/\mathrm{m}$. The reverse configuration was accidental but the operating with this field condition gave very important insights into high-gradient behaviour and a comprehensive analysis has been carried out. A particular attention was paid to the characterization of the distribution of BD positions along the structure and within a cell.

A macroscopic traffic flow model considering the velocity difference between adjacent vehicles on uphill and downhill slopes

In this paper, we deduced a macroscopic traffic model on the uphill and downhill slopes by employing the transformation relation from microscopic variables to macroscopic ones based on a microscopic car-following model considering the velocity difference between adjacent vehicles. The angle [Formula: see text] of the uphill and downhill and the gravitational force have a great impact upon the stability of traffic flow. The linear stability analysis for macroscopic traffic model yielded the stability condition. The Korteweg–de Vries (KdV) equation is derived by nonlinear analysis and the corresponding solution to the density wave near the neutral stability line is obtained. By using the upwind finite difference scheme for simulation, the spatiotemporal evolution patterns of traffic flow on the uphill and downhill are attained. The unstable region is shrunken with slope of the gradient increasing and backward-traveling density waves gradually decrease and even disappear on uphill. Conversely, the unstable region on downhill is extended and density waves propagate quickly backward to the whole road with slope of the gradient increasing.

One-end protection algorithm for offshore wind farm HVDC transmission based on travelling waves and cross-alienation

This paper develops a novel one-ended non-communication protection scheme for offshore wind farm connected to the public grid through HVDC cables. The relay is installed at the DC side of the inverter station. The proposed technique for both fault detection and location along HVDC cable is based on evaluating the arriving instants of fault induced backward travelling wave and its subsequence. These instants are determined using the cross-alienation coefficients between the backward travelling waves and a unity reference function. Cross-alienation coefficients values are used for fault detection, while the difference time between two sequential coefficients peaks after fault inception are used for fault location. The required fault detection and location time is very small of milliseconds range. This will enhance the system reliability due to fast DC cables protection. The proposed protection scheme is based on continuous tracking of sampled current and voltage signals. Thereby, the proposed technique is applied on sampled movable windows of the calculated backward travelling wave and the reference function. The simulation results show that the proposed protection scheme is simple, fast, accurate, and not sensitive to fault resistance or load variations. Also its implementation is economically as no need for extra devices or communication links.

Single-ended fault location for direct distribution overhead feeders based on characteristic distribution of traveling waves along the line

To decrease the location error brought by reflected wave crest identification for direct distribution overhead feeders in non-effective grounding system based on single-ended traveling location method, a novel location method is proposed in this paper. Taking advantage of transient information measured at line terminals, Bergeron Transfer Equation is utilized for calculation of voltage and current traveling waves along the faulty feeder. A location function is then built by the forward and backward traveling-wave components to describe the energy distribution of traveling-wave catastrophe. The method is of high precision and feasible for direct distribution overhead feeders through PSCAD simulation. Various measurement conditions are also taken into consideration.