Plasma wake inhibition at the collision of two laser pulses in an underdense plasma

An electron injector concept for laser-plasma accelerator was developed in ref [1] and [2] ; it relies on the use of counter-propagating ultrashort laser pulses. In [2], the scheme is as follows: the pump laser pulse generates a large amplitude laser wakefield (plasma wave). The counter-propagating injection pulse interferes with the pump laser pulse to generate a beatwave pattern. The ponderomotive force of the beatwave is able to inject plasma electrons into the wakefield. We have studied this injection scheme using 1D Particle in Cell (PIC) simulations. The simulations reveal phenomena and important physical processes that were not taken into account in previous models. In particular, at the collision of the laser pulses, most plasma electrons are trapped in the beatwave pattern and cannot contribute to the collective oscillation supporting the plasma wave. At this point, the fluid approximation fails and the plasma wake is strongly inhibited. Consequently, the injected charge is reduced by one order of magnitude compared to the predictions from previous models.Comment: 4 pages, 4 figure

Concept of a laser-plasma based electron source for sub-10 fs electron diffraction

We propose a new concept of an electron source for ultrafast electron diffraction with sub-10~fs temporal resolution. Electrons are generated in a laser-plasma accelerator, able to deliver femtosecond electron bunches at 5 MeV energy with kHz repetition rate. The possibility of producing this electron source is demonstrated using Particle-In-Cell simulations. We then use particle tracking simulations to show that this electron beam can be transported and manipulated in a realistic beamline, in order to reach parameters suitable for electron diffraction. The beamline consists of realistic static magnetic optics and introduces no temporal jitter. We demonstrate numerically that electron bunches with 5~fs duration and containing 1.5~fC per bunch can be produced, with a transverse coherence length exceeding 2~nm, as required for electron diffraction

Quasimonoenergetic electron beams produced by colliding cross-polarized laser pulses in underdense plasmas

The interaction of two laser pulses in an underdense plasma has proven to be able to inject electrons in plasma waves, thus providing a stable and tunable source of electrons. Whereas previous works focused on the "beatwave" injection scheme in which two lasers with the same polarization collide in a plasma, this present letter studies the effect of polarization and more specifically the interaction of two colliding cross-polarized laser pulses. It is shown both theoretically and experimentally that electrons can also be pre-accelerated and injected by the stochastic heating occurring at the collision of two cross-polarized lasers and thus, a new regime of optical injection is demonstrated. It is found that injection with cross-polarized lasers occurs at higher laser intensities.Comment: 4 pages, 4 figure

Where Fail-Safe Default Logics Fail

Reiter's original definition of default logic allows for the application of a default that contradicts a previously applied one. We call failure this condition. The possibility of generating failures has been in the past considered as a semantical problem, and variants have been proposed to solve it. We show that it is instead a computational feature that is needed to encode some domains into default logic

Laser-plasma interactions with a Fourier-Bessel Particle-in-Cell method

A new spectral particle-in-cell (PIC) method for plasma modeling is presented and discussed. In the proposed scheme, the Fourier-Bessel transform is used to translate the Maxwell equations to the quasi-cylindrical spectral domain. In this domain, the equations are solved analytically in time, and the spatial derivatives are approximated with high accuracy. In contrast to the finite-difference time domain (FDTD) methods that are commonly used in PIC, the developed method does not produce numerical dispersion, and does not involve grid staggering for the electric and magnetic fields. These features are especially valuable in modeling the wakefield acceleration of particles in plasmas. The proposed algorithm is implemented in the code PLARES-PIC, and the test simulations of laser plasma interactions are compared to the ones done with the quasi-cylindrical FDTD PIC code CALDER-CIRC.Comment: submitted to Phys. Plasma

SO(4) Invariant States in Quantum Cosmology

The phenomenon of linearisation instability is identified in models of quantum cosmology that are perturbations of mini-superspace models. In particular, constraints that are second order in the perturbations must be imposed on wave functions calculated in such models. It is shown explicitly that in the case of a model which is a perturbation of the mini-superspace which has $S^3$ spatial sections these constraints imply that any wave functions calculated in this model must be SO(4) invariant. (This replaces the previous corrupted version.)Comment: 15 page