  A recent characterization of core turbulence carried out with a Doppler
reflectometer in the optimized stellarator Wendelstein 7-X (W7-X) found that
discharges achieving high ion temperatures at the core featured an ITG-like
suppression of density fluctuations driven by a reduction of the gradient ratio
$\eta_i = L_n/L_{T_i}$ [D. Carralero et al., Nucl. Fusion, 2021]. In order to
confirm the role of ITG turbulence in this process, we set out to establish
experimentally the relation between core density fluctuations, turbulent heat
flux and global confinement. With this aim, we consider the scenarios found in
the previous work and carry out power balance analysis for a number of
representative ones, including some featuring high ion temperature. As well, we
evaluate the global energy confinement time and discuss it in the context of
the ISS04 inter-stellarator scaling. We find that, when turbulence is
suppressed as a result of a reduction of $\eta_i$, there is a reduction of ion
turbulent transport, and global performance is improved as a result. This is
consistent with ITG turbulence limiting the ion temperature at the core of
W7-X. In contrast, when turbulence is reduced following a decrease in
collisionality, no changes are observed in transport or confinement. This could
be explained by ITG modes being combined with TEM turbulence when the later is
destabilized at low collisionalities.
