Beyond the intelligent shell concept: the clean-mode-control

Due to their discrete nature, a grid of active coils for the feedback control of plasma instabilities produces an infinite sequence of sideband harmonics in the magnetic field. If the magnetic sensors have the same periodicity as the coils, as in the intelligent shell scheme (Bishop 1989 Plasma Phys. Control. Fusion 31 1179), the aliasing of the sidebands determines a systematic error on the Fourier analysis of the measurements. Since the intelligent shell scheme relies on raw measurements, it does not recognize the aliasing effect. This is a drawback for error field compensation and for the control of those perturbations that cannot be suppressed by the feedback, but only reduced in their saturation level, such as the non-linear tearing modes (TMs) involved in the dynamo process of reversed field pinch machines. We have derived analytical formulae, valid in cylindrical geometry, for the subtraction of the sideband effect, and we implemented them in a real-time correction algorithm of the Fourier analysis of the magnetic measurements. The Fourier harmonics so obtained are used as the feedback variable of a new control scheme named clean-mode-control (CMC). The first tests of CMC in RFX-mod have given some interesting results in the control of the saturated TMs: besides a reduction of the radial field at the edge and the consequent plasma surface distortion, a systematic rotation of these perturbations with frequencies of up to 100 Hz is seen for the first time. This brings a mitigation of the phase-locking and wall-locking phenomena giving the possibility of operating safely at high currents.

[1]  Paolo Zanca,et al.  MHD linear stability analysis in the setting up phase of the RFX-mod experiment , 2006 .

[2]  E. Yu,et al.  Feedback stabilization of resistive shell modes in a reversed field pinch , 1999 .

[3]  Sergio Ortolani,et al.  Machine modification for active MHD control in RFX , 2003 .

[4]  Paolo Zanca,et al.  Phase-locking of tearing modes in the reversed field experiment , 2002 .

[5]  M. Cavinato,et al.  Distributed real time control in RFX-mod nuclear fusion experiment: commissioning and first results , 2005, 14th IEEE-NPSS Real Time Conference, 2005..

[6]  V. Toigo,et al.  Active MHD control at high currents in RFX-mod , 2007 .

[7]  Chris C. Hegna,et al.  Effect of a resistive vacuum vessel on dynamo mode rotation in reversed field pinches , 1999 .

[8]  Paolo Zanca,et al.  Plasma wall interactions in RFX-mod with virtual magnetic boundary , 2007 .

[9]  Harold P. Furth,et al.  Finite‐Resistivity Instabilities of a Sheet Pinch , 1963 .

[10]  V. Toigo,et al.  The power supply system for the active control of MHD modes in RFX , 2003 .

[11]  Ming-Sheng Chu,et al.  Interaction of an external rotating magnetic field with the tearing mode in a plasma surrounded by a resistive wall , 2001 .

[12]  R. Fitzpatrick,et al.  Interaction of tearing modes with external structures in cylindrical geometry (plasma) , 1993 .

[13]  C. Bishop.  An intelligent shell for the toroidal pinch , 1989 .

[14]  A A Newton,et al.  RFP stability with a resistive shell in HBTX1C , 1989 .

[15]  C. G. Gimblett,et al.  On free boundary instabilities induced by a resistive wall , 1986 .

[16]  Sergio Ortolani,et al.  Active MHD control experiments in RFX-mod , 2006 .

[17]  M. Ottaviani,et al.  Saturation of a tearing mode in zero-β full magnetohydrodynamics , 2007 .

[18]  M. G. Rusbridge,et al.  The relationship between the 'tangled discharge' and 'dynamo' models of the magnetic relaxation process , 1991 .

[19]  G Serianni,et al.  Active-feedback control of the magnetic boundary for magnetohydrodynamic stabilization of a fusion plasma. , 2006, Physical review letters.

[20]  D. Gregoratto,et al.  Feedback control of resistive wall modes in reversed field pinches , 2002 .

[21]  Sergio Ortolani,et al.  Experimental and theoretical studies of active control of resistive wall mode growth in the EXTRAP T2R reversed-field pinch , 2004 .