An efficient and portable SIMD algorithm for charge/current deposition in Particle-In-Cell codes

In current computer architectures, data movement (from die to network) is by far the most energy consuming part of an algorithm (10pJ/word on-die to 10,000pJ/word on the network). To increase memory locality at the hardware level and reduce energy consumption related to data movement, future exascale computers tend to use more and more cores on each compute nodes ("fat nodes") that will have a reduced clock speed to allow for efficient cooling. To compensate for frequency decrease, machine vendors are making use of long SIMD instruction registers that are able to process multiple data with one arithmetic operator in one clock cycle. SIMD register length is expected to double every four years. As a consequence, Particle-In-Cell (PIC) codes will have to achieve good vectorization to fully take advantage of these upcoming architectures. In this paper, we present a new algorithm that allows for efficient and portable SIMD vectorization of current/charge deposition routines that are, along with the field gathering routines, among the most time consuming parts of the PIC algorithm. Our new algorithm uses a particular data structure that takes into account memory alignement constraints and avoids gather/scatter instructions that can significantly affect vectorization performances on current CPUs. The new algorithm was successfully implemented in the 3D skeleton PIC code PICSAR and tested on Haswell Xeon processors (AVX2-256 bits wide data registers). Results show a factor of $\times 2$ to $\times 2.5$ speed-up in double precision for particle shape factor of order $1$ to $3$. The new algorithm can be applied as is on future KNL (Knights Landing) architectures that will include AVX-512 instruction sets with 512 bits register lengths (8 doubles/16 singles).Comment: 36 pages, 5 figure

Optimization of laser-plasma injector via beam loading effects using ionization-induced injection

Simulations of ionization induced injection in a laser driven plasma wakefield show that high-quality electron injectors in the 50-200 MeV range can be achieved in a gas cell with a tailored density profile. Using the PIC code Warp with parameters close to existing experimental conditions, we show that the concentration of $\mathrm{N_2}$ in a hydrogen plasma with a tailored density profile is an efficient parameter to tune electron beam properties through the control of the interplay between beam loading effects and varying accelerating field in the density profile. For a given laser plasma configuration, with moderate normalized laser amplitude, $a_0=1.6$ and maximum electron plasma density, $n_{e0}=4\times 10^{18}\,\mathrm{cm^{-3}}$, the optimum concentration results in a robust configuration to generate electrons at 150~MeV with a rms energy spread of 4\% and a spectral charge density of 1.8~pC/MeV.Comment: 13 pages, 10 figure

Optical Transverse Injection in Laser-Plasma Acceleration

International audienceLaser-wakefield acceleration constitutes a promising technology for future electron accelerators. A crucial step in such an accelerator is the injection of electrons into the wakefield, which will largely determine the properties of the extracted beam. We present here a new paradigm of colliding-pulse injection, which allows us to generate high-quality electron bunches having both a very low emittance (0.17  mm·mrad) and a low energy spread (2%), while retaining a high charge (∼100  pC) and a short duration (3 fs). In this paradigm, the pulse collision provokes a transient expansion of the accelerating bubble, which then leads to transverse electron injection. This mechanism contrasts with previously observed optical injection mechanisms, which were essentially longitudinal. We also specify the range of parameters in which this new type of injection occurs and show that it is within reach of existing high-intensity laser facilities

Numerical growth of emittance in simulations of laser-wakefield acceleration

International audienceTransverse emittance is a crucial feature of laser-wakefield accelerators, yet accurately reproducing its value in numerical simulations remains challenging. It is shown here that, when the charge of the bunch exceeds a few tens of picocoulombs, particle-in-cell (PIC) simulations erroneously overestimate the emittance. This is mostly due the interaction of spurious Cherenkov radiation with the bunch, which leads to a steady growth of emittance during the simulation. A new computational scheme is proposed, which is free of spurious Cherenkov radiation. It can be easily implemented in existing PIC codes and leads to a substantial reduction of the emittance growth

Observation of longitudinal and transverse self-injections in laser-plasma accelerators

Laser-plasma accelerators can produce high quality electron beams, up to giga-electronvolts in energy, from a centimeter scale device. The properties of the electron beams and the accelerator stability are largely determined by the injection stage of electrons into the accelerator. The simplest mechanism of injection is self-injection, in which the wakefield is strong enough to trap cold plasma electrons into the laser wake. The main drawback of this method is its lack of shot-to-shot stability. Here we present experimental and numerical results that demonstrate the existence of two different self-injection mechanisms. Transverse self-injection is shown to lead to low stability and poor quality electron beams, because of a strong dependence on the intensity profile of the laser pulse. In contrast, longitudinal injection, which is unambiguously observed for the first time, is shown to lead to much more stable acceleration and higher quality electron beams.Comment: 7 pages, 7 figure

Transverse dynamics of an intense electron bunch traveling through a pre-ionized plasma

International audienceThe propagation of a relativistic electron bunch through a plasma is an important problem in both plasma-wakefield acceleration and laser-wakefield acceleration. In those situations, the charge of the accelerated bunch is usually large enough to drive a relativistic wakefield, which then affects the transverse dynamics of the bunch itself. Yet to date, there is no fully relativistic, fully electromagnetic model that describes the generation of this wakefield and its feedback on the bunch. In this article, we derive a model which takes into account all the relevant relativistic and electromagnetic effects involved in the problem. A very good agreement is found between the model and the results of particle-in-cell simulations. The implications of high-charge effects for the transport of the bunch are discussed in detail

Angular momentum evolution in laser-plasma accelerators

The transverse properties of an electron beam are characterized by two quantities, the emittance which indicates the electron beam extend in the phase space and the angular momentum which allows for non-planar electron trajectories. Whereas the emittance of electron beams produced in laser- plasma accelerator has been measured in several experiments, their angular momentum has been scarcely studied. It was demonstrated that electrons in laser-plasma accelerator carry some angular momentum, but its origin was not established. Here we identify one source of angular momentum growth and we present experimental results showing that the angular momentum content evolves during the acceleration

Investigation of the dynamics of ionization induced injected electrons under the influence of beam loading effects

In laser-driven wakefield, ionization induced injection is an efficient way to inject electrons in the plasma wave. A detailed study on the beam dynamics under the influence of beam loading effects, which can be controlled by the concentration of nitrogen impurity introduced in the hydrogen gas was conducted. For a specific value of this percentage, the final energy of the high-energy electron bunch becomes nearly independent of the trapped positions, thus leading to a small energy dispersion. We also show that the final beam emittance is mainly determined by the injection process

An ultracompact X-ray source based on a laser-plasma undulator

International audienceThe capability of plasmas to sustain ultrahigh electric fields has attracted considerable interest over the last decades and has given rise to laser-plasma engineering. Today, plasmas are commonly used for accelerating and collimating relativistic electrons, or to manipulate intense laser pulses. Here we propose an ultracompact plasma undulator that combines plasma technology and nanoengineering. When coupled with a laser-plasma accelerator, this undulator constitutes a millimetre-sized synchrotron radiation source of X-rays. The undulator consists of an array of nanowires, which are ionized by the laser pulse exiting from the accelerator. The strong charge-separation field, arising around the wires, efficiently wiggles the laser-accelerated electrons. We demonstrate that this system can produce bright, collimated and tunable beams of photons with 10-100 keV energies. This concept opens a path towards a new generation of compact synchrotron sources based on nanostructured plasmas

Warp-X: a new exascale computing platform for beam-plasma simulations

Turning the current experimental plasma accelerator state-of-the-art from a promising technology into mainstream scientific tools depends critically on high-performance, high-fidelity modeling of complex processes that develop over a wide range of space and time scales. As part of the U.S. Department of Energy's Exascale Computing Project, a team from Lawrence Berkeley National Laboratory, in collaboration with teams from SLAC National Accelerator Laboratory and Lawrence Livermore National Laboratory, is developing a new plasma accelerator simulation tool that will harness the power of future exascale supercomputers for high-performance modeling of plasma accelerators. We present the various components of the codes such as the new Particle-In-Cell Scalable Application Resource (PICSAR) and the redesigned adaptive mesh refinement library AMReX, which are combined with redesigned elements of the Warp code, in the new WarpX software. The code structure, status, early examples of applications and plans are discussed