Coupled-mode analysis for two-dimensional coaxial Bragg structures with helical ripples

Peculiarities of mode spectrum in two-dimensional Bragg structures

Two-dimensional (2D) Bragg structures have been suggested to realize 2D distributed feedback. The 2D Bragg structures can be obtained by providing 2D periodic perturbations of the waveguide surface. Such a structure can be used in a wide variety of applications including high-power microwave electronics and integrated optics. It has been suggested that such structures can be used as passive components such as narrow band mirrors, filters and multiplexers. In this work we present the results of the studies of the properties of such structures at microwave frequencies. It is important to note however that the results are applicable throughout the electromagnetic spectrum.

A coaxial backward wave oscillator with a two-dimensional Bragg structure mounted at the exit from the interaction space is studied theoretically and experimentally. Azimuthal wave fluxes arising in this structure permit one to synchronize radiation of different fractions of a tubular relativistic electron beam with a large diameter. A narrowband radiation of the 8-mm range with a megawatt level of power has been obtained in the experimental prototype of the backward wave oscillator. The prototype was implemented on the basis of the Saturn accelerator (300 keV/200 A/1 μs), as the perimeter of the system constituted more than 15 wavelengths.

A new compact helical system CHS-qa has been designed based on the advanced stellarator concept of the quasi-axisymmetry in the magnetic field configuration. This new concept of quasi-axisymmetry makes it possible to realize the two-dimensional vacuum magnetic field structure (a tokamak-like configuration) without using plasma current. In this system, the neoclassical transport is greatly reduced compared with conventional helical systems because the helical ripples are very small. Good MHD stability for the interchange and ballooning modes are obtained. The possibility of finding the confinement improvement is also discussed from the viewpoint of plasma rotation and maximum-J criterion.

In this paper, we present a general analytical modeling of the steady-state temperature distribution within a two-dimensional multilayered structure heated by arbitrary multi-heaters. Possible heat flux distributions on the heater–layer interface are discussed in detail. For a double-layer device, the selection of these distributions shows negligible influence on the total energy consumption and the average temperature across each heater–layer interface. The effects of the heat conduction inside the heaters are investigated according to the heater–layer thermal conductivity contrast and the thickness-to-width aspect ratio of heaters. Based on this model, we have developed a new method that can directly compute the electricity input powers to maintain required temperatures in specified regions. The cross talk within the heater array can be easily evaluated with this method. Moreover, its application in the thermal design of microfluidic devices is demonstrated by analyzing the experimental results in [2]. The linear relationship between the temperatures of the microchannel and corresponding heater is proven with our analytical model. The solution is also extended to structures with arbitrary embedded thin-film heaters and utilized to evaluate the surface temperature linearity of a microdevice with thin-film heaters that are covered with an insulating SiO2 layer.

Photocurrent generated by ionizing radiation represents a threat to microelectronics in radiation environments. Circuit simulation tools that employ compact models for individual electrical components (SPICE, e.g.) are often used to analyze these threats. Historically, many photocurrent compact models have suffered from accuracy issues due to the use of empirical assumptions, or physical approximations with limited validity. In this paper, an analytic model is developed for epitaxial diode structures that have a heavily-doped sub-collector. The analytic model is compared with both numerical TCAD calculations and the compact model described in reference [1]. The new analytic model compares well against TCAD over a wide range of operating conditions, and is shown to be superior to the older compact model [1]. The methods put forth in this paper could also be applied to model devices with similar physics, such as photonic and power devices.

We present analytical and numerical models of a cluster wind flow resulting from the interaction of stellar winds of massive stars with a supermassive black hole (SMBH). We consider the motion of the stars as well as the gravitational force of the SMBH. In the numerical simulations we consider two cases: the first one with the stars in circular orbits around the SMBH, and the second one with the stars in eccentric orbits. We found that after the system reaches an equilibrium, the circular and elliptical cases are very similar. We found a very good agreement between the analytical and numerical results, not only from our numerical simulations but also from other high-resolution numerical calculations. The analytical models are very interesting, since the properties of such complex systems involving strong winds and a massive compact object can be rapidly inferred without the need of a numerical calculation.

The transport of a currentless ECRH plasma in Heliotron E is studied numerically, using a 1-D transport code with an assumed RF power deposition profile. The investigated parameter range is as follows: 300 ⪅ Te ⪅ 1000 eV, Ti ⪅ 120 eV, 2 < n̄e< 10 X 1012 cm−3 and PRF ≈ 90 kW at B = I T. Four typical models for the transport coefficients are used in the numerical calculations. It is found that a neoclassical model results in a remarkable discrepancy of the time evolution of the electron temperature Te(r) and density ne(r) profiles with regard to the measured values. The neoclassical model which includes the effects of the helical field ripple seems to simulate the saturation of Te(0), in agreement with the experimental results. This saturation is due to the dependence of . However, a consistent description of Te(r) and ne(r) cannot be given by this model. In order to reproduce the measured profiles, it is necessary to use radially increasing transport coefficients. Adding Alcator-like anomalous transport terms (independent of Te) to the neoclassical transport model, it is found that n̄e and PRF dependences of Te(0) can be adequately explained. The fourth model investigated has a temperature-dependent . For ions the n̄e dependence of an anomaly factor, ,is also examined.

An analytic formula is derived for the loss cone in toroidal helical systems. Particular emphasis is put on the loss region for the particles that are barely trapped in the helical ripples. Effects of the radial electric field and shifts of the magnetic axis are discussed. Comparison with the numerical calculation confirms that the simple analytic formula gives good approximations.

Neoclassical diffusion coefficients, D, are evaluated in an ℓ = 2 helical system with ∊h, ≳ ∊t by using the Monte-Carlo method as developed by Boozer and Kuo-Petravic, where ∊h and ∊t are the helical and the toroidal ripple, respectively. An elliptic deformation of the magnetic surface, d, introduces an additional ℓ= 1 helical ripple on the assumption of d ≪ 1. Numerical calculations show that D ∞ l/ν with a coefficient depending on d, where ν is the collision frequency. By adding an ℓ = 3 helical component with no substantial effect on neoclassical transport to the ℓ = 1 and ℓ = 2 helical fields, D ∞ (1+1.2d)/ν is obtained analytically, as a result of helically trapped particles. This will bring about an increase in D of 30–50%, as compared to the assumption of a circular averaged magnetic surface.

Band-gap overlap always occurs when an overmoded coaxial Bragg structure operates in the Terahertz frequency spectrum, which may cause serious competition of undesired modes with the operating mode at the operating frequency. In the present paper it is revealed that band-gap overlap of modes can be efficiently separated by setting the phase difference between the outer- and inner-conductor corrugations to be π. Moreover, the residual side-lobes interaction of the involved modes in a coaxial Bragg structure can be entirely eliminated by applying Hamming-window distribution to both the outer-conductor and inner-rod corrugations. These peculiarities provide potential advantage in constructing a coaxial Bragg cavity with high quality factor for single higher-order-mode operation of a high-power free-electron maser in the Terahertz frequency range.

Moore’s Law scaling has last for almost half century, leading to a tremendous performance/area improvement and cost reduction. Unfortunately, in the past decade, severe challenges have been faced by the CMOS technology as the technology node enters sub-100nm region. To sustain the growing transistor performance and density, many novel device technologies are proposed in the past decade to augment or even replace the conventional Si CMOS technology with Cu interconnect. For the device innovation, ever since the discovery of graphene with excellent physical and electrical properties, much interest has been drawn in the electron-device community and graphene-based FETs have been introduced. Based on the property of the angular dependent transmission probability of electrons observed in GPNJs, an enhanced device structure is presented. More elaborate physical models are also developed to better evaluate the upper limit of delay and power consumptions of GPNJ circuits, including ON resistance, leakage current, contact resistance, and footprint area. For the low-power applications, TFETs show promise in overcoming the power wall faced by thermionic FETs by allowing significant reduction in the supply voltage. The analytical device-level models for TFETs are developed to efficiently evaluate the overall system-level metrics. It is demonstrated that the IV characteristics of a TFET behaves like a Si CMOS switch, but it provides much lower leakage current and ultra-low supply voltage at the cost of low ON current. For the interconnect technology, as the conventional Cu interconnect scales, the resistance per unit length increases dramatically because of 1) the smaller cross-sectional area and 2) the severe size effects at sub-20nm nodes. To alleviate the interconnect challenge, a novel local interconnect structure and hybrid Al-Cu interconnect architecture are proposed and benchmarked against their copper counterpart in terms of energy and energy-delay product. Alternatively, carbon-based interconnects, such as graphene sheets and carbon nanotubes, are also potential candidates because of their outstanding electrical properties. However, graphene is a two-dimensional structure, and increasing the interconnect pitch does not lower the resistance as fast as it does in copper interconnects. Hence, comparing graphene and copper interconnects strongly depends on the interconnect pitch. As a result, system-level analyses are essential to better understand and evaluate the overall benefits of graphene interconnects. Different from existing system-level performance simulators that are based on cycle-accurate simulations, the present methodology employs three hierarchies of compact analytical models on material, device and interconnect, and system levels, respectively. Significant acceleration is achieved in the simulation speed, making multi-parameter optimization feasible. This run-time efficiency is crucial because many novel device concepts have fundamentally different operational principles, and their on/off currents and input capacitances vary drastically in accordance with their design parameters. This methodology allows technologies to evaluate various trade-offs among key design parameters and to maximize the overall chip throughputs or energy efficiencies of processors in a highly efficient way. For the validation of the proposed methodology, simulation results are compared and well matched with eight commercially available Intel multi-core processors across three technology generations from 65nm to 32nm technology nodes. For the GPNJ-based processors, the proposed design methodology is applied to efficiently perform device-, circuit-, and system-level co-optimization. For given power density and die size area budgets, various device-level parameters, including supply voltage, control voltage, gap distance, and oxide thickness, are optimized for a GPNJ core, where 2.1X throughput improvement is observed for a sharp-corner GPNJ core. This advantage is predominantly because of the smaller output resistance, which reduces both device and interconnect delay and saves the power for repeaters. For the TFET based processors, the results indicate that TFETs have excellent performance at the low power density range due to the low supply voltage. The limitations imposed by the interconnects and the large leakage current at high supply voltage restrict the driving current, leading to a lower performance of the TFETs at a high power density. For the emerging interconnect technology, the proposed Al-Cu hybrid interconnect technology is evaluated by replacing short narrow local signal interconnects by Al interconnects. Six interconnect architecture options are analyzed and their optimal aspect ratio and chip frequency are predicted for five technology generations. The optimization and benchmarking results indicate that the potential improvement in chip clock frequency can be between 50 to 100% for the 7nm technology node. A comprehensive optimization/benchmarking is also performed for the multi-layer graphene interconnects. The results show that a single core using graphene interconnects have a higher throughput within the same power density and die size area because of the power saving offered by the low capacitance graphene interconnects.

The first operation of a coaxial free-electron maser (FEM) based on two-dimensional (2D) distributed feedback has been recently observed. Analytical and numerical modeling, as well as measurements, of microwave radiation generated by a FEM with a cavity defined by coaxial structures with a 2D periodic perturbation on the inner surfaces of the outer conductor were carried out. The two-mirror cavity was formed with two 2D periodic structures separated by a central smooth section of coaxial waveguide. The FEM was driven by a large diameter (7 cm), high-current (500 A), annular electron beam with electron energy of 475 keV. Studies of the FEM operation have been conducted. It has been demonstrated that by tuning the amplitude of the undulator or guide magnetic field, modes associated with the different band gaps of the 2D structures were excited. The Ka-band FEM generated 15 MW of radiation with a 6% conversion efficiency, in good agreement with theory.

Nematic liquid crystals integrated with metallic resonators (metamaterials) are intriguing hybrid systems, which not only offer added optical functionalities, but also promote strong light-matter interactions. In this report, we show with an analytical model that the electric field generated by a conventional oscillator-based terahertz time domain spectrometer is strong enough to induce partial, all-optical switching of nematic liquid crystals in such hybrid systems. Our analysis provides a robust theoretical footing for the mechanism of all-optical nonlinearity of liquid crystals, which was recently hypothesised to explain an anomalous resonance frequency shift in liquid crystal-loaded terahertz metamaterials. The integration of metallic resonators with nematic liquid crystals offers a robust approach to explore optical nonlinearity within such hybrid material systems in the terahertz range; paves the way towards increased efficiency of existing devices; and broadens the range of applications of liquid crystals in the terahertz frequency range.

The distributions of charge and current density resulting from ballistic electrons incident on a two-dimensional parabolic repeller barrier are obtained. Formulae for incidence at an angle are derived, utilizing as initial electron state a wavepacket locating the particle at the source and carrying the electron momentum. Numerical calculations have been performed with expected electron energies below the barrier height so that the transmitted current is of tunnelling origin. When the electrons leave the source at zero speed the resulting distributions are symmetrical about the perpendicular drawn from the source to the barrier axis. The case of incidence at angle pi /3 and a small initial momentum is also studied. Polar diagrams depicting the current field components and the charge distributions round various circles centred at the point of incidence are given. For each circle there are two current maxima, one corresponding to transmission and the other to reflection. The locus of the positions round the various circles where the magnitude of the current field acquires maximum values coincides with the corresponding locus for the charge distribution, apart from a region in the vicinity of the point of incidence in which case the charge distribution does not exhibit distinct maxima in the transmitted region. The hydrodynamic picture obtained for the electron cloud may be useful to experimenters involved in two-dimensional ballistic arrangements, as can be attained at the interface of certain semiconductors.