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SUMMARYAn underground nuclear explosion (UNE) couples mechanical energy into crustal rock, which propagates as seismic and acoustic waves. These different physical phenomena transport, by different pathways, to standoff detectors at varying distances. The transport pathways attenuate the original signal but in different ways. Enabled by correct statistical weighting, signal attenuation models can be used to combine these disparate sensor data to estimate the yield of an UNE. Contemporaneous statistical models, used in yield estimation, can be improved with an advanced partition of error for these physical signal propagation models. We present an advanced multivariate approach to error modelling of multiphenomenology physical signatures. In addition to measurement error, our error model represents physical model biases as random with a physics-based covariance structure. To illustrate this proposed framework, we demonstrate the estimation of explosion yield using openly available seismic and acoustic data from chemical single-point explosions.

We present an electrostatic Particle-In-Cell (PIC) code on multi-block, locally structured, curvilinear meshes called Curvilinear PIC (CPIC). Multi-block meshes are essential to capture complex geometries accurately and with good mesh quality, something that would not be possible with single-block structured meshes that are often used in PIC and for which CPIC was initially developed. Despite the structured nature of the individual blocks, multi-block meshes resemble unstructured meshes in a global sense and introduce several new challenges, such as the presence of discontinuities in the mesh properties and coordinate orientation changes across adjacent blocks, and polyjunction points where an arbitrary number of blocks meet. In CPIC, these challenges have been met by an approach that features: (1) a curvilinear formulation of the PIC method: each mesh block is mapped from the physical space, where the mesh is curvilinear and arbitrarily distorted, to the logical space, where the mesh is uniform and Cartesian on the unit cube; (2) a mimetic discretization of Poisson's equation suitable for multi-block meshes; and (3) a hybrid (logical-space position/physical-space velocity), asynchronous particle mover that mitigates the performance degradation created by the necessity to track particles as they move across blocks. The numerical accuracy of CPIC was verified using two standard plasma–material interaction tests, which demonstrate good agreement with the corresponding analytic solutions. Compared to PIC codes on unstructured meshes, which have also been used for their flexibility in handling complex geometries but whose performance suffers from issues associated with data locality and indirect data access patterns, PIC codes on multi-block structured meshes may offer the best compromise for capturing complex geometries while also maintaining solution accuracy and computational efficiency.

Predicting variations in the near-Earth space environment that can lead to spacecraft damage and failure is one example of “space weather” and a big space physics challenge. A project recently funded through the Los Alamos National Laboratory (LANL) Directed Research and Development (LDRD) program aims at developing a new capability to understand, model, and predict Space Hazards Induced near Earth by Large Dynamic Storms, the SHIELDS framework. The project goals are to understand the dynamics of the surface charging environment (SCE), the hot (keV) electrons representing the source and seed populations for the radiation belts, on both macro- and micro-scale. Important physics questions related to particle injection and acceleration associated with magnetospheric storms and substorms, as well as plasma waves, are investigated. These challenging problems are addressed using a team of world-class experts in the fields of space science and computational plasma physics, and state-of-the-art models and computational facilities. A full two-way coupling of physics-based models across multiple scales, including a global MHD (BATS-R-US) embedding a particle-in-cell (iPIC3D) and an inner magnetosphere (RAM-SCB) codes, is achieved. New data assimilation techniques employing in situ satellite data are developed; these provide an order of magnitude improvement in the accuracy in the simulation of the SCE. SHIELDS also includes a post-processing tool designed to calculate the surface charging for specific spacecraft geometry using the Curvilinear Particle-In-Cell (CPIC) code that can be used for reanalysis of satellite failures or for satellite design.

Conformal (or body-fitted) electromagnetic particle-in-cell (EM-PIC) numerical solution schemes are reviewed. Included is a chronological history of relevant particle physics algorithms often employed in these conformal simulations. Brief mathematical descriptions of particle-tracking algorithms and current weighting schemes are provided, along with a brief summary of major time-dependent electromagnetic solution methods. Several research areas are also highlighted for recommended future development of new conformal EM-PIC methods.

The sampling weights and approximate stability criteria for two general fourth-order accurate FDTD schemes were previously derived by Smith et al. However, inconsistencies between several of their governing equations and corresponding solutions were recently discovered. Upon rederiving the full 3-D, fourth-order FDTD dispersion relation, two separate errors were identified in the original work. This paper details these errors and provides corrected sampling weights. Exact stability criteria are also derived and stated for the first time.

A subwavelength aperture-microbolometer sensor was simulated using a finite-difference time domain (FDTD) numerical method. The system consisted of a focusing bullseye grating with a central subwavelength aperture. A thin vanadium oxide sensing layer was suspended behind the aperture for sensing purposes, backed by a thin layer of silicon dioxide. The maximum electric field across the vanadium oxide was measured as a function of incident field. Total noise equivalent power (NEP) was estimated to be 180 pW/√Hz at a frequency of 0.665 THz, comparable to similar current micro sensors. The formation of surface plasmonics greatly increases its sensitivity to particular incident wavelengths. This fact, along with its compact size, make this system a promising candidate for numerous low-cost, small-scale terahertz sensing applications.

Electromagnetic particle-in-cell (EM-PIC) numerical simulation frameworks are often based upon the finite-difference (FD) method. Although computationally efficient and easily implemented, their accurate treatment of curved or slanted boundaries is limited to the staircase approximation, which results in both an inaccurate electromagnetic field solution and particle behavior. Finite volume, finite element (FE), and Discontinuous Galerkin EM-PIC schemes have all been developed in attempts to overcome this limitation. While such methods promise better accuracy in the presence of curved boundaries, their corresponding meshes can be denser than their FD counterparts, and their codes more complex. As a result, the total number of system unknowns, computation time, and production time can be much greater.

This paper presents a plastic, surface plasmon (SP) enhanced bullseye antenna for millimeter wave and terahertz (THz) frequency range. A low refractive index plastic layer is used to confine incident electromagnetic energy to the structure. A metal grating layer focuses the incident wave through a subwavelength aperture via SP. The electromagnetic propagation characteristics were simulated and analyzed using both FDTD and commercial FEM software tools. The structure was also fabricated, and experimental data will be presented at the time of the conference.

Microwave plasma-assisted chemical vapor deposition (PACVD) reactors are used for the production of high quality diamond films. Microwave PACVD systems have traditionally been operated at pressures between 10 and 150 Torr, resulting in diamond growth rates of up to 5 um/hr.

Two approaches for coupling terahertz radiation to surface plasmonic waveguides are proposed and simulated. The first approach utilizes a rectangular waveguide with a tapered plasmonic feed, which allows for the gradual confinement of terahertz waves into a plasmonic structure. The second method employs a ribbon dielectric waveguide, which edge couples terahertz radiation onto a surface plasmonic structure. Coupling designs are presented along with simulation results.

Self-consistent simulation of Microwave PACVD reactors at higher pressures is challenging as it involves coupling many different physical phenomena that are highly nonlinear. This paper presents a solution for these problems. Two major components of the simulation include a finite-difference frequency domain (FDFD) electromagnetic model, and a steady-state convective plasma flow model. These two components are run concurrently while converging toward a single, self-consistent solution. To our knowledge, this is the first model to describe in detail the various physical, chemical, and thermal processes occurring during Microwave PACVD diamond film growth at higher pressures (up to 40% atmosphere). Detailed results and comparisons with experimental data will be presented at the conference.

Summary form only given. Microwave plasma-assisted chemical vapor deposition (PACVD) reactors have been used extensively for the growth of diamond films. The design geometric features of these reactors vary to enable control and shaping of the electromagnetic fields and plasma discharge. In particular, the design and tuning of various geometric parameters is known to affect not only the electromagnetic field structure, but also the plasma shape during operation. For example, the positioning of the substrate height is known to greatly affect the plasma characteristics when changed as little as a few millimeters. In the past, empirical experience has often guided decisions for changing these physical parameters during design and operation. A more detailed numerical study of these effects related to the geometry and the interaction of the microwave fields and plasma discharge is required. This study may also lead to more efficient reactor designs and ultimately faster deposition rates.

This paper presents planar metal patterned structures that support surface plasmon polaritons (SPPs) like propagation mode. Four unit cells, each having a solid ground plane, were designed that support SPPs. To optimize return loss, these structures were first simulated as infinitely periodic in two dimensions. These were then further optimized by designing waveguide structures. Using multi-band unit cell designs, THz circuits (transmission line and power splitter) were fabricated and tested. A new approach to probe these planar plasmonic devices is presented using wide band THz dielectric probes. Measured results of THz circuits match closely with simulation results. It is also shown through simulations that these structures can be used in the design of THz chemical and biological sensors.

Summary form only given. Microwave plasma-assisted chemical vapor deposition (PACVD) reactors have been used extensively for the growth of synthetic diamond. Simulations of such reactors have been developed in order to aid in the testing of new designs and parameters. Since this type of diamond growth has historically been carried out at relatively low pressures (less than 100 Torr), the plasma transport properties have been approximated as purely diffusive. However, recent experiments citing numerous advantages of growing at higher pressures (100-300 Torr) have suggested this approximation to be insufficient. Thus, a more advanced transport model accurately predicting complex convective plasma flows is required. This paper details a self-consistent multi-physics model that simulates microwave PCAVD diamond reactors at higher pressures of 100-300 Torr. As with previous simulations, a finite-difference electromagnetic simulation is coupled to a plasma fluid model, converging on a self-consistent solution. However, this new simulation includes a time-dependent fluid flow plasma model which includes diffusion, conduction, and convection processes. Moreover, a reactor geometry temperature profile model is also inserted into the solution scheme. The absorbed power within the plasma is passed to the plasma model, while the neutral species temperature, and electron temperature and density are converted to conductivity and passed to the electromagnetic simulation. This process is iterated until a stable solution is achieved. Numerical results of electromagnetic power distribution, species concentration, temperature profiles, and temporal solution convergence will be presented. These results will also be compared to selected experimental data.

In this paper, thin metamaterial-inspired structures are investigated for wide-band absorption of stray signals for THz packages. The absorber itself consists of a low-index, low-loss dielectric sandwiched between a patterned, two-dimensionally periodic, thin metallic layer, and a metal backing. Numerical simulations were performed using both the finite element (FEM) and finite-difference time domain (FDTD) numerical methods. Design, fabrication and tests were carried out for absorbers having center frequencies of 0.2 THz and 0.4 THz. Ultra-wide bandwidth and strong absorption were obtained by taking advantage of skin-effect losses in metamaterial structures, and through multi-stacking of these structures. Absorbers having high absorption coefficients and bandwidth (> 1THz) can easily be fabricated using the approach demonstrated in this paper. Two measurement approaches are applied to characterize these structures, details of which are presented in this paper.

Surface Plasmons (SPs) are often defined as coherent oscillations of free charge bound to the surface of a lossy dielectric, and are extremely sensitive to their surrounding environment at resonance. Similarly, externally-driven resonance of the two-dimensional electron gas (2DEG) within High Electron Mobility Transistors (HEMTs) has been observed [1, 2]. Introducing SP-based electric fields into a HEMT would likely influence such carrier resonance. Since SPs are sensitive to the external environment, we hypothesize that this would, in turn, affect electron behavior in the 2DEG region. If this hypothesis is valid, then one can build extremely sensitive device level sensors. To our knowledge, no such study of the influence of electromagnetic radiation on the quantum mechanical behavior of electrons in HEMTs has been proposed or conducted. This understanding would likely lead to numerous SP-HEMT device applications and technologies. Therefore, this multi-physics framework must be designed and simulated.