RESUMEN
Magnetic feedback control of the resistive-wall mode has enabled the DIII-D tokamak to access stable operation at safety factor q(95) = 1.9 in divertor plasmas for 150 instability growth times. Magnetohydrodynamic stability sets a hard, disruptive limit on the minimum edge safety factor achievable in a tokamak, or on the maximum plasma current at a given toroidal magnetic field. In tokamaks with a divertor, the limit occurs at q(95) = 2, as confirmed in DIII-D. Since the energy confinement time scales linearly with current, this also bounds the performance of a fusion reactor. DIII-D has overcome this limit, opening a whole new high-current regime not accessible before. This result brings significant possible benefits in terms of fusion performance, but it also extends resistive-wall mode physics and its control to conditions never explored before. In present experiments, the q(95) < 2 operation is eventually halted by voltage limits reached in the feedback power supplies, not by intrinsic physics issues. Improvements to power supplies and to control algorithms have the potential to further extend this regime.
RESUMEN
In this Letter, the linear stability of the resistive wall modes (RWMs) in toroidal geometry for a reversed field pinch (RFP) plasma is studied. Three computational models are used: the cylindrical code ETAW, the toroidal MHD code MARS-F, and the CarMa code, able to take fully into account the effects of a three-dimensional conducting structure which mimics the real shell geometry of a reversed field pinch experimental device. The computed mode growth rates generally agree with experimental data. The toroidal effects and the three-dimensional features of the shell, like gaps, allow a novel interpretation of the RWM spectrum in RFP's and remove its degeneracy. This shows the importance of making accurate modeling of conductors for the RWM predictions also in future devices such as ITER.
RESUMEN
The fundamental question of how the flow velocity of the background plasma can influence the motion of magnetohydrodynamics instabilities and, in the ultimate analysis, their stability is addressed. The growth of resistive-wall-mode instabilities in toroidal confinement devices well represents one example of such a problem. In this Letter, we illustrate a new strategy that allowed, for the first time in a reversed field pinch experiment, a fully controlled rotation of a nonresonant instability by means of a set of active coils and how the new findings compare with numerical modeling.
RESUMEN
Stable operation with control on magnetohydrodynamic modes has been obtained in the modified reversed field experiment employing a set of 192 feedback controlled saddle coils. Improvements of plasma temperature, confinement (twofold), and pulse length (threefold) and, as a consequence of the magnetic fluctuation reduction, strong mitigation of plasma-wall interaction and mode locking are reported.
RESUMEN
We present the results of the first successful experiments aimed to control in a quasistationary way the magnetic fluctuation dynamics in a reversed field pinch (RFP) device. This is done by the application of an oscillating poloidal electric field to the plasma edge. Although the additional power input is negligible, a stationary positive effect on the electron temperature is obtained. Temperature increases up to 50% over the standard values. This experiment demonstrates that in principle a stationary current profile and magnetic fluctuation control technique is feasible in the RFP.
RESUMEN
Active feedback stabilization of multiple independent resistive wall modes is experimentally demonstrated in a reversed-field pinch plasma. A reproducible simultaneous suppression of several nonresonant resistive wall modes is achieved. Coupling of different modes due to the limited number of the feedback coils is observed in agreement with theory. The feedback stabilization of nonresonant RWMs also has an effect on tearing modes that are resonant in the central plasma, leading to a significant prolongation of the discharge pulse.