Experiments and simulations investigating filamentary instabilities in ns-duration-laser-driven expanding plasmas

Theoretically predicted to exist in plasmas with an anisotropic electron distribution function and later confirmed in computational studies, the electron Weibel instability is observed experimentally in an expanding high-energy-density (HED) plasma generated with a modest intensity ~2e14 W/cm², ~1 ns laser. The observed structures have wavelengths ~150-220 μm and growth rates 0.4-1.0 1/ns, consistent with an electron-driven Weibel instability where the anisotropy in the electron distribution is small, A~0.002. This mechanism is found to be a better match to observations than other field generation mechanisms typically found in HED plasmas, including counter-streaming ion Weibel and magnetothermal instabilities. These observations show experimentally that the electron Weibel instability must be considered alongside other magnetic field generation and amplification mechanisms in expanding ablation plasmas ubiquitous in HED research, and possibly large-scale astrophysical plasmas. Additionally, inspection of the scaling of the magnetic power spectrum shows a possible scaling match to analytic gyrokinetic predictions of turbulence: B ∝ k^-16/3, which motivates future investigation. Simulations of future experimental conditions are presented to guide design considerations.