Session BI3: Particle Acceleration, Radiation, Relativistic Plasmas

Flying Focus: Spatiotemporal control of intensity for laser applications

A chromatic focusing system combined with chirped laser pulses was used to create a “flying focus” [1]. This advanced focusing scheme provides unprecedented spatiotemporal control over the laser focal volume by enabling a small-diameter laser focus to propagate over 100x its Rayleigh length. Furthermore, the flying focus decouples the speed at which the peak intensity propagates from the group velocity of the laser pulse, allowing the laser focus to co- or counter-propagate along its axis at any velocity. Experiments have demonstrated a nearly constant intensity over 4.5 mm, while the velocity of the focus ranged from subluminal (0.01c) to superluminal (15c). When increasing the laser intensity above the ionization threshold of the background gas, an ionization wave was measured to track the ionization threshold intensity isosurface as it propagates and ionization waves of arbitrary velocity were demonstrated [2],[3]. All backward and all superluminal cases mitigated the issue of ionization-induced refraction that typically challenges the formation of long, contiguous plasma channels. These properties provide the opportunity to overcome current fundamental limitations in laser-plasma amplifiers [4], laser-wakefield accelerators, photon accelerators, ion accelerators, THz generation, and high-order frequency conversion.

Are we close to transfering optical power into gamma-rays and pairs?

The next generation of lasers is expected to reach powers on the order of 10 PW, which opens new horizons for exploring quantum effects in laser-matter interactions. On one hand, this offers a new experimental platform to probe matter at fundamental levels, while on the other hand, this technology can be potentially used to design novel gamma-ray and electron-positron beam sources. Relativistic electron beams and lasers with intensities above I>10^20 W/cm^2 allow entering the radiation-dominated and quantum regime of interaction. Electron energy loss due to the classical radiation reaction could be measured by colliding Wakefield accelerated electron beams with state-of-the-art laser pulses [1]. This was demonstrated in two recent experiments [2]. Using quasi-monoenergetic electron beams, one can further expect to measure signatures of quantum radiation reaction in the electron energy spread and divergence [3]. There is also a practical benefit of radiation reaction: a radiation reaction-induced particle trapping at extreme intensities can assist efficient electron acceleration in plasma channels [4]. With the next generation of lasers, laser-electron beam scattering configurations will generate electron-positron pairs in conditions where they can be immediately accelerated to multi-GeV energies [5]. Multiple-laser optical traps can be used to confine the self-generated electron-positron pairs in the region of the highest laser intensity, resulting in an efficient energy conversion into gamma-rays and dense pair plasma [5] containing approximately the same number of electrons and positrons [6]. [1] M. Vranic et al, PRL (2014) [2] J. M. Cole et al, PRX (2018); K. Poder et al, ArXiv (2017) [3] M. Vranic et al, NJP (2016) [4] M. Vranic et al, PPCF (2018) [5] M. Vranic et al, PPCF (2017); T. Grismayer et al, POP(2016); T. Grismayer et al, PRE (2017) [6] M. Vranic et al, S. Rep. (2018).   

Towards experimental measurements of strong-field QED effects with high-intensity lasers

Strong-field quantum electrodynamics (QED) processes are predicted to play a dominant role in the interaction of next-generation high-intensity (> 10^23W/cm^2) laser pulses with matter. In particular quantum radiation reaction will play a major role in the motion of the electrons and positrons in the plasma created in the laser focus. Until recently quantum radiation reaction had not been studied in the relevant regime in the laboratory.  First measurements of radiation reaction in the collision of a high-intensity (>10^20W/cm^2) laser-pulse with a laser wakefield accelerated electron beam will be discussed which hint at the importance of quantum corrections.  Recent work will be presented further elucidating the signatures of quantum radiation reaction in this all optical collider set-up and pointing the way to a definitive test of the quantum model of radiation reaction in future experiments. In particular we will show a new theoretical (quantum kinetic) framework in which the degree of broadening of the energy spectrum of the beam due to quantum stochasticity may be quantified. We will then discuss signatures of quantum radiation reaction in laser-plasma interactions as intensities surpass 10^22W/cm^2. Radiation reaction can lead to strong absorption of the laser pulse and quantum corrections strongly affect this. We will compare radiation-reaction mediated laser absorption in PIC simulations using quantum and classical frameworks for the first time, demonstrating the importance of quantum radiation reaction effects in this type of interaction.