Abstract. We describe OSIRIS, a three-dimensional, relativistic, massively parallel, object oriented particle-in-cell code for modeling plasma based accelerators. Developed in Fortran 90, the code runs on multiple platforms (Cray T3E, IBM SP, Mac clusters) and can be easily ported to new ones. Details on the code's capabilities are given. We discuss the object-oriented design of the code, the encapsulation of system dependent code and the parallelization of the algorithms involved. We also discuss the implementation of communications as a boundary condition problem and other key characteristics of the code, such as the moving window, open-space and thermal bath boundaries, arbitrary domain decomposition, 2D (cartesian and cylindric) and 3D simulation modes, electron sub-cycling, energy conservation and particle and field diagnostics. Finally results from three-dimensional simulations of particle and laser wakefield accelerators are presented, in connection with the data analysis and visualization infrastructure developed to post-process the scalar and vector results from PIC simulations.

3module. A 28.5 GeV electron beam with 1:8 10 10 electrons is compressed to 20 m longitudinally and focused to a transverse spot size of 10 m at the entrance of a 10 cm long column of lithium vapor with density 2:8 10 17 atoms=cm 3 . The electron bunch fully ionizes the lithium vapor to create a plasma and then expels the plasma electrons. These electrons return one-half plasma period later driving a large amplitude plasma wake that in turn accelerates particles in the back of the bunch by more than 2.7 GeV.Plasmas have extraordinary potential for advancing the energy frontier in high-energy physics due to the large focusing and accelerating fields that are generated.Beam-plasma interactions have demonstrated focusing gradients of MT=m [1] while laser plasma interactions have demonstrated GeV=cm accelerating gradients [2 -7] over distances of a few mm. Beam-driven plasmawakefield accelerators (PWFA) have recently demonstrated acceleration and focusing of both electrons [8,9] and positrons [10,11] in meter scale plasmas.The experiment described in this Letter uses an ultrarelativistic electron bunch to simultaneously create a plasma in lithium vapor and drive a large amplitude plasma wave. When the electron bunch enters the lithium vapor, the electric field of the leading portion of the bunch ionizes the valence electron of each lithium atom in its vicinity leaving fully ionized neutral plasma for the remainder of the bunch [12,13]. The plasma electrons are then expelled from the beam volume and return one-half plasma period later. The returning plasma electrons form density concentrations on axis behind the bunch leading to a large accelerating field for the particles in the back of the bunch.In linear plasma theory [14] the wakefield amplitude increases as N= 2 z , provided the plasma density is increased such that k p z 2 p where N is the number of electrons in the bunch, z is the bunch length, and k p ! p =c is the inverse of the plasma collisionless skin depth. The nonlinear or blowout regime is reached when the electron bunch density n b N= 2 3=2 z 2 r is greater than the plasma density n p and the beam radius satisfies r c=! p . In the blowout regime, for bunch lengths on the order of the plasma wavelength, the plasma electrons are expelled from the beam volume to a radius r c 2 N= 2 3=2 z n p q leaving behind a pure ion column. This experiment is in a regime in which the electron bunch radius, bunch length, ion channel radius, and plasma wavelength are all on the same order. Although the experiments described here are on the edge of the blowout regime, numerical simulations indicate the N= 2 z increase in plasma-wakefield amplitude can still be realized [15]. Verification of the dramatic increase in accelerating gradient predicted for short bunches is a critical milestone for the application of plasma-wakefield accelerators to future high-energy accelerators and colliders.A single 28.5 GeV bunch of 1:8 10 10 electrons from the Stanford Linear Accelerator Center (SLAC) linac enters the Final Focus Test B...

The onset of trapping of electrons born inside a highly relativistic, 3D beam-driven plasma wake is investigated. Trapping occurs in the transition regions of a Li plasma confined by He gas. Li plasma electrons support the wake, and higher ionization potential He atoms are ionized as the beam is focused by Li ions and can be trapped. As the wake amplitude is increased, the onset of trapping is observed. Some electrons gain up to 7.6 GeV in a 30.5 cm plasma. The experimentally inferred trapping threshold is at a wake amplitude of 36 GV=m, in good agreement with an analytical model and PIC simulations.

The electron hosing instability in the blow-out regime of plasma-wakefield acceleration is investigated using a linear perturbation theory about the electron blow-out trajectory in Lu et al. [in Phys. Rev. Lett. 96, 165002 (2006)]. The growth of the instability is found to be affected by the beam parameters unlike in the standard theory Whittum et al. [Phys. Rev. Lett. 67, 991 (1991)] which is strictly valid for preformed channels. Particle-in-cell simulations agree with this new theory, which predicts less hosing growth than found by the hosing theory of Whittum et al. Recent experiments have shown amazing progress for both plasma-wakefield acceleration (PWFA) and laser wakefield acceleration (LWFA) [1][2][3] in the electron blow-out regime. In this regime, plasma electrons are completely evacuated by the space charge force of an electron beam or the ponderomotive force of a laser pulse, forming an ion channel on the axis of the system with a laminar sheath at the channel boundary carrying large concentrations of relativistic electrons. However, the electron hosing instability [4 -7] of the drive and/or trailing beam remains a major concern for PWFA/LWFA concepts. The hosing instability results from the interaction between the electron sheath and the self-injected or externally injected electron beam. It leads to spatiotemporally growing oscillations of the beam centroid at each axial slice thus limiting the useful acceleration length and making it difficult to aim the beam. Existing standard theory [4,6] predicts rapid growth for this instability. However, recent experiments [1,2] have shown little evidence of hosing.In this Letter, we present a more general hosing theory based on a perturbation method to the zeroth order trajectory [8] for the ion-channel/electron-sheath boundary. The initial hosing growth predicted by the linearized coupling is found to be affected by the nonconstant channel radius, relativistic mass corrections, and the longitudinal velocity of electrons in the plasma sheath. We verify this theory using particle-in-cell (PIC) simulations and compare it to the standard theory.The existing work [4,6] focused on the hosing in a long ion channel with a radius near the charge neutralization radius, i.e., r c r neu n b R 2 b =n p q , where n b , n p are the beam and plasma density, respectively, and R b is the beam radius. Such a channel is either preformed or adiabatically formed. The electrons in the sheath layer are assumed to be at rest, i.e., the nonrelativistic limit; therefore, they do not generate or feel the magnetic fields. This adiabatic (referring to the channel formation), nonrelativistic (referring to the plasma sheath motion) limit is appropriate for a beam where, s is the propagation distance into the plasma, ct ÿ z is the location within the beam, and is the beam Lorentz factor. For a PWFA, the ''short-pulse'' limit of these equations is relevant, i.e., k s ! 0 , and the asymptotic solution for a linear tilt in x b is x b =x b0 0:341A ÿ3=2 e A cos k s ÿ A= 3 p =4 [6], where ...

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