Abstract: Understanding the effect of magnetic field geometry on turbulence is
important for both tokamaks and stellarators. The introduction of a
magnetic island to a toroidal plasma might influence turbulence in
several ways: directly through geometric effects or indirectly through
modifications to gradients that drive turbulence or flows that
suppress turbulence. In an experiment on DIII-D tokamak, we studied
the response of ion-gyro-radius scale density fluctuations using the
Beam Emission Spectroscopy (BES) diagnostic. Our results confirm
qualitative predictions of prior turbulence simulations, especially
the reduction of turbulent fluctuations at the center of the island.
This motivates dedicated simulations to understand the relative
contribution of the various mechanisms by which magnetic geometry
impacts turbulence in this situation.