Traveling-wave Structures Research Articles

Steady toroidal current drive with m=1 radio frequency travelling wave structures

The authors report on experimental measurement of toroidal current Iphi up to about 1 kAmp driven by m=1 helical antennas fed at 330 kHz. Poloidal currents I0 are driven simultaneously by the same structure. The measurements are interpreted in terms of the nonlinear Hall effect terms being the active driving terms in the generalized Ohm's law.

Experimental study of MESFET travelling-wave structures

Abstract Wave impedances and propagation constants of waves travelling along the electrodes of a long FET structure were experimentally investigated. For this purpose, a GaAs MESFET with large gate width was fabricated. Connecting pads on both ends of the gate and drain electrodes make it possible to measure the four-port S-parameters of the structure. Using these results, it is shown that a great improvement in the gain is possible, if the normally open ends of the FET are connected to the optimum terminations.

Design Principles for Microwave Heating and Sintering

AbstractMicrowave heating equations are shown to have a complex functional dependence on material and process parameters such as composition, density, temperature, frequency and geometry. Power flow and its deposition in the presence of simple material shapes are used to illustrate relevant electromagnetic boundary conditions. Various types of applicator design using standing wave and travelling wave structures are discussed with respect to the results on power density, process efficiency and controllability. The problem of measuring and controlling the temperature inside a microwave system is addressed with emphasis on the use of thermocouples and infrared thermometry. Some of the problems peculiar to high temperature processing are considered. A review of the dielectric and magnetic property variation with temperature and frequency is given in order to understand the complexity and difficulty of uniformity of heating even with microwaves. The use of combination processing, for example, microwaves and infrared heating, is discussed. A short bibliography is presented listing some important publications in the field.

Experiments on Stochastic Cooling of 200 MeV Protons

Equipment for the stochastic cooling of 200 MeV protons in the Fermilab cooler ring has been installed and operated. Vertical and longitudinal cooling systems were installed by early 1980 and had successfully operated by May 1980. Traveling-wave structures particularly effective for the sub-relativistic beam velocities were used for the pickup and kicker electrodes.

The Traveling Wave IMPATT Mode: Part II -- The Effective Wave Impedance and Equivalent Transmission Line

The coupling between a microstrip and a distributed IMPATT diode was investigated in a field analysis. An effective wave impedance in the traveling wave diode can be defined as the ratio of the space-average transverse electric and magnetic fields. This impedance is related to an effective characteristic impedance by a geometry factor. Thus the coupling question is reduced to the coupling between two transmission lines. In addition the diode is modeled in an equivalent transmission line. The equivalent series impedance and shunt admittance are found. The shunt admittance is approximately equal to the admittance (per unit length) of a discrete diode of identical doping profile. The coupling analysis presented here seems applicable to microstrip interfaces to traveling wave structures other than the IMPATT diode.

Computer Calculations of Traveling-Wave Periodic Structure Properties

The versatility and accuracy of programs such as LALA and specially SUPERFISH to calculate the rf properties of standing-wave cavities for linacs and storage rings is by now well established. Such rf properties include the resonant frequency, the phase shift per periodic length, the E- and H-field configurations, the shunt impedance per unit length and Q. While other programs such as TWAP have existed for some time for traveling-wave structures, the wide availability of SUPERFISH makes it desirable to extend the use of this program to traveling-wave structures as well. That is the purpose of this paper. In the process of showing how the conversion from standing waves to traveling waves can be accomplished and how the group velocity can be calculated, the paper also attempts to clear up some of the common ambiguities between the properties of these two types of waves. Good agreement is found between calculated results and experimental values obtained earlier.

Scattering of electromagnetic waves by traveling-wave structures in the ionosphere

The diffraction of electromagnetic waves, incident obliquely on an ionospheric layer with density inhomogeneities of the traveling-wave type, is investigated by S. M. Rytov's method of smooth perturbations. The spatial dependencies of the properties of the ionosphere and of the wavelike inhomogeneities are taken into consideration. The diffraction patterns are analyzed for several variants of height profiles of the wave perturbations. The solutions obtained make it possible to estimate the effect of the form of ionospheric profile and the distribution of the level of disturbance of the wavelike inhomogeneities with respect to altitude on the diffraction effects.

The Traveling-Wave IMPATT Mode

The small-signal analysis of a distributed IMPATT diode ifdicates the existence of a traveling-wave mode. The severe power-frequency limitation as well as the associated low impedance level of the discrete diode appear avoidable. No external resonant circuitry is needed. It is shown that the TEM parallel-plate waveguide mode of the junction is modified by the injection of electrons at the p+ -n junction (or Schottky contact). The transverse electric field takes on a traveling-wave nature in the transverse direction tracking the injected electrons, and a small longitudinal electric field will also be present. In previous papers on IMPATT traveling-wave structures, the IMPATT effect was lumped into an effective complex permittivity in a composite layer model or into an effective shunt admittance in a transmission line model. The current work attempts to incorporate the IMPATT mechanism into the wave model and considers the actual carrier field interaction. The srnall-signal analysis yields an analytic field solution and a characteristic equation for the complex propagation constant. Solutions are found and documented for various frequencies and bias current densities. For the particular structure considered, at 12 GHz with a bias current density of 1000 A/cm/sup 2/ a gain of 72 dB/cm was found.

Beam chopper for subnanosecond pulses in scanning electron microscopy

A beam deflection system for an AUTOSCAN scanning electron microscope is described. The primary electrons (PE) are deflected in the electrostatic field of a symmetrical deflection system consisting of two trough-type travelling-wave structures with a bandwidth of 8 GHz. For routine measurements the system is driven with pulse generators limiting the PE pulse width to 350 ps; it is optimised like the electron gun to an acceleration voltage of 2.5 kV and can be externally adjusted. The degradation of local resolution due to defocusing and the virtual shift of the crossover is largely avoided by applying deflection voltages of opposite polarity to the two structures and by limiting the beam path within the deflection system.

Design considerations for resonant travelling wave IMPATT oscillators†

It is shown theoretically that distributed IMPATT structures can be used as resonant lengths of active transmission line if particular care is taken to choose the substrate thickness and resistivity correctly in order to minimize losses. In contrast to discrete IMPATT structures, a relatively high substrate resistivity is required for the travelling wave structures. Such self-resonant oscillating structures can have an input admittance of the same order as the total depletion layer negative conductance, since the shunting effect of the depletion layer capacitance has automatically been tuned out. Thus, for a given value of circuit impedance the distributed structure can be much larger in area, and output power than the corresponding discrete IMPATT. It is shown theoretically possible to produce a 450 W pulsed device with an input impedance of −1Ω, although a 100 W device with an input impedance of −4Ω might be a more practicable objective.

Beam Loading of RF Cavities

An analysis of the microwave properties of an RF cavity, including the effects of beam interaction, is given. The analysis includes the effect of input coupling mis-match and transient conditions. The principal result is that the energy gain must be represented by a cubic expression rather than the quadratic form applicable to traveling wave structures.

The deflection sensitivity of traveling-wave electron-beam deflection structures

A combination experimental-analytical method for easily determining the frequency-dependent deflection sensitivity of traveling-wave deflection structures is described. Experimental measurements have been made to verify the accuracy of the proposed method.

Multimode propagation on radiating traveling-wave structures with glide-symmetric excitation

Standing-wave measurements on a periodic traveling-wave array of dipoles driven with glide-symmetric excitation from a coaxial transmission line show the existence of multimode propagation in certain frequency bands. This phenomenon is explianed qualitatively in terms of coupling of symmetric and antisymmetric modes of the dipole array by the excitation from an unbalanced line. It has not been found when the dipoles are driven from a balanced two-conductor line. The avoidance of such multimode phenomena is crucial to the self-balancing property of successful log-periodic dipole arrays.

The electronic theory of tape-helix traveling-wave structures

An electronic theory of the operation of tape-helix traveling-wave structures is given. This includes the case of traveling-wave amplifiers and backward-wave oscillators operating on any space harmonic. The method of solution is based upon an integral equation for the longitudinal electric field which is solved approximately for the case of small beam currents. This method is equally applicable to thin beams, thick beams, and annular beams with or without a transverse variation of dc density or velocity. Explicit expressions are obtained for the impedance and space-charge parameter of each space harmonic.

The electronic theory of tape–helix traveling–wave structures

An electronic theory of the operation of tape-helix traveling-wave structures is given. This includes the case of traveling-wave amplifiers and backward-wave oscillators operating on any space harmonic. The method of solution is based upon an integral equation for the longitudinal electric field which is solved approximately for the case of small beam currents. This method is equally applicable to thin beams, thick beams, and annular beams with or without a transverse variation of dc density or velocity. Explicit expressions are obtained for the impedance and space-charge parameter of each space harmonic.

A Traveling-Wave Electron Deflection System

Several types of traveling-wave electron deflection structures that can be used in microwave oscillographs are described and compared. An interaction structure consisting of a folded wire over a plane is considered in detail, both theoretically and experimentally. A general analysis of the interaction of electrons with sinusoidally varying transverse electric fields is presented and is applied to traveling-wave deflection systems. This analysis gives quantitative information about the interdependence of deflection and drift space lengths, beam velocities, frequencies and phase velocities along the structure. Limitations on the design and performance of traveling-wave deflection systems can be determined from this analysis.