Paper

This paper deals with electron acceleration by a laser pulse in a plasma with a static uniform magnetic field B∗. The laser pulse propagates perpendicular to the magnetic field lines with the polarization chosen such that (E_laser⋅B_∗)=0. The focus of the work is on the electrons with an appreciable initial transverse momentum that are unable to gain significant energy from the laser in the absence of the magnetic field due to strong dephasing. It is shown that the magnetic field can initiate an energy increase by rotating such an electron, so that its momentum becomes directed forward. The energy gain continues well beyond this turning point where the dephasing drops to a very small value. In contrast to the case of purely vacuum acceleration, the electron experiences a rapid energy increases with the analytically derived maximum energy gain dependent on the strength of the magnetic field and the phase velocity of the wave. The energy enhancement by the magnetic field can be useful at high laser amplitudes, a₀≫1, where the acceleration similar to that in the vacuum is unable to produce energetic electrons over just tens of microns. A strong magnetic field helps leverage an increase in a₀ without a significant increase in the interaction length.

Even in the situation where an electron interacts with a single plane wave, the well-known dynamical adiabaticity can be broken when an applied magnetic field is present, which will act to increase the dephasing rate of the electron during the interaction. Here we demonstrate this for the case where there is a uniform static magnetic field which is oriented either parallel or perpendicular to the electric field of the incident plane wave, and perpendicular to the direction of its propagation. The described energy gain phenomenon has direct relevance to laser-plasma interactions that involve external magnetic fields generated by laser-driven capacitor coils.

Conventionally, friction is understood as a mechanism depleting a physical system of energy and as an unavoidable feature of any realistic device involving moving parts. In this work, we demonstrate that this intuitive picture loses validity in nonlinear quantum electrodynamics, exemplified in a scenario where spatially random friction counter-intuitively results in a highly directional energy flow. This peculiar behavior is caused by radiation friction, i.e., the energy loss of an accelerated charge due to the emission of radiation. We demonstrate analytically and numerically how radiation friction can dramatically enhance the energy gain by electrons from a laser pulse in a strong magnetic field that naturally arises in dense laser-irradiated plasma. We find the directional energy boost to be due to the transverse electron momentum being reduced through friction whence the driving laser can accelerate the electron more efficiently. In the considered example, the energy of the laser-accelerated electrons is enhanced by orders of magnitude, which then leads to highly directional emission of gamma-rays induced by the plasma magnetic field.

A relatively simple model problem where a single electron moves in two relativistically strong obliquely intersecting plane wave-packets is studied using a number of different numerical solvers. It is shown that, in general, even the most advanced solvers are unable to obtain converged solutions for more than about 100 fs in contrast to the single plane wave problem, and that some basic metrics of the orbit show enormous sensitivity to the initial conditions. At a bare minimum, this indicates an unusual degree of nonlinearity, and may well indicate that the dynamics of this system are chaotic.

Laser-driven ion acceleration has been an active research area in the past two decades with the prospects of designing novel and compact ion accelerators. Many potential applications in science and industry require high-quality, energetic ion beams with low divergence and narrow energy spread. Intense laser ion acceleration research strives to meet these challenges and may provide high charge state beams, with some successes for carbon and lighter ions. Here we demonstrate the generation of well collimated, quasi-monoenergetic titanium ions with energies ~145 and 180 MeV in experiments using the high-contrast (<10⁻⁹) and high-intensity (6 x 10²⁰ W cm^-2) Trident laser and ultra-thin (~100 nm) titanium foil targets. Numerical simulations show that the foils become transparent to the laser pulses, undergoing relativistically induced transparency (RIT), resulting in a two-stage acceleration process which lasts until ~2 ps after the onset of RIT. Such long acceleration time in the self-generated electric fields in the expanding plasma enables the formation of the quasi-monoenergetic peaks. This work contributes to the better understanding of the acceleration of heavier ions in the RIT regime, towards the development of next generation laser-based ion accelerators for various applications.

We present an analytical theory that reveals the importance of the longitudinal laser electric field in the course of the resonant acceleration of relativistic electrons by a tightly confined laser beam. It is shown that this laser field component always counteracts the transverse one and effectively decreases the final energy gain of electrons via the direct laser acceleration (DLA) mechanism. This effect is demonstrated by carrying out particle-in-cell simulations of the DLA of the electrons injected into the accelerating phase of the plasma wake. It is shown that the electron energy gain from the wakefield is substantially compensated by the quasiresonant energy loss to the longitudinal laser field component. The analytically obtained scalings and estimates are in good agreement with the results of the numerical simulations.

The interaction of micro-bubbles with ultra-intense laser pulses has been shown to generate ultra-high proton densities and correspondingly high electric fields. We investigate the possibility of using such a combination to study the fundamental physical phenomenon of vacuum polarization. With current or near-future laser systems, measurement of vacuum polarization via the bending of gamma rays that pass near imploded micro-bubbles may be possible. Since it is independent of photon energy to within the leading-order solution of the Heisenberg–Euler Lagrangian and the geometric optics approximation, the corresponding index of refraction can dominate the indices of refraction due to other effects at sufficiently high photon energies. We consider the possibility of its application to a transient gamma-ray lens.

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.

We examine the impact of the ion dynamics on laser-driven electron acceleration in an initially empty channel irradiated by an ultra-high intensity laser pulse with I > 10²² W cm⁻². The negative charge of the accelerated electrons inside the channel generates a quasi-static transverse electric field that causes gradual ion expansion into the channel. Once the ions fill the channel, the pinching force from the quasi-static magnetic field generated by the accelerated electrons becomes uncompensated due to the reduction of the quasi-static transverse electric field. As a result there are two distinct populations of accelerated electrons: those that accelerate ahead of the expanding ion front while moving predominantly forward and those that accelerate in the presence of the ions in the channel while performing strong transverse oscillations. The ions diminish the role of the longitudinal laser electric field, making the transverse electric field the dominant contributor to the electron energy. The ion expansion also has a profound impact on the gamma-ray emission, causing it to become volumetrically distributed while reducing the total emitted energy. We formulate a criterion for the laser pulse duration that must be satisfied in order to minimize the undesired effect from the ions and to allow the electrons to remain highly collimated. We demonstrate the predictive capability of this criterion by applying it to assess the impact of a given pre-pulse on ion expansion. Our results provide a guideline for future experiments at multi-PW laser facilities with ultra-high intensities.

A solid density target irradiated by a high-intensity laser pulse can become relativistically transparent, which then allows it to sustain an extremely strong laser-driven longitudinal electron current. The current generates a filament with a slowly varying MT-level azimuthal magnetic field that has been shown to prompt efficient emission of multi-MeV photons in the form of a collimated beam required for multiple applications. This work examines the feasibility of using an x-ray beam from the European x-ray free electron laser for the detection of the magnetic field via the Faraday rotation. Post-processed three dimensional particle-in-cell simulations show that, even though the relativistic transparency dramatically reduces the rotation in a uniform target, the detrimental effect can be successfully reversed by employing a structured target containing a channel to achieve a rotation angle of 10⁻⁴rad. The channel must be relativistically transparent with an electron density that is lower than the near-solid density in the bulk. The detection setup has been optimized by varying the channel radius and focusing the laser pulse driving the magnetic field. We predict that the Faraday rotation can produce 10³ photons with polarization orthogonal to the polarization of the incoming 100 fs long probe beam with 5 × 10¹² x-ray photons. Based on the calculated rotation angle, the polarization purity must be much better than 10⁻⁸ in order to detect the signal above the noise level.