Traveling-wave Structures Research Articles

Creation of spatial structure by an electric field applied to an ionic cubic autocatalator system

The effects of applying electric fields to a reactor with kinetics based on an ionic version of the cubic autocatalator are considered. Three types of boundary condition are treated, namely (constant) prescribed concentration, zero flux and periodic. A linear stability analysis is undertaken and this reveals that the conditions for bifurcation from the spatially uniform state are the same for both the prescribed concentration and zero-flux boundary conditions, suggesting bifurcation to steady structures, whereas, for periodic boundary conditions, the bifurcation is essentially different, being of the Hopf type, leading to travelling-wave structures. The various predictions from linear theory are confirmed through extensive numerical simulations of the initial-value problem and by determining solutions to the (non-linear) steady state equations. These reveal, for both prescribed concentration and zero-flux boundary conditions, that applying an electric field can change the basic pattern form, give rise to spatial structure where none would arise without the field, can give multistability and can, if sufficiently strong, suppress spatial structure entirely. For periodic boundary conditions, only travelling waves are found, their speed of propagation and wavelength increasing with increasing field strength, and are found to form no matter how strong the applied field.

Complex dynamics and phase synchronization in spatially extended ecological systems.

Population cycles that persist in time and are synchronized over space pervade ecological systems, but their underlying causes remain a long-standing enigma. Here we examine the synchronization of complex population oscillations in networks of model communities and in natural systems, where phenomena such as unusual '4- and 10-year cycle' of wildlife are often found. In the proposed spatial model, each local patch sustains a three-level trophic system composed of interacting predators, consumers and vegetation. Populations oscillate regularly and periodically in phase, but with irregular and chaotic peaks together in abundance-twin realistic features that are not found in standard ecological models. In a spatial lattice of patches, only small amounts of local migration are required to induce broad-scale 'phase synchronization, with all populations in the lattice phase-locking to the same collective rhythm. Peak population abundances, however, remain chaotic and largely uncorrelated. Although synchronization is often perceived as being detrimental to spatially structured populations, phase synchronization leads to the emergence of complex chaotic travelling-wave structures which may be crucial for species persistence.

Characteristics of microstrip leaky wave antenna using the method of moments

An efficient MoM algorithm is introduced to analyse large travelling-wave structures, exemplified with a microstrip leaky wave antenna. The method utilises a combination of travelling-wave and subdomain basis functions for the current expansion to minimise the number of unknowns in the analysis. The propagation constants which characterise the modal expansions are evaluated efficiently by the algorithm for the implementation of the travelling-wave basis functions. As a result the model overcomes computer memory problems encountered by the conventional MoM model in analysing large antennas where a large impedance matrix is needed. The algorithm is capable of generating results as accurate as that of the conventional method while achieving substantial reduction in computation time and resources.

A novel analytical expression of saturation intensity of InGaAsP tapered traveling-wave semiconductor laser amplifier structures

We have proposed a novel approximate analytical expression for saturation intensity for tapered traveling-wave semiconductor laser amplifier structures. The application of this analytical expression of saturation intensity has been demonstrated by considering the effect of gain saturation on polarization sensitivity of two tapered amplifier structures, linear and exponential tapered amplifier structures. It is found that polarization sensitivity of the tapered amplifier structure is several decibels higher than that of passive tapered waveguides in unsaturated condition. Polarization sensitivity of the two tapered amplifier structures has also been investigated in a highly saturated condition. The combined effects of mode conversion and gain saturation on fundamental TE gain have also been investigated using the proposed analytical expression for saturation intensity.

The use of active traveling-wave structures in GaAs MMIC's

A coplanar waveguide has been fabricated on a modulation doped GaAs substrate in order to evaluate the potential of traveling-wave structures in microwave applications. The use of a Schottky contact center conductor enables the line to function as a slow wave structure in which the rf propagation characteristics can be modified with a dc bias. Measurements are reported at 10 GHz on simple structures, some of which incorporated an additional dielectric layer. Results show that slow-wave factors of between 8 and 24 are readily obtained with a loss per slow-wave factor of about 0.7 dB/mm. The practical issues relating to the application of such structures in phase shifters, chip size reduction, compact active filters and resonators are examined.

Theory of inductively detuned traveling wave structures

As high power rf extraction cavities, traveling wave structures (TWS) have demonstrated significant advantages due to their inherent low field gradients. For applications involving long, multi-cavity devices such as a relativistic klystrons two-beam accelerator (RK-TBA), the extraction cavities must be inductively detuned to maintain longitudinal beam stability. In this paper, the theory of inductively detuned traveling wave cavities is developed within the framework of a coupled-cavity circuit description of TWS. We determine the output cell parameters (eigenfrequency ω 0 and quality factor Q) required for proper matching to avoid unwanted reflections. We then derive the power balance equation which quantifies the generation and the transmission of electromagnetic energy in each cell of the TWS. An analytic formula for predicting the power output from a TWS is obtained. Finally, using the analytic results derived we check the applicability of the computer code RKS for inductively detuned TWSs.

Efficient full-wave analysis of stratified planar structures and unbiased TW-FET's

We have developed a rigorous full-wave analysis technique capable of characterizing quasiplanar travelling wave structures, consisting of multilayer dielectrics and conductors of finite thickness, also taking into account dielectric and conductor losses. We have studied boxed embedded microstrips and another complex passive structure, namely the T-gate TW-FET, devoting particular attention to the distribution of current inside the metallization. All structures considered were simulated by means of a desktop computer. We have tested our program by comparing our results with experimental data of embedded microstrips and employed it for the characterization of planar and T-type gates of the FET's without bias.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Propagation of electromagnetic and space-charge waves in quasiperiodic structures

An analytic method to calculate the propagation of electromagnetic and space-charge waves in a quasiperiodic disk-loaded waveguide including input and output arms, is presented. The approach relies on Cauchy’s residue theorem to formulate the transmission, reflection, or the Green’s function of a system composed of radial arms and grooves which are attached to a cylindrical waveguide. The only constraint of this method is that the inner radius has to be constant; all the other parameters of the system can be arbitrarily changed. This method is particularly useful for the analysis of input and output section of a high-power traveling wave structures.

Theory and observation of spontaneous oscillatory contractions in skeletal myofibrils.

At low levels of activation, an isometrically-held myofibrillar preparation on the descending limb may exhibit persistent oscillations of period 1-6 s in tension and sarcomere lengths. We propose a sarcomeric theory of spontaneous oscillatory contraction, based on the phenomena of force creep and delayed length activation. The time delay leads to oscillations and controls their period. A computer model using these ideas simulates spontaneous oscillatory contraction for fixed-end fibres only if isometric tension capacity varies slightly along the fibre. The form of this inhomogeneity controls a diversity of spontaneous oscillatory contraction behaviour: the tension waveform can vary from large and sinusoidal to small-amplitude pulses or chaotic behaviour, and these variations are observed in slow-twitch soleus fibres from the same animal (rat). The model predicts that oscillatory and quiescent regions coexist in the fibre, with large-amplitude sawtooth waveforms in sarcomere length in the former as observed. It can also generate travelling-wave structures, similar to those found by the Tokyo group, in oscillating regions when there is a spatial gradient in isometric tension capacity. Phase discontinuities in sarcomere length occur near the oscillatory-quiescent boundary. Predictions for the Ca2+ concentrations and sarcomere lengths in which spontaneous oscillatory contraction occurs and for differences in the spontaneous oscillatory contraction frequencies of fast- and slow-twitch fibres compare well with experiment. Spontaneous oscillatory contraction is also predicted under isotonic conditions.

Active and nonlinear wave propagation devices in ultrafast electronics and optoelectronics

We describe active and nonlinear wave propagation devices for generation and detection of (sub)millimeter wave and (sub)picosecond signals. Shock-wave nonlinear transmission lines (NLTL's) generate /spl sim/4-V step functions with less than 0.7-ps fall times. NLTL-gated sampling circuits for signal measurement have attained over 700-GHz bandwidth. Soliton propagation on NLTL's is used for picosecond impulse generation and broadband millimeter-wave frequency multiplication. Picosecond pulses can also be generated on traveling-wave structures loaded by resonant tunneling diodes. Applications include integration of photodetectors with sampling circuits for picosecond optical waveform measurements and instrumentation for millimeter-wave waveform and network (circuit) measurements both on-wafer and in free space. General properties of linear and nonlinear distributed devices and circuits are reviewed, including gain-bandwidth limits, dispersive and nondispersive propagation, shock-wave formation, and soliton propagation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Particle micromanipulator consisting of two orthogonal channels with travelling-wave electrode structures

Abstract In the paper experimental and numerical results for a simple particle micromanipulator fabricated in silicon technology are presented. It consists of three orthogonal liquid filled channels above meander-shaped planar electrode strips. By applying appropriate alternating, rotating or travelling electric fields in the chamber dielectric forces acting on particles suspended in the liquid are induced allowing trapping, movement and separation of them. The efficient manipulation of small particles with typical dimensions of several micrometers using electrogradient techniques (EGT) requires electrode structures of the same size. Due to the complex electrode geometry a numerical procedure is used for calculation of particle trajectories and optimising the design. In the micro range small fabrication defects are likely to cause large changes of the properties of the manipulator. Therefore, a test procedure based on electrode processes in aqueous media and the pH-dependent fluorescence intensity of a marker solution which enabled us to visualise the working states, surface coatings and fabrication defects of microstructured electrodes via a microscope is introduced.

Extended Theory of Beam Loading in Electron Linac

A new class of disk-loaded traveling-wave structures is proposed. This class is derived from direct generalization of conventional structures, and contains the constant-impedance (CZ) and constant-gradient (CG) structures as members. A beam-loading theory extended for this class implies that within the class, there exists a structure better than the conventional structures in field uniformity and other aspects, when a large-current beam is accelerated. A structure, taken as an example for the case of 10-MeV acceleration of a 1-A beam, is 12% shorter in dimension and 7% more efficient in microwave-to-beam power conversion than the constant-gradient structure.

Ponderomotive focusing in axisymmetric rf linacs.

In many present and planned compact electron sources and linacs, very large rf accelerating fields are present in the linac structures. A charged particle whose trajectory is parallel to the axis receives no net focusing from the periodic transverse forces arising from the axisymmetric rf wave. However, if the lowest-order periodic transverse motion of the particles is included, a net ponderomotive focusing force is obtained when the momentum transfer is averaged over a period of the motion. Since this force is second order in the electric-field amplitude, the resultant focusing can be non-negligible in high gradient structures. In order to produce and preserve high-brightness beams in rf linacs it is necessary to obtain a better understanding of all the forces acting on the beam and the subsequent beam dynamics. To this end we examine the lowest-order ponderomotive rf focusing of electrons in axisymmetric standing- and traveling-wave linac structures. We obtain a generalized paraxial envelope equation which includes the effects of the ponderomotive focusing and longitudinal acceleration to describe the transverse beam envelope evolution. The implications of our results on beam dynamics in electron linacs are examined, and extensions of the results to positron and proton linacs are discussed.

Strongly interacting travelling waves and quasiperiodic dynamics in porous medium convection

Buoyancy-driven convection in a square container of porous medium, related to Rayleigh-Bénard convection, is studied in the parameter regime (Rayleigh parameter) where periodic and quasiperiodic dynamics occur. It is shown to have a branch structure determined by simple pairwise interactions of traveling waves. The simultaneous existence of two separate branches of traveling waves (stable or unstable) distinguished by temporal and spatial character (frequency and wavenumber) leads to a connecting branch of quasiperiodic motions. Furthermore, the spatial structure of the quasiperiodic convection is fixed by the wavenumbers of the two traveling wave structures to which it is connected. A simple bifurcation diagram built from several such pairwise interactions accounts for all the behavior, including stable tori, “reverse transitions” and hysteresis, reported in previous simulations and predicts behavior verified by simulations first reported here. The interaction picture for the square container is inferred from the behavior for a special shallow container (width/height near 2.5) where two traveling waves simultaneously come into existence. At this double Hopf bifurcation center manifold and normal form techniques with coefficients supplied by the “exact” equations (more than 100 ODEs) unravel the nonlinear interactions. It turns out that only a slight generalization of this weakly nonlinear picture is sufficient to infer the bifurcation diagram for the square box.

Analysis and design of multi-gigahertz oscilloscope deflection structures

A numerical model for multigigahertz traveling-wave oscilloscope deflection structures is presented. The model uses the exact electric fields obtained from solving the Laplace equation for the specific structure geometry. A particle-tracking code based on this model serves as a research and design tool for the traveling-wave structures. The code predicts structure characteristics such as bandwidth, distortions, defocusing, etc. Comparison with the experimentally measured performance of a 17-GHz-bandwidth oscilloscope designed with the help of this code shows excellent agreement with the calculated predictions. The code operates under the VAX/VMS system, but it could be adapted to PC systems.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Traveling waves and pulses in a two-dimensional large-aspect-ratio system.

Near a bifurcation point a system far from thermal equilibrium can be described by use of generalized Ginzburg-Landau equations. We present a systematic method to derive the nonlinear interaction terms of these equations in real space reflecting the selection rules as well as the stabilization of different patterns intrinsic in the basic equations of the systems under consideration. Our work treats the case of periodic instabilities of a homogeneous state in space as well as in time, where the interacting patterns are represented by traveling-wave trains having arbitrary orientations in a two-dimensional plane. Numerical solutions of two-dimensional pattern formation and wave propagation are presented using a system that allows for a backward Hopf bifurcation as is the case for the convection instability of a binary fluid mixture. The stability of the emerging traveling-wave structures is discussed in terms of phase-diffusion equations.

Modern circuit models for traveling-wave structures with nearest neighbor coupling

A method of analyzing traveling-wave structures based on a coupled-line physical model and a commercial frequency-domain microwave-circuit-modeling program is described. This approach provides significant insight into the sources of pulse distortion observed in real structures. The model is compared to a real structure to benchmark the model's various components. >

Propagation in periodically loaded corrugated waveguides

The analysis and characterization of corrugated periodic structures in dielectric loaded waveguides are presented. The Floquet theorem and mode-matching techniques are used to perform the analysis. The characteristic equation, which gives the dispersion relation for the periodic structure, is derived. Phase and group velocities of waves on these periodic structures are then obtained from the dispersion curves. These results have potential applications as slow wave structures for traveling-wave tubes and possible filtering structures at millimeter-wave frequencies. >

Traveling-Wave IMPATT Amplifiers and Oscillators

Traveling-wave IMPATT oscillators and amplifiers are analyzed using a Iarge-signal transmission-line model. A specific case for a GaAs structure at 33.7 GHz is examined in detail. General equations relating to the design and expected power output from these devices are also developed. It is concluded that more RF power can he generated by traveling-wave structures than is obtainable from discrete IMPATT devices.

New wideband GaAs travelling-wave device: linear gate transistor

A new device called the linear the gate transistor (LGT) is described which promises in excess of 20 GHz flat-band performance in a single compact structure. The LGT is designed and modelled using a unique software package developed for all travelling-wave structures. Results from a prototype LGT are reported.