Breaking the 100-MeV barrier for proton acceleration will help elucidate fundamental physics and advance practical applications from inertial confinement fusion to tumour therapy. Herein we propose a novel concept of bubble implosions. A bubble implosion combines micro-bubbles and ultraintense laser pulses of 1020–1022 W cm−2 to generate ultrahigh fields and relativistic protons. The bubble wall protons undergo volumetric acceleration toward the centre due to the spherically symmetric Coulomb force and the innermost protons accumulate at the centre with a density comparable to the interior of a white dwarf. Then an unprecedentedly high electric field is formed, which produces an energetic proton flash. Three-dimensional particle simulations confirm the robustness of Coulomb-imploded bubbles, which behave as nano-pulsars with repeated implosions and explosions to emit protons. Current technologies should be sufficient to experimentally verify concept of bubble implosions.
For ultra-high-intensity lasers irradiating nanometer-sized targets, Coulomb explosion (CE) is one of the main ion acceleration schemes. Previous studies have shown that the CE of solid nanospheres can produce quasi-monoenergetic ions. However, the development of optimized hollow nanospheres has yet to be achieved. Currently, the technology for the production of various types of hollow nanospheres has already been established. Still, the applications of hollow nanospheres are more inclined toward energy storage. This study shows that CE-based ion acceleration is another plausible application for these hollow nanospheres. Different nanosphere designs were analyzed using simple analytical models, which showed the possibility of using them to produce quasi-monoenergetic ions. This was then confirmed using one-dimensional particle–particle simulations in spherical coordinates. Overall, the results showed that hollow nanospheres are viable targets that can be used for the production of quasi-monoenergetic ions via spherical CE. Furthermore, the new proposed target design substantially improved the energy coupling efficiency.
Laser intensity scalings are investigated for accelerated proton energy and attainable electrostatic field using microbubble implosion (MBI). In MBI, the bubble wall protons are subject to volumetric acceleration toward the center due to the spherically symmetric electrostatic force generated by hot electrons filling the bubble. Such an implosion can generate an ultrahigh density proton core of nanometer size on the collapse, which results in an ultrahigh electrostatic field to emit energetic protons in the relativistic regime. Three-dimensional particle-in-cell and molecular dynamics simulations are conducted in a complementary manner. As a result, underlying physics of MBI are revealed such as bubble-pulsation and ultrahigh energy densities, which are higher by orders of magnitude than, for example, those expected in a fusion-igniting core of inertially confined plasma. MBI has potential as a plasma-optical device, which optimally amplifies an applied laser intensity by a factor of two orders of magnitude; thus, MBI is proposed to be a novel approach to the Schwinger limit.
Breaking the 100-MeV barrier for proton acceleration will help elucidate fundamental physics and advance practical applications from inertial confinement fusion to tumor therapy. A novel concept of "microbubble implosion (MBI)" is proposed. In the MBI concept, bubble implosion combines micro-bubbles and ultraintense laser pulses of 10 20-10 22 Wcm-2 to generate ultrahigh fields and relativistic protons. The bubble wall protons are subject to volumetric acceleration toward the center due to the spherically symmetric electrostatic force generated by hot electrons filling the bubble. Such an implosion can generate an ultrahigh density proton core of nanometer size on the collapse, which results in an ultrahigh electrostatic field to emit energetic protons in the relativistic regime. Laser intensity scaling is investigated for accelerated proton energy and attainable electrostatic field using MBI. Three-dimensional particle-in-cell and molecular dynamics simulations are conducted in a complementary manner. As a result, underlying physics of MBI are revealed such as bubble-pulsation and ultrahigh energy densities, which are higher by orders of magnitude than, for example, those expected in a fusion-igniting core of inertially confined plasma. MBI has potential as a plasma-optical device, which optimally amplifies an applied laser intensity by a factor of two orders of magnitude; thus, MBI is proposed to be a novel approach to the Schwinger limit.
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