In low Earth orbit (LEO) partially-ionized space plasmas exist in the vicinity of orbiting objects, such as satellites as well as debris, which travel at speeds much higher than the ion thermal speed, and much slower than the electron thermal speed. Characterizing the local plasma conditions is important for understanding not only charging of the objects but also details of the complex nonneutral plasmas density that affect satellite sensors, communications, and in the case of debris, that might provide a signature that can be used to identify and track them. In such an environment, much of the dynamics happen in the electrostatic approximation limit, and the MICHELLE beam optics code [1]–[3] is an excellent platform for modeling these conditions. Its ability to run in the electrostatic steady-state (SS) domain as well as time-domain (TD) with its unstructured mesh and proven ability to resolve small structures and fine details and regions make it suitable for complex structures where plasma sheaths may exist.

We present advances in the photoemission and secondary emission models and implementations in the MICHELLE charged particle beam optics code. MICHELLE has been developed to include a wide variety of emission models designed to suit a robust set of applications and be customizable to use cases by institutions and groups. In the case of photoemission, there are upgrades to our computational methods to evaluate the Moments model for enhanced computational efficiency. In the case of secondary emission, MICHELLE employs a table-lookup approach that captures the moments of the secondary emission. Upgrades includes not only a more robust secondary particle distribution but a methodology employing the Sandia ITS suite to populate secondary emission tables for materials of interest. The paper highlights these models and features that have evolved over time and identifies cases where they can be applied to improve simulation fidelity or enable new problems to be approached.

We present advances, key features, and advanced capability in the MICHELLE charged particle beam optics code. Although the MICHELLE code was originally developed to support the vacuum electronics and accelerator physics communities, over the years it has had some significant developments that make it a truly general purpose charged particle beam optics code with many advanced models and features. This paper discusses some of the advanced models and features that we find useful in modern design and optimization workflow. These include capability for multi-faceted meshing (2D, 3D, unstructured, structured-multiblock, hybrid, all element shapes), various solution algorithms (steady-state, time-domain, Boltzmann electron, circuit model), latest advances in emission physics (generalized thermionic, general thermal-field with self-consistent heating effects, photoemission, secondary emission), thermal beam models, adjoint analysis integration, multi-physics analysis environments (CAD, meshing, solution, post-processing, pipelining, optimization, sensitivity), inter-code operability within user workflows, resolving highly-disparate feature scales and sizes, and computer platform support (laptop-to-HPC). The paper highlights these models and features that have evolved over time and identifies cases where they can be applied to improve simulation fidelity or enable new problems to be approached.

The status of first principles modeling and simulation tools for vacuum electronics (VEs) is presented. These tools, when combined with parametric beam-wave interaction tools, comprise the suite of tools that enable the design and optimization of VE devices, such as RF amplifiers. Although many basic algorithms discussed in the article have been around for decades, the advances in electromagnetic and electrostatic particle-in-cell and meshless beam codes have revolutionized the VE device development process where first-pass design has become the standard. These include advances in emission physics, disparate-mesh geometry resolution, advanced boundary conditions, intercode operability, multimodule simulation pipelines, exploitation of computer hardware, optimization and sensitivity analysis, and advanced user simulation environments with multimodule pipelines. The article presents these advances.

We report on a new process under development in recent years that enables bringing the effects of detailed meso-scale thermionic emission properties to inform full-scale electron gun simulations. Specifically, detailed material properties and electrical properties of emitter samples are measured and processed, leading to a mapping of the work function across the emitter surface patch that is statistically applicable to the larger surface areas existing in a gun simulation. The measurements and processing subsequently leads to the development of Miram and IV curves for the material, as well as a prediction of the particle energy distribution and intrinsic emittance. Having this, we can then use these properties in simulations for high fidelity gun modeling for design applications. The process includes EBSD measurements, DFT simulations, then fast simulations using new 2-1/2D models from U. Wisconsin and U. Michigan 3D in collaboration with “first-principles” MICHELLE simulations, and will be presented.

We present new advances in the MICHELLE charged particle beam optics code in area of advancing emission models and algorithms that capture more electron emission physics. The new capability covered in this paper concerns the implementation of a new general thermal field (GTF) model that predicts current density in the transition region between temperature limited emission and field emission. In the case of electron thermionic emission where the effective AK gap is not large compared with the potential minimum distance that forms in front of the cathode, we employed a modified and improved thermionic emission model Coupled to these models are a new thermal beam model that better control macro-particle count while maintaining simulation fidelity, as well as new meshing methodologies that improve emission fidelity in the regime of disparate spatial scales, especially useful for field emitter fibers. Lastly, also for field emission fibers, we will present results from the implementation of a self-consistent thermal model. To support such widely ranging spatial scales, effort has been put into disparate meshing capabilities. The presentation provides an overview of the new capabilities that has been achieved.

Radio signals are strongly impacted by natural and artificial ionospheric disturbances which can be challenging to model. This is especially true of D- Region chemistry which is affected from below by shocks, acoustic-gravity waves and thunderstorm electric fields, and from above by solar x-rays, high-energy protons, and precipitating electrons. Detailed chemistry schemes have been developed by various research groups, with many 10s of species and many 100s of reactions, that have been used to calculate RF absorption during geomagnetic events. In contrast, advanced studies of VLF signal propagation typically employ reduced D-Region chemistry schemes [1]–[2], e.g., the four species Glukhov-Pasko-Inan scheme [3], that rely upon simplified parameterizations of electron attachment, detachment and recombination.

New advanced models and applications of the MICHELLE electrostatic particle-in-cell code are presented. MICHELLE continues to follow a strong development plan, advancing the level of physics-based models to improve simulation fidelity in more complex regimes. As a charged particle beam optics code MICHELLE [1] now includes a new model for thermionic emission that more accurately transitions through the knee of the Miram curve as a cathode crosses from temperature-limited to fully space-charge limited emission. This is fortified by a new thermal beam model to greatly reduce computation resources in the transition regime. The model correctly treats small scale geometries where the potential minimum is resolved as well as the large, cm-scale diodes. Another area that has undergone much development is MICHELLE's ability to mesh with various user design environments, including the state-of-the-art Cadence Analyst-MP [2] toolset as well as the new COMPASS II environment that enables the stitching together of native and third-party simulation tools and utility tools, all supporting UNIX & Windows, up through Windows 10. These environments and tool sets work together with the NRL Vacuum Electronics tools, including CHRISTINE-Z, TESLA-Z [3] and NEPTUNE [4] large signal and first principle codes, respectively. Lastly, high energy beam sources require high-fidelity self-magnetic fields to be calculated. Several examples and applications that stress codes such as MICHELLE will be presented, illustrating how the new capability can be used by the device designer.

We present improved numerical algorithms for thermionic emission in the finite-element gun code MICHELLE. The improved algorithms employ newly implemented Child-Langmuir special functions and robust iterative solvers that quickly yield near double precision machine accuracy in analytic problems. The implementation in MICHELLE yields substantially improved current densities, reducing the 1% error observed in some cases to less than 0.25% error.

We present new visualization tools for the recently developed High Performance Computing (HPC) capability for the MICHELLE code. New HPC support has been implemented in the MICHELLE code in support of rapid device design and optimization. The adoption of HPC tools and the porting of the code to HPC platforms allows for high -resolution, large-scale models and simulations of multiple beam devices, such as MBK guns and MB-IOTs. Visual analysis and postprocessing of simulation results from such large-scale models is a computational challenge that requires advanced scientific visualization tools such as Para View, which was chosen for our program due to its flexibility to plug-in customization. To enhance usability and efficiency for new and existing MICHELLE users, a GUI plug-in to ParaView with MICHELLE-specific tools has been developed using the Qt GUI toolkit and Python. Here we present a description of the Qt plug-in with example problems, and a review of some Para View capabilities. Also presented is real-world benchmarking of HPC visualization performance using Para View's in client-server mode.

The advancement of modern vacuum electronics into millimeter and sub-millimeter wavelengths places a growing demand on sensitivity analysis which in turn plays an increasingly important role in improving production yield. This results in higher computational demand on simulation software, often pushing it into the realm of High Performance Computing (HPC). Domain decomposition provides one means for increasing simulation resolution and/or decreasing simulation execution time by breaking the volumetric mesh into sub-domains (blocks) which can be distributed over memory and multiple processors.

We present new visualization tools for the recently developed High Performance Computing (HPC) capability for the MICHELLE code. New HPC support has been implemented in the code and its environment over the past two years in support of rapid device design and optimization. Central to this is the adoption of HPC tools and the porting of the code to HPC platforms, which allows for accurate, high-resolution, large-scale models and simulations of multiple beam devices, such as MBK guns and MB-IOTs. Visual analysis and post-processing of simulation results from such large-scale models is a computational challenge that requires advanced scientific visualization tools such as ParaView, which was chosen for our program due to its flexibility to plug-in customization. To enhance usability and efficiency for new and existing MICHELLE users, a GUI plug-in to ParaView with MICHELLE-specific tools has been developed using the Qt GUI toolkit and Python. Here we present a description of the Qt plug-in with example problems, and a review of some ParaView capabilities. Also presented is real-world benchmarking of HPC visualization performance using ParaView in client-server mode.

Progress has been made in the area of domain decomposition using the NRL CAPSTONE mesher and the MICHELLE beam optics code. Domain decomposition, being the standard for parallelization for mesh-based solvers, has been implemented in the MICHELLE code environment. CAPSTONE was chosen as the volumetric mesher for a variety of reasons, but for this purpose it is suited to the production of Domain-Decomposed meshes. The approach taken to support domain-decomposition from the mesher through to the solution and visualization is presented.

New capability has been implemented in the MICHELLE code and its environment over the past two years in support of rapid device design and optimization. Central to this is the adoption of High Performance Computing (HPC) tools and the porting of the code to HPC clusters. The solver now supports distributed-memory, MPI-based domain decomposition. The development also supports per-node accelerators such as GPUs and multicore CPUs. The code can handle largescale models supporting the ability to simulate multiple beam devices, such as MBK guns and MB-IOTs, and at adequate resolution to properly resolve the dynamics. Significant to this development has been the integration of MICHELLE with the CAPSTONE mesher, the ParaView data visualizer, and the GSB/DAKOTA execution environment and optimization library, which enables the rapid device design and optimization process. With the current development program in its final stage, this presentation provides an overview of the new capability that has been achieved.

The MICHELLE charged particle beam optics code [1], [2] has been extended to perform on HPC class clusters in support of rapid device design and optimization. Central to the work is that the code now supports new distributed-memory domain decomposition and per-node GPU accelerators as well as multicore processing. For use on the HPC, MICHELLE has been integrated with the CAPSTONE mesher [3], the Para View data visualizer, and the GSB/DAKOTA execution environment and optimization library. This talk presents the current status of this program.

The MICHELLE ES PIC [1,2] code is being extended with new distributed-memory domain decomposition and per-node accelerators such as Graphics Processing Unit (GPU) and multicore processing. In addition, new interfaces are being built between MICHELLE and existing DOD software tools such as CAPSTONE [3], Galaxy Simulation Builder (GSB), and ParaView to form the next generation framework for efficient design and optimization workflow.

The existing HPC capabilities in the MICHELLE ES PIC code is being supplanted with new distributed-memory, MPI-based domain decomposition and per-node accelerators such as GPUs and multicore processing. New interfaces are also being built between MICHELLE and existing DOD software tools such as CAPSTONE, GSB, and ParaView to form the next generation framework for efficient design and optimization workflow. This is an evolutionary process, and this paper reports on the latest progress and discusses applicable algorithms and implementations.

The next generation of the MICHELLE ES PIC code is to improve its parallelization and leverages a number of existing and emerging DOD HPC architectures and software including distributed memory clusters, multicore, and computational accelerators such as GPUs and Intel Xeon Phi co-processors. The ongoing project supported by the DOD HASI program also aims to build interfaces between MICHELLE and existing HPC tools such as CAPSTONE, GSB, ParaView, and VisIt for efficient design and optimization workflow. This paper reports on the latest progress and discusses applicable algorithms and implementations.