Search Results (3 total)

Recently reported observations of filamentation during high power microwaves breakdown of near-atmospheric pressure gas are explained using a one-dimensional fluid model coupled to a theoretical wave-plasma model. This self-consistent treatment allows for time-dependent effects, plasma growth and diffusion, and partial absorption and reflection of waves. Simulation results, consistent with experiments, show the evolution of the plasma filaments spaced less than one-quarter wavelength, the sequential discrete light emission propagating back toward the source, and the diffusion and decay of the plasma. The model allows examination of many features not easily obtained experimentally, including dependence on field strength and frequency, pressure, and gas composition, which influence the breakdown and emission properties, including the spacing and speed of propagation of the filaments.

We present 2D 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 \(∼10¹⁶ W/cm²\) and high \(∼10¹⁸ W/cm²\) 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 to XOOPIC required by this work, and summarize the issues relevant to modeling relativistic electron-neutral collisions in a particle-in-cell code.

High-frequency waves, which are driven by a strong electron beam and propagate along a density gradient, can form spatially concentrated “HF spikes” which extend typically one wavelength (1 cm) in the direction along the beam. Experiments and computer simulations show that the spike is a standing wave with nodes at boundaries to regions with propagating waves. The spikes only form in a plasma density gradient, and attempts to produce them in homogeneous plasma have failed. They form without trapping of the waves in density cavities and remain stable after the formation, i.e., there is no tendency towards collapse of the structure.