Plasma Diagnostics

The MEC endstation at SLAC recently commissioned their new MEC X-ray Imager (MXI) diagnostic. By looking at changes in the transmitted X-ray signal, this can visualize changes within the target on picosecond time scales and with unprecedented <200nm spatial resolution. We used these new capabilities to image the interaction of the high-intensity MEC short pulse laser with 10 μm copper wire targets. With additional Compound Refractive Lenses (CRLs) behind the target, we can observe the phase contrast of them system, increasing our sensitivity to small density variation.

Our results show the evolution of the interaction.

•  At early times we see filaments forming, as precited by the current filamentation instability. 
• At later times (>20 ps) we see a laser driven shock wave form, travelling at ~ 80 km/s.
• Once the shock front has passed the target rear, we observe thermal expansion of the heated region. 

This is the first time that the asymmetric current-filamentation instability has been seen in a wire target. This instability is due to the high-intensity laser ionizing and accelerating a small fraction of highly energetic electrons with propagate into the wire. To maintain neutrality, this hot electron current requires a return current of cold electrons. Such counter-propagating electron populations are subject to streaming instabilities, most notably the current filamentation, or Weibel, instability. This leads to the generation of current filaments as electrons are deflected by magnetic field perturbations of the background plasma. The transverse deflection further amplifies present magnetic field modes and forms filament structures consistent with those observed in our experiment. Due to their fundamental role in plasma physics, such as laser absorption, plasma heating, electron transport, compression dynamics and resulting ion beam quality, streaming instabilities such as the current filamentation instability are of high interest to a plethora of different fields. 

Given the high-density of the plasma and small spatio-temporal scales on which the instability evolves, past studies have so far almost exclusively relied on its characterization via numerical simulations and analytical models. However, while numerous theoretical and numerical efforts suggest a strong dependence of the instability on background plasma resistivity, electrostatic effects, and asymmetries in the counter-streaming electron populations, the specific dependence and associated microphysics remains poorly understood. This highlights the necessity for experimental measurements of wavelength and density perturbations associated with the instability evolution to study the instability growth and its dependence under realistic conditions.

For instance, based on the experimental observations, 2D, kinetic, PIC simulations performed by our group revealed that ion dynamics are required to preserve quasi-neutrality and allow the growth of asymmetric filaments, i.e. alternating low- and high-density regions, consistent with the experimental observations. Additionally, the target resistivity strongly increases the dominant filament wavelength. Thus, the combination of experimental results and subsequent analysis via numerical simulations and theoretical models allows to deliver a significantly improved and more complete picture of streaming instabilities by delivering 2D attenuation maps associated with the evolution of target density over time.