Theoretical refinements to an existing model for the loss of ions by drifting across the last closed flux surface are presented. V C 2014 AIP Publishing LLC. [http://dx.The collisionless loss of energetic ions on orbits through the X-region which carried them into the divertor seems to have been first suggested in Ref. 1 and confirmed by Monte Carlo analysis of measured particle and heat fluxes in JET. 2 Collisional loss of ions on banana orbits near the boundary was then explored as a cause for the H-mode transition, 3,4 and direct loss of ions on banana orbits that crossed the separatrix was discussed in Refs. 5-7. A theory was developed 8-10 for the L-H transition based on the bifurcation of the poloidal flow velocity due to the effect of ion orbit loss on viscosity, and later the experimental observation of large rotation velocities in MAST were explained in terms of the torques produced by return currents compensating the ion orbit loss of energetic beam ions. 11,12 Recently, the intrinsic rotation observed in DIII-D in the absence of an external torque has been explained by ion orbit loss of thermalized ions. [13][14][15][16][17] There is a school of thought which holds that the physics of the edge plasma is determined in large part by ion orbit loss-the free-streaming of ions from inner flux surfaces along drift orbits that cross the last closed flux surface and are lost to the plasma. There are two different basic mechanisms for ion orbit loss in the edge plasma. The most general is the loss of ions on passing or banana-trapped orbits that leave the plasma by drifting outward across the last closed flux surface (as developed, e.g., in Refs. 7 and 13-15 or 18-22). Both thermalized plasma ions and energetic neutral beam ions (and fusion alpha particles) can be lost in this manner. This type of ion orbit loss will be referred to as "ion orbit loss." A second ion orbit loss mechanism ("X-loss," as developed, e.g., in Refs. 23-29) is the ion loss through the X-point in diverted plasmas produced by the fact that ions on orbits that pass near the X-point where the poloidal magnetic field is very small have a very small poloidal displacement in time and are essentially trapped in the poloidal vicinity of the X-point, where they are subject to vertical curvature and grad-B drifts which take them outward across the last closed flux surface and eventually into the divertor. Such ion orbit loss effects could significantly alter the results of most of the ongoing work on edge plasma physics experimental interpretation and prediction, but they are not yet routinely taken into account in such calculations.We have recently developed a model 18-22 for (i) the calculation of the radially cumulative fractions of ion particles F orb , momentum M orb , and energy E orb in a tokamak plasma flowing outward across an internal flux surface that are on drift orbits that would carry them immediately outward across the last closed flux surface (i.e., be ion orbit lost) and (ii) for the calculation of how these ion orbit losses reduc...