Particle in Cell

The Inertial Electrostatic Confinement Fusion Reactor (IEC Fusion) is a potential method for controlled fusion power. The IEC fusion reactor confines plasma via a strong electrostatic potential between two concentric grids as ions stream towards the center. The electrostatic potential between the outer grid and the inner grid is on the order of magnitude of tens of kilo volts, and the outer grid is usually grounded. Gas species such as Deuterium and Tritium are injected into a vacuum chamber housing the grids, usually at pressures on the order of a millitorr. Once ionized by the electric field, the ions accelerate radially toward the central grid, colliding with other ions of sufficient relative velocity to undergo fusion. Those ions that shoot through the center without fusing will decelerate and fallback towards the center, making another pass. This method of confinement therefore has two advantages: Plasma recirculates around the central grid making multiple passes and hence increasing the likelihood of fusion. Optimal fusion cross section is on the order of hundred million Kelvins, while difficult to achieve conventionally, it translates to only tens of kilo electron volts, which is fairly easy to achieve via electrostatics. There are also difficulties associated with IEC fusion. Several loss mechanisms limit its ultimate power output, the most significant being Coulomb interactions, causing bremsstrahlung radiation and thermalization of ion velocities, and ion-grid collisions. This thesis investigates the fusion performance of a cylindrical IEC device, this will be done via computational modelling, and iterative numerical simulations of performance parameters. A set of optimum design parameters will be derived from the simulations, and a preliminary design for an IEC device will be given. The results were not surprising, there is an optimum operating voltage of approximately 190 kV. The optimum pressure and chamber size is approximately 1 meter at 1 millitorr. The grid transparency has a dramatic effect on the performance as well, but this is material limited to 98%. A novel method of reducing grid collisions is being researched by the US Navy currently, in which electrons are trapped to form a virtual cathode, using magnetic fields. This is termed the Polywell.

This paper is the first in a series of three papers to summarize the recent work of an European-wide collaborationwhich is ongoing since about one decade using Particle-in-Cell (PIC) methods in low temperature plasma physics. In the present first paper the main aspects of this computational technique will be presented. In the second paper, an overview of applications in low-temperature plasma modelling will be given, whereas the third part will put emphasis on the specific results of modelling ion thrusters. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Molecular motors perform active movements along cytoskeletal filaments and drive the traffic of organelles and other cargo particles in cells. In contrast to the macroscopic traffic of cars, however, the traffic of molecular motors is characterized by a finite walking distance (or run length) after which a motor unbinds from the filament along which it moves. Unbound motors perform Brownian motion in the surrounding aqueous solution until they rebind to a filament. We use variants of driven lattice gas models to describe the interplay of their active movements, the unbound diffusion, and the binding/unbinding dynamics. If the motor concentration is large, motor-motor interactions become important and lead to a variety of cooperative traffic phenomena such as traffic jams on the filaments, boundary-induced phase transitions, and spontaneous symmetry breaking in systems with two species of motors. If the filament is surrounded by a large reservoir of motors, the jam length, i.e., the extension of the traffic jams, is of the order of the walking distance. Much longer jams can be found in confined geometries such as tube-like compartments.

In this phd dissertation thesis I present results from simulations of streaming plasmas in relativistic collisionless shocks, which are believed to produce Gamma-Ray Burst afterglows. Further, results from simulations of detailed photon-plasma interaction effects during prompt Gamma-Ray Burst - Circumburst Medium interaction are presented. It is argued that inclusion of plasma effects and detailed kinetic modeling may prove imperative in order to gain better understanding of burst evolution and observed behaviour of Gamma-Ray Bursts.

Several applications of PIC simulations for understanding basic physics phenomena in low-temperature plasmas are presented: capacitive radiofrequency discharges in Oxygen, dusty plasmas and negative ion sources for heating of fusion plasmas. The analysis of these systems based on their microscopic properties as accessible with PIC gives improved insight into their complex behavior. These studies are results of joint efforts over about one decade of research groups from Greifswald University, Germany; Bari University, Italy; Keio University, Japan and Innsbruck University, Austria. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The prospects for achieving net energy production with pB11 fuel have recently considerably brightened. Studies have shown that the multi-GG field potentially obtainable with modest dense plasma focus devices have the effect of reducing the flow of energy from the ions to the electrons and thus suppressing bremsstrahlung radiation that cools the plasma. We report here on new simulations that indicate that net energy production may be achievable in high-magnetic-field devices at peak currents as low as 2.3 MA. While these simulations only model the dense plasmoid formed in the focus, new simulation techniques can allow a full particle-in-cell simulation of DPF functioning over the wide range of time and space scales needed. Such simulations will be of great value in the next round of experiments that will use pB11 fuel.

Plasma-based accelerators can sustain accelerating gradients on the order of 100 GV/m. If the plasma is not fully ionized, fields of this magnitude will ionize neutral atoms via electron tunneling, which can completely change the dynamics of the plasma wake. Particle-in-cell simulations of a high-field plasma wakefield accelerator, using the OOPIC code [D. L. Bruhwiler et al., Phys. Rev. ST Accel. Beams 4, 101302 (2001)], which includes field-induced tunneling ionization of neutral Li gas, show that the presence of even moderate neutral gas density significantly degrades the quality of the wakefield. The tunneling ionization model in OOPIC has been validated via a detailed comparison with experimental data from the l'OASIS laboratory [W.P. Leemans et al., Phys. Rev. Lett. 89, 174802 (2002)]. The properties of a wake generated directly from a neutral gas are studied, showing that one can recover the peak fields of the fully ionized plasma simulations, if the density of the electron drive bunch is increased such that the bunch rapidly ionizes the gas.

We provide a guide to implementing the particle-in-cell algorithm, which is useful for simulating diverse phenomena in plasmas. We focus on two-dimensional systems which have vector fields with three Cartesian components but depend only on two spatial coordinates. We describe the algorithm in detail, including particle-to-grid interpolation, the fast Fourier transform, the Boris algorithm, and the use of dimensionless units. As an example, we discuss a simulation of the two-stream instability, which occurs in a plasma system composed of two counter-streaming electrons and an ion background at rest.

The construction of the initial part of the J-PARC linac has been started at KEK for beam tests before moving to the JAERI Tokai campus, where J-PARC facility is finally to be constructed. The RFQ and MEBT (Medium Energy Beam Transport) have already been installed at KEK, and the beam test has been performed successfully. In this paper, the experimental

The concept for Electron Fast Ignition Inertial Confinement Fusion demands sufficient laser energy be transferred from the ignitor pulse to the assembled fuel core via MeV electrons. We have assembled a suite of diagnostics to characterize such transfer. Recent experiments have simultaneously fielded absolutely calibrated extreme ultraviolet multilayer imagers at 68 and 256eV; spherically bent crystal imagers at 4 and