Laser acceleration of monoenergetic protons in a self-organized double layer from thin foil

Abstract

We present a theory for the acceleration of monoenergetic protons, trapped in a self-organized double layer, by short pulse laser irradiation on a thin foil with the specific thickness suggested by the simulation study of Yan et al (2008 Phys. Rev. Lett. 100 135003). The laser ponderomotive force pushes the electrons forward, leaving the ions behind until the space charge electric field balances the ponderomotive force at a distance . For the optimal target thickness D = > c/ω p , the electron sheath is piled up at the rear surface and the sheath electric field accelerates the protons until they are reflected by the inertial force in the accelerated frame. These protons are therefore trapped by the combined forces of the electrostatic field of the electron sheath and the inertial force of the accelerating target. Together with the electron layer, they form a double layer and are collectively accelerated by the laser ponderomotive force, leading to monoenergetic ion production.

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 restoring force of the space-charge electric field does not reach the intensity where it can balance the ponderomotive force of the laser pressure to keep the two peaks at a constant distance. In Figure 5f, the initial ion peak has almost disappeared, and the figure now is very close to what is discussed in Figure 2 of Tripathi et al (2009), and in Macchi et al (2010), showing a trapped ion population in the negatively charged electron solitary structure. This transition of the double-layer structure from a shape similar to Figures 5d and 5e, to a shape similar to Figure 5f has been also reported in Figure 7 of Shoucri (2012), for more moderate values of a 0 = 25/ 2 √ and density n = 25n cr .…”

“…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.…”

“…In the case with a 0 = 100, the whole thin foil is transformed into a double layer and the ponderomotive force acceleration persists beyond the transition through the initial target volume. Electrons pile up in a thin layer at the rear surface (Tripathi et al, 2009;Macchi et al, 2010), and only the ions in this electron layer are effectively accelerated, this layer ultimately leading to a neutral plasma jet. In these first two cases, there is no transmission of the laser beam across the moving target.…”

Dynamic study of a compressed electron layer during the hole-boring stage in a sharp-front laser interaction region

“…High-energy ions accelerated by such intense laser interacting with the solid target can be prospectively applied in fast ignition for inertial confinement fusion [3,4], medical therapy [5,6], and proton imaging [7], among others. Radiation pressure acceleration (RPA) using circularly polarized (CP) laser pulses provides a promising way to obtain a high-energy ion beam with monoenergetic spectrum in a much more efficient manner, compared with target normal sheath acceleration [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. Yan et al [12] proposed the phase-stable-acceleration (PSA) mechanism in the RPA regime [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] to synchronously accelerate and bunch ions within skin depth of the laser pulse to get a monoenergetic ion beam in the longitudinal direction.…”

“…These conditions, which have recently been investigated analytically [16,21] and applied in experiments [18], provide static solutions at the end of the hole-boring stage, neglecting the effects of ion motion. The dynamics of the CEL and ions during the hole-boring stage is also important for the stable state formation of the CEL and stable ion acceleration.…”