Session BI01: Particles, Beams and Coherent Radiation

Ultrafast X-ray imaging of filamentation instabilities in high-intensity laser-solid interactions

The interaction of ultraintense lasers with solid-density targets is actively being explored for a wide range of applications, from compact ion acceleration to fusion energy. Current instabilities driven by the laser accelerated relativistic electrons in the dense plasma are thought to play an important role in electron transport and plasma heating. The same instabilities play an important role in magnetic field amplification in astrophysical plasmas. Despite their interest, so far, the direct observation of these instabilities has been challenged by the associated ultrafast temporal and small spatial scales and by the high density of the plasma for which conventional optical diagnostics are not appropriate. We present the results of experiments conducted at SLAC's Matter in Extreme Conditions (MEC) end station that combine a high-intensity optical laser with the Linac Coherent Light Source (LCLS) X-ray laser. Using the new MEC X-ray Imager, we have successfully imaged for the first time the solid-density region of the laser-target interaction, achieving a spatial resolution of <200 nm and following in detail the plasma evolution from sub-ps to >100 ps. In particular, we resolve the development and evolution of the current filamentation instability caused by the momentum anisotropy of laser-accelerated relativistic electrons and the subsequent background return current. The wavelength and density modulation of the filaments increases in time due to the nonlinear evolution and filament merging. The results will be discussed and compared with theory and simulations providing an unprecedented characterization of high-intensity laser-solid interactions and associated relativistic plasma phenomena.   

Efficient Raman Amplification past Wavebreaking

.We present experimental results from the Raman amplification experimental platform at the University of Rochester’s Laboratory for Laser Energetics (LLE). This platform explores Raman amplification in a unique parameter space which includes a multi-joule pump and an adjustable-energy seed with seed intensities up to 7 x 10¹⁵ W/cm², which is significantly more intense than prior experiments. Initial experiments have demonstrated single-pass Raman amplification in multiple focal configurations with energy gain factors as high as 30x, energy transferred to the seed exceeding 220 mJ, and record efficiencies as high as 11.7%. The efficiency is observed to scale with seed intensity.  For intense seed pulses, efficient amplification is seen well into the wavebreaking regime, as predicted by particle-in-cell simulations [1].

 

References:

[1] M. R. Edwards et al., Phys. Plasmas 22, 074501 (2015).   

Evidence of suppressed beam-plasma instabilities in a laboratory analogue of blazar-induced pair jets

Here we report on an experimental platform at the HiRadMat facility, within CERN’s accelerator complex aimed at recreating a laboratory analogue of ultra-relativistic blazar-induced pair jets propagating into the intergalactic tenuous plasma. More than 10¹³ electron-positron pairs are produced by irradiating a target with 440 GeV protons from the Super Proton Synchrotron. The pair yield and plasma extent are orders of magnitude larger than currently achievable at laser facilities, producing for the first time pair plasma conditions necessary for the study of relativistic kinetic plasma instabilities. In our experiment, the pair beams are remarkably stable as they propagate through 1-m of plasma. Linear theory predicts that the growth of kinetic instabilities is strongly suppressed when non-idealized beam conditions are assumed, such as the inclusion of a small transverse temperature, and particle-in-cell simulations suggest that beam divergences of a few percent are enough to significantly suppress the instability. An experimentally inferred growth rate, when scaled to blazar's jets, is comparable to the inverse-Compton cooling time of the pairs on the cosmic microwave background. Given that a cascade of GeV inverse-Compton scattered photons is not observed from blazar's jets, our results imply that such an absence must be the related to the presence of intervening magnetic fields in the intergalactic plasma of primordial origin.   

Ultrarelativistic spin-polarized plasma

Ultrarelativistic plasma is an extreme state associated with strong field QED effects such as radiation reaction, gamma-ray photon emission, and positron generation. The ultrarelativistic plasma can be polarized due to radiative spin flips within high-energy photon emission. Considering spin is an intrinsic property of electrons, spin-polarized plasma might open a new avenue for the controlled generation of polarized electron beams. The latter has crucial applications in material science, nuclear structure measurements, and fundamental high-energy physics. Additionally, due to the new degree of freedom of information carried by the spin signal, ejected polarized particles can also be used to retrieve transient plasma dynamics that are hard to achieve by conventional diagnostics.

This talk will focus on i) how ultrarelativistic spin-polarized plasma is produced and ii) the correlation between transient dynamics and plasma’s spin-polarization signal. By inspecting unsymmetric electron polarization, we identified a nontrivial magnetic island structure accompanied by current vortexes. In addition, we found that the angular polarization pattern of emitted gamma photons could facilitate deciphering electrons’ in-situ acceleration status. Furthermore, we elucidated that electron polarization in current filamentation instabilities is correlated with the filament’s nonlinear transverse motion.   

Ultrafast spatiotemporal control of photoionization and THz generation

.A laser pulse composed of a fundamental and properly phased second harmonic exhibits an asymmetric electric field that can drive a time-dependent current of photoionized electrons. The current produces an ultrashort burst of terahertz (THz) radiation. When driven by a conventional laser pulse, the THz radiation is emitted into a cone with an angle determined by the dispersion of the medium. Here we demonstrate that the programmable velocity intensity peak of a spatiotemporally structured laser pulse can be used to control the emission angle, focal spot, and spectrum of the THz radiation. Specifically, a subluminal ultrafast flying focus can drive a highly focusable, single-cycle THz pulse ideal for probing picosecond dynamics or pumping new states of matter. The recently demonstrated ultrafast flying focus employs an axiparabola to focus different radial locations in the near field to different axial locations in the far field and an echelon to adjust the relative timing of the foci. While effective for THz generation, the optical configuration required for the ultrafast flying focus constrains properties of the intensity peak, such as the transverse profile and orbital angular momentum (OAM). Recently proposed nonlinear flying focus techniques offer additional opportunities for spatiotemporal control that can overcome these constraints. The self-flying focus combines temporal pulse shaping with the inherent nonlinearity of a medium, avoiding the need for custom optics. The flying focus X applies cross-phase modulation in a Kerr lens to create an ultrashort flying focus with a customized radial profile, with or without OAM. The resulting pulses can drive arbitrary trajectory ionization fronts that mitigate ionization refraction and allow for the formation of long, contiguous plasma channels—a critical component of advanced particle and photon accelerators.    

Kinetically unstable distributions as a result of radiative damping in strong electromagnetic fields

We have identified a novel physical process whereby plasmas can become kinetically unstable when subjected to strong electromagnetic fields and radiative cooling. This can occur under different conditions, such as emittance damping for beams experiencing betatron oscillations in ion-channels and synchrotron cooling for plasmas in strong magnetic fields. Unlike the Lorentz force, the radiation reaction force responsible for cooling the momentum distribution does not conserve momentum space volume and has a preferred cooling direction. This differential cooling can significantly affect the shape of the momentum distribution, resulting in "bunching" in momentum space and strong anisotropies with an energy population inversion under certain electromagnetic field configurations. We analytically examine the case of a collisionless plasma undergoing synchrotron cooling in a strong constant magnetic field and demonstrate a general condition for the development of rings in momentum space, which is fulfilled for many common initial momentum distributions. Such ring momentum distributions are known to be unstable to kinetic instabilities. Specifically, the electron cyclotron maser instability for the case of magnetised plasmas, which leads to coherent radiation emission as it diffuses the ring momentum distribution. This population inversion mechanism and subsequent maser instability are relevant to astrophysical plasmas and coherent radiation mechanisms.

The development of ring distributions and the onset of kinetic instabilities are confirmed with simulations performed with the Particle-in-cell OSIRIS code.