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We present 2-D simulations of both beam-driven and laser-driven plasma wakefield accelerators, using the object-oriented particle-in-cell code XOOPIC, which is time explicit, fully electromagnetic, and capable of running on massively parallel supercomputers. Simulations of laser-driven wakefields with low (~10^16 W/cm^2) and high (~10^18 W/cm^2) peak intensity laser pulses are conducted in slab geometry, showing agreement with theory and fluid simulations. Simulations of the E-157 beam wakefield experiment at the Stanford Linear Accelerator Center, in which a 30 GeV electron beam passes through 1 m of preionized lithium plasma, are conducted in cylindrical geometry, obtaining good agreement with previous work. We briefly describe some of the more significant modifications of XOOPIC required by this work, and summarize the issues relevant to modeling relativistic electron-neutral collisions in a particle-in-cell code.

We demonstrate the transition characteristics and electron kinetics of microdischarges in a microgap with a cathode having microstructures using two-dimensional particle-in-cell/Monte Carlo collision (2d3v) simulations. It is found that the microdischarge is inside the hollow cavity at lower pressures, forming microhollow cathode discharges (MHCDs), while the dominant discharge moves outside the cavity at higher pressures, locating above the cathode rectangular protrusion tip. The spatial distributions of the microdischarge parameters (e.g., electric potential and electron density) are presented by capturing the transition characteristics. The electron kinetics of the microdischarges are examined based on the moment analysis of the electron Boltzmann equation. The collisional and non-collisional components of the electron power absorption are decomposed, which were seldomly investigated for microdischarges previously. The results provide fundamental insights into MHCD formation with a structured electrode at varying pressure conditions, which could be beneficial for the design and fabrication of microplasma devices in practical applications. [ABSTRACT FROM AUTHOR]

A common approach to modeling kinetic problems in plasma physics is to represent the plasma as a set of Lagrangian macro-particles which interact through long-range forces. In the well-known particle-in-cell (PIC) method, the particle charges are interpolated to a mesh and the fields are obtained using a fast Poisson solver. The advantage of this approach is that the electrostatic forces can be evaluated in time O(N log N), where N is the number of macro-particles, but the scheme has difficulty resolving steep gradients and handling nonconforming domains unless a sufficiently fine mesh is used. The current work describes a grid-free alternative, the boundary integral/treecode (BIT) method. Using Green's theorem, we express the solution to Poisson's equation as the sum of a volume integral and a boundary integral which are computed using particle discretizations. The treecode replaces particle-particle interactions by particle-cluster interactions which are evaluated by Taylor expansions. In addition, the Green's function is regularized and adaptive particle insertion is implemented to maintain resolution. Like PIC, the operation count is O(N log N), but BIT avoids using a regular grid, so it can potentially resolve steep gradients and handle complex domains more efficiently. We applied BIT to several bounded plasma problems including a one-dimensional (1-D) sheath in direct current (dc) discharges, 1-D virtual cathode, cold two-stream instability, two-dimensional (2-D) planar and cylindrical ion optics, and particle dynamics in a Penning-Malmberg trap. Some comparisons of BIT and PIC were performed. These results and ongoing work will be reviewed. Index Terms--Boundary integral method, Coulomb potential, grid-free, ion optics, multipole expansion, particle-in-cell method, Penning-Malmberg trap, Poisson solver, sheath formation, treecode algorithm, two-stream instability, virtual cathode.

From the characteristics of current and voltage, the capacitive coupled external electrode fluorescent lamps are found to be similar to cold cathode fluorescent lamps having the ballast capacitors at both ends of the lamps. From the current-voltage characteristic for two types of lamps, the discharge mechanism is similar with respect to discharge process: initially a dark current, Townsend discharge with a firing voltage, followed by a normal glow. The slope and magnitude of the dark current have the same values since the gas species in the lamps are identical. At the Townsend discharge, the lamp current increases abruptly with a firing voltage of about 1 kV. In the normal glow region, the cathode fall voltage is sustained at about 600 V in the cold cathode discharge tube after Townsend breakdown at about 1 kV, while the voltage increases linearly as the current increases without negative resistance in both the external electrode lamp and the cold cathode lamp including the ballast capacitors. Index Terms--Ballast, cold cathode fluorescent lamp, discharge, external electrode fluorescent lamp.

A description of the progress in the field, and explaining the interesting physical effects and improvements in the Plasma-assisted slow-wave oscillators (pasotrons) performance without going into details of either the theoretical formalism or experimental parameters is presented. The 50% efficient operation at the 0.5-MW power level and more than 30% efficient operation at more than a 1-MW level is reported.

An efficient grid-free numerical method is developed for particle simulations in plasma dynamics. The method relies on a treecode algorithm to solve the Poisson equation for interacting charged particles. Such algorithms have been extensively used in astrophysics, fluid dynamics, and molecular dynamics, and our aim is to assess their capability in plasma dynamics in comparison with traditional mesh-based methods such as particle-in-cell (PIC). It is well-known that PIC simulations have difficulty resolving local particle interactions within a grid cell, and we expect the grid-free treecode algorithm to have an advantage for problems involving small-scale features and highly nonuniform particle distributions. To illustrate this point we apply direct summation, treecode, and PIC methods to simulate a virtual cathode in one dimension. The treecode and PIC methods are found to be much faster than direct summation, but the treecode yields the correct solution while spurious features are present in the PIC results. The present work treats a one-dimensional problem, but the treecode algorithm generalizes directly to higher dimensions. Index Terms--Coulomb potential, grid-free, multipole expansion, particle-in-cell method, Poisson solver, treecode algorithm, virtual cathode.

A planar one-dimensional (1-D) particle-in-cell (PIC) simulation with Monte Carlo collisions (MCC) has been used to study the effect of including metastable argon on the characteristics of a 13.56-MHz argon discharge. Reactions such as metastable creation, ionization from the metastable state, metastable quenching to resonant level, and metastable pooling were taken into account. At low pressure (0.05 torr), metastables were found to have little influence on the discharge. However, at a pressure of 1 torr, ionization from the metastable state and collisions between two metastables represent a significant percentage of the total ionization rate, strongly influencing the electron heating and the discharge potential. The effect of pressure and applied voltage on the discharge parameters is studied and a comparison between discharges with and without metastables is presented. A PIC-MCC simulation was compared to the fluid model of Lymberopoulos and Economu [33] using their published parameters, showing that using PIC simulation the main process of metastable loss is metastable pooling. Index Terms--Argon metastable atoms, one-dimensional planar simulation, Particle-in-cell (PIC) with Monte Carlo collisions.

The Curnow equivalent circuit was used to predict the dispersion of cold coupled-cavity traveling-wave tubes, as well as the voltage and current characteristics for lossless and lossy multicavity circuits. The equivalent circuit is extended to have three ports. The added beam port allows the future modeling of the interaction between beam and cavity. Losses are introduced into the circuit as resistors in series with the corresponding inductors. The time-domain solution to the multicavity circuit is developed. It can be applied to the full-spectrum signal. It is also useful for the transient analysis for both single frequency and full-spectrum signals, including the turn-on transients. Numerical methods to solve the time-domain equations are discussed; a second-order leap-frog method and a fourth-order Runge-Kutta method are implemented and analyzed. Simulation results from both codes are compared, and match well with the theory. Index Terms--Circuit simulation, equivalent circuit, microwave tubes, time-domain finite-difference, traveling wave tubes.