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 nonaxisymmetric 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.

No abstract

We present results of the first experiments on resistive tearing mode control in a reversed field pinch by an external m 0 magnetic perturbation. In this way the helical deformation of the plasma column produced by resonant m 1 modes can be localized in a preselected toroidal position. We also show that a continuous toroidal rotation of this deformation can be induced by a rotating external field. From the study of such toroidal rotations the nonlinear origin of the torque acting on the internally resonant modes clearly emerges: in particular, we demonstrate the occurrence of phase conjugation and harmonic generation phenomena. PACS numbers: 52.35.Py, 52.35.Mw, 52.55.Hc Reconnection of field lines by nonideal effects in a magnetized hot plasma is responsible for a variety of phenomena observed in nature [1], such as solar flares and magnetosphere substorms. Magnetic reconnection also plays a central role in the behavior of toroidal plasmas used in thermonuclear fusion studies, since it is responsible for the onset of resistive instabilities usually referred to as tearing modes [2].Tearing modes are, however, essential to the functioning of reversed field pinch (RFP) devices of present days, since they provide a MHD dynamo that sustains the poloidal plasma current needed for field reversal [3]. In the RFP many tearing modes, resonant on closely spaced rational surfaces in the plasma core, can be simultaneously unstable. They have poloidal mode number m 0 and 1 and toroidal mode number n ranging between 2 and 3 times the device aspect ratio.A distinguishing feature of the RFP is that the mode amplitudes are sufficiently high for their nonlinear interaction to become important, and, indeed, the dynamo results from the coherent excitation of magnetic field and flow velocity fluctuations. This behavior can be described to a large extent by three wave interactions [4] satisfying the conservation rules n 3 n 1 2 n 2 and m 3 m 1 2 m 2 . Therefore the main m 1 modes are effectively coupled together through m 0 modes with low n number which are all resonant at the reversal surface where the toroidal field vanishes.Control of the nonlinear dynamics of tearing modes is the central issue of today's RFP research effort. In fact, although they sustain the configuration, these modes are responsible for two phenomena that severely limit the RFP potential as a thermonuclear reactor. First, the excitation of many resonant modes leads to stochasticization of magnetic field lines over a large portion of the plasma core, thereby degrading the plasma thermal isolation. Second, the nonlinear interaction between the tearing modes leads to their phase locking and results in a localized helical deformation (LHD) of the plasma column. The LHD in its turn is easily locked to the wall by error fields, causing localized plasma wall interaction and, consequently, impurity release and wall damage.It has long been known that the magnetic reconnection rate can be changed by a resonating external field [5], obtaining a stabilizing or destabilizi...

The modified RFX is a very flexible device used for a variety of control schemes for MHD instabilities and for advanced reverse field pinch scenarios. Relative to the previous machine, RFX-mod has a thin Cu shell with vertical field penetration time τS, lowered from 450 to 50 ms and shell/plasma proximity from b/a = 1.24–1.1. Toroidal equilibrium is feedback-controlled and new power supplies provide better Bϕ control. Newly designed graphite tiles protect the vessel from localized power deposition. A mesh of 192 external saddle coils, supervised by a digital feedback system, controls radial fields due to field errors and MHD modes. The paper presents an overview of the very encouraging results obtained using both new and ‘standard’ advanced operational modes in the current range 0.3–1 MA. A dramatic improvement of plasma performance was obtained by using the saddle coils to cancel all the radial field components, an operation mode dubbed virtual shell (VS). The toroidal voltage was lowered by more than 25% and the pulse length was tripled, up to 7 times the τS. Steady-state RFP pulses are now limited only by the applied volt-seconds. The improved magnetic boundary also has an effect on the tearing modes underlying the sustainment of the RFP, whose core amplitude is more than halved. The VS combined with new schemes for the active rotation of the MHD dynamo modes has allowed us to obtain reliable and well-controlled long RFP pulses in the MA current range. This results in a 100% increase in the particle and energy confinement time relative to the previous experiment and opens the possibility of exploring the machine performance in the 2 MA design range.