Overview of the RFX fusion science program

Abstract

This paper summarizes the main achievements of the RFX fusion science program in the period between the 2008 and 2010 IAEA Fusion Energy Conferences. RFX-mod is the largest reversed field pinch in the world, equipped with a system of 192 coils for active control of MHD stability. The discovery and understanding of helical states with electron internal transport barriers and core electron temperature >1.5 keV significantly advances the perspectives of the configuration. Optimized experiments with plasma current up to 1.8 MA have been realized, confirming positive scaling. The first evidence of edge transport barriers is presented. Progress has been made also in the control of firstwall properties and of density profiles, with initial first-wall lithization experiments. Micro-turbulence mechanisms such as ion temperature gradient and micro-tearing are discussed in the framework of understanding gradient-driven transport in low magnetic chaos helical regimes. Both tearing mode and resistive wall mode active control have been optimized and experimental data have been used to benchmark numerical codes. The RFX programme also provides important results for the fusion community and in particular for tokamaks and stellarators on feedback control of MHD stability and on three-dimensional physics. On the latter topic, the result of the application of stellarator codes to describe three-dimensional reversed field pinch physics will be presented.

Three-dimensional simulations of plasma turbulence in the RFX-mod scrape-off layer and comparison with experimental measurements

“…25 While RFX-mod plasma discharges have been performed mainly in the reversed field pinch (RFP) configuration, recent developments allow now operating the device also with magnetic geometries that feature inner-wall limited and diverted ohmic tokamaks. 26,28,29 Using a toroidal magnetic field on axis B u ' 0:6 T and a plasma current up to I p ' 150 kA, it is possible to perform plasma pulses longer than 1 s with integrated plasma densities n e ! 10 19 m À3 and core electron temperatures T e !…”

“…The RFX-mod device can access such low safety factors thanks to an advanced feedback magnetic boundary control a) Electronic mail: fabio.riva@ukaea.uk system, which allows stabilizing resistive wall modes and performing plasma discharges with q LCFS % 2 without disruptions. 26 We then carefully compare the simulation results with RFX-mod experimental measurements and we analyze the nature of the turbulence in the SOL of RFX-mod.…”

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.…”

Magnetic turbulence suppression by a helical mode in a cylindrical geometry

“…2,4 The QSH state is favored by large current. In RFX-mod 5 the system oscillates between QSH and multiple helicity states, spending more time in QSH relative to the multiple helicity configuration if the current is high. 3 A Hamiltonian theory 6 of the magnetic field has shown that a large single helicity fluctuation relative to other helicities results in a unitary helical equilibrium core without an x-point, as observed in experiment.…”