Session PO4: AB: Strong Field and Relativistic Intensity Laser-Plasma Interactions

Electron-positron QED cascades in the collision of tightly focused lepton beams

Electron-positron QED cascades are important in extreme astrophysical environments, such as pulsar magnetospheres, and are of fundamental interest in strong-field quantum electrodynamics. Recently, there has been a significant effort to study the conditions for the onset of pair cascades in the laboratory, using ultra-intense laser fields. We will present the results of QED-PIC simulations that show for the first time that QED cascades can also be studied in the head-on collision of tightly focused 10 GeV-class electron-positron beams. In the moderate disruption regime (D = 1 - 4), these beams are strongly compressed during the interaction, leading to very large field amplification that can exceed the Schwinger field by a factor $>$ 100 in the frame of the beams, and to a QED cascade that reaches a multiplicity of 10. We have derived an analytical model and show that it is able to recover the cascade rate and saturation time as a function of the beam parameters.   

Signature of collective effects of pair plasmas

Existing technology has enabled creating electron-positron pair plasma at high densities using, e.g. electron beam interaction with the field of heavy atoms [1]. The promise of new physics emerging in dense pair plasmas challenges the community to find a means to verify experimentally pair plasma behavior. This talk suggests a possibility to observe the collective effects of the pair plasma by studying pair dynamics induced by the electromagnetic wave field when and after they are created. The plasma frequency could be experimentally deduced so as to inform unambiguously on the pair density. 1. G. Sarri, et. al., Nature Communications (6), 6747 (2015).   

Ultrafast Polarization of an Electron Beam in an Intense Bi-chromatic Laser Field

Recent high-intensity laser-plasma experiments provided evidence for quantum radiation reaction effects due to hard photon emission. In this talk I will discuss the radiative spin-polarization of the electrons as a manifestation of quantum radiation reaction affecting the spin-dynamics. It is demonstrated that radiative polarization of high-energy electron beams can be achieved in collisions with bi-chromatic laser pulses, by employing both a Boltzmann kinetic approach and a Monte-Carlo algorithm within the quasi-classical approximation of intense field QED. I will present simulations for a near-term experimentally feasible scenario of a 8 GeV electron beam scattering from a 1 PW laser pulse. Aspects of spin dependent radiation reaction are also discussed, with spin polarization leading to a measurable splitting of the energies of spin-up and spin-down electrons.   

Beam-beam collision in the high-disruption regime

The collision of high energy particle beams presents a platform where quantum electrodynamics (QED) theory can be accurately tested. Bright $\gamma$ rays and nonlinear Breit-Wheeler pair production can also be copiously produced in these collisions. However, disruption effects may arise, including the focus/deflection of beam particles by the strong self-consistent collective field. These disruption effects significantly change the key parameters at the interaction point, in particular for long and dense beams. Here, we present a numerical study on the beam-beam collisions in high-disruption regime using the particle-in-cell (PIC) simulation (OSIRIS), exploring how photon emission and pair production differ from the low disruption limit. The beam energy loss and the increased luminosity due to the disruption effects are also considered. The trade-off between the luminosity enhancement and the beam depletion is analysed in the context of upcoming particle colliders.   

Strong energy enhancement in a laser-driven magnetic filament through stochastic radiation friction

It has been previously shown that a high intensity laser pulse can propagate through a classically overdense plasma while generating a magnetic filament with a strong quasi-static azimuthal magnetic field [PRL116, 185003]. This azimuthal magnetic field can significantly enhance the energy gain by laser-accelerated electrons, but this requires a sufficiently strong longitudinal plasma current [arXiv:1811.00425]. One might expect that the inclusion of the radiation friction at higher intensities would make the restriction on the current even more severe. Counterintuitively, the radiation friction allows the electrons to enter an otherwise inaccessible regime of acceleration. As a result of the radiation friction, the energy of the laser-accelerated electrons is enhanced by orders of magnitude, as the laser generates a well-collimated beam of energetic electrons and gamma-rays [arXiv:1905.02152]. Our results suggest that this effect could be accessible at next-generation laser facilities.   

Forward sliding-swing acceleration: electron acceleration by high-intensity lasers in strong plasma magnetic fields

A high-intensity laser beam propagating through a dense plasma drives a strong current that robustly sustains a strong quasi-static Mega Tesla-level azimuthal magnetic field. The transverse laser field efficiently accelerates electrons in such a field that confines the transverse motion and deflects the electrons in the forward direction, establishing the novel \textit{forward-sliding swing acceleration} mechanism. Its advantage is a threshold rather than resonant behavior, accelerating electrons to high energies for sufficiently strong laser-driven currents. We study the electrons' dynamics by a simplified model analytically, specifically deriving simple relations between the current, the particles' initial transverse momenta and the laser's field strength classifying the energy gain. We confirm the model's predictions by numerical simulations, indicating Mega ampere-level threshold currents and energy gains two orders of magnitude higher than achievable without the magnetic field [arXiv:1811.00425 https://arxiv.org/pdf/1811.00425.pdf].   

Novel approach to the Schwinger limit using micro bubble implosion

We propose a novel principle to approach the Schwinger limit using micro-bubble implosion (MBI) [1, 2]. Being supported by the three-dimensional particle simulations, we have developed a semi-analytical 1D model to estimate the attainable electric field with MBI to derive the limiting curve and the relevant scaling in terms of the applied laser intensity, the spatial degree of ionization, and the bubble radius. Introducing high-Z materials as the target composition or a surface coating should realize higher attainable electric fields by MBI. It is shown that pair creation becomes active at around the maximum compression of MBI, when the ultrahigh-energy-density nano-sphere is formed at the center. [1] M. Murakami et al., Sci. Rep. 8, 7537 (2018). [2] M. Murakami et al., Plasmas 26, 043112 (2019).   

Developing a high-intensity laser-plasma experimental capability for the Pair Plasma Discovery Science campaign on NIF-ARC

The Advanced Radiographic Capability (ARC) laser at the NIF produces high-energy pulses (up to 4 kJ) but has a large focal spot with sub-relativistic intensities (1e18 W/cm2) below that needed for applications such as radiography, and pair creation. Results from the first high-intensity shots on ARC demonstrated a surprisingly high electron temperature of 2.2 MeV [1]. 2D PIC simulations show a dephasing acceleration mechanism where electrons sample a large area of changing laser phase, only achievable using long pulse durations with large spatial scales. The electron temperature was then increased by using parabolic focusing plasma optics [2] where inferred intensity was an order of magnitude compared to a flat target. This enabled the observation of pairs on ARC for the first time [3]. This newly developed high-intensity platform benefits a range of short-pulse, high-intensity laser applications at NIF.   

Controlling Interaction of Pair-plasmas with Laser-plasmas: Laser Positron Accelerator

Laser electron accelerators [1], utilizing CPA laser driven collective plasma modes are now recognized as a means for centimeter-scale acceleration of multi-GeV electron beams with various applications. Unfortunately, even with the rapid development of laser electron accelerators, laser acceleration of exotic particles like positrons remains unexplored. We propose to pioneer the first-ever prototype of a Laser Positron Accelerator that will pave the way for centimeter-scale acceleration of tunable positron beams at numerous laser facilities worldwide with new applications such as crystal channeling [2] and acceleration [3]. This work uses an innovative two-laser two-stage model [4] where a laser-driven plasma (stage 2) is used to post-process $e^+$-$e^−$ pair-plasmas produced (stage 1) in a target by laser accelerated electrons. It is timely due to the success of two recent experiments: (a) all-optical shower production [5]; (b) multistage laser electron acceleration [6]. The goal is to tune the pair-plasma characteristics in order to match them with the chosen post-processing stage properties, specifically to trap and accelerate a spectrally peaked positron beam through controlled interaction [4] between pair-plasma and laser-driven plasma waves.   

Self force and radiation reaction from a uniformly accelerated charge

In 1938, Dirac had derived an equation of motion for charged particles, which is called `Lorentz-Abraham-Dirac (LAD) equation.' However, due to its causality violation, the classical electrodynamics (CED) is still insufficient to explain the exact `dynamics' of point charges which must be the basic elements of the CED system. Especially, the hyperbolic (or uniformly accelerated) motions of charged particles have not been successfully described by LAD or its modification such as Landau-Lifshitz equation, because their radiation reaction terms vanish, apparently violating the energy conservation. Recently, this old problem is getting interest owing to the emergence of ultra-intense laser facilities, which can generate laser pulses with $10^{24}~\mathrm{W/cm}^2$ intensity. In such a strong field regime, the radiation reaction from the particles is expected to be observed. Hence, an uncontradictory theory model to explain the exact motions of point charges is now required. In this talk, we present a unique charge distribution that we named `point-like conductor (PLC),' which vanishes its electromagnetic field inside, but generates the same field outside as that from a uniformly accelerated point charge. We show that the self-force exerting on the PLC leads to the radiation reaction.   

Effects of radiation-reaction on resonant phase locking in laser driven auto-resonant particle acceleration scheme

It is well known that relativistic motion of a charged particle in the presence of a laser field and an external axial magnetic field, under resonance condition, results in unbounded energy gain by the particle. Technological advancements achieved in recent times have resulted in laser intensities $\sim 10^{22} W/cm^2$ and uniform magnetic field $\sim$ kilo Tesla, which not only make this scheme attractive but also leads to a regime where the effect of radiation-reaction becomes important. It has been recently shown that [Phys. Plasmas 22, 123102 (2015)] the inclusion of radiation-reaction in the particle's equation of motion, leads to saturation of the particle energy in the resonant case and a net energy gain in the non-resonant case. These results were obtained numerically for a monochromatic wave by solving the model equations of motion suggested by Landau-Lifshitz. In order to obtain a deeper insight and to verify whether the above results are model independent, in the present work, we have studied the above problem using the Hartmann equation of motion [Phys. Rev. Lett. 74, 1107 (1995)]. Our studies firstly show that the above results are indeed model independent and secondly there exist parameter regimes where the energy gain by the particle under non-resonant condition.   

Tunnel Ionization in Tightly Focused Laser Fields at Intensity up to 3 x 10$^{\mathrm{23\thinspace }}$W/cm$^{\mathrm{2}}$

Experimental tests of the Ammosov-Delone-Krainov (ADK) tunnel ionization model above 10$^{\mathrm{20}}$ W/cm$^{\mathrm{2}}$ should provide insight into relativistic effects in the tunneling process. We present simulations of the ion yields, ion dynamics, and electron dynamics in near-infrared laser fields with intensities ranging from 10$^{\mathrm{20\thinspace }}$W/cm$^{\mathrm{2}}$ to 3 x 10$^{\mathrm{23}}$ W/cm$^{\mathrm{2}}$ and how our results will influence the design of future experiments. We included the effects of the $f$/1 focal geometry required to reach 3 x 10$^{\mathrm{23}}$ W/cm$^{\mathrm{2\thinspace }}$in the near future, incorporating nonparaxial corrections to the laser fields up to fifth order in the diffraction angle. Simulations of the ion energy gained from the ion-laser interaction demonstrate the need to develop new ionization yield measurement techniques, as the Wiley-McLaren time-of-flight methods used previously have insufficient energy resolution when laser intensity exceeds 10$^{\mathrm{21}}$ W/cm$^{\mathrm{2}}$. When considering the ionization of Kr$^{\mathrm{35+}}$ at 3 x 10$^{\mathrm{23}}$ W/cm$^{\mathrm{2}}$, we find that the ponderomotive expulsion of ions from the laser focus will decrease the ionization yield. The highly charged krypton ions and their above-threshold ionization electrons can be accelerated to energies above 2 MeV/nucleon and 1.4 GeV, respectively.   

Experimental determination of peak laser intensity via relativistic Thomson scattering

In this work, we report on the development of a high-power laser intensity diagnostic capable of determining the peak laser intensity in-situ based on relativistic Thomson scattering. Electrons ionized in a low-pressure gas at the laser focus are injected into the peak laser intensity via barrier suppression ionization and the radiation emitted by the electrons oscillating relativistically in the laser focus produces harmonic emission with characteristic spatial and spectral signatures dependant on the laser intensity. Advanced simulations of the interactions will be presented and compared to an initial experimental study in which an intensity of 6E18Wcm-2 was directly measured based on spectral scattering measurements using the 200TW VEGA laser at CLPU in Salamanca, Spain. Future studies will focus on measuring both the spatial and spectral signatures of relativistic Thomson scattering to obtain an accurate in-situ measurement of the peak laser intensity.   

Mid-Infrared High-order Laser-Plasma Interactions in Solids

Relativistic laser-solid interactions are capable of driving high energy electron, ion, and x-ray emission. Despite favorable scalings towards longer wavelength, most solid high-order harmonic generation experiments have been with near-infrared lasers. High-energy, longer-wavelength experiments are now capable of driving such interactions enabling enhanced diagnostics. We report an in depth investigation of these interactions at 1.3 and 2.1 micron wavelengths, with supporting Particle-in-Cell simulations.   

High harmonic generation and QED effects induced by relativistic oscillating mirror

The laser-plasma interactions are dominated by the QED regime since intensities of the forthcoming laser facilities are approaching 10$^{\mathrm{23-24}}$ W/cm$^{\mathrm{2}}$. Here we present the high brightness $\gamma $-photon emission and e$^{\mathrm{+}}$e$^{\mathrm{-}}$ pair creation accompanied with the high harmonic generation. Relativistic oscillating mirror reflects the incident laser field and generates the focused attosecond pulse with enhanced intensity. A large number of high energy photons are emitted by the collisions between the trapped electrons and the high harmonic pulses. The corresponding photons are counter-propagating through the strong field which provides a large cross section for pair creation. Relativistic positron bunches are generated and further accelerated in the reflected laser field. The peak intensity of the $\gamma $-ray reaches 0.74 PW with the brilliance of 2×10$^{\mathrm{24}}$ s$^{\mathrm{-1}}$mm$^{\mathrm{-2\thinspace }}$mrad$^{\mathrm{-2}}$ (0.1{\%}BW)$^{\mathrm{-1}}$ (at 58 MeV). A GeV positron beam is obtained with a particle number of 5.6×10$^{\mathrm{9}}$. [1] Y. J. Gu, O. Klimo, S. V. Bulanov, S. Weber, Communications Physics, 1, 93 (2018) [2] Y. J. Gu, S. Weber, Opt. Express, 26, 19932 (2018).