A 3D approach to equilibrium, stability and transport studies in RFX-mod improved regimes

Abstract

The full three-dimensional (3D) approach is now becoming an important issue for all magnetic confinement configurations. It is a necessary condition for the stellarator but also the tokamak and the reversed field pinch (RFP) now cannot be completely described in an axisymmetric framework. For the RFP the observation of self-sustained helical configurations with improved plasma performances require a better description in order to assess a new view on this configuration. In this new framework plasma configuration studies for RFX-mod have been considered both with tools developed for the RFP as well as considering codes originally developed for the stellarator and adapted to the RFP. These helical states are reached through a transition to a very low/reversed shear configuration leading to internal electron transport barriers. These states are interrupted by MHD reconnection events and the large T e gradients at the barriers indicate that both current and pressure driven modes are to be considered. Furthermore the typically flat T e profiles in the helical core have raised the issue of the role of electrostatic and electromagnetic turbulence in these reduced chaos regions, so that a stability analysis in the correct 3D geometry is required to address an optimization of the plasma setup. In this view the VMEC code proved to be an effective way to obtain helical equilibria to be studied in terms of stability and transport with a suite of well tested codes. In this work, the equilibrium reconstruction technique as well as the experimental evidence of 3D effects and their first interpretation in terms of stability and transport are presented using both RFP and stellarator tools.

Helically symmetric extended magnetohydrodynamics: Hamiltonian formulation and equilibrium variational principles

“…For example, 3D equilibrium states with internal helical structures with toroidicity, e.g., helical cores, have been observed experimentally (Weller et al 1987;Pecquet et al 1997) and simulated (Cooper et al , 2011 in Tokamaks and RFPs (e.g. Lorenzini et al (2009);Bergerson et al (2011); Puiatti et al (2009) ;Terranova et al (2010)). Another example of helical structures that emerge from plasma instabilities, such as the resistive or collisionless tearing modes, or as a result of externally imposed symmetry-breaking perturbations are magnetic islands (Waelbroeck 2009).…”

Helical core tokamak MHD equilibrium states

“…Bifurcated equilibrium states in tokamaks were first obtained by adding a forcing term to the second variation of an energy principle, then removing it to get a static solution with perturbations around rational surfaces that were interpreted as indications of the formation of magnetic islands [16]. More global structures like the SHAx equilibria were computed with the application of the VMEC code [17,18] in the reversed field pinch RFX-mod with seven-fold toroidally periodic structures [19]. In tokamak systems with an imposed axisymmetric boundary, internal helical equilibrium states similar to saturated ideal m = 1, n = 1 kink modes have been calculated using the ANIMEC code [20], a modified version of the VMEC code [18], in model configurations of the TCV tokamak [21], ITER [22], MAST [23] and JET [24].…”

Overview of the RFX-mod fusion science programme

“…As an example Fig. 6 shows the remapping of the electron temperature profile on the equilibrium reconstruction by V3FIT done using as a constraint the experimental pressure profile [28,29].…”