Increased efficiency of short-pulse laser-generated proton beams from novel flat-top cone targets

Abstract

Ion-driven fast ignition (IFI) may have significant advantages over electron-driven FI due to the potentially large reduction in the amount of energy required for the ignition beam and the laser driver. Recent experiments at the Los Alamos National Laboratory’s Trident facility employing novel Au flat-top cone targets have produced a fourfold increase in laser-energy to ion-energy efficiency, a 13-fold increase in the number of ions above 10MeV, and a few times increase in the maximum ion energy compared to Au flat-foil targets. Compared to recently published scaling laws, these gains are even greater. If the efficiency scales with intensity in accordance to flat-foil scaling, then, with little modification, these targets can be used to generate the pulse of ions needed to ignite thermonuclear fusion in the fast ignitor scheme. A proton energy of at least 30MeV was measured from the flat-top cone targets, and particle-in-cell (PIC) simulations show that the maximum cutoff energy may be as high as 40–45MeV at modest intensity of 1×1019W∕cm2 with 20J in 600fs. Simulations indicate that the observed energy and efficiency increase can be attributed to the cone target’s ability to guide laser light into the neck to produce hot electrons and transport these electrons to the flat-top of the cone where they can be heated to much higher temperatures, creating a hotter, denser sheath. The PIC simulations also elucidate the critical parameters for obtaining superior proton acceleration such as the dependence on laser contrast/plasma prefill, as well as longitudinal and transverse laser pointing, and cone geometry. These novel cones have the potential to revolutionize inertial confinement fusion target design and fabrication via their ability to be mass produced. In addition, they could have an impact on the general physics community studying basic electron and radiation transport phenomena or as better sources of particle beams to study equations of state and warm dense matter, or for hadron therapy, as new radioisotope generators, or for compact proton radiography sources.

Effects of the Micro-hole Target Structures on the Laser-driven Energetic Proton Generation

“…Also, several ideas on target design to generate high-quality, high-efficiency and high-energy protons have been proposed. Flippo et al used flat-top cone targets to get high-efficiency proton beams [8]. By using particle-in-cell (PIC) simulations, Liu et al showed that energetic collimated ion bunches can be achieved with a thin concave target [9].…”

Reaching high flux in laser-driven ion acceleration

“…This scheme, however, suffers from a number of severe drawbacks [11], including (i) a disfavorable scaling of achievable ion energies with increasing laser intensity [12], limiting the scheme's efficiency at higher laser intensities, (ii) a limited control over the acceleration, as is apparent from the mostly thermal ion energy spectrum, hindering the precise tuning of the target ion energies, as well as (iii) a low efficiency of the acceleration mechanism at high laser powers [13]. Many of these drawbacks, however, can be counteracted to some extent by specially designed targets [14][15][16][17] and laser pulse shapes [18,19]. Thus, the comparatively simple design and operation of TNSA accelerators renders it a promising mechanism for the construction of reliable and stable lower-energy laser ion-accelerators.…”

Energy partitioning and electron momentum distributions in intense laser-solid interactions

“…Various studies have recently been performed of specially designed targets and laser pulse shapes [23][24][25][26][27][28][29][30][31][32][33][34][35]. However, it is well known that the TNSA scheme has several shortcomings, such as intrinsic angular and energy spread of the accelerated ions.…”