Physical Mechanism of the Transverse Instability in Radiation Pressure Ion Acceleration

Abstract

The transverse stability of the target is crucial for obtaining high quality ion beams using the laser radiation pressure acceleration (RPA) mechanism. In this letter, a theoretical model and supporting two-dimensional (2D) Particle-in-Cell (PIC) simulations are presented to clarify the physical mechanism of the transverse instability observed in the RPA process. It is shown that the density ripples of the target foil are mainly induced by the coupling between the transverse oscillating electrons and the quasi-static ions, a mechanism similar to the transverse two stream instability in the inertial confinement fusion (ICF) research. The predictions of the mode structure and the growth rates from the theory agree well with the results obtained from the PIC simulations in various regimes, indicating the model contains the essence of the underlying physics of the transverse break-up of the target. [6] and so on. Ideal one dimensional (1D) simulations show monoenergetic ion acceleration in the RPA process using a circularly polarized (CP) laser pulse[7-13] with high energy conversion efficiency. In reality, however, the finite transverse witdth of the laser pulse can deform the target shape, leading to electron heating and energy spectrum broading of the accelerated ions [10,11,14]. At the same time, 2/3D simulations also show that transverse density ripples can grow significantly, leading to some of the laser energy through and breaking up the target [10,11,[14][15][16][17][18][19]. This phenomenon shows up even for a laser pulse of infinite width and uniform intensity profile [11,19,20]. Various mechanisms have been proposed to explain the structrue of these ripples, such as Rayleigh-Taylor like (RT-like) instability [10,11,17,[19][20][21][22][23], Weibel like instability [16,18] and so on. However, these models have not been able to give accurate predictions of the mode structure and its growth rates for a wide range of laser and plasma parameters.In this letter, we show through theoretical analysis and PIC simulations that these surface ripples are more likely induced by the coupling between the transverse oscillating electrons and the quasi-static ions within the high density layer formed by the laser radiation pressure pushing the surface plasma forward in a process often called 'hole-boring" (H-B) [7,24]. As shown in Fig. 1(a), during this H-B process, soon after the laser impinges on the front surface of the target, a dynamic equilibrium between the laser pressure and the electrostatic field within the plasma is built, forming a quasi-static high density structure co-moving with the laser pulse [7]. Within this layer, the CP laser field oscillates at the laser frequency along both transverse directions albeit pi radians out of phase. A very small transverse ion density fluctuation can couple with the oscillating laser field to excite an electron oscillation. This oscillation in turn can couple with the oscillating laser field to generate a ponderomotive force with spatial variation, driving the electrons to...

Kinetic and finite ion mass effects on the transition to relativistic self-induced transparency in laser-driven ion acceleration

“…In realistic (3D) geometries, further complicating factors may play an interesting role. Transverse, instabilities developing at the laser-plasma interaction surface [30,34,[49][50][51] can trigger additional electron heating [52,53] thus modifying the threshold density in a complex way. It is therefore important that these 3D effects are taken into account in future works and that mitigation strategies for transverse instabilities relying, for example, on tuning the laser polarization [54] or intensity profile [55] be employed.…”

Surface modulation and back reflection from foil targets irradiated by a Petawatt femtosecond laser pulse at oblique incidence

“…However, the distribution of point sources of secondary waves are not completely random. There are number of articles devoted to the analysis of the plasma density profile formed through the development of various kinds of instabilities (Rayleigh-Taylor instabilities [21], two-stream instabilities [22], and generation of surface waves [23,24]), but we see the formation of quasiperiodic structures of the surface having certain (emphasize) scale.…”

High quality ion acceleration through the interaction of two matched counterpropagating transversely polarized Gaussian lasers with a flat foil target

“…However, there are also some problems in the RPA regime, such as the maximum energy limitation of the accelerated ions [39] and the transverse instability. There have been many researches about the transverse instability [22,[24][25][26], and a recent work indicates that the ripples on the target is induced by the coupling between oscillating electrons and quasistatic ions [42]. On the other hand, in the high dimension situation, the amplitude profile of the normal transversely Gaussian laser has a transverse distribution, which means a transversely nonuniform radiation on the foil.…”