Geometric dependencies of the mean ExB shearing rate in negative triangularity tokamaks*

Negative triangularity(NT) tokamaks exhibit better confinement than positive triangularity(PT) tokamaks. ExB shearing is well known to reduce turbulence and improve confinement. We present a comparative study of the poloidal distribution of mean ExB shearing rate for PT and NT tokamaks. The effects of flux surface up-down asymmetry due to asymmetric upper and lower triangularities is also considered. Both direct eddy straining and effects on Shafranov shift feedback loops are examined. Shafranov shift increases the shearing rate at all poloidal angles for all triangularities, due to flux surface compression. The maximum shearing rate bifurcates at a critical triangularity δ_crit ≤0. Thus, the shearing rate is maximal off the outboard mid-plane for NT, while it is maximal on the outboard mid-plane for PT. For up-down asymmetric triangularity, the usual up-down symmetry of the shearing rate is broken. The shearing rate at the out board mid-plane is lower for NT than for PT, suggesting that the shearing efficiency in NT is reduced. Implications for turbulence stabilization and confinement improvement in high-β_p NT and ITB discharges and experimental suggestions will be discussed.