Sudhakar Mahalingam

Modeling Breakdown Processes in RF Cavities Using Particle-in-cell (PIC) Codes

Design of future accelerators can benefit from the use of high gradient RF electric fields (>50 MV/m) and operating under strong applied magnetic fields (>2.5 T). These large fields help to minimize the number of costly accelerating elements needed for the neutrino experiments. However a main limitation in applying such large fields in the design of accelerators is the physical breakdown of metallic structures. We have developed computer models of the process of breakdown using OOPIC Pro, a 2-dimensional particle-in-cell (PIC) code in order to understand the physical processes that are responsible for breakdown and test methodologies to mitigate the problem. We describe here the results of our numerical experiments including the effects of applied magnetic field strength and direction on the breakdown process, sensitivity of breakdown triggers on field emission parameters, and the potential to measure the onset of breakdown by examining impurity radiation.

Computational speed up techniques for particle-in-cell-Monte Carlo collision simulations of an ion engine discharge chamber

Summary form only given. Next generation ion thrusters such as NASAs Evolutionary Xenon Thruster (NEXT) are being considered for in-space propulsion applications to meet the future space mission needs of travel to asteroids and for satellite maneuvers. NASA has been actively pursuing new designs of ion thrusters to address the high power and high-thrust propulsion needs. Computational simulations of ion engine discharge chamber plasmas will help to understand the operation and performance of these high power ion thrusters. In this work, we describe a two-dimensional Particle-in-cell Monte Carlo Collision (PIC-MCC) model developed to simulate the ion engine discharge chamber plasma processes. We utilize VSimPD, a simulation package for plasma discharges based on the Vorpal computing engine. In this model, the electrons, singly charged xenon ions, doubly charged xenon ions and xenon neutrals are tracked as kinetic particles which includes the effects of both electric and magnetic fields. Also, the model solves the electric fields every time step based on the charge particle distributions. This detailed PIC-MCC model was benchmarked on NASAs NEXT ion engine discharge chamber and the simulation results are in good agreement with experimental plasma measurements. Recently we have focused on improving the numerical algorithms in VSimPD to speed up this discharge chamber model. New particle splitting and merging procedures are implemented. These procedures preserve the charge, energy, momentum and also the electron energy distribution functions (EEDF). We will discuss these numerical algorithms and compare the accuracies of these simulation results and provide speed up results. In addition, we will also provide speed up results from the parallel processing option and the convergence procedures developed with the numerical parameters considered in these simulations.

Particle-in-cell (PIC) tools for simulation of electrodynamic bare tether plasma interactions

Summary form only given. Electrodynamic tethers (EDTs) can be used for near Earth space missions to produce thrust to raise and lower a satellite orbit or change its inclination, and, in drag inducing (de-orbit) mode, EDTs can provide electrical power to the satellite. Recently there has been renewed interest in bare wire EDTs that operate at larger current and voltage levels, and hence it is becoming important to understand current collection along the positively biased regions of the tether and the plasma interactions these high current EDTs will induce. In this work, we describe numerical and experimental studies of EDTs with the goal to develop and validate computational tools for researchers who are designing tethers for future missions. In our numerical studies we will utilize two particle-in-cell codes VORPAL and OOPIC Pro. VORPAL is a 1-D, 2-D and 3-D massively parallel electromagnetic simulation code and OOPIC Pro is a 2-D parallel electromagnetic simulation code with user-friendly GUI interfaces. We will develop suitable boundary conditions such as quasi-neutrality and open boundaries to simulate accurately the tethers operation in space. We will benchmark our 2-D electrostatic numerical simulations against other published analytical and numerical results. In addition, we will conduct full electromagnetic simulations to consider self-induced magnetic field effects. On the experimental side, tests on thin tether tapes will be conducted in a vacuum facility equipped with a plasma source developed by Rubin et al. that replicates the streaming plasma encountered by satellites in low Earth orbit. These experiments will characterize electron collection to the tape tethers under varying electrical bias and angle of attack.

Modeling of Multipacting in RF Structures Using VORPAL

VORPAL is a simulation framework which can model electromagnetic fields and charged particle dynamics in RF structures such as those encountered in high gradient accelerating cavities. Multiple models exist in VORPAL to determine the secondary electron yield (SEY) for an impact of a primary electron on the cavity surface. Combining these models allows VORPAL to be used to study electron multipacting in radio frequency cavity structures. VORPAL also supports the tracking of individual multipacting trajectories as well as recording the impact history of multipacting electrons. We present simulation results of multipacting simulations in various RF structures including accelerating cavities as well as coupler and waveguide structures. Benchmarking results are given comparing VORPAL results from other codes as well as experimental data.

Modeling Breakdown and Electron Orbits in High‐Gradient Accelerating Cavities

Next‐generation rf accelerating cavities will employ very high‐gradient electric fields, greater than 100 MV/m, as well as strong magnetic fields. However, breakdown of accelerating structures due to high field gradients is a major limitation on these accelerating cavities. One possible mechanism for breakdown initiation is the rapid buildup of electrons due to field emission coupled with secondary electron emission. Multipacting may enhance this effect. In order to understand the physical processes of breakdown initiation and the effectiveness of potential mitigation techniques, researchers in the Muon Accelerator Program are experimenting with a simplified cavity, referred to as the Box Cavity, in which they will measure breakdown under high‐gradient rf with strong externally applied magnetic fields with different orientations. We present here simulation results for the box cavity including the effects of rf (805 MHz), magnetic fields, field‐dependent emission, secondary electron emission, and space cha...

Preserving Gauss's law during particle emission from general boundaries in electromagnetic particle codes

Preserving Gausss law during particle emission in electromagnetic codes is key for obtaining proper solutions in high emission current electromagnetic simulations. Tech-Xs electromagnetic PIC code VORPAL has recently been extended to cut cell geometries with arbitrary emission regions defined for a variety of emitters including (among others) thermal-field emitters and secondary emission. In this paper we present the general algorithms used for Gauss preserving particle emission (which can be applied to all sorts of EM particle simulation schemes) along with recent results.