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Center for Nonlinear Studies |
NOTE TIME CHANGE Part 1: The sawtooth instability is important in tokamaks at many levels, having both positive and negative effects. In addition to sometimes triggering NTMs (which can trigger disruptions), by relaxing temperature and density peaked profiles, sawtooth cycles transiently reduce the energetic content of the core plasma, which could decrease the average fusion power in a large size machine like ITER. But sawteeth also provide a mechanism for flushing out the accumulating He ash, as well as the heavy impurities such as Tungsten. Contrary to the temperature evolution however, density evolution is not well understood. I have investigated the sawtooth-crash induced particle transport using the implicit 3D non-linear 2-fluid MHD code XTOR-2F, which has the capability to simulate sawtooth cycles at almost realistic resistivity (Lundquist number S=10^7). The transport mechanism is shown to be dominated by the perpendicular flows induced by the kink mode. Fine density structures observed with fast-sweeping reflectometry are thus reproduced and explained. The model is extended to simulate the impact of a sawtooth crash on impurity profiles. The simulations show that despite a different mechanism, the relaxed profiles are similar to those obtained with the rules of a Kadomtsev model. Peaked profiles are efficiently flattened, while hollow profiles lead to significant penetration in the core. Part 2: In stellarators, despite better MHD stability properties compared to tokamaks, the pressure collapse of the plasma core is sometimes observed due to the interchange mode. It happens in particular when the mode locks in the error field or in externally applied RMPs. Understanding precisely mode locking requires to understand the rotation of the mode. Unfortunately, the interchange modes are usually observed to rotate in the electron direction in the plasma frame, which seems to contradict the theory (at least for ideal modes), and in any case is not well understood. With use of simulations with an explicit 3D extended MHD code, an eigenvalue 1D solver and analytical arguments, I will show that the heat conductivity and the viscosity, which are often neglected in MHD studies, can dramatically modify the behaviour of the mode rotation induced by diamagnetic effects.
Host: Luis Chacon