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

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.

Feedback control of dynamo tearing modes in a reversed field pinch: comparison between out-vessel and in-vessel active coils

“…In fact CMC operations at high currents are characterized by transient quasi-singlehelicity (QSH) conditions, where the innermost resonant TM grows at the expense of the others [10]. The experimental results have stimulated the development of a MHD model able to simulate the evolution of TM phases and edge amplitudes under CMC action [11,12]. The closed-loop system formed by the single-fluid motion equation, and the equations describing the TM radial field diffusion across the shell(s), the electro-technical modelling of the coils, the acquisition system of measurements and the CMC feedback law is solved using a semianalytical approach.…”

“…In this case, the self-consistent inclusion of evolution of TM amplitudes at the resonant surfaces would require a simulation of the dynamo mechanism, which is beyond the possibility of the model. The model can deal with a multiple shell layout, but in [11,12] a single shell, acting as a vacuum vessel, has been taken in order to simplify the parametric analysis discussed therein. Moreover, coils have been located outside the vacuum vessel, as in the present RFP experiments, while sensors have been placed inside it.…”

Advanced feedback control of magnetohydrodynamic instabilities: comparison of compensation techniques for radial sensors

“…The minimum of ζ m,n is as much closer to the ideal-shell limit, i.e. the value with an ideal shell in the place of the resistive one, as smaller is the ratio between the delay of the feedback and the shell time constant, at least in the limit of relatively thin shell (see figure 5 of [46]). Therefore, the ultimate CMC possibilities are fixed by the plasma-shell proximity.…”

“…The analysis [17,46] illustrated this, adopting the simplifications of a single shell placed at a finite distance from the plasma, and a continuous-time feedback, with the delays of acquisition and coils' amplifiers modeled by one-pole filter laws. The minimum of ζ m,n is as much closer to the ideal-shell limit, i.e.…”

Upgrades of the RFX-mod reversed field pinch and expected scenario improvements

“…In particular, the feedback does not modify the amplitude rampup curve, but changes significantly the rampdown curve making possible the return of the spontaneous rotation with a modest amplitude decrease. This behavior is due to the fact that the feedback mimics a quasiideal boundary at the plasmafacing conducting structure radius [86], thus low ering the phase delay between eddy currents in the vacuum vessel and TM current at the resonant surface. Therefore, for a given tearing mode amplitude, the braking torque between the vacuum vessel and the tearing mode is much lower when feedback is acting, leading to the higher unlocking threshold compared to the nonfeedback case.…”