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Beyond the Intelligent-shell Concept: the Clean-mode-control for Tearing Perturbations

Paolo ZANCA

Consorzio RFX, Associazione Euratom-ENEA sulla Fusione, Padova, Italy

(Received 24 July 2008 / Accepted 29 January 2010 / Published 25 May 2010)

Abstract

The Intelligent Shell scheme, where a grid of active coils counteracts in a feedback scheme the measurements provided by an identical grid of sensors, has shown some limitations in the control of the dynamo tearing modes in RFX-mod. The origin of the problem is the aliasing on the measurements coming from the high periodicity sideband harmonics produced by the discrete nature of the active coils. A more efficient feedback on tearing modes is obtained by removing the sidebands from the measurements, thereby counteracting the true tearing Fourier modes. In this scheme, named Clean-Mode-Control, the sidebands are computed in real time from the coils currents using the cylindrical geometry approximation. The Clean-Mode-Control significantly alleviates the wall-locking of tearing modes in RFX-mod, giving the possibility of operating at a plasma current (1.5 MA) never reached before in a RFP machine. These features are well explained by a MHD model describing the tearing mode dynamic under the viscous torque due to the fluid motion and the electromagnetic torques produced by the feedback, the conductive structures surrounding the plasma and the non-linear interaction between the different modes [P. Zanca, Plasma Phys. Control. Fusion 51, No. 1, 015006 (2009)]. Here some new results obtained with this model are discussed. In particular we will show that the edge radial field control improves by reducing the ratio between the delay introduced by the digital acquisition of the measurements and the time constant of the shell that contains the plasma. In this formulation the active coils are assumed to be located outside the shell.

Copyright (c) 2010 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords

dynamo tearing mode, proportional-derivative feedback controller, sideband, phase-locking, wall-locking, resistive shell

DOI: 10.1585/pfr.5.017

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This paper may be cited as follows:

Paolo ZANCA, Plasma Fusion Res. 5, 017 (2010).