Active MHD control at high currents in RFX-mod

Abstract

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.

Equilibrium reconstruction for single helical axis reversed field pinch plasmas

“…Such conditions are generally called Quasi Single Helicity (QSH) states, since they differ from the theoretical SH ones due to the fact that the secondary modes still have a non-negligible amplitude. In particular, in the RFX-mod device (R = 2 m, a = 0.459 m) it has been observed, when feedback control of the radial magnetic field at the edge is applied [19,20], that the duration and frequency of occurrence of QSH states increases with plasma current [9,21,22]. The secondary mode amplitude, normalized to the total magnetic field, is also found to decrease with increasing plasma current, so that experimental QSH states are progressively approaching the ideal SH condition [9,21,22].…”

Mechanical measurements in RFX-mod experiment

“…RFX-mod is the renewed and upgraded version of the former RFX device, and started its operation in December 2004 [6]. The original RFX thick (which had a magnetic field penetration time constant shell = 500 ms) was replaced with a thinner one, whose time constant for penetration of vertical magnetic field (≈50 ms) is ≈10 times shorter than pulse duration.…”

Effect of resonant magnetic perturbations on three dimensional equilibria in the Madison Symmetric Torus reversed-field pinch

“…The resonant magnetic perturbation (RMP) is used routinely in the tokamak, [1][2][3][4][5][6][7][8][9][10][11][12] the stellarator, 13,14 the heliotron, 15,16 and the reversed-field pinch (RFP). [17][18][19][20][21] The resonant magnetic perturbation is also considered to be of fundamental importance for the ITER tokamak advanced scenario. 3,11,22,23 Resonant magnetic perturbations are used, e.g., for the control of ELMs, the stabilization of detached plasmas, the suppression of high-energy electrons, and the control of tearing mode rotation.…”