Neoclassical analysis of impurity transport following transition to improved particle confinement

Abstract

Strongly peaked impurity density profiles have been observed in Alcator C after frozen hydrogen pellet injection. More recent experiments in ASDEX, PBX, TEXT, JET, and TFTR have exhibited similar impurity accumulation during regimes of improved confinement. In this context, we present calculations of the neoclassically predicted equilibrium profiles of the intrinsic impurities in Alcator C. These theoretical calculations were performed for comparison with the experimentally determined peaked profiles observed after pellet fueling. Profiles of the main impurities in Alcator, carbon (C) and molybdenum (Mo), were measured using soft x-ray diagnostics. C exists in the plateau collisionality regime, and its transport is dominated by collisions with the hydrogen background ions and temperature gradient effects. Mo is in the Pfirsch-Schliiter regime, and it is driven mostly by collisions with C inside r/a ~ 0.25 and temperature gradients outside this radius. The rigorous multi-ion, mixed regime calculation necessary for the Mo transport is shown here. The predicted C profile is in excellent agreement with observation, and the profile predicted for Mo (which is not as close to equilibrium as C) is in fair agreement with observation.

Core impurity transport in Alcator C-Mod L-, I- and H-mode plasmas

“…Shown in Fig.2 [69,7], so the impurity transport in L-mode is regarded as anomalous. Examination of the subsequent injections with increasing ICRF heating power (and electron temperature) reveals a systematic decrease in the core impurity confinement time.…”

“…Understanding impurity transport is important in its own right, since impurity accumulation can lead to ion dilution and radiative collapse. In the enhanced confinement operational regimes (H-mode and ITB plasmas) envisioned for future devices, long impurity confinement times are widely seen, with impurity transport approaching neo-classical levels in the barrier region [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]. Since the neo-classical impurity density profile peaking factor increases strongly with atomic charge, H-mode and ITB plasmas in devices with tungsten (Z = 74) walls will potentially have a serious problem with tungsten accumulation and subsequent radiation.…”

Impurity behavior during sawtooth activity in tokamak plasmas

“…This is of course, however, not true at all during the quiescent MHD free evolution of the plasma, where strong dependence of the neoclassical and turbulent transport coefficients are found theoretically and numerically, and observed experimentally. [29][30][31] …”

Modelling of confinement degradation in the radiative improved mode caused by a strong gas puff

“…This implies that the diffusivities of electrons and ions are close to each other. For the pinch velocity of impurity particles we adopt a neoclassical one, which is caused by the density and temperature gradients of the background ions (Wenzel and Sigmar 1990)…”