Paper

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.

The impact of longitudinal electric fields that are present in intense focusing and defocusing electromagnetic pulses on electron acceleration is investigated. These fields are typically much weaker than the transverse fields, but it is shown that they can have a profound effect on electron energy gain. It is shown that the longitudinal electric field of a defocusing pulse is directed backward along the trajectory of an accelerated electron, which leads to a continuous net energy gain. At the same time, the effect of the transverse oscillating electric field in a defocusing pulse is to reduce the electron energy over multiple oscillations. In contrast to a well-known interaction with a plane wave, the electron is able to retain a substantial amount of energy following its interaction with a defocusing pulse. The roles of the transverse and longitudinal electric fields are reversed in a focusing pulse, which leads to a reduction in the energy retention. The present analysis underscores the importance of relatively weak oscillating electric fields in focusing and defocusing pulses.

Powerful nanosecond laser-plasma processes are explored to generate discharge currents of a few 100  kA in coil targets, yielding magnetostatic fields (B-fields) in excess of 0.5 kT. The quasi-static currents are provided from hot electron ejection from the laser-irradiated surface. According to our model, which describes the evolution of the discharge current, the major control parameter is the laser irradiance I_lasλ²_las. The space-time evolution of the B-fields is experimentally characterized by high-frequency bandwidth B-dot probes and proton-deflectometry measurements. The magnetic pulses, of ns-scale, are long enough to magnetize secondary targets through resistive diffusion. We applied it in experiments of laser-generated relativistic electron transport through solid dielectric targets, yielding an unprecedented 5-fold enhancement of the energy-density flux at 60  μm depth, compared to unmagnetized transport conditions. These studies pave the ground for magnetized high-energy density physics investigations, related to laser-generated secondary sources of radiation and/or high-energy particles and their transport, to high-gain fusion energy schemes, and to laboratory astrophysics.

The ability of an intense laser pulse to propagate in a classically over-critical plasma through the phenomenon of relativistic transparency is shown to facilitate the generation of strong plasma magnetic fields. Particle-in-cell simulations demonstrate that these fields significantly enhance the radiation rates of the laser-irradiated electrons, and furthermore they collimate the emission so that a directed and dense beam of multi-MeV gamma-rays is achievable. This capability can be exploited for electron–positron pair production via the linear Breit–Wheeler process by colliding two such dense beams. Presented simulations show that more than 10³ pairs can be produced in such a setup, and the directionality of the positrons can be controlled by the angle of incidence between the beams.

Spherical tokamak plasmas are typically overdense and thus inaccessible to externally-injected microwaves in the electron cyclotron range. The electrostatic electron Bernstein wave (EBW), however, provides a method to access the plasma core for heating and diagnostic purposes. Understanding the details of the coupling process to electromagnetic waves is thus important both for the interpretation of microwave diagnostic data and for assessing the feasibility of EBW heating and current drive. While the coupling is reasonably well–understood in the linear regime, nonlinear physics arising from high input power has not been previously quantified. To tackle this problem, we have performed one- and two-dimensional fully kinetic particle-in-cell simulations of the two possible coupling mechanisms, namely X-B and O-X-B mode conversion. We find that the ion dynamics has a profound effect on the field structure in the nonlinear regime, as high amplitude short-scale oscillations of the longitudinal electric field are excited in the region below the high-density cut-off prior to the arrival of the EBW. We identify this effect as the instability of the X wave with respect to resonant scattering into an EBW and a lower-hybrid wave. We calculate the instability rate analytically and find this basic theory to be in reasonable agreement with our simulation results.

A laser-driven azimuthal plasma magnetic field is known to facilitate electron energy gain from the irradiating laser pulse. The enhancement is due to changes in the orientation between the laser electric field and electron velocity caused by magnetic field deflections. Transverse elec- tron confinement is critical for realizing this concept experimentally. Using analytical theory, we show that the phase velocity of the laser pulse has a profound impact on the maximum transverse size of electron trajectories. The transverse size remains constant only below a threshold energy that depends on the degree of the superluminosity, and it increases with the electron energy above the threshold. We illus- trate this finding using 3D particle-in-cell simulations. The described increase can cause electron losses in tightly focused laser pulses, so it should be taken into account when designing high-intensity experiments at high-power laser facilities.

Since the invention of chirped pulse amplification, which was recognized by a Nobel Prize in physics in 2018, there has been a continuing increase in available laser intensity. Combined with advances in our understanding of the kinetics of relativistic plasma, studies of laser–plasma interactions are entering a new regime where the physics of relativistic plasmas is strongly affected by strong-field quantum electrodynamics (QED) processes, including hard photon emission and electron–positron (e⁻–e⁺) pair production. This coupling of quantum emission processes and relativistic collective particle dynamics can result in dramatically new plasma physics phenomena, such as the generation of dense e⁻–e⁺ pair plasma from near vacuum, complete laser energy absorption by QED processes, or the stopping of an ultra-relativistic electron beam, which could penetrate a cm of lead, by a hair's breadth of laser light. In addition to being of fundamental interest, it is crucial to study this new regime to understand the next generation of ultra-high intensity laser-matter experiments and their resulting applications, such as high energy ion, electron, positron, and photon sources for fundamental physics studies, medical radiotherapy, and next generation radiography for homeland security and industry.

Using three-dimensional kinetic simulations, we examine the emission of collimated γ -ray beams from structured laser-irradiated targets with a prefilled cylindrical channel and its scaling with laser power (in the multi-PW range). The laser power is increased by increasing the laser energy and the size of the focal spot while keeping the peak intensity fixed at 5×10²² W/cm². The channel radius is increased proportionally to accommodate the change in laser spot size. The efficiency of conversion of the laser energy into a beam of MeV-level γ rays (with a 10^o opening angle) increases rapidly with the incident laser power P before it roughly saturates above P ≈ 4 PW. Detailed particle tracking reveals that the power scaling is a result of enhanced electron acceleration at higher laser powers. One application that directly benefits from such a strong scaling is pair production via two-photon collisions. We investigate two schemes for generating pairs through the linear Breit-Wheeler process: colliding two γ-ray beams and colliding one γ-ray beam with black-body radiation. The two scenarios project up to 10⁴ and 10⁵ pairs, respectively, for the γ -ray beams generated at P = 4 PW. A comparison with a regime of laser-irradiated hollow channels corroborates the robustness of the setup with prefilled channels.

Magnetized high energy density physics offers new opportunities for observing magnetic field-related physics for the first time in the laser-plasma context. We focus on one such phenomenon, which is the ability of a laser-irradiated magnetized plasma to amplify a seed magnetic field. We performed a series of fully kinetic 3D simulations of magnetic field amplification by a picosecond-scale relativistic laser pulse of intensity 4.2×10¹⁸ W/cm² incident on a thin overdense target. We observe axial magnetic field amplification from an initial 0.1 kT seed to 1.5 kT over a volume of several cubic microns, persisting hundreds of femtoseconds longer than the laser pulse duration. The magnetic field amplification is driven by electrons in the return current gaining favorable orbital angular momentum from the seed magnetic field. This mechanism is robust to laser polarization and delivers order-of-magnitude amplification over a range of simulation parameters.

One of the paradigm-shifting phenomena triggered in laser–plasma interactions at relativistic intensities is the so-called relativistic transparency. As the electrons become heated by the laser to relativistic energies, the plasma becomes transparent to the laser light even though the plasma density is sufficiently high to reflect the laser pulse in the non-relativistic case. This paper highlights the impact that relativistic transparency can have on laser-matter interactions by focusing on a collective phenomenon that is associated with the onset of relativistic transparency: plasma birefringence in thermally anisotropic relativistic plasmas. The optical properties of such a system become dependent on the polarization of light, and this can serve as the basis for plasma-based optical devices or novel diagnostic capabilities.