William L. Langston

Impedance, axial-ratio, and receive-power bandwidths of microstrip antennas

Closed-form expressions for the impedance and axial-ratio bandwidths of a single-feed circularly polarized microstrip antenna have been derived. Receive-power bandwidths for both linearly and circularly polarized microstrip antennas have also been derived, which quantify the bandwidths when the microstrip antennas are used as receivers. The microstrip antennas are assumed to be probe fed and are modeled as RLC circuits (neglecting the probe inductances). For circular polarization, the probe feed is assumed to be along a 45/spl deg/ line across the microstrip antenna such that two orthogonal modes are identical in magnitude and in phase quadrature at the center frequency. Although derived for microstrip antennas, the expressions for impedance, axial-ratio, and receive-power bandwidth are also valid for other high-Q resonant antenna structures having an impedance behavior that is approximated by the same type of RLC circuits.

Doping tunable resonance: Toward electrically tunable mid-infrared metamaterials

We demonstrate metamaterials at the mid-infrared (mid-IR) wavelengths (8–12 μm) that can be widely tuned by doping in adjacent semiconductor epilayers. The metamaterials are based on metallic split ring resonators (SRRs) fabricated on doped indium antimonide (InSb). Finite integral time-domain simulation results and measured transmission data show that the resonance blueshifts when the semiconductor electron carrier concentration is increased while keeping the split ring geometry constant. A resonant wavelength shift of 1.15 μm is achieved by varying the carrier concentration of underlying InSb epilayer from 1×1016 to 2×1018 cm−3. This work represents the first step toward active tunable metamaterials in the mid-IR where the resonance can be tuned in real time by applying an electric bias voltage to control the effective carrier density.

Spurious radiation from a practical source on a leaky covered microstrip line

The radiated fields from the currents induced on a covered microstrip transmission line by a finite-gap voltage source are presented. The behavior of the bound-mode and continuous-spectrum fields is studied. It is determined that leaky-mode fields can contribute to cross-talk and other interference effects near the source and within an angular leakage region, while bound-mode radiation fields are the predominant mechanism for these effects further away from the gap source outside the leakage region.

Perturbation Theory in the Design of Degenerate Rectangular Dielectric Resonators

The design of resonators with degenerate magnetic and electric modes usually requires the ability to perturb one or both types of modes in order to induce alignment of magnetic and electric properties. In this paper perturbation theory is used to identify difierent types of inclusions that can be used to realize fundamental- mode degeneracy in a rectangular dielectric resonator and thus, can ultimately be used in the design of negative-index metamaterials. For reasons associated with fabrication in the infrared-frequency regime, rectangular resonator designs are of particular interest.

A Quick and Easy Simulation Procedure to Aid in Metamaterial Unit-Cell Design

In this letter, a simple simulation procedure is presented and used to design a negative-index metamaterial unit cell. The procedure is based upon full-wave simulations of a single unit cell where electric and magnetic drives are separated to significantly simplify the interpretation of the effective-media response. More specifically, by extracting polarizabilities from the far-field response of the resonator under these drive conditions, the effective-media parameters are shown to be nicely correlated with the resonant responses of the resonator. For the purposes of demonstrating this simulation procedure, a negative-index metamaterial design based on a composite unit cell containing a split-ring-resonator and z-dipole is employed as a straightforward example.

Perturbation Theory in the Design of Degenerate Spherical Dielectric Resonators

The design of resonators with degenerate magnetic and electric modes for negative-index metamaterial applications usually requires the ability to perturb one or both types of modes in order to induce alignment of the negative magnetic and electric properties. The incentive behind the resonator designs presented in this paper is to minimize both the losses and the size of the unit cell, particularly in the case of high frequencies such as the infrared or visible. Thus, the designs discussed in this paper are based on degenerate-mode (the fundamental magnetic and electric modes are aligned in frequency) spherical dielectric resonators which rely on only a single-particle resonator and thereby do not require physical or electrical extensions of the unit cell, as might be seen with other negative-index unit cell designs. Cavity-perturbation techniques are used to arrive at the types of inclusions (in terms of material, polarization, and placement) that are necessary to realize a degenerate spherical dielectric resonator, as well as to derive simple formulas which can be used for the design of these types of resonators. Rigorous electromagnetic simulations of the degenerate resonators are also provided for comparison to the theoretical derivations.

High-Frequency Pulse Distortion on a Lossy Microstrip Line With a Top Cover

This paper studies the time-domain propagation and dispersion of a pulse propagating on a microstrip line with a metallic top cover. A gap voltage source is used to model a practical excitation on the line. High-frequency distortion effects are observed that cannot be accounted for by conventional transmission-line theory, since they are due to the simultaneous excitation of the bound mode and a strong leaky mode. The bound-mode and leaky-mode components of the pulse are identified and separately studied to aid in the physical interpretation of the pulse distortion. The excitation of a dominant leaky mode gives rise to an interesting pulse-splitting phenomenon, due to the different velocities of the bound mode and the leaky mode. The influence of dielectric and conductor losses on the pulse shape is also studied.

Computer simulations of the magnetically insulated transmission lines and post-hole convolute of ZR

An important consideration for the success of the ZR project, refurbishing the Z accelerator at Sandia National Laboratories, is limiting current loss in the vacuum section, ideally to no worse than the 5 – 10% seen on Z. The primary source for this loss is electrons flowing into the post-hole convolute from the four magnetically insulated transmission lines (MITLs). The MITLs on ZR have larger gaps to reduce the electron flow to values comparable to Z when operating at ∼40% higher voltage and ∼30% higher current. Electron flow in the vacuum section is analyzed with electromagnetic, particle-in-cell simulations, using two complementary simulation setups. First, the exact MITL profiles are modeled with high-resolution 2-D simulations out to large radius (typically r = 60 cm), providing accurate values for the electron flow into the convolute. Second, the convolute is modeled in 3-D, but with MITLs extending out only to r ∼ 30 cm. The 3-D MITL geometry is modified to provide the same electron flow into the convolute as the 2-D simulations. The 3-D simulations have detailed diagnostics for current loss and surface deposition heating in the convolute.

Recyclable transmission line (RTL) and linear transformer driver (LTD) development for Z-pinch inertial fusion energy (Z-IFE) and high yield.

Z-Pinch Inertial Fusion Energy (Z-IFE) complements and extends the single-shot z-pinch fusion program on Z to a repetitive, high-yield, power plant scenario that can be used for the production of electricity, transmutation of nuclear waste, and hydrogen production, all with no CO{sub 2} production and no long-lived radioactive nuclear waste. The Z-IFE concept uses a Linear Transformer Driver (LTD) accelerator, and a Recyclable Transmission Line (RTL) to connect the LTD driver to a high-yield fusion target inside a thick-liquid-wall power plant chamber. Results of RTL and LTD research are reported here, that include: (1) The key physics issues for RTLs involve the power flow at the high linear current densities that occur near the target (up to 5 MA/cm). These issues include surface heating, melting, ablation, plasma formation, electron flow, magnetic insulation, conductivity changes, magnetic field diffusion changes, possible ion flow, and RTL mass motion. These issues are studied theoretically, computationally (with the ALEGRA and LSP codes), and will work at 5 MA/cm or higher, with anode-cathode gaps as small as 2 mm. (2) An RTL misalignment sensitivity study has been performed using a 3D circuit model. Results show very small load current variations for significant RTL misalignments. (3) The key structural issues for RTLs involve optimizing the RTL strength (varying shape, ribs, etc.) while minimizing the RTL mass. Optimization studies show RTL mass reductions by factors of three or more. (4) Fabrication and pressure testing of Z-PoP (Proof-of-Principle) size RTLs are successfully reported here. (5) Modeling of the effect of initial RTL imperfections on the buckling pressure has been performed. Results show that the curved RTL offers a much greater buckling pressure as well as less sensitivity to imperfections than three other RTL designs. (6) Repetitive operation of a 0.5 MA, 100 kV, 100 ns, LTD cavity with gas purging between shots and automated operation is demonstrated at the SNL Z-IFE LTD laboratory with rep-rates up to 10.3 seconds between shots (this is essentially at the goal of 10 seconds for Z-IFE). (7) A single LTD switch at Tomsk was fired repetitively every 12 seconds for 36,000 shots with no failures. (8) Five 1.0 MA, 100 kV, 100 ns, LTD cavities have been combined into a voltage adder configuration with a test load to successfully study the system operation. (9) The combination of multiple LTD coaxial lines into a tri-plate transmission line is examined. The 3D Quicksilver code is used to study the electron flow losses produced near the magnetic nulls that occur where coax LTD lines are added together. (10) Circuit model codes are used to model the complete power flow circuit with an inductive isolator cavity. (11) LTD architectures are presented for drivers for Z-IFE and high yield. A 60 MA LTD driver and a 90 MA LTD driver are proposed. Present results from all of these power flow studies validate the whole LTD/RTL concept for single-shot ICF high yield, and for repetitive-shot IFE.

Cable Braid Electromagnetic Penetration Model.

The model for penetration of a wire braid is rigorously formulated. Integral formulas are developed from energy principles and reciprocity for both self and transfer immittances in terms of potentials for the fields. The detailed boundary value problem for the wire braid is also setup in a very efficient manner; the braid wires act as sources for the potentials in the form of a sequence of line multipoles with unknown coefficients that are determined by means of conditions arising from the wire surface boundary conditions. Approximations are introduced to relate the local properties of the braid wires to a simplified infinite periodic planar geometry. This is used in a simplified application of reciprocity to be able to treat nonuniform coaxial geometries including eccentric interior coaxial arrangements and an exterior ground plane.