Loss ion orbits at the tokamak edge

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Abstract
In divertor tokamak plasmas with intense additional heating a large fraction of the power deposited at the target has been attributed to the collisionless losses of energetic ions from a region of about twice an ion poloidal Larmor radius inside the plasma boundary. These energetic ion losses can also strongly effect the formation of the radial electric field at the plasma edge, and may be responsible for the L to H transition. This paper presents the first systematic analysis of loss ion orbits at the tokamak edge. A particle following code has been developed to assess the detailed shape of the loss cone of escaping ions in velocity space, its transformation in the presence of a radial electric field, the shape of the loss orbits and the effect of the reversal of the toroidal magnetic field. The results obtained allow one to calculate the total amount of direct ion losses, which is expected to be substantially less than that estimated from the simplified assumptions used, because of the narrowness of the loss cone. The complex response of the ion loss cone to changes in radial electric field can potentially create a condition of instability (like the L to H transition) with rapid growth of the negative potential inside the plasma boundary. Numerical simulations have shown a relatively weak dependence of the energetic ion behaviour on the exact profile of the potential, suggesting that the resonant effects of the ion interaction with the magnetic field, expected from the theory of an idealized large aspect ratio tokamak (r/R<<1) and leading, for instance, to the effect of banana orbit squeezing, may not exist