Double layer acceleration by laser radiation

Abstract

It is shown that it is possible to accelerate micro-foils to velocities from 108 cm/s up to relativistic velocities without the disturbance of the Rayleigh-Taylor instability. The acceleration occurs due to the radiation pressure of proper high power lasers. In these systems, the ablation force is negligible relative to the ponderomotive force that dominates the acceleration. The laser irradiances of 1017 W/cm2 < IL < 1021 W/cm2 with a pulse duration of the order of 10 picoseconds can accelerate a micro-foil by the laser radiation pressure to velocities as high as 109 cm/s before breaking by Rayleigh Taylor (RT) instability. Similarly, laser irradiances of IL > 1021 W/cm2 with pulse duration of the order of 10 femtoseconds can accelerate a micro-foil to relativistic velocities without RT breaking. Due to the nature of the accelerating ponderomotive force, in both the relativistic and non-relativistic cases, the structure of the accelerated target contains a double layer (DL) at the interface of the laser-target interaction. The DL acts as a piston during the acceleration process. The influence of the DL surface tension on the RT instability is also analyzed in this paper.

A numerical study of ponderomotive ion acceleration in a dense plasma driven by a circularly polarized high-intensity laser beam normally incident on thin foils

“…The rapid build-up of the ion density at the target surface has been reported and discussed in several publications (Macchi et al, 2005(Macchi et al, , 2010Shoucri, 2010Shoucri, , 2012Shoucri & Afeyan, 2010;Shoucri et al, 2011). In Figure 1d at t ≈ 23.2, in Figure 1e at t ≈ 24.8 and in Figure 1f at t ≈ 26.4, we see that the ion peak is decreasing slightly leaving a small ion tail behind it, and a double-layer structure is formed, a geometry close to what has been discussed in Figure 1 of Schlegel et al (2009), and in Figure 2 of Eliezer et al (2014). This doublelayer structure is supported by the radiation pressure of the ponderomotive force of the laser beam.…”

“…This generates a charge separation and an electric field at the plasma edge which accelerates the ions. Under the effect of the radiation pressure acting on the electron surface, the ion peak and the electron peak separate, forming a double-layer structure [very close to what is described in Figure 1 of Schlegel et al (2009) and in Figure 2 of Eliezer et al (2014)], with an electric field between the two peaks providing a restoring force, with an additional ion population trapped in the electron thin layer. The laser ponderomotive force pushes the electron density peak and its trapped ion population forward, while the initial ion peak is slowly decaying and following accordingly behind.…”

“…In addition, laser technology makes it possible now to generate extremely short (femtosecond) and intense (>10 22 W/cm 2 ) laser pulses (Mourou et al, 2006;Borghesi et al, 2007). Under these conditions, the formation of a double layer in the dense target plasmas by the ponderomotive force of these intense electromagnetic pulses has been considered in several publications (see, for instance, Naumova et al, 2009;Eliasson et al, 2009;Schlegel et al, 2009;Tripathi et al, 2009;Eliezer et al, 2014). Initially, the radiation pressure (the ponderomotive force per unit area, or the flow of delivered momentum by the wave to the target per unit time and surface) of the incident laser beam pushes the electrons at the target surface producing a sharp density gradient at the surface of the target.…”

“…The entire structure at the interface of the laser-target interaction is accelerated as a whole. Eliezer et al (2014) have recently studied the stability of such double-layer structure and shown that the laser radiation pressure can accelerate this double-layer structure to very high velocities without breaking it by the Rayleigh-Taylor instability. A recent study and review on radiation pressure acceleration can be found in Schmidt et al (2015a, b).…”

Ultrafast ignition with relativistic shock waves induced by high power lasers

“…In this domain of laser intensities the force acts on the electrons that are accelerated and the ions that follow accordingly. This model describes our piston model [37,38] as summarized schematically in Fig Figure 2(b) the system of the negative and positive layers is called a double layer (DL), n e and n i are the electron and ion densities respectively, E x is the electric field, λ DL is the distance between the positive and negative DL charges, and δ is the solid density skin depth of the foil. The DL is geometrically followed by a neutral plasma where the electric field decays within a skin depth and a shock wave is created.…”