The X-point scrape-off plasma in JET with L- and H-modes

The JET scrape-off plasma during X-point discharges with L- and H-modes is described. There is a flow of current in the scrape-off layer which can influence equilibrium, transport, heating and impurity generation in the scrape-off layer. A local measurement of Dperpendicularto is obtained in the divertor region. The saturated flux of ions flowing parallel to the magnetic field, ISAT, is ≅ 10 A/cm2 at the divertor separatrix in ohmic discharges. In L-mode with NBI it increases by a factor ≤2, depending on injected power level (PNBI<8 MW) and decreases with H-mode. The e-folding decay length, λISAT, is the same in ohmic and L-mode but decreases with H-mode. The electron temperatures at the divertor target are ≅ 30 eV (OH) and ≅ 50-60 eV (H-mode). Measurements of Te during L-mode are not reported because of the high level of fluctuations. An enhancement in the erosion of carbon due to ion-stimulated desorption at the divertor target is discussed. Flow reversal in the scrape-off layer is demonstrated. (orig.)