Jean-François Luciani

Curvature effects on the dynamics of tearing modes in tokamaks

The curvature effects on the dynamics of magnetic island evolution in tokamaks are investigated both theoretically and numerically. By taking into account perpendicular and parallel heat diffusion, a new dispersion relation is derived for tearing modes that match the linear and nonlinear results. This evolution equation allows a quantitative description over the whole range of island sizes. It predicts a nonlinear instability, i.e., growing magnetic islands in linearly stable magnetic configurations. All these predictions are in excellent agreement with full tridimensional linear and nonlinear magnetohydrodynamic (MHD) computations with the latest version of XTOR [K. Lerbinger and J. F. Luciani, J. Comput. Phys. 97, 444 (1991)]. These results have important consequences on the onset of neoclassical tearing modes because they predict a resistive MHD threshold.

A new semi-implicit method for MHD computations

Abstract An efficient semi-implicit method for the solution of the nonlinear, three-dimensional, resistive MHD equations is presented. The method is unconditionally stable with respect to the compressional fast magnetoacoustic and shear Alfven waves. The time step is limited instead by the nonlinear physical plasma phenomena. Furthermore, the method allows a high spatial resolution. Physically relevant test cases are presented and the feasibility of longtime simulations is discussed.

XTOR-2F: A fully implicit Newton-Krylov solver applied to nonlinear 3D extended MHD in tokamaks

XTOR-2F solves a set of extended magnetohydrodynamic (MHD) equations in toroidal tokamak geometry. In the original XTOR code, the time stepping is handled by a semi-implicit method [1-3]. Moderate changes were necessary to transform it into a fully implicit one using the NITSOL library with Newton-Krylov methods of solution for nonlinear system of equations [4]. After addressing the sensitive issue of preconditioning and time step tuning, the performances of the semi-implicit and the implicit methods are compared for the nonlinear simulation of an internal kink mode test case within the framework of resistive MHD including anisotropic thermal transport. A convergence study comparing the semi-implicit and the implicit schemes is presented. Our main conclusion is that on one hand the Newton-Krylov implicit method, when applied to basic one fluid MHD is more computationally costly than the semi-implicit one by a factor 3 for a given numerical accuracy. But on the other hand, the implicit method allows to address challenging issues beyond MHD. By testing the Newton-Krylov method with diamagnetic modifications on the dynamics of the internal kink, some numerical issues, to be addressed further, are emphasized.

Diamagnetic thresholds for sawtooth cycling in tokamak plasmas

The cycling dynamics of the internal kink mode, which drives sawtooth oscillations in tokamak plasmas, is studied using the three dimensional, non-linear magnetohydrodynamic (MHD) code XTOR-2F [H. Lutjens and J.-F. Luciani, J. Comput. Phys. 229, 8130 (2010)]. It is found that sawtooth cycling, which is characterized by quiescent ramps and fast crashes in the experiment, can be recovered in two-fluid MHD provided that a criterion of diamagnetic stabilization is fulfilled. The simulation results indicate that diamagnetic effects alone may be sufficient to drive sawteeth with complete magnetic reconnection in high temperature Ohmic plasmas.

The XTOR code for nonlinear 3D simulations of MHD instabilities in tokamak plasmas

The latest version of the XTOR code which solves a set of the extended magnetohydrodynamic (MHD) equations in toroidal geometry is presented. The numerical method is discussed with particular emphasis on critical issues leading to numerical stability and robustness. This includes the time advance algorithm, the choice of variables and the boundary conditions. The physics in the model includes resistive MHD, anisotropic thermal diffusion and some neoclassical effects. The time advance method used in XTOR is unconditionally stable for linear MHD. First, both the ideal and the resistive MHD parts of the equations are advanced semi-implicitly and then the thermal transport part full-implicitly, using sub-stepping [H. Lutjens, Comp. Phys. Commun. 164 (2004) 301]. The time steps are only weakly limited by the departure of the nonlinear MHD dynamics from the linear one and are automatically defined by a set of nonlinear stability criteria. The robustness of the method is illustrated by some numerically difficult simulations, i.e. sawtooth simulations, the nonlinear destabilization of ballooning instabilities by an internal kink, and the dynamics of a neoclassical tearing mode in International Thermonuclear Experimental Reactor (ITER) [R. Aymar, V.A. Chuyanov, M. Huguet, et al., Nucl. Fusion 41 (2001) 1301] like geometry about its nonlinear stability threshold.

Linear and nonlinear thresholds of neoclassical tearing modes in tokamaks

The understanding of the physics of neoclassical tearing modes (NTM) is important for the dimensioning of current drive systems used to stabilize these modes in International Thermonuclear Experimental Reactor (ITER) plasmas [R. Aymar et al., Nucl. Fusion 41, 1301 (2001)]. Predictions by theoretical models for the dynamics of NTM’s are compared with full scale numerical magnetohydrodynamical simulations including bootstrap current and transport effects. It is shown that curvature currents are sufficient to generate a nonlinear stability threshold for NTM’s. Furthermore, it is emphasized that at large resistivity NTM’s behave as an ordinary linear instability, which suppresses this nonlinear stability threshold.

Non-linear magnetohydrodynamic simulations of density evolution in Tore Supra sawtoothing plasmas

The plasma density evolution in sawtooth regime on the Tore Supra tokamak is analyzed. The density is measured using fast-sweeping X-mode reflectometry which allows tomographic reconstructions. There is evidence that density is governed by the perpendicular electric flows, while temperature evolution is dominated by parallel diffusion. Postcursor oscillations sometimes lead to the formation of a density plateau, which is explained in terms of convection cells associated with the kink mode. A crescent-shaped density structure located inside q = 1 is often visible just after the crash and indicates that some part of the density withstands the crash. 3D full MHD nonlinear simulations with the code XTOR-2F recover this structure and show that it arises from the perpendicular flows emerging from the reconnection layer. The proportion of density reinjected inside the q = 1 surface is determined, and the implications in terms of helium ash transport are discussed.

Nonlinear magnetohydrodynamic simulation of Tore Supra hollow current profile discharges

Magnetohydrodynamic (MHD) activity often undermines the realization of fully noninductive plasma discharges in the Tore Supra tokamak [J. Jacquinot, Nucl. Fusion 45, S118 (2005)], by producing large degradation of electron energy confinement in the plasma core and the bifurcation to a regime with permanent MHD activity. The nonlinear evolution of MHD modes in these hollow current density profile discharges is studied with the full-scale three-dimensional MHD code XTOR [K. Lerbinger and J.-F. Luciani, J. Comput. Phys. 97, 444 (1991)] and compared with experimental features. Large confinement degradation is predicted when q(0) is close to 2. This derives either from the full reconnection of an unstable double-tearing mode, or from the coupling between a single tearing mode and adjacent stable modes in a region with reduced magnetic shear.

Bistability driven by correlated noise: Functional integral treatment

A complete study of non-Markovian effects induced by correlated noise applied to a bistable dynamical system is presented. Starting from the exact functional integral solution of the stochastic equation, it is possible to show that the customary expansion in powers of the characteristic correlation time gives wrong asymptotic results. Other approaches based on a Fokker-Planck equation with a modified diffusion coefficient also fail in reproducing the right long-time behavior of the system. Using a generalized version of instanton calculus of functional integrals, explicit expressions of the invariant measure and transition time between stable fixed points are obtained, in the limit of small noise intensity but arbitrary correlation time. In particular, an original method for extracting the collective degrees of motion has been developed. These analytical results fit, for a large range of parameters, with numerical calculations, giving confidence in the formalism employed.

Saturation levels of neoclassical tearing modes in International Thermonuclear Experimental Reactor plasmas

For the future ITER tokamak (International Thermonuclear Experimental Reactor) plasmas [R. Aymar et al., Nucl. Fusion 41, 1301 (2001)] a simple and robust theoretical model for the prediction of the dynamics of neoclassical tearing modes (NTM) is a crucial topic. Presently, this theory is incomplete. Using full magnetohydrodynamic simulations, saturated NTM island widths significantly smaller than those predicted by any existing NTM theory are found. Nevertheless, these islands are sufficiently large to potentially alter the plasma confinement. Some reasons for the departure of the simulation results from the theoretical predictions are suggested and issues to be addressed to achieve a quantitative model are indicated.