Magnetic Self Organization, MHD Active Control and Confinement in RFX-mod

RFX-mod is a reversed field pinch (RFP) experiment equipped with a system that actively controls the magnetic boundary. In this paper we describe the results of a new control algorithm, the clean mode control (CMC), in which the aliasing of the sideband harmonics generated by the discrete saddle coils is corrected in real time. CMC operation leads to a smoother (i.e. more axisymmetric) boundary. Tearing modes rotate (up to 100 Hz) and partially unlock. Plasma-wall interaction diminishes due to a decrease of the non-axisymmetric shift of the plasma column. With the ameliorated boundary control, plasma current has been successfully increased to 1.5 MA, the highest for an RFP. In such regimes, the magnetic dynamics is dominated by the innermost resonant mode, the internal magnetic field gets close to a pure helix and confinement improves

Analysis and modelling of the magnetic and plasma profiles during PPCD experiments in RFX

In this paper, we analyse the main features of the pulsed poloidal current drive (PPCD) technique, used in the reversed field pinch configuration to achieve improved confinement conditions. In the RFX experiment, PPCD corresponds to a decrease of the magnetic fluctuations, to a peaking of the temperature profile, and to a reduced transport and plasma-wall interaction. A three-dimensional MHD nonlinear code and one-dimensional time-dependent transport models have been applied to study the effect of PPCD on the magnetic and plasma profiles. The three-dimensional MHD simulations show that the external inductive drive pinches and peaks the current profile driving the configuration through a transient phase, where the spontaneous turbulent dynamo action is quenched. The one-dimensional transport codes indicate that the experimental profile modifications associated with PPCD are consistent with a reduction of the stochastic transport

Overview of RFX-mod results with active MHD control

Plasma experiments resumed in December 2004 on RFX-mod. The machine now has a thin (3mm) Cu shell with one overlapped poloidal gap and one toroidal gap. Shell penetration time for Bv has been lowered from 450 to 50 ms and shell/plasma proximity from b/a=1.24 to 1.1. Toroidal equilibrium is feedback-controlled and new power supplies provide a better control of the toroidal field. Newly designed graphite tiles protect the vessel from highly localized power deposition. The MHD Control System, MHD-CS, a set of 192 external saddle coils controlled by a digital feedback system, is used to control radial fields due to field errors, MHD modes and Resistive Wall Modes (RWMs). A dramatic improvement of plasma performance was obtained by using the MHD-CS to cancel all of the radial field components, an operational mode dubbed Virtual Shell (VS). The toroidal loop voltage was lowered by more than 40% and the plasma pulse duration tripled. In practice, steady state RFP pulses are now limited only by the applied volt-seconds. Hence RFX-mod initial operation demonstrated the possibility to operate a large RFP without a thick conducting shell, and opened enhanced RFP scenarios. Indeed the improved magnetic boundary in VS mode, which mimics an ideal closely fitting shell, has an effect on the tearing modes underlying the sustainment of the RFP configuration, the so-called dynamo modes, which are also responsible for field line stochastization in the plasma core and confinement limitation. With the VS the amplitude of such modes in the plasma centre was nearly halved. As expected, this led to improved particle and energy confinement. For instance, peak electron temperature in reference pulses at 600 kA was increased from 200 to 300 eV with more peaked profiles, which corresponds to a reduction of the thermal conductivity by a factor 2 in the region r/a < 0.9. The MHD-CS is extremely flexible and can be used for a variety of mode control experiments. The most important result already obtained was the demonstration of the active control of RWMs. We found that full VS control completely inhibits the growth of RWMs, whereas such modes are indeed seen to grow in agreement with the theoretical prediction if the MHD-CS operated in Selective VS mode, i.e. leaving one or more mode helicity uncontrolled

New Insights into MHD Dynamics of Magnetically Confined Plasmas from Experiments in RFX

The experimental and theoretical activity performed in the RFX device has allowed a deeper insight into the MHD properties of the reversed field pinch (RFP) configuration. A set of successful experiments has demonstrated the possibility of influencing both the amplitude and the spectrum of the magnetic fluctuations which characterize the RFP configuration. A new regime (quasi-single-helicity states) where the dynamo mechanism works in a nearly laminar way and a helical core plasma is produced has been investigated. With these studies a reduction of magnetic chaos has been obtained. The continuous rotation of wall locked resistive tearing modes has been obtained by an m = 0 rotating perturbation. This perturbation induces rotation of m = 1 non-linearly coupled modes

Improvement of the magnetic configuration in the reversed field pinch through successive bifurcations

The reversed \ufb01eld pinch (RFP) is a magnetic con\ufb01guration alternative to the tokamak that can be considered for a second generation of reactors. In this paper new remarkable results obtained in the RFP experiment RFX-mod are presented, showing that an internal transport barrier delimitates a large fraction of the plasma volume in a RFP when the current is raised to 1.5 MA. The formation of this transport barrier is related to a profound, spontaneous modi\ufb01cation of the magnetic topology. Due to the occurrence of a saddle node bifurcation the plasma enters in the single helical axis state, which is theoretically known to be more resilient to chaos. This bifurcation is driven by the amplitude of the helical perturbation which dominates the mode spectrum