Particle and heat deposition in the X-point region at JET

In several experiments (DIII-D, ASDEX, PBX, JET), the H-mode has been associated with a favourable heat and particle flow pattern at the edge due to the X-point configuration within the tokamak vessel. The observations of these flows have been made with CCD cameras and Langmuir Probes and have been interpreted in the conventional way as heat conduction by electrons along the magnetic field lines. This is appropriate for high collisionality plasmas. However, in low collisionality plasmas as is the case in many JET discharges heat convection by hot ions is more important. A magnetic separatrix affects dramatically the convected heat flux, because as particles enter a region of vanishing poloidal field the magnetic drift and diamagnetic drift dominate over the parallel flux term. Thus for a given particle energy, the particle drift orbit is further displaced from the magnetic surface on which it started, as Bp decreases. The drift becomes purely vertical for Bp≅0. The application of the conventional picture has led to some misinterpretation when comparing the plasma strike zones with the location of the magnetic separatrix deduced by the magnetic equilibrium reconstruction code. We consider here a set of discharges where careful CCD camera observations were made and demonstrate how the correct interpretation of the observed heat strike zones on the vessel is obtained by the detailed analysis of loss cone and particle orbit effects near a magnetic stagnation point. (author) 4 refs., 6 figs