A new paradigm for RFP magnetic self-organization: results and challenges

Abstract

This paper reports the most recent experimental results on quasi-single helicity (QSH) reversed field pinch (RFP) plasmas. QSH is considered a key element towards the full experimental realization of the theoretically predicted single helicity (SH) RFP. The SH RFP, where an individual resistive kink mode and its harmonics drive the dynamo electric field, is predicted to have superior confinement performance with respect to the standard multiple helicity (MH) state. Magnetic chaos is in fact strongly reduced in the SH RFP, which therefore retains all the positive features of the RFP configuration without the problems connected with the magnetic turbulence typical of the MH scenario. Data from the RFX-mod device, presented here, provide a more complete description of QSH states, indicate a positive synergy between the growth of the dominant resistive mode and the decrease in the secondary modes (with reduction of magnetic chaos and hints of confinement improvement outside the helical domain), and show a promising scaling with plasma current. Initial experiments on active control of QSH states in RFX-mod are presented.

High current regimes in RFX-mod

“…RFX-mod: a device with feedback controlled magnetic boundary missions are to explore high current RFP plasmas, up to 2 MA [Sonato 2003, Ortolani 2006, Marrelli 2007, Martin 2007, and to contribute to the community effort on MHD stability active control.…”

Physics of magnetic confinement fusion

“…In this way the probability for the otherwise low-probability 3-body process from 4 He to 12 C is strongly increased. These resonances were brilliantly concluded by Fred Hoyle from the cosmic abundance of 12 C (similar with 16 O and 20 Ne) before they were experimentally verified.…”

“…In this way, 16 O forms out of 12 C, 20 Ne from 16 O and so on. These processes continue ultimately up to 56 Fe.…”

“…The plasma self-heals a defect improving confinement thereby. Figure 8, right, shows this configurational change with an island in the poloidal cross-section containing nested flux-surfaces giving rise to the benefit of a distinct local increase in temperature [16].…”

Use of helical fields to allow a long pulse reversed field pinch

“…However, RFP plasmas in the axisymmetric Reversed Field eXperiment (RFX) [6] are observed [7], [8] to self-organize into a long-lived helical state with greatly improved transport coefficients. The ability to design helically-shaped plasmas to achieve given physics goals has greatly improved over the last quarter century [9], [10], [11].…”