Session TM10: Mini-Conference: Multi-Petawatt Physics II

Stimulated Photon–Photon Scattering: Experimental Designs and Predicted Results for Multipetawatt Facilities

We present designs for an experiment using a multipetawatt facility to measure stimulated photon–photon scattering (SPPS), providing a first direct measurement of the nonlinearity of the quantum electrodynamic (QED) vacuum. SPPS is a prediction of strong-field QED for the response of the vacuum to intense electromagnetic fields, in which virtual electron–positron pairs mediate scattering between photons. The scattering is a fourth-order QED process and has never been directly observed due to its low cross section. The proposed experiments will test the hypothesis that the Euler–Heisenberg Lagrangian accurately predicts the nonlinear vacuum response to intense electromagnetic fields, and will begin to address the broader scientific question: how can we harness the nonlinearity of the quantum vacuum? Preliminary simulations using a realistically temporally and spatially shaped laser model predict a three-beam interaction using one 25-PW beam and two 10-PW beams will produce a detectable signal of >1000 scattered photons per shot. We present predicted results for proposed experimental configurations of the NSF OPAL facility, including realistic laser-focusing geometries with and without second-harmonic conversion of two input beams. From these predictions, we derive requirements for the laser facility, including experimental geometry, beam polarization, co-timing, and co-pointing. Requirements and potential technologies for the detector system are also discussed. By varying the relative power and, ultimately, focal geometry of the beams, we will rigorously assess the probability of SPPS and explore the scattering, frequency shifting, and birefringent properties of the nonlinear QED vacuum. This material is based upon work supported by the Department of Energy [National Nuclear Security Administration] University of Rochester “National Inertial Confinement Fusion Program” under Award Number DE-NA0004144 and Department of Energy under Award Number DE-SC0020431.   

Testing strong-field QED with the avalanche precursor

The NFS OPAL facility will deliver extreme optical fields that will allow probing, for the first time, a fundamentally new regime of the electromagnetic radiation interaction with matter when the dynamics of electrons is dominated by quantum radiation reaction and light can be transformed into high-brilliance gamma radiation and even electron-positron pairs. Electrons injected into the mutual focus of two multi-PW counter-propagating laser pulses can be repeatedly accelerated by the extreme electric field to GeV-scale energies at a sub-cycle time, with a rapidly increasing probability rate to emit high-energy photons causing strong recoil. With 3D PIC-QED simulations, we show that in two laser pulses with a total power>40PW, the energy transferred to high-energy photons exceeds that of electrons by an order of magnitude. At increasing power, a fractionof photons can also create secondary electron-positron pairs. This renders the precursor of avalanche-type (or selfsustained) QED cascades characterised by an exponential particle number growth. To achieve this, we propose to focus strongly two NSF OPAL Alpha-beams with the peak power in a (transparent for the laser) gas jet of heavy-atomic gas (e.g. Argon). The initial electrons will result from the ionization process. The study of outgoing photon radiation and electron spectral features will allow identifying the radiation-domination regime, whilst registering positrons will give strong evidence of a QED avalanche onset.   

Worldline description of scattering processes in ultra intense lasers: plane waves and beyond

The strong field and non-perturbative regime necessitated by the modeling of quantum field theory in the presence of an ultra-intense laser background requires new techniques beyond the diagrammatic approach well describing quantum electrodynamics (QED). I discuss an application of the worldline formalism, in which second-quantized fields are studied using an inherently non-perturbative first-quantized representation, to exactly model electron-photon scattering processes in plane waves and non-null backgrounds with arbitrary intensity. For realistic modeling of lasers with an optical carrier frequency, non-null fields result from interactions with a background plasma being treated as a refractive index.

I present formulae for an arbitrary number of N-photon tree-level scattering processes for plane waves [1] and non-null “shockwave” fields [2]; by virtue of the worldline formalism these take on a compact form and are exact to all orders in the background field. A low-energy limit of the N-photons in a constant crossed field is also considered, wherein I show a remarkably simple form arises. Ongoing studies on higher-loop corrections will briefly be discussed.   

Phase Control of Nonlinear Breit-Wheeler Pair Creation

High-power laser facilities produce intense laser pulses that when collided with a relativistic electron beam trigger the strong-field QED process of nonlinear Breit-Wheeler pair creation. Despite the successful generation of Breit-Wheeler pairs in past experiments, detection remains a challenge due to the low number of created pairs. In this work, a laser pulse composed of a fundamental and second harmonic is used for both Breit-Wheeler pair creation and to spatially separate the positrons from the electrons, which could facilitate experimental detection. An analytical model of the interaction between the created pairs and a plane-wave laser pulse is used to determine the phase difference and relative amplitudes of the two harmonics that optimize the positron-electron separation. Particle-in-cell simulations verify this model and show that the phase difference can indeed be used to control the separation. Aside from facilitating experimental detection of positrons without the need for large magnets, the asymmetry of a two-color electromagnetic field can also be used to spin-polarize the pairs. Combining these effects could provide an easy-to-detect polarized positron bunch.   

Using XUV laser light to study the breakdown of perturbative strong-field QED

Multi-petawatt laser facilities are currently running experiments colliding lasers with electron beams to study strong-field quantum electrodynamics (SFQED) processes, i.e., nonlinear Compton scattering and nonlinear Breit-Wheeler pair creation. In these experiements the quantum nonlinearity parameter will reach values χ∽10 where perturbative SFQED applies. It was conjectured by Ritus and Narozhny that under even more extreme conditions perturbative SFQED will break down and the scaling of the SFQED processes will change. Reaching this regime using optical laser light is well outside the capabilities of current multi-petawatt lasers; however, increasing the laser frequency greatly increases the possible χ and the probability of electrons reaching the maximum χ. Theory and simulation demonstrate that high-intensity XUV laser pulses will provide the χ necessary to study non-perturbative SFQED with measurable signatures in the gamma ray spectra. Sufficient laser frequencies and field strengths to begin studying this regime may be achieved through photon acceleration of a pulse in a plasma wakefield and subsequent focusing using a plasma lens.   

Strong field QED in crystals

Ultrarelativistic particles in strong crystalline fields of the order 10¹¹ V/cm enable investigations of processes in fields of the order the QED critical field E₀=1.32·10¹⁶ V/cm. Utilizing the relativistic invariance of the parameter χ=γE/E₀, where E is the local electric field, the field may become critical in the rest frame of an electron penetrating a single crystal. In the framework of the CERN NA63 experiment we have obtained experimental results on e.g. quantum synchrotron radiation emission, coherent pairs, radiation reaction and recently trident production in critical fields. Such observations are similar and complementary to what may be obtained from high-power lasers interacting with electrons. An overview of some of the results from the CERN NA63 experiment is presented.   

Progress of Probing Strong-Field Laser Electron Interaction at FACET-II

The 1990’s SLAC Experiment-144  measured the onset of  QED nonlinearities in laser-electron interactions, including multi-photon Compton scattering and electron-positron pair production [1, 2]. E-144 has since stimulated a worldwide endeavor to access the transition from the perturbative multiphoton to the non-perturbative tunneling regime of QED, which is still unexplored experimentally [3]. Recent advances in laser technology combined with the Lorentz boost of multi-GeV electron beams enables us to experimentally explore this strong-field QED (SFQED) regime, where the laser intensity reaches or even exceeds the QED critical (Schwinger) scale in the center-of-momentum frame.

Here, we report recent progress of the SLAC Experiment 320 that investigates the SFQED regime at the FACET-II facility [4]. Our scheme is based on the collision of ~10 TW laser pulses with the FACET-II 10 GeV e-beam at a ~150^o crossing angle.  We describe our experimental approaches, specifically the technique of fine tuning of the spatial-temporal overlap. We also present recent  experimental results from the latest FACET-II run that successfully demonstrate higher order nonlinear Compton scattering signals from the laser-e-beam collision, as well as future plans and experimental development.

[1]     C. Bula et al., “Observation of nonlinear effects in Compton scattering,” Phys. Rev. Lett. 76, 3116-3119 (1996).

[2]     D. Burke et al., “Positron production in multi-photon light by light scattering,” Phys. Rev. Lett. 79, 1626-1629 (1997).

[3]     S. Meuren et al., “Semiclassical picture for electron-positron photoproduction in strong laser fields,” Phys. Rev. D 93, 085028 (2016).

[4]     V. Yakimenko, et al., “FACET-II facility for advanced accelerator experimental tests,” Phys. Rev. Accel. Beams 22, 101301 (2019).   

Electron acceleration and electron-photon collision by multi-PW lasers for extreme field science

We present the energy scaling of electrons accelerated via laser-wakefield acceleration driven by multi-10-PW laser systems. We show that with 25 PW (5 kJ) laser pulse, electrons can be accelerated up to 500 GeV over the distance of 100 m. Upon collision with counter-propagating 25 PW laser, strong-field QED limits can be approached.