  In this work, we highlight an issue that may reduce the accuracy of many
local nonlinear gyrokinetic simulations - turbulent self-interaction through
the parallel boundary condition. Given a sufficiently long parallel correlation
length, individual turbulent eddies can span the full domain and "bite their
own tails," thereby altering their statistical properties. Such
self-interaction is only modeled accurately when the simulation domain
corresponds to a full flux surface, otherwise it is artificially strong. For
Cyclone Base Case parameters and typical domain sizes, we find that this
mechanism modifies the heat flux by roughly 40% and it can be even more
important. The effect is largest when using kinetic electrons, low magnetic
shear, and strong turbulence drive (i.e. steep background gradients). It is
found that parallel self-interaction can be eliminated by increasing the
parallel length and/or the binormal width of the simulation domain until
convergence is achieved.
