Reversed field pinch operation with intelligent shell feedback control in EXTRAP T2R

Abstract

Discharges in the thin shell reversed field pinch (RFP) device EXTRAP T2R without active feedback control are characterized by growth of non-resonant m = 1 unstable resistive wall modes (RWMs) in agreement with linear MHD theory. Resonant m = 1 tearing modes (TMs) exhibit initially fast rotation and the associated perturbed radial fields at the shell are small, but eventually TMs wall-lock and give rise to a growing radial field. The increase in the radial field at the wall due to growing RWMs and wall-locked TMs is correlated with an increase in the toroidal loop voltage, which leads to discharge termination after 3-4 wall times. An active magnetic feedback control system has been installed in EXTRAP T2R. A two-dimensional array of 128 active saddle coils (pair-connected into 64 independent m = 1 coils) is used with intelligent shell feedback control to suppress the m = 1 radial field at the shell. With feedback control, active stabilization of the full toroidal spectrum of 16 unstable m = 1 non-resonant RWMs is achieved, and TM wall locking is avoided. A three-fold extension of the pulse length, up to the power supply limit, is observed. Intelligent shell feedback control is able to maintain the plasma equilibrium for 10 wall times, with plasma confinement parameters sustained at values comparable to those obtained in thick shell devices of similar size.

Achieving high flux amplification in a gun-driven, flux-core spheromak

“…As the magnetic energy within the flux conserver decreases only slightly during ABR, the magnetic energy to be removed is that in the gun and exit volumes; for each, ABR is also consistent with the control of the tilt and shift modes by feedback stabilization such as an "intelligent" wall [23] rather than by a highly conduction flux conserver. Recent experimental studies in the reversed-field pinch with a thin, resistive wall demonstrate successful active stabilization of non-resonant resistive wall modes and locking of resonant tearing modes over the duration of the discharge [24]. Detailed examination of this physics in the spheromak is beyond the present study.…”

Measurements and phenomenological modeling of magnetic flux buildup in spheromak plasmas

“…To date, however, the model is only phenomenological and not quantitative, due to a lack of measurements and precision that would allow the estimation of the total helicity, the helicity decay rate, and the reconnection details at the open and closed flux surfaces. Better understanding of these parameters would be required for spheromak reactor scenarios in which poloidal flux is amplified and sustained by periodically pulsing the gun current and controlling the flux conserver response (such as the so-called "intelligent" wall, described by Bishop [25] and recently tested in a reversed-field pinch by Brunsell et al [26]). …”

High current regimes in RFX-mod

“…Research on Reversed Field Pinch (RFP) configurations has experienced in the recent years substantial progress [Marrelli 2007, Wyman 2008, Brunsell 2006, Hirano 2006, with significant advances both in terms of plasma performance and control, and of understanding of relevant physics. The aim of this paper is to add to the line of such progress new interesting physics results, which are emerging from the reliable operation of RFX-mod in the regime with plasma current above 1 MA.…”