Micro-sphere layered targets efficiency in laser driven proton acceleration

Abstract

Proton acceleration from the interaction of high contrast, 25 fs laser pulses at >1019 W/cm2 intensity with plastic foils covered with a single layer of regularly packed micro-spheres has been investigated experimentally. The proton cut-off energy has been measured as a function of the micro-sphere size and laser incidence angle for different substrate thickness, and for both P and S polarization. The presence of micro-spheres with a size comparable to the laser wavelength allows to increase the proton cut-off energy for both polarizations at small angles of incidence (10∘). For large angles of incidence, however, proton energy enhancement with respect to flat targets is absent. Analysis of electron trajectories in particle-in-cell simulations highlights the role of the surface geometry in the heating of electrons.

Laser-driven high-energy proton beam with homogeneous spatial profile from a nanosphere target

“…The difference in angular distribution can be explained by trajectories of electrons on the front surface of the targets as shown in Ref. [25], and it agrees well with our previous numerical results obtained at a lower laser intensity [13]. Hot electrons can be generated when the electric field associated with the laser pulse, which forces electrons to oscillate in vacuum, is shielded by the plasma (ionized target).…”

“…Since the diameter of the spheres is several times smaller than the focal spot size, the result is not sensitive to the distribution of the nanospheres owing to the laser focal spot position. In a real 3D geometry, the electrons traveling on a line parallel to the surface would experience a spatial periodicity equal to the size of the sphere only along a limited set of directions [25]. According to their initial acceleration direction, electrons can cross any vacuum region between any couple of spheres or can travel along a different path, showing an even more complicated periodicity of the system.…”

Toward high-energy laser-driven ion beams: Nanostructured double-layer targets

“…Up to now these issues have been addressed separately. In the last few years several target concepts have been developed with the aim of increasing the maximum ion energy: ultrathin foils [3,4], reduced mass targets [5], nanosphere targets [6][7][8], and grating targets [9]. Recently energies up to 20 MeV=u for C 6þ ions and 29 MeV for protons were obtained using ultrathin diamond-like carbon (DLC) foils covered with a layer of carbon nanotubes [10].…”

Increased efficiency of ion acceleration by using femtosecond laser pulses at higher harmonic frequency

“…Such strong enhancement of ion acceleration can surpass other enhancements which propose to use special targets like the ones with microstructures [33][34][35], foam [36], or grating [37] on the surface. Moreover, flat foil targets can be more easily produced than other special targets, which is important from the point of view of future applications of high repetition rate femtosecond lasers.…”