Overview of the RFX-mod contribution to the international Fusion Science Program

Abstract

The RFX-mod device is operated both as a reversed field pinch (RFP), where advanced regimes featuring helical shape develop, and as a tokamak. Due to its flexibility, RFX-mod is contributing to the solution of key issues in the roadmap to ITER and DEMO, including MHD instability control, internal transport barriers, edge transport and turbulence, isotopic effect, high density limit and three-dimensional (3D) non-linear MHD modelling. This paper reports recent advancements in the understanding of the self-organized helical states, featuring a strong electron transport barrier, in the RFP configuration; the physical mechanism driving the residual transport at the barrier has been investigated. Following the first experiments with deuterium as the filling gas, new results concerning the isotope effect in the RFP are discussed. Studies on the high density limit show that in the RFP it is related to a toroidal particle accumulation due to the onset of a convective cell. In the tokamak configuration, q(a) regimes down to q(a) = 1.2 have been pioneered, with (2,1) tearing mode (TM) mitigated and (2,1) resistive wall mode (RWM) stabilized: the control of such modes can be obtained both by poloidal and radial sensors. Progress has been made in the avoidance of disruptions due to the (2,1) TM by applying q(a) control, and on the general issue of error field control. The effect of externally applied 3D fields on plasma flow and edge turbulence, sawtooth control and runaway electron decorrelation has been analysed. The experimental program is supported by substantial theoretical activity: 3D non-linear visco-resistive MHD and non-local transport modelling have been advanced; RWMs have been studied by a toroidal MHD kinetic hybrid stability code.

Kinetic micro-instabilities in the presence of impurities in toroidal magnetized plasmas

“…The non-adiabatic response h s derived from the gyrokinetic equation for ions in a RFP configuration satisfies [39] The integral dispersion equation for study of ITG modes in toroidal RFP plasmas is extended to include a second (impurity) ion species [32]. Hydrogen ions are referred as main ion species, while all the rest subdominant ion species, such as carbon, oxygen, lithium and boron, are referred as impurity ions in the real RFX-mod experiment [40][41][42]. The full kinetic characteristics, including finite Larmor radius, magnetic gradient and curvature drifts, of the main and impurity ions are taken into account.…”

Edge plasma properties with 3D magnetic perturbations in RFX-mod

“…The aim of this paper is to characterize the response of the edge plasma to a 3D magnetic field with several helicities (resonant and non-resonant) in the RFX-mod experiment (R = 2 m, a = 0.459 m), operated both in reversed field pinch (RFP) and tokamak configurations [7,8]. It will be shown that the edge pressure, floating potential and perpendicular flow follow the temporal and spatial structure of the MP only as a first approximation: even if an almost monochromatic m = 1 MP is applied, the poloidal modulation of the plasma edge is characterized by sidebands, which reflect the complex (and not monochromatic) structure of the magnetic connection length to the wall.…”

Design of a new reflectometric system for real time plasma position control on the RFX-mod2 device

“…Even though the toroidal experiment has been designed to operate in reversed field pinch configuration, thanks to an easily reconfigurable winding and power supply systems and to a very flexible control system, RFX-mod operated also as a low plasma current (I p < 150 kA) Tokamak. Several experimental campaigns related to low-q (q edge < 2) operations [17], sawtoothing mitigation and runaway decorrelation through 3D applied perturbations [18,19], L-H transition in circular and D-shaped plasmas [20], have been successfully run.…”