Persistent Non-Gaussian Structure in the Image of Sagittarius A* at 86
  GHz
  Observations of the Galactic Center supermassive black hole Sagittarius A*
(Sgr A*) with very long baseline interferometry (VLBI) are affected by
interstellar scattering along our line of sight. At long radio observing
wavelengths ($\gtrsim1\,$cm), the scattering heavily dominates image
morphology. At 3.5 mm (86 GHz), the intrinsic source structure is no longer
sub-dominant to scattering, and thus the intrinsic emission from Sgr A* is
resolvable with the Global Millimeter VLBI Array (GMVA). Long-baseline
detections to the phased Atacama Large Millimeter/submillimeter Array (ALMA) in
2017 provided new constraints on the intrinsic and scattering properties of Sgr
A*, but the stochastic nature of the scattering requires multiple observing
epochs to reliably estimate its statistical properties. We present new
observations with the GMVA+ALMA, taken in 2018, which confirm non-Gaussian
structure in the scattered image seen in 2017. In particular, the ALMA-GBT
baseline shows more flux density than expected for an anistropic Gaussian
model, providing a tight constraint on the source size and an upper limit on
the dissipation scale of interstellar turbulence. We find an intrinsic source
extent along the minor axis of $\sim100\,\mu$as both via extrapolation of
longer wavelength scattering constraints and direct modeling of the 3.5 mm
observations. Simultaneously fitting for the scattering parameters, we find an
at-most modestly asymmetrical (major-to-minor axis ratio of $1.5\pm 0.2$)
intrinsic source morphology for Sgr A*.
