  We present experimental observations of electron-scale structures in an
expanding high-energy-density (HED) plasma generated with a modest intensity
$\sim 2 \times 10^{14}$ W/cm$^2$, $\sim1$ ns laser. The observed structures
have wavelengths ($\sim 150-220 \mu m$) and growth rates ($\sim 0.4-1.0$
ns$^{-1}$) consistent with an electron-driven Weibel instability where the
anisotropy in the electron distribution is small, $A\sim 0.002$. This
instability is found to be a better match to the observed phenomena than other
typical field-generation mechanisms found in HED plasmas, including
counter-streaming ion Weibel and magnetothermal instabilities. These
observations experimentally demonstrate for the first time that the electron
Weibel instability must be considered alongside other magnetic field generation
and amplification mechanisms in expanding ablation plasmas, which are
ubiquitous in HED research. They also provide physics insight into the
generation of magnetic fields in large-scale astrophysical plasmas.
Additionally, inspection of the magnetic power spectrum shows a possible
scaling match to analytic gyrokinetic predictions, $|B_k|^2\propto k^{-16/3}$,
at scales below the electron Larmor radius.
