Simulation Of Charged Particles Research Articles

SMALL-SCALE GRADIENTS OF CHARGED PARTICLES IN THE HELIOSPHERIC MAGNETIC FIELD

Using numerical simulations of charged-particles propagating in the heliospheric magnetic field, we study small-scale gradients, or "dropouts," in the intensity of solar energetic particles seen at 1 AU. We use two turbulence models, the foot-point random motion model and the two-component model, to generate fluctuating magnetic fields similar to spacecraft observations at 1 AU. The turbulence models include a Kolmogorov-like magnetic field power spectrum containing a broad range of spatial scales from those that lead to large-scale field-line random walk to small scales leading to resonant pitch-angle scattering of energetic particles. We release energetic protons (20 keV–10 MeV) from a spatially compact and instantaneous source. The trajectories of energetic charged particles in turbulent magnetic fields are numerically integrated. Spacecraft observations are mimicked by collecting particles in small windows when they pass the windows at a distance of 1 AU. We show that small-scale gradients in the intensity of energetic particles and velocity dispersions observed by spacecraft can be reproduced using the foot-point random motion model. However, no dropouts are seen in simulations using the two-component magnetic turbulence model. We also show that particle scattering in the solar wind magnetic field needs to be infrequent for intensity dropouts to form.

A multiple-orbit time-of-flight mass spectrometer based on a low energy electrostatic storage ring

The results are presented for an electrostatic storage ring, consisting of two hemispherical deflector analyzers (HDA) connected by two separate sets of cylindrical lenses, used as a time-of-flight mass spectrometer. Based on the results of charged particle simulations and formal matrix model, the Ion Storage Ring is capable of operating with multiple stable orbits, for both single and multiply charged ions simultaneously.

Gauge properties of the guiding center variational symplectic integrator

Variational symplectic algorithms have recently been developed for carrying out long-time simulation of charged particles in magnetic fields [H. Qin and X. Guan, Phys. Rev. Lett. 100, 035006 (2008); H. Qin, X. Guan, and W. Tang, Phys. Plasmas (2009); J. Li, H. Qin, Z. Pu, L. Xie, and S. Fu, Phys. Plasmas 18, 052902 (2011)]. As a direct consequence of their derivation from a discrete variational principle, these algorithms have very good long-time energy conservation, as well as exactly preserving discrete momenta. We present stability results for these algorithms, focusing on understanding how explicit variational integrators can be designed for this type of system. It is found that for explicit algorithms, an instability arises because the discrete symplectic structure does not become the continuous structure in the t→0 limit. We examine how a generalized gauge transformation can be used to put the Lagrangian in the “antisymmetric discretization gauge,” in which the discrete symplectic structure has the correct form, thus eliminating the numerical instability. Finally, it is noted that the variational guiding center algorithms are not electromagnetically gauge invariant. By designing a model discrete Lagrangian, we show that the algorithms are approximately gauge invariant as long as A and φ are relatively smooth. A gauge invariant discrete Lagrangian is very important in a variational particle-in-cell algorithm where it ensures current continuity and preservation of Gauss’s law [J. Squire, H. Qin, and W. Tang (to be published)].

The direct numerical simulation of charged particles' acceleration in turbulent magnetized plasmas

The direct numerical simulation of charged particles' acceleration in turbulent magnetized plasmas

Low perveance confined-flow Pierce gun for a 0.14 THz broadband folded waveguide traveling wave tube

An approach for designing an electron gun for a high efficiency, high linearity 0.14 THz traveling wave tube (TWT), has been presented. A Pierce electron gun of beam perveance 0.0047 μP has been designed for a high gain, high linearity and high efficiency 0.14 THz TWT using CST STUDIO SUITE Charged Particle Simulation soft and electronic optical software TAU. The initial gun geometry, as obtained from the Vaughan iterative synthesis method, has been used as input for simulation of the electron gun. An M-type dispenser cathode of diameter 0.8 mm has been used for cathode loading of 2.75 A/cm 2. The magnetic focusing with integral-pole-piece barrel assembly and periodic-permanent magnets (PPM) have been designed using CST and TAU. The practical problem of linking requisite cathode flux to the cathode for confined flow of the electron beam with low convergence factor has been sorted out by gradually increasing the PPM magnetic field. The magnetic field has been increased in steps from the gun and over the first five magnets varying from Brillouin field ( B B) value to twice B B for achieving the electron beam with scalloping less than 10%. Simulation results show that the design method is reasonable for obtaining a 0.14 THz TWT electron gun and periodic-permanent magnets (PPM).

Electron-Hop-Funnel Measurements and Comparison With the Lorentz-2E Simulation

Electron hop funnels have been fabricated using a low-temperature cofired ceramic. The measurements of the hop-funnel I-V curve and electron energy distribution have been made using gated field emitters as the electron source. The charged particle simulation Lorentz 2E has been used to model the hop-funnel charging and to predict the - and energy characteristics. The results of this comparison indicate that the simulation can be used to design hop-funnel structures for use in various applications.

A method for incorporating the Kerr–Schild metric in electromagnetic particle-in-cell code

An algorithm is presented that incorporates the Kerr–Schild metric in an electromagnetic particle-in-cell code. This paper describes the algorithm and implementation for the simulation of charged particles in the region of a spinning black hole. A description is given of the method used to calculate fields, currents, and particle motion using the tensor formalism. We test the overall model by using a ‘toy’ black hole and accretion disk system in a uniform magnetic field to produce bipolar jets.

An electrostatic storage ring for low kinetic energy electron collisions

The criteria are presented for stable multiple orbits of charged particles in a race-track shaped storage ring and applied to an electrostatic system consisting of two hemispherical deflector analyzers (HDA) connected by two separate sets of cylindrical lenses. The results of charged particle simulations and the formal matrix theory, including aberrations in the energy-dispersive electrostatic 'prisms', are in good agreement with the observed experimental operating conditions for this Electron Recycling Spectrometer (ERS).

Stability criteria for a passive electrostatic non-relativistic charged particle storage device

The general conditions are determined for stable multiple orbits of non-relativistic charged particles in a race-track-shaped optical system. These conditions are then considered in the context of an electrostatic storage ring consisting of two 180° hemispherical deflector analysers connected by two separate sets of cylindrical lenses. The race-track configuration of this type has already been constructed and demonstrated to achieve storage of low-energy (tens of electronvolts) electrons (Tessier et al 2007 Phys. Rev. Lett. 99 253201). Incorporating the aberrations of the energy-dispersive electrostatic prisms is found to modify and restrict the general stability conditions. This modified formal matrix theory and the results of charged particle simulations described in this study are in excellent agreement with the observed experimental operating conditions for this electron recycling spectrometer.

Bmad: A relativistic charged particle simulation library

Bmad is a subroutine library for simulating relativistic charged particle beams in high-energy accelerators and storage rings. Bmad can be used to study both single and multi-particle beam dynamics using routines to track both particles and macroparticles. Bmad has various tracking algorithms including Runge–Kutta and symplectic (Lie algebraic) integration. Various effects such as wakefields, and radiation excitation and damping can be simulated. Bmad has been developed in a modular, object-oriented fashion to maximize flexibility. Interface routines allow Bmad to be called from C / C ++ as well as Fortran programs. Bmad is well documented. Every routine is individually annotated, and there is an extensive manual.

Auxiliary field simulation and Coulomb's law

We review a family of local algorithms that permit the simulation of charged particles with purely local dynamics. Molecular dynamics formulations lead to discretizations similar to those of “particle-in-cell” methods in plasma physics. We show how to formulate a local Monte-Carlo algorithm in the presence of the long ranged Coulomb interaction. We compare the efficiencies of our molecular dynamics and Monte-Carlo codes.

Memoryless control of boundary concentrations of diffusing particles

Flux between regions of different concentration occurs in nearly every device involving diffusion, whether an electrochemical cell, a bipolar transistor, or a protein channel in a biological membrane. Diffusion theory has calculated that flux since the time of Fick (1855), and the flux has been known to arise from the stochastic behavior of Brownian trajectories since the time of Einstein (1905), yet the mathematical description of the behavior of trajectories corresponding to different types of boundaries is not complete. We consider the trajectories of noninteracting particles diffusing in a finite region connecting two baths of fixed concentrations. Inside the region, the trajectories of diffusing particles are governed by the Langevin equation. To maintain average concentrations at the boundaries of the region at their values in the baths, a control mechanism is needed to set the boundary dynamics of the trajectories. Different control mechanisms are used in Langevin and Brownian simulations of such systems. We analyze models of controllers and derive equations for the time evolution and spatial distribution of particles inside the domain. Our analysis shows a distinct difference between the time evolution and the steady state concentrations. While the time evolution of the density is governed by an integral operator, the spatial distribution is governed by the familiar Fokker-Planck operator. The boundary conditions for the time dependent density depend on the model of the controller; however, this dependence disappears in the steady state, if the controller is of a renewal type. Renewal-type controllers, however, produce spurious boundary layers that can be catastrophic in simulations of charged particles, because even a tiny net charge can have global effects. The design of a nonrenewal controller that maintains concentrations of noninteracting particles without creating spurious boundary layers at the interface requires the solution of the time-dependent Fokker-Planck equation with absorption of outgoing trajectories and a source of ingoing trajectories on the boundary (the so called albedo problem).

CPM: a deformable model for shape recovery and segmentation based on charged particles

A novel, physically motivated deformable model for shape recovery and segmentation is presented. The model, referred to as the charged-particle model (CPM), is inspired by classical electrodynamics and is based on a simulation of charged particles moving in an electrostatic field. The charges are attracted towards the contours of the objects of interest by an electrostatic field, whose sources are computed based on the gradient-magnitude image. The electric field plays the same role as the potential forces in the snake model, while internal interactions are modeled by repulsive Coulomb forces. We demonstrate the flexibility and potential of the model in a wide variety of settings: shape recovery using manual initialization, automatic segmentation, and skeleton computation. We perform a comparative analysis of the proposed model with the active contour model and show that specific problems of the latter are surmounted by our model. The model is easily extendable to 3D and copes well with noisy images.

A continuum, O(N) Monte Carlo algorithm for charged particles.

We introduce a Monte Carlo algorithm for the simulation of charged particles moving in the continuum. Electrostatic interactions are not instantaneous as in conventional approaches, but are mediated by a constrained, diffusing electric field on an interpolating lattice. We discuss the theoretical justifications of the algorithm and show that it efficiently equilibrates model electrolytes and polar fluids. In order to reduce lattice artifacts that arise from the interpolation of charges to the grid we implement a local, dynamic subtraction algorithm. This dynamic scheme is completely general and can also be used with other Coulomb codes, such as multigrid based methods.

Particle identification using the time-over-threshold method in the ATLAS Transition Radiation Tracker

Test-beam studies of the ATLAS Transition Radiation Tracker (TRT) straw tube performance in terms of electron–pion separation using a time-over-threshold method are described. The test-beam data are compared with Monte Carlo simulations of charged particles passing through the straw tubes of the TRT. For energies below 10 GeV , the time-over-threshold method combined with the standard transition-radiation cluster-counting technique significantly improves the electron–pion separation in the TRT. The use of the time-over-threshold information also provides some kaon–pion separation, thereby significantly enhancing the B-physics capabilities of the ATLAS detector.

Simulation of flows within an electrostatic ion thruster for space missions

AbstractIon thrusters are well suited for near-Earth and deep space missions because of their exceptionally high specific impulse. Those already flying are also proving to be very reliable. The flow of neutral and charged particles within the different components of these devices is very complex and although they have been studied for several years, little of the detail of their internal operation is properly understood. Numerical simulations of these flows can potentially yield an improved understanding of the physical phenomena involved and in turn this should assist in the optimising of new designs and in achieving higher specific impulses. The external plume, which can potentially cause damage to other spacecraft components such as solar panels, has been the object of some studies but the flow within the internal chambers where many complex phenomena are suspected to occur has been seriously neglected. For this reason the present fully kinetic neutral and charged particle simulations, which take account of the ionisation processes and the applied magnetic field, have been conducted. The studies have been made on one specific electrostatic ion thruster - the T5 model produced by the Space Department at the Defence and Evaluation Research Agency (DERA, Farnborough, UK). Two regions within the thruster have been concentrated on, namely the exit area of the hollow cathode and the main chamber.

A finite‐volume particle‐in‐cell method for the numerical treatment of Maxwell‐Lorentz equations on boundary‐fitted meshes

A new conceptual framework solving numerically the time-dependent Maxwell–Lorentz equations on a non-rectangular quadrilateral mesh in two space dimensions is presented. Beyond a short review of the applied particle treatment based on the particle-in-cell method, a finite-volume scheme for the numerical approximation of the Maxwell equations is introduced using non-rectangular quadrilateral grid arrangements. The coupling of a high-resolution FV Maxwell solver with the PIC method is a new approach in the context of self-consistent charged particle simulation in electromagnetic fields. Furthermore, first simulation results of the time-dependent behaviour of an externally applied-B ion diode developed at the Forschungszentrum in Karlsruhe are presented. Copyright © 1999 John Wiley & Sons, Ltd.

A dedicated circuit for charged particles simulation using the Monte Carlo method

We present a dedicated integrated circuit for the simulation of charged particles based on Monte Carlo method. The Monte Carlo method leads to the solution of a particular form of the integro-differential Boltzmann equation (non-linear charge transport in semiconductors) permitting a direct statistical computation of the simulated particles distribution function in the phase space. This circuit should be the building block of a semiconductor device hardware simulator. Because of the complexity of microdynamical transport in semiconductors, the physical model used needs to be simplified in order to achieve a more simple circuit of small size. Starting from a chosen model we describe here how all the arithmetics involved in the problem has been set up for resolving the Boltzmann equation. Moreover, the binary format for the various physical quantities involved is discussed in view of the desired result accuracy: mainly, the drift velocity in a static uniform electric field taking into account all hot carrier effects.

High energy neutron and charged particle irradiation effects on thermomechanical properties of carbon–carbon composites for divertor applications

To assess the degradation of carbon–carbon (C/C) composite materials for the divertor structure, 14 MeV neutron and charged particle simulation irradiations were performed on several grades of C/C composites. For the charged particle irradiation the damage to be caused was also estimated using EDEP-1 code. Materials used were several grades of C/C composites, all of which were used in the JT-60 as divertor armor tiles. The measurement of thermal diffusivity up to 1600 K, electrical resistivity during 14 MeV neutron irradiation and the micro-indentation test at room temperature were performed. The strength and Young's modulus were evaluated on the basis of the result of micro-indentation test. Main results are: (1) The maximum of the microhardness was found near the maximum projected range of the C/C composite for charged particles. (2) After the irradiation of 10 MeV protons (<0.1 dpa), 10–90% increases in tensile strength and Young's modulus were observed, depending on the grades of C/C composites. (3) Softening of C/C composites was observed in the micro-indentation test when they were irradiated by 14 MeV neutrons up to a fluence of 10 19 n/m 2 .

An Iterative PPPM Method for Simulating Coulombic Systems on Distributed Memory Parallel Computers

We describe results obtained from a new implementation of Hockney's Particle-Particle Particle-Mesh (PPPM) method for evaluation of Coulomb energies and forces in simulations of charged particles. Rather than taking the usual approach, solving Poisson's equation by means of a Fourier transformation, we use an iterative Poisson solver. In a molecular dynamics (MD) simulation the solution from the previous time-step provides a good starting point for the next solution. This reduces the number of iterations per time-step to acceptable values. The iterative scheme has a complexity O(N), and, in contrast with the Fourier transform based approach, it is easily implemented on a parallel architecture with a minimum of communication overhead. We examine the origin of the errors in the algorithm and find that reasonable accuracies in the Coulomb interaction can best be attained by making the charge density profile as smooth as possible. This involves spreading the particle charges over a large number of grid points. Assigning these charges then becomes the most time consuming part of the algorithm. We show how we can then gain a considerable saving in computing time by employing a diffusion equation as a charge spreading mechanism. The effect of employing the algorithm with an accuracy less than that typically tolerated in an Ewald summation is studied by computing, from an MD simulation of silica, quantities that are sensitive to the long range part of the Coulomb interaction. These results are compared to full Ewald sum reference simulations and found to be within the statistical error.