  A systematic study of the impact of impurities on the turbulent heat fluxes
is presented for Wendelstein 7-X. By means of nonlinear multispecies
gyrokinetic simulations, it is shown that impurities, depending on the sign of
their density gradient, can significantly enhance or reduce turbulent heat
losses. For the relevant scenario of turbulence reduction, heat fluxes have a
local minimum at a certain impurity concentration. This result demonstrates the
potential of impurities for controlling turbulence and accessing enhanced
confinement regimes in stellarators.
