  We present an observational and theoretical study of the primary ionizing
agents (cosmic rays and X-rays) in the TW Hya protoplanetary disk. We use a set
of resolved and unresolved observations of molecular ions and other molecular
species, encompassing eleven lines total, in concert with a grid of disk
chemistry models. The molecular ion constraints comprise new data from the
Submillimeter Array on HCO$^+$, acquired at unprecedented spatial resolution,
and data from the literature, including ALMA observations of N$_2$H$^+$. We
vary the model incident CR flux and stellar X-ray spectra and find that TW
Hya's HCO$^+$ and N$_2$H$^+$ emission are best fit by a moderately hard X-ray
spectra, as would be expected during the "flaring" state of the star, and a low
CR ionization rate, $\zeta_{\rm CR}\lesssim10^{-19}$ s$^{-1}$. This low CR rate
is the first indication of the presence of CR exclusion by winds and/or
magnetic fields in an actively accreting T Tauri disk system. With this new
constraint, our best fit ionization structure predicts a low turbulence
"dead-zone" extending from the inner edge of the disk out to $50-65$ AU. This
region coincides with an observed concentration of millimeter grains, and we
propose that the inner region of TW Hya is a dust (and possibly planet) growth
factory as predicted by previous theoretical work.
