Modeling of neutral particle distributions at the L to H transition in DIII-D

In order to study possible effects of neutrals on the power threshold for transition from L-mode to H-mode confinement in DIII-D, the problem of reconstructing neutral particle distributions inside the separatrix from available edge, scrape-off layer, and divertor plasma diagnostic data is addressed. Neutral particle profiles in the shear layer are reconstructed primarily with the DEGAS code using plasma parameters obtained by fitting discharge data with the B2.5 code. A series of discharges in which the ion grad B drift direction is toward the X-point is analyzed. For reversed B_t, the L to H transition power threshold is 2-4 times higher than normal B_t discharges at the same density: one discharge of this type is analyzed. The variation of the L-H transition power threshold divided by the density is seen to be correlated with the neutral density scale length in the shear layer for both directions of the magnetic field. Radial and poloidal neutral particle profile results for three low density discharges in which the L-H transition power threshold varies from 1.3 to 5.3 MW are presented.