The stochastic diffusion process in reversed-field pinch

Abstract

The stochastic diffusion process has been carefully analysed for the Reverse Field Pinch ͑RFP͒ equilibrium configuration. Three different approaches to the problem have been used, i.e. the direct calculation of the stochastic diffusion coefficient (D st ) from the radial evolution of an ensemble of field lines, an indirect determination using a Gaussian-fit to the field line probability distribution function and finally the relationship of D st with the maximum Lyapunov exponent of the system. Different perturbation spectra have been also considered and compared and the quasi-linear approximation has been tested.

Chaos Healing by Separatrix Disappearance and Quasisingle Helicity States of the Reversed Field Pinch

“…In most experimental conditions, several such modes are simultaneously present with similar amplitudes of the order of a few percent of the axisymmetric magnetic field; such plasma states are dubbed multiple helicity (MH) states. Owing to the close mode spacing, this produces magnetic chaos in the plasma core [2] by resonance overlap [3]. Soft x-ray (SXR) tomographic measurements of MH states performed in RFX, the largest present RFP, show a rather axisymmetric thermal structure with its hottest point at the secondary axis of the plasma annulus [1].…”

Transport Barrier inside the Reversal Surface in the Chaotic Regime of the Reversed-Field Pinch

“…The dominant one is negative: the modes produce magnetic chaos in the q > 0 region [3,9] which implies poor confinement. Furthermore, if m 1 modes are phase locked, they also produce a toroidally localized bulging of the magnetic field lines and of the chaotic sea which increases transport as observed in experiments [10].…”

A statistical analysis of pulsed poloidal current drive in the Reversed Field eXperiment

“…The fluctuations in the magnetic field and plasma velocity produce a poloidal electric field, E ,dyn and, therefore, convey the energy externally supplied to the toroidal electric field E by the toroidal transformer. [2][3][4] In the MH regime, magnetic fluctuations are nonetheless detrimental to confinement: They cause magnetic chaos in the plasma core, [5][6][7] which allows for an enhanced particle and energy transport. 8,9 This might not be the case in the single helicity ͑SH͒ regime, when a single mϭ1 mode takes place, therefore, allowing for a more laminar ͑and less turbulent͒ dynamo mechanism.…”