Interaction of external n  =  1 magnetic fields with the sawtooth instability in low-q RFX-mod and DIII-D tokamaks

Abstract

External n = 1 magnetic fields are applied in RFX-mod and DIII-D low safety factor Tokamak plasmas to investigate their interaction with the internal MHD dynamics and in particular with the sawtooth instability. In these experiments the applied magnetic fields cause a reduction of both the sawtooth amplitude and period, leading to an overall stabilizing effect on the oscillations. In RFX-mod sawteeth eventually disappear and are replaced by a stationary m = 1, n = 1 helical equilibrium without an increase in disruptivity. However toroidal rotation is significantly reduced in these plasmas, thus it is likely that the sawtooth mitigation in these experiments is due to the combination of the helically deformed core and the reduced rotation. The former effect is qualitatively well reproduced by nonlinear MHD simulations performed with the PIXIE3D code. The results obtained in these RFX-mod experiments motivated similar ones in DIII-D L-mode diverted Tokamak plasmas at low q 95. These experiments succeeded in reproducing the sawtooth mitigation with the approach developed in RFX-mod. In DIII-D this effect is correlated with a clear increase of the n = 1 plasma response, that indicates an enhancement of the coupling to the marginally stable n = 1 external kink, as simulations with the linear MHD code IPEC suggest. A significant rotation braking in the plasma core is also observed in DIII-D. Numerical calculations of the neoclassical toroidal viscosity (NTV) carried out with PENT identify this torque as a possible contributor for this effect.

Helical flow in RFX-mod tokamak plasmas

“…It is noteworthy that a helical deformation of magnetic surfaces associated with the internal 1/1 kink mode is observed in SXR data in the presence of 3D magnetic fields [26].…”

“…Experimentally, the sawtooth period and its amplitude decrease and a stationary 1/1 helical equilibrium forms. The PIXIE3D code is able to describe such dynamics, as documented in [26]. (c) the normalized radial magnetic field amplitude and the corresponding phase of the 2/1 RWM (in black) and internal 1/1 kink mode (in green), (d) O V toroidal flow, (e) C V poloidal flow as measured in the HFS (in blue) and in the LFS (in red) and ( f ) m = 1 poloidal flow component of a q(a) < 2 plasma experiment in which a 2/1 triangular shaped magnetic field perturbation rotating at 10 Hz is applied through the magnetic feedback control system.…”

“…In RFX-mod experiments without externally applied magn etic field perturbations, the presence of these 1/1 convective cells could not be detected by the Doppler spectroscopic diag nostic since the time integration of the rotation signals is larger than the sawtooth period. However, the application of the 3D magnetic fields allows the amplitude of the internal 1/1 kink mode and the corresponding convective cell to increase, and the sawtooth intermittent behaviour is replaced by a stationary 1/1 helical equilibrium [26]. In such condition, we are able to drive the rotation of the helical core, maintaining it into rotation at the desired frequency, and the Doppler spectroscopic diagnostic is able to measure the flow modulation quite accurately, as described in the previous section.…”

“…From a theoretical point of view, the helical magnetic structures correspond to a magnetohydrodynamic minimum energy state accessed through a bifurcation process, characterized by an internal 3D helical magnetic equilibrium [25], with the same helicity as the experimental perturbation. For example, in RFX-mod tokamak experiments, helical equilibria are associated with an internal 1/1 kink mode, which produces a helical distortion in the plasma core [26]. Conversely, in reversed field pinch (RFP) RFX-mod plasmas, helical equilibria are linked to the innermost resonant TM, which sustains the magn etic field configuration [27,28].…”

“…Here, the signature of helical flow in the experimental data has been investigated. In particular, the 3D fully non-linear PIXIE3D code, which has been extensively validated against RFX-mod tokamak plasmas [24,26,35], has been used to interpret the origin of the observed flow oscillations in combination with ad-hoc 2D flow modelling code, which allows the reconstruction of rotation measurements. Inputs to the 2D flow modelling are the PIXIE3D flow map, the ion radial emission profiles as calculated by a 1D collisional radiative impurity transport code [36] and a synthetic passive spectroscopy diagnostic with the same geometry implemented in RFX-mod.…”