Temporal evolution of particle transport of Super Dense Core plasma in LHD

Abstract

A highly peaked density profile was obtained in pellet-injected discharges in LHD. The peaking factor, which is the ratio of the central to volume-averaged densities, increased from around 0.8 in the gas puff fuelled phase, up to more than 2.0 after multiple pellet injection. The core density reached to several times 1020m-3. Temporal changes of density profiles were measured using a CO2 laser imaging interferometer. The change of the particle flux was estimated from temporal variations of measured density profiles. The diffusion coefficient (D) and the convection velocity (V) were then obtained from temporal traces of the relationship between the normalized density gradient and the normalized particle flux. Thus-obtained diffusion coefficients increased monotonically toward the plasma edge, while the convection velocity was inwardly directed. These observations are clearly different from those in the low collision regime, where D was spatially almost constant and V was outwardly directed. Three different regimes of particle transport were identified after the pellet injection. The first phase is characterized by a constant density peaking just after the pellet injection. In this phase, negative slopes between the normalized particle flux and the normalized density gradient were found, implying that D and V could not be determined. The second phase is at an additional density peaking, where the normalized particle flux increased with an increase in the normalized density gradient. In this phase, the core diffusion coefficient (ρ < 0.6) was reduced compared with that before the pellet injection, and an inwardly directed pinch was observed. The third phase is with the density broadening, where the normalized particle flux was reduced with a decrease in the normalized density gradient. Here, the core diffusion coefficients were further reduced, and the core pinch velocity (ρ < 0.8) became close to zero. The transition from the first (the constant peaking phase) to the second phase (the additional peaking phase) occurred almost simultaneously in the entire region of the plasma, while the transition from the second to third phase (the broadening phase) occurred from the outer region to the inner region of the plasma. The collisionality dependence of D and V at ρ = 0.4∼0.7 was studied in the wide range of collisionality (νb* = 0.3∼30) from density modulation experiments and the SDC analysis. A minimum value of D was likely to exist in the plateau regime (νb* = 1∼5) together with almost zero convection velocity. This may be favourable for future high density reactor operations of the heliotron reactor. © 2008 IOP Publishing Ltd.