An active feedback recovery technique from disruption events induced bym= 2,n= 1 tearing modes in ohmically heated tokamak plasmas

Abstract

We present experimental results of magnetic feedback control on the m=2, n=1 tearing mode in RFX-mod operated as a circular ohmically heated tokamak. The feedback suppression of the non-resonant m=2, n=1 Resistive Wall Mode (RWM) in q(a)<2 plasmas is a well-established result of RFX-mod. The control of the tearing counterpart, which develops in q(a)>2 equilibrium, is instead a more difficult issue. In fact, the disruption induced by a growing amplitude m=2, n=1 tearing mode can be prevented by feedback only when the resonant surface q=2 is close to the plasma edge, namely 2<q(a)<2.5, and the electron density does not exceed approximately half of the Greenwald limit. A combined technique of tearing mode and q(a) control has been therefore developed to recover the discharge from the most critical conditions: the potentially disruptive tearing mode is converted into the relatively benign RWM by suddenly decreasing q(a) below 2. The experiments demonstrate the concept with 100% of successful cases. The q(a) control has been performed through the plasma current, given the capability of the toroidal loop-voltage power supply of RFX-mod. We also propose a path for controlling q(a) by acting on the plasma shape, which could be applied to medium size elongated tokamaks.

Overview of the RFX-mod fusion science activity

“…The experiments have demonstrated that magnetic feedback realized by active saddle coils placed outside the 100ms time-constant copper shell is able to push a wall locked m=2, n=1 mode into slow rotation with frequency of the order of several tens of Hertz. Such result has been shown to remain valid when replicated in DIII-D diverted, D-shaped, high-β plasmas, controlled by coils placed inside the 5ms time-constant vacuum vessel [58,59]. Simulations with the RFXlocking electromagnetic code indicate the improved efficiency of the feedback control with respect to the feedforward one [58].…”

Helical flow in RFX-mod tokamak plasmas

“…The flow oscillates at the same frequency as the MHD modes, i.e. the 2/1 RWM and the internal 1/1 kink mode, which are maintained into rotation by the magnetic feedback control system [5,23]. By correlating flow measurements with radial magnetic field data, the presence of helical flow and m = 1, n = 1 convective cells has been inferred.…”

“…The novelty of the experiments reported in this paper is that, in the presence of rotating 3D magnetic fields, oscillations have been observed in the plasma flow, measured by the Doppler shift of spectral lines from several ion species. The flow oscillates at the same frequency of the MHD modes, which are maintened into rotation by the magnetic feedback [8,11]. By correlating flow measurements with radial magnetic field data, m = 1, n = 1 (with m and n poloidal and toroidal mode numbers, respectively) convective cells have been inferred in the flow pattern, which are a signature of helical flow.…”

“…The RFX-mod experiment [20], when operated as a tokamak [5,[21][22][23], can contribute to the study of the effects responsible for momentum injection and transport in the absence of external heating systems, and of the impact of externally applied 3D magnetic fields on plasma rotation. The RFX-mod device is equipped with a sophisticated feedback control system, made up of 192 active coils, independently driven and fully covering the torus surface, which can induce a wide range of m/n 3D magnetic field perturbations (with m and n poloidal and toroidal mode numbers, respectively).…”

Comparative studies of electrostatic turbulence induced transport in presence of resonant magnetic perturbations in RFX-mod

“…The maximum plasma current achieved in tokamak operations is I p = 0.15 4 MA, with a maximum toroidal magnetic field B φ = 0.55 T, corresponding to an edge safety factor q a = 1.7. This low-q operations are possible only by means of a feedback control on the (m = 2, n = −1) mode, that resonates close to the wall and would otherwise grow too large and disrupt the discharge [33]. In the following sections we present the results obtained by analyzing and reconstructing the topology in a typical tokamak discharge of RFX-mod, with q a 2.1 and where a resonant (2, -1) perturbation with a rotation frequency of 10 Hz is applied through the feedback control system.…”