Contrary to common wisdom, we argue that the Shafranov shift is destabilizing for the ambient ballooning-type instabilities, which account for most of the transport flux in tokamak core plasmas. Higher Shafranov shift indeed reduces the magnetic drift frequency , i.e. improves the bad curvature mildly, around the outboard midplane. However, this improvement is limited to a very narrow spatial region while is increased over the remaining poloidal space. The eigenfunction averaged is effectively enhanced due to the finite mode width of the eigenfunction and is thus destabilizing to the ballooning-type mode, as demonstrated by gyrokinetic simulations with the CGYRO code using local Miller equilibrium geometry. The predicted nonlinear flux also increases with Shafranov shift, consistent with linear simulations. The reduced transport model TGLF can capture the physics reasonably well.
This paper argues that the Shafranov shift, contrary to common belief, actually destabilizes the ambient ballooning-type instabilities that drive most of the transport in tokamak core plasmas. The Shafranov shift increases the magnetic drift frequency over most of the poloidal space, outweighing the mild improvement in bad curvature around the outboard midplane. Gyrokinetic simulations and reduced transport models confirm this destabilizing effect and the predicted increase in nonlinear flux.