Studies of classical radiation emission from plasma wave undulators

The authors examine the characteristics of the classical radiation emitted by a relativistic electron beam that propagates perpendicularly through a large amplitude relativistic plasma wave. Such a study is useful for evaluating the feasibility of using relativistic plasma waves as extremely short wavelength undulators for generating short wavelength radiation. The electron trajectories in a plasma wave undulator are obtained using perturbation techniques and are then compared to numerical simulation results. The frequency spectrum and angular distribution of the spontaneous radiation emitted by a single electron and the stimulated radiation gain are obtained analytically, and are then compared to 3-D numerical simulations. The characteristics of the plasma wave undulator are compared to the AC free-electron laser (FEL) undulator and the conventional FEL     

Modeling single-frequency laser-plasma acceleration usingparticle-in-cell simulations: the physics of beam breakup

We investigate electron acceleration from space-charge waves driven by single-frequency lasers using a fully explicit particle-in-cell (PIC) model. The two dimensional (2-D) simulations model ~100 fs pulses at densities near n=4×10¹⁹ cm^-3 for 1-μm lasers. The pulses are found to break up due to a combination of longitudinal and transverse bunching of the laser intensity via Raman forward scattering type instabilities. The ponderomotive force of these intensity modulations generates large amplitude plasma waves. Large numbers of self-trapped electrons and multiple Raman forward scattering satellites are observed. The relevance of these simulations to experiments is discussed     

A plasma klystron for generating ultra-short electron bunches

A technique for producing ultra-short electron bunches (e.g., ⩽100 fs) from a continuous electron beam using a short plasma wave section and a drift space is explored. The bunches are a fraction of a plasma wavelength long and are spaced by a plasma wavelength, making them of interest for injection into plasma accelerators or for driving a klystron-like structure to produce infrared radiation     

Frequency upshifting by an ionization front in a magnetized plasma

The interaction of a light wave with a relativistic ionization front in the presence of an applied DC magnetic field which is perpendicular or parallel to the incident wave is considered. In both cases, four transmitted modes are generated in the magnetized plasma by an incident linearly polarized wave. The frequency upshifts of the various modes are calculated and compared to the unmagnetized case. The corresponding reflection and transmission coefficients are also obtained. Finally, the density ripple associated with the free streaming mode in a magnetized plasma for the perpendicular case is discussed     

Hosing instability in the blow-out regime for plasma-wakefield acceleration

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)10.1103/PhysRevLett.96.165002]. 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)10.1103/PhysRevLett.67.991] 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.     

Nonlinear theory for relativistic plasma wakefields in the blowout regime

We present a theory for nonlinear, multidimensional plasma waves with phase velocities near the speed of light. It is appropriate for describing plasma waves excited when all electrons are expelled out from a finite region by either the space charge of a short electron beam or the radiation pressure of a short intense laser. It works very well for the first bucket before phase mixing occurs. We separate the plasma response into a cavity or blowout region void of all electrons and a sheath of electrons just beyond the cavity. This simple model permits the derivation of a single equation for the boundary of the cavity. It works particularly well for narrow electron bunches and for short lasers with spot sizes matched to the radius of the cavity. It is also used to describe the structure of both the accelerating and focusing fields in the wake.     

Ionization-induced electron trapping in ultrarelativistic plasma wakes

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.     

Enhancing parallel quasi-static particle-in-cell simulations with a pipelining algorithm

A pipelining algorithm to overcome the limitation on scaling quasi-static particle-in-cell models of relativistic beams in plasmas to a very large number of processors is described. The pipelining algorithm uses multiple groups of processors and optimizes the job allocation on the processors in parallel computing. The algorithm is implemented on the quasi-static code QuickPIC and is shown to scale to over 10³ processors and increased the scale and speed by two orders of magnitude over the non-pipelined model. The new approach opens the door to performing full scale 3D simulations of future plasma wakefield accelerators or full lifetime models of beam interaction with electron clouds in circular accelerators such as the Large Hadron Collider (LHC) at CERN.