Comment on “Electron Temperature Scaling in Laser Interaction with Solids”

International audienceA Comment on the Letter by T. Kluge et al., Phys. Rev. Lett. 107, 205003 (2011). The authors of the Letter offer a Reply

Comment on “Transition to the Relativistic Regime in High Order Harmonic Generation”

International audienceIn [Phys. Rev. Lett. 98, 103902 (2007)], Tarasevitch et al. demonstrate the existence of two generation mechanisms for laser high-order harmonicsfrom overdense plasmas. One of these mechanisms leads to harmonics with frequencies up to the maximum plasmafrequency of the target and occurs even at nonrelativistic laser intensities. We show that the mechanism responsiblefor these harmonics is coherent wake emission (CWE), a process that significantly differs from thequalitative model proposed by these authors, and it leads toa different interpretation of several essential features of this emission

Self-generation of megagauss magnetic fields during the expansion of a plasma

International audienceThe expansion of a plasma slab into a vacuum is studied using one-dimensional and two-dimensional particle-in-cell simulations. As electrons transfer their longitudinal kinetic energy to ions during the expansion, the electron temperature becomes anisotropic. Once this anisotropy exceeds a threshold value, it drives the Weibel instability, leading to magnetic fields in the megagauss range. These fields induce energy transfer between the longitudinal and transverses directions, which influences the expansion. The impact of a cold electron population on this phenomenon is also investigated. Plasma expansion is a fundamental process which occurs in very different fields, such as astrophysics ͓1,2͔, laser-plasma ion acceleration ͓3–5͔ and thin-film deposition ͓6͔. This phenomenon is usually described by simple one-dimensional models ͓7–9͔. Yet, even when the system is translation-invariant along the plasma surface, several effects ͑e.g., Coulomb collisions ͓10͔͒ can induce momentum transfer between the longitudinal and transverse directions. The purely one-dimensional ͑1D͒ description is thus, in general, inaccurate. In this paper, we show that self-generated magnetic fields can lead to such momentum transfer during the expansion of a collisionless plasma slab. This study is of particular interest in the context of laser-plasma ion acceleration , where an intense laser pulse is focused on a thin foil to create a hot electron population that transfers progressively its energy to ions via the ambipolar electric field at the plasma surface ͓11͔. We assume here that the electron distribution is initially Maxwellian with an isotropic temperature. As the plasma expands, the longitudinal temperature T ʈ decreases and the anisotropy parameter A = T Ќ / T ʈ − 1 increases, which eventually leads to the growth of the Weibel instability ͓12–18͔. The most unstable modes are obtained for k = k x e x , where e x is a unit vector normal to the plasma surface. In this case, the maximum unstable wave vector is k

Regimes of expansion of a collisional plasma into a vacuum

International audienceThe effect of elastic Coulomb collisions on the one-dimensional expansion of a plasma slab is studied in the classical limit, using an electrostatic particle-in-cell code. Two regimes of interest are identified. For a collision rate of few hundreds of the inverse of the expansion characteristic time $\tau_e$ the electron distribution function remains isotropic and Maxwellian with a homogeneous temperature, during all the expansion. In this case, the expansion can be approached by a three-dimensional version of the hybrid model developed by Mora [P. Mora, Phys. Rev. E 72, 056401 2005]. When the collision rate becomes somewhat greater than $10^4 \tau_e^{-1}$ the plasma is divided in two parts: an inner part which expands adiabatically as an ideal gas and an outer part which undergoes an isothermal expansion

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

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

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

Phase Properties of Laser High-Order Harmonics Generated on Plasma Mirrors

International audienceAs a high-intensity laser-pulse reflects on a plasma mirror, high-order harmonics of the incident frequency can be generated in the reflected beam. We present a numerical study of the phase properties of these individual harmonics, and demonstrate experimentally that they can be coherently controlled through the phase of the driving laser field. The harmonic intrinsic phase, resulting from the generation process, is directly related to the coherent sub-laser-cycle dynamics of plasma electrons, and thus constitutes a new experimental probe of these dynamics

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