Session PO03: Beams: Relativistic and Strong-Field Effects

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Linearly polarized orbital angular momentum (OAM) modes have recently been demonstrated with intensities exceeding 3E19Wcm-2 at focus using off-axis spiral phase mirrors [1]. With the ability now to generate such modes, we have explored the possibility of generating kilo-Tesla level axial magnetic fields numerically with OAM modes that can be obtained in high-power laser facilities. 3D PIC simulations run using the EPOCH code confirm the analytic model of the inverse Faraday effect [2], and show kilo-Tesla fields generated over lengths greater than 100 microns, and with decay times on the order of picoseconds using laser pulses of length 100 femtoseconds. We discuss the coupling of linearly polarized OAM modes to plasma through nonlinear mechanisms, scaling laws of the inverse Faraday effect, magnetic field persistence, and length scales. [1]. A. Longman et al, Opt. Lett. 48(8), 2187-2190, 2020 [2]. S. Ali et al. Phys. Rev. Lett. 105(035001), 2010   

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QED cascades can generate electron-positron pairs when the electric field or magnetic field substantially exceeds the Schwinger limit. Signatures of collective pair plasma effects in these QED cascades are shown to appear in exquisite detail through, e.g., plasma-induced frequency upshifts in the laser spectrum. Remarkably, these signatures can be detected even in small plasma volumes moving at relativistic speeds. Strong-field quantum and collective pair plasma effects can thus be explored with existing technology, provided that ultra-dense electron beams were co-located with multi-PW lasers. 1. K. Qu, S. Meuren, and N. J. Fisch, arXiv:2001.02590 (2020)   

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By taking the spin and polarization of the electrons, positrons and photons into account in the strong-field QED processes of nonlinear Compton emission and pair production, we find that the growth rate of QED cascades in ultra-intense laser fields is modified, up to $25$ \%. While this means that fewer particles are produced, we also found them to be highly polarized. We further find that the high-energy tail of the particle spectra is polarized opposite than that expected from Sokolov-Ternov theory, which results from ``spin-straggling''. We employ a kinetic equation approach for the electron, positron and photon distributions, each of them spin/polarization-resolved, with the QED effects of photon emission and pair production modelled by a spin/polarization dependent Boltzmann-type collision operator. For photon-seeded cascades, depending on the photon polarization, we find an excess or a shortage of particle production in the early stages of cascade development, which provides a controllable experimental signature.   

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Apart from maximizing the strength of optical electromagnetic fields achievable at high-intensity laser facilities, the collision of several phase-matched laser pulses has been identified theoretically as a trigger of and way to study various phenomena. These range from the basic processes of strong-field quantum electrodynamics to Cherenkov radiation, emitted by an ultrarelativistic electron in a vacuum due to an induced strong electromagnetic field refraction index larger than unity. We report here on a systematic analysis of different regimes and opportunities, including a synergetic Cherenkov-Compton process, achievable with the concept of multiple colliding laser pulses, for both current and upcoming laser facilities.   

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When a relativistically intense laser interacts with an over-dense plasma, the electrons will be accelerated to relativistic velocities within a single optical cycle. The associated mass increase leads to a reduction of the critical plasma frequency, allowing the laser to propagate through the plasma. This effect, often referred to as relativistic induced transparency (RIT), plays an important role in many solid density laser-plasma experiments. Despite its prevalence, few direct experimental studies of the dynamics of this effect exist (for example, Bagnoud et al. PRL 118 255003(2017) and Palaniyappan et al. Nat. Phys. 8, 763--769(2012)), and none for \textless 100 fs pulses. Here we present results of a pump-probe study of RIT carried out at the Scarlet Laser Facility at The Ohio State University. A p-polarized pump with peak intensity \textgreater 10$^{\mathrm{21}}$ W/cm$^{\mathrm{2}}$ was incident on \textasciitilde 23 nm liquid crystal targets at 45 degrees, simultaneously a 200 mJ pick-off with linear 45 degree polarization probed the target from the rear side at near normal incidence. The transmission of the pump and probe (resolving s and p polarized components) was measured over a delay range of 15 ps. An increase in probe transmission was observed at zero delay which persists for \textasciitilde 300 fs. The plasma then becomes opaque again before expansion leads to classical transparency after about 2.5 ps. This is the first direct measure of temporal dynamics of RT in the \textless 100 fs regime.   

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Induced transparency during relativistically intense laser-matter interactions is still not a fully understood phenomenon. There have been a few direct measurements of this process (for example, Palaniyappan, \textit{et al., Nature Physics}\textbf{ 8},~763 (2012) and Bagnoud, \textit{et al., }Phys. Rev. Lett.~\textbf{118}, 255003 (2017)) however recent theory and computational results (Stark, \textit{et al.,} Phys. Rev. Lett. \textbf{115}, 025002 (2015)) predict a polarization dependent transmission that has not been experimentally verified. We utilized a pump-probe configuration at the OSU Scarlet Laser Facility to measure the polarization dependent transmission through \textless 25 nm thick liquid crystal targets with femtosecond temporal resolution. 2D and 3D LSP PIC simulations of the experiment were performed for different probe polarizations and temporal delays between pump and probe. We compare the simulation and experimental results and then use the simulations to analyze conditions of the induced transparency within the target.   

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High temporal pulse contrast is critical for experiments using ultraintense laser pulses. Prepulses or a slowly rising pulse pedestal can impair experiments using solid density targets. One method to improve the contrast is the implementation of a self-triggering plasma mirror (PM). A PM typically consists of a low reflectance substrate that is precisely placed such that only the main peak of the pulse will ionize it and is reflected, resulting in a higher contrast pulse. Here we report the use of ultrathin, free standing liquid crystal (LC) films formed \textit{in-situ} to implement a double plasma mirror (DPM) configuration at the BELLA PW facility. We characterize the DPM system including nanosecond and picosecond contrast enhancement, total reflection, far-field wavefront quality, focal spot mode, and beam pointing using 7 J p-polarized pulses. Notably, we observed \textasciitilde 80{\%} reflection and negligible deterioration of the focus. We also describe a new analytical model that predicts the total reflection as a function of intensity incident on each PM. This model was also validated against a previous PM experiment using LC.   

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With the availability of intense pulse lasers, it is now possible to construct accelerators in plasmas, that harness the energy of the laser and transfer it to energetic particles. Here we consider direct laser acceleration in plasma channels, analytically and with particle-in-cell simulations. Our study of electron acceleration provides compact scaling laws applicable to the regime of extreme laser intensities when particle motion is affected by radiation reaction. Counter-intuitively, the radiation emission can be beneficial for particle energy gain through radiative trapping and by increasing a fraction of particles that can achieve the betatron resonance. We have shown that electrons can be accelerated to energies > 10 GeV, in a single-stage experimental setup using near-future laser technology. The presented scaling laws can be used to optimize the acceleration strategy, and predict the output radiation content. A similar technique, with several modifications, can be used to accelerate positrons. We propose a viable configuration for experimental generation and acceleration of lepton beams using the near-future laser technology.   

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TV/m electric fields present in focus of intense short-pulse laser has the potential of accelerating electrons to ultra-relativistic energy. Despite its merit in boosting electron energy, experimental demonstration of direct electron acceleration to 10's MeV energy by laser field, however, remains elusive. We demonstrate the two-stage acceleration of electron during a 40 fs, second-harmonic, high-contrast (\textgreater 10$^{\mathrm{-12}})$, relativistic intense laser pulse irradiating 5 - 20 nm thin foils. For thin foils undergoing relativistic transparency, electrons at the target front surface are first heated by the radiation pressure of incident laser pulse. This is followed by electrons gaining additional energy while co-moving with the transmitted laser field. The extent of electron heating showed a strong dependence on the fraction of transmitted laser field. The spatial profile of electron beam also shows an annular profile due to the ponderomotive force from the laser pulse. PIC simulation reproduced the experimental results and reveals the detailed mechanism of electron gaining momentum in the transmitted laser field.   

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Laser-driven ion beams have made significant increases in recent times and maximizing yield and energy of ions for a given laser configuration would be beneficial. However, there exist limitations to enhancing ion beams that are due to technical challenges such as limited laser energy, intensity, and pulse duration. Here, we present computational studies on a new feasible scheme of laser-driven ion acceleration that utilizes the synergetic effects of laser-induced target transparency and continuous field acceleration. By employing precisely shaped or double laser pulses, the onset of target transparency and driving a continuous electric field can be efficiently achieved compared to using a single pulse. Once a target becomes transparent by the first pulse, a longer pulse, lower intensity, second pulse is beneficial to generate super-ponderomotive electrons, since the pulse can interact with largely developed under-dense plasma. With this enhanced electron temperature, a strong electric field continuously accelerates ions. This results in an increase of maximum ion energy by a factor of 2.5-3 compared to a typical TNSA for the given laser intensity. Detailed simulation results including systematic comparison with different laser parameters will be presented.   

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Next-generation high-power laser with intensities exceeding $10^{23}$ W/cm$^2$ are enabling new physics regimes and applications. In strong EM fields, the motion of charged particles and their spin is effected by radiation reaction (RR). Standard PIC codes using operator-splitting methods to advance the particle 6D phase space have been shown to fail in the strong field regime. In addition, some problems require tracking the particle spin which means that the phase space expands to nine-dimensional now. Therefore, numerical algorithms that enable high-fidelity modeling of 9D phase space in strong fields are required. We present a new particle pusher based on the analytical solutions to the equation of motion, together with the semi-classical form of RR in Landau-Lifshitz, and Bargmann-Michel-Telegdi equation for the evolution of the spin. The analytical solutions are obtained by only assuming a locally uniform and constant field during a time step. Owing to the analytical integration of particle trajectory and spin orbit, the constraint on the time step can be greatly reduced. We present examples of single-particle tracking and full PIC simulations to show the proposed particle pusher can greatly improve the accuracy of particle trajectory in 9D phase space for given fields.