Session CI2: High Energy Density Physics I

Extreme high field plasmonics: electron acceleration and XUV harmonic generation from ultrashort surface plasmons

Propagating surface plasmons (SP) are collective electromagnetic (EM) modes localized across a sharp interface between vacuum and a metal or plasma. Plasmonics exploits SP for EM field confinement and enhancement with several applications. In a series of experiments and simulations [1] it has been shown that "relativistic" SP of high amplitude can be excited in the interaction of intense sub-picosecond laser pulses with solid targets.

By using a femtosecond laser system with extremely high contrast, SP have been excited on "grating" (periodically engraved) targets. "Surfing" acceleration of high charge (up to 650 picoCoulomb) electron bunches along the SP has been demonstrated [2,3]. The extreme ultraviolet (XUV) harmonics diffracted by the grating are strongly enhanced in the grazing direction [4], quasi-collinear to the electron bunches. Simulations show that the XUV harmonics are boosted by nanobunching in the SP field of the electrons which scatter the laser field, similarly to free electron laser operation. Ongoing work include the effect of shortening the SP down to the single-cycle limit by using a laser driver with rotating wavefront [5].

[1] A. Macchi, "Surface plasmons in superintense laser-solid interactions", Phys. Plasmas 25 (2018) 031906
[2] L. Fedeli et al, "Electron acceleration by relativistic surface plasmons in laser-grating interaction", Phys. Rev. Lett. 116 (2016) 015001
[3] G. Cantono et al, "Extensive study of electron acceleration by relativistic surface plasmons", Phys. Plasmas 25 (2018) 031907
[4] G. Cantono et al, "Extreme ultraviolet beam enhancement by relativistic surface plasmons", Phys. Rev. Lett. 121 (2018) 264803
[5] F. Pisani, L. Fedeli, A. Macchi, "Few-cycle Surface Plasmon Polariton Generation by Rotating Wavefront Pulses", ACS Photonics 5 (2018) 1068
  

Shock-driven discrete vortex evolution on a high-Atwood number oblique interface

The shock acceleration of interfaces drives hydrodynamic instability growth, which in turn drives mixing across interfaces. This is an important yield degradation mechanism in inertial confinement fusion (ICF) implosions, where mixing across interfaces leads to the injection of high-Z material into the hot spot, quenching ignition. When shocks are oblique (i.e. the shock front is non-normal to the interface), a shear flow is driven across the post-shock interface, altering the subsequent instability growth. For some combinations of tilt, perturbation amplitude, and perturbation wavelength, the resulting instability growth exhibits mixed characteristics of the Richtmyer-Meshkov (RM) and Kelvin-Helmholtz (KH) instabilities. The growth is impulsive early in time, like RM, but exhibits the morphology and late-time behavior of KH. I will present new theory, simulations, and data showing that this complex instability growth can be understood as a consequence of the vorticity distribution created by the oblique shock-interface interaction. This work opens the door to future experiments in the HED regime where control over initial interface structure can be used to create arbitrary vorticity distributions on interfaces. Such a schema can used to create vortices with independently varying spatial scale, strength, and sign, which will enable a new generation of experiments studying the vortex-merger dynamics which dominate late-time behavior of mixing layers.