C. Ren

Nonlinear and three-dimensional theory for cross-magnetic field propagation of short-pulse lasers in underdense plasmas

The nonlinear and finite spot size effects for short laser pulses propagating in a plasma across a constant magnetic field (ordinary and extraordinary modes) have been studied. Starting from a fluid Lagrangian for magnetized plasmas with immobile ions, we derive the envelope equation for the laser and also the equation for the plasma wake in a three-dimensional geometry. The derived equations reveal that the external magnetic field reduces the strength of ponderomotive self-focusing, causes astigmatic self-focusing, and leads to the possibility of deflecting a short and narrow laser pulse in a magnetized plasma.

On the mutual interaction between laser beams in plasmas

The nonlinear interaction between light beams in a plasma is studied. In particular, nonlinearities due to relativistic mass corrections and density modulations from a plasma wave wake are considered; but the results can be generalized for other nonlinearities. A simple physical picture using the nonlinear phase velocity of the light wave in a plasma is developed to show that when two laser beams are coherent, the force can be repulsive or attractive, depending on their relative phase. When the two laser beams are polarized in mutually perpendicular directions, the force is always attractive. Using a variational method, a simple analytical expression for this attractive force is derived for Gaussian beams. The centers of the lasers move analogously to point masses under this attractive force with the laser power playing the role of the mass. Under an attractive force, solutions exist where the two lasers can spiral around each other. It is also shown that the plasma wave wake can cause the two spiraling l...

Physical picture for the laser hosing instability in a plasma

The hosing of a light beam centroid is caused by upward or downward tilting of the local wave fronts due to the transverse phase velocity difference across the wave fronts. The phase velocity gradient is caused by the plasma density perturbation, which in turn is driven by the ponderomotive force of the hosing light beam. The hosing equations in both long- and short-wavelength regimes can be heuristically derived from this physical picture.

Modeling of Ionization Physics with the PIC Code OSIRIS

When considering intense particle or laser beams propagating in dense plasma or gas, ionization plays an important role. Impact ionization and tunnel ionization may create new plasma electrons, altering the physics of wakefield accelerators, causing blue shifts in laser spectra, creating and modifying instabilities, etc. Here we describe the addition of an impact ionization package into the 3‐D, object‐oriented, fully parallel PIC code OSIRIS. We apply the simulation tool to simulate the parameters of the upcoming E164 Plasma Wakefield Accelerator experiment at the Stanford Linear Accelerator Center (SLAC). We find that impact ionization is dominated by the plasma electrons moving in the wake rather than the 30 GeV drive beam electrons. Impact ionization leads to a significant number of trapped electrons accelerated from rest in the wake.

Further Developments For A Particle‐In‐Cell Code For Efficiently Modeling Wakefield Acceleration Schemes: QuickPIC

We describe the present status of the development of a three‐dimensional fully parallel Particle‐In‐Cell (PIC) code, called QuickPIC. QuickPIC embeds a two‐dimensional plasma simulation code in a three‐dimensional beam and/or laser simulation code utilizing the quasi‐static approximation [1,2]. In this paper, we will provide a brief review of the quasi‐static approximation and the current algorithm in QuickPIC, as well as the future plans to improve the code. Preliminary results will be presented, which include comparing QuickPIC Plasma wakefield simulations with OSIRIS [3] simulations, and which include laser propagation simulations.

Simulation of Electron‐Cloud Instability in Circular Accelerators using Plasma Models

The interaction between a low density electron cloud in a circular particle accelerator with a circulating charged particle beam is considered. The particle beam’s space charge attracts the cloud, enhancing the cloud density near the beam axis. Beam‐ cloud interaction is studied with a plasma wakefield accelerator simulation code and the results are benchmarked against an existing code. The restoring force on the off‐centered beam due to the cloud’s space charge is studied. The force is stronger at the tail than it is at the head due to the cloud compression near the tail of the beam. The beam dynamics over 20Km of the SPS ring at CERN is studied and the head‐tail dephasing is observed.

3‐D Simulations of Plasma Wakefield Acceleration with Non‐Idealized Plasmas and Beams

3‐D Particle‐in‐cell OSIRIS simulations of the current E‐162 Plasma Wakefield Accelerator Experiment are presented in which a number of non‐ideal conditions are modeled simultaneously. These include tilts on the beam in both planes, asymmetric beam emittance, beam energy spread and plasma inhomogeneities both longitudinally and transverse to the beam axis. The relative importance of the non‐ideal conditions is discussed and a worstcase estimate of the effect of these on energy gain is obtained. The simulation output is then propagated through the downstream optics, drift spaces and apertures leading to the experimental diagnostics to provide insight into the differences between actual beam conditions and what is measured. The work represents a milestone in the level of detail of simulation comparisons to plasma experiments.