J.H. Booske

Folded waveguide traveling-wave tube sources for terahertz radiation

Microfabricated folded waveguide traveling-wave tubes (TWTs) are potential compact sources of wide-band, high-power terahertz radiation. We present feasibility studies of an oscillator concept using an amplifier with delayed feedback. Simulations of a 560-GHz oscillator and experimental evaluation of the concept at 50 GHz are presented. Additionally, results from various fabrication methods that are under investigation, such as X-ray lithography, electroforming, and molding (LIGA), UV LIGA, and deep reactive ion etching are presented. Observations and measurements are reported on the generation of stable single-frequency oscillation states. On varying the feedback level, the oscillation changes from a stable single-frequency state at the threshold to multifrequency spectra in the overdriven state. Simulation and experimental results on amplifier characterization and dynamics of the regenerative TWT oscillator include spectral evolution and phase stability of the generated frequencies. The results of the experiment are in good agreement with the simulations.

Microwave ponderomotive forces in solid-state ionic plasmas*

Numerous observations have been reported in the literature of enhanced mass transport and solid-state reaction rates during microwave heating of a variety of ceramic, glass, and polymer materials. An explanation for these controversial observations has eluded researchers for over a decade. This paper describes a series of recent experimental and theoretical investigations that provide an explanation for these intriguing observations in terms of ponderomotive forces acting on mobile ionic species. The ponderomotive phenomenon, like its conventional-plasma analog, can be described in the continuum model limit by combining the continuity, Poisson’s, and transport equations. However, the solid-state plasma version typically manifests as a result of gradients in mobile charge mobility (e.g., near physical surfaces or interfaces), whereas the conventional plasma ponderomotive transport is typically a consequence of gradients in the radiation field intensity. Both cases can be captured in a single, general, mathematical articulation developed in terms of the mobile particle fluxes.

Microfabrication and Characterization of a Selectively Metallized W-Band Meander-Line TWT Circuit

Vacuum electronic devices offer significant potential for increased power and performance at millimeter-wave frequencies. However, new approaches are required to reliably manufacture the miniature electromagnetic circuits used at these high frequencies. In this paper, we describe the design, fabrication, and testing of an innovative meander-line slow-wave structure for a W-band traveling-wave tube (TWT). The unique challenge of metallizing only the top of a high-aspect-ratio serpentine dielectric ridge using conventionally planar microfabrication techniques is overcome using a novel selective masking and metallization process. The procedure is demonstrated by fabricating a W-band meander-line circuit for a 10-W continuous-wave TWT. Cold-test S -parameter measurements are presented.

TWO-PLANE FOCUSING OF HIGH-SPACE-CHARGE SHEET ELECTRON BEAMS USING PERIODICALLY CUSPED MAGNETIC FIELDS

Numerical and theoretical analyses show that stable, two-plane focusing of finite width, elliptical cross section, sheet electron beams with high space charge (low voltage, high current density) can be accomplished using periodically cusped-magnetic (PCM) fields. Magnetic field strength requirements for focusing high-space-charge sheet beams are within technological capabilities of modern permanent magnet technology. Both an offset-pole PCM stack and a PCM stack combined with a periodic quadrupole magnet (PQM) edge array are shown to be effective for two-plane sheet beam confinement. The PCM-PQM hybrid configuration offers inherent advantages for independent adjustment of confinement fields to achieve beam matching (minimum ripple) in both transverse dimensions. The offset-pole configuration offers the advantage of open-side access for applications such as vacuum electronic microwave devices. It is also shown that PCM-focused sheet beam envelope stability obeys criteria equivalent to that previously ident...

Wave dispersion and growth analysis of low‐voltage grating Čerenkov amplifiers

A theoretical and computational investigation of an electron sheet beam propagating over a grating structure in a rectangular waveguide is carried out. Regimes for low voltage Cerenkov amplifier operation are sought by examining the complex dispersion relation for hybrid waveguide modes in the slow wave structure, which includes sheet beam space‐charge effects. A computer code is developed to examine the complex dispersion relation and growth rates for the wave modes. Mode competition is considered and methods to reduce it are presented. Briggs’ criteria is utilized to examine absolute and convective wave growth for the forward wave, backward wave, and transition mode regimes of operation as a function of the beam, hybrid mode, and slow wave grating characteristics. An examination of the effects of beam spread on absolute and convective wave growth to determine regimes for amplifier operation is carried out. A modest Maxwellian beam spread is found to yield a regime of effective backward convective amplif...

Modeling and numerical simulations of microwave-induced ionic transport

A numerical model was developed to simulate and study microwave-induced transport in ionic solids. The model is based on continuum equations, is very general, and could be applied to many materials. The assumptions, boundary conditions, initial conditions, and numerical techniques used in the model are described. Results are presented from a study of microwave driven defect transport in sodium chloride. Static, high-frequency, and quasistatic results show that ponderomotive rectification of vacancy fluxes will act to deplete the vacancies in a near-surface region and will continue to pull vacancies to the surface through diffusion kinetics. The ponderomotive driving force for this transport is characterized over a wide range of variable space. The magnitude of the driving force falls right in the range such that it can explain why microwave-enhanced mass transport is observed in some experimental cases but not in others.

Experimental and theoretical investigations of a rectangular grating structure for low-voltage traveling wave tube amplifiers

A periodic, rectangular grating slow-wave-structure is considered for forward and backward wave low-voltage (⩽10 kV) Ku-band traveling wave tube (TWT) amplifiers. For forward wave operation, it is required that the ratio of groove depth, d, to grating period, p, be large (i.e. d/p⩾5) while small values of d/p allow backward wave operation. For large d/p, skin effect losses in the grating slots are large and can substantially reduce the growth rate produced by the beam-slow wave interaction. Phase and amplitude measurements of the grating structures utilizing a slotted line and a fast Fourier transform (FFT) analysis have been carried out. The results show that the measured dispersion relations for both shallow (d/p= 0.446) and deep groove gratings (d/p= 7.43) agree very well with the theoretical dispersion relations. For amplifier experiments, a round “probe” beam (10 kV, 0.25 A, 1 mm radius) from a Litton Pierce electron gun (model M707) is utilized. The beam is confined by means of a 1 kG focusing solen...

Novel TWT interaction circuits for high frequency applications

Summary form only given. The initial focus of this program is on the development of Ka-band TWTs producing 10 W of RF power. These devices would potentially be used as RF sources for phased array antennas. This requires innovative TWT designs, which result in improved repeatability, increased yield and reliability, and reduced cost over existing Ka-band devices. To do this, the batch nature of micro-electro-mechanical systems (MEMS) fabrication techniques is ideal. However, many TWT interaction circuits, such as the conventional helix, are not compatible with MEMS techniques. Thus, Calabazas Creek Research, Inc. (CCR) has computationally investigated several innovative TWT interaction circuits based on MEMS fabrication. These include the square helix, planar helix and modified folded waveguide circuits.

MEASUREMENT OF ELECTRON ENERGY DISTRIBUTION FUNCTION IN AN ARGON/COPPER PLASMA FOR IONIZED PHYSICAL VAPOR DEPOSITION

The electron energy distribution function (EEDF) has been measured under a variety of conditions in an Ar/Cu plasma for ionized physical vapor deposition. The EEDF is directly measured in a system including a direct-current magnetron sputter source for copper and a radio frequency (rf) induction plasma, using a Langmuir probe with a modulated bias voltage in combination with a lock-in amplifier. The experimental data indicate that at fixed rf ionization power, the electron population in the tail of the EEDF is depleted by the introduction of copper vapor, and the electron average energy decreases slightly. Observed changes in the EEDF are attributed to inelastic collisions with copper atoms, which have lower threshold energies for excitation and ionization as well as larger cross sections as compared to argon, and the resulting reduction in the measured plasma potential.

Absolute densities of long lived species in an ionized physical vapor deposition copper–argon plasma

Optical absorption spectroscopy has been used to measure absolute, average gas phase densities of neutral copper, ground and metastable states, and neutral argon, metastable and resonance states, in an ionized physical vapor deposition plasma. Spectroscopic measurements were carried with a xenon arc lamp as a high intensity, continuum light source, and an optical multichannel detector. Copper radiative transitions in the wavelength range of 324.8–510.6 nm and argon radiative transitions in the 706.7–811.5 nm range were employed. The curve of growth method has been used to calculate the absolute line average densities from fractional absorption data. For a copper–argon plasma of neutral pressure 30 and 10 mTorr copper metastable state densities were found to lie in the range of 1010–1012 cm−3. Comparison of these densities with neutral copper densities derived from independent measurements of neutral copper flux at the substrate indicate gas phase temperatures greater than 1500 K under certain experimental...