L-H power threshold scaling with magnetic geometry on NSTX and the role of ion orbit loss

The L-H power threshold (P$_{LH})$ on the National Spherical Torus Experiment varies with X-point radius (R$_{X})$, plasma current (I$_{p})$, the direction of the ion grad-B drift and the amount of lithium evaporated on the divertor surfaces. The edge T$_{e}$ and T$_{i}$ (where T$_{e} \sim $ T$_{i})$ just prior to the time of the L-H transition vary with the magnetic geometry, but are fairly independent of the neutral fueling rate and lithium conditioning. These observations are consistent with the X-transport theory, which describes the mean edge radial electric field (E$_{r})$ profile required to prevent non-ambipolar ion loss in a diverted plasma. A guiding-center orbit calculation in the absence of electric fields, collisions and flows provides insight into the dependence of the ion loss, and thus E$_{r}$, on the magnetic geometry and edge T$_{i}$. For example, the number of ion loss orbits remains constant as R$_{X}$ is reduced from 0.64m to 0.47m only if the edge T$_{i}$ increases by 60{\%}. This is in agreement with self-consistent calculations of E$_{r}$ using the neoclassical XGC0 code and experiments that measured edge T$_{e}$ and T$_{i}$ to be 40 -- 60{\%} larger. Similar agreement is also observed between guiding-center calculations, XGC0 results and the measured P$_{LH}$ versus I$_{p}$ and ion grad-B direction.