Increased laser-accelerated proton energies via direct laser-light-pressure acceleration of electrons in microcone targets

Abstract

We present experimental results showing a laser-accelerated proton beam maximum energy cutoff of 67.5 MeV, with more than 5 × 106 protons per MeV at that energy, using flat-top hollow microcone targets. This result was obtained with a modest laser energy of ∼80 J, on the high-contrast Trident laser at Los Alamos National Laboratory. From 2D particle-in-cell simulations, we attribute the source of these enhanced proton energies to direct laser-light-pressure acceleration of electrons along the inner cone wall surface, where the laser light wave accelerates electrons just outside the surface critical density, in a potential well created by a shift of the electrostatic field maximum with respect to that of the magnetic field maximum. Simulations show that for an increasing acceleration length, the continuous loading of electrons into the accelerating phase of the laser field yields an increase in high-energy electrons.

On the origin of super-hot electrons from intense laser interactions with solid targets having moderate scale length preformed plasmas

“…Others have investigated the role of stochastic fields on electron acceleration [23][24][25] . PIC modeling was used to identify direct laser acceleration (DLA) as the mechanism responsible for the experimentally observed enhancement to electron and ion energy spectra when a high-contrast short-pulse laser interacts with the wall of a flat-top cone 26,27 . For this case, electrons were injected directly into the intense laser field by the side of the cone wall, thus producing the enhanced spectra.…”

Radiation pressure acceleration: The factors limiting maximum attainable ion energy

“…Most of the laser ion acceleration experimental results were obtained in the TNSA regime [5] with the maximum proton energy around 70 MeV [16]. Using a high contrast, 200 TW, femtosecond-pulse laser irradiating a micronthickness Al foil target, the TNSA regime has produced 40 MeV protons [34].…”

“…This interest is also due to the recent availability of ultrahigh power lasers with focused intensity up to 10 22 W/cm 2 [14] and laser pulse cleaning techniques that allow a temporal intensity contrast of 14 orders of magnitude [15]. New and efficient acceleration regimes were proposed, and some of them tested experimentally, giving rise to proton beams with the energy of about 100 MeV from nm-scale foils of solid density [16][17][18].…”

“…Each mechanism is characterized by the operational parameter range and requires specially designed targets in order to maximize the advantages of the particular mechanism and compensate for different limitations. For TNSA special target designs were considered, such as pizza-cone targets [16], nano structured targets [48], thin foil targets with a thin layer deposited on the back, and thin foils with a low density slabs attached at the front [29,31]. Recently several target designs were proposed for other mechanisms, including mass limited targets, RPA in a tube, and double shock formation in a gas jet for MVA [49].…”

Ion motion effects on the generation of short-cycle relativistic laser pulses during radiation pressure acceleration

“…These events jeopardize the relativistic interaction of the ultra-thin target. Thus, high-contrast [9][10][11][12][13][14][15][16][17][18][19][20] and short-duration laser pulses [21,22] are needed. Plasma mirrors may be a feasible method by which to solve these problems.…”