r_advection

do a single stage time advance (potentially as part of a multi-stage RK scheme)

1D false r advection, intended to provoke ITG in 1D. Despite there being no real r advection, we insist that a radial temperature gradient exists, and add the ExB drift term to the DKE anyway to bring in temperature gradient drive. Since f is not known along r, and hence its temperature dependence is not known, it is assumed that f is roughly a Maxwellian with an R dependent temperature only. This means we can control L_T through this term.

calculate the advection speed in the r-direction at each grid point