High density physics in reversed field pinches: comparison with tokamaks and stellarators

Abstract

Reversed Field Pinches (RFPs) share with Tokamaks and Stellarators the experimental evidence of an upper limit for the maximum value of the electron density at which they can operate. Above a certain density level, well described by the Greenwald law for Tokamaks and RFPs, a radiative collapse with strong plasma cooling is observed, predominantly due to processes occurring at the plasma boundary. In the RFX-mod RFP close to the density limit a radiating belt, poloidally symmetric and toroidally localized, develops in the region where the plasma is shrunk as an effect of the m=0 tearing modes. The phenomenology recalls that of MARFES or plasma detachment, though, unlike Tokamaks, the appearance of the radiating belt is associated to a soft landing of the plasma discharge. The paper reports the experimental pattern of the RFX-mod plasmas close to the density limit, including density and radiation profiles, plasma flow and turbulence. Particles are toroidally conveyed towards the region of maximum shrinking of the plasma column where they accumulate. The interpretation is related to the topology of MHD m=0 and m=1 modes: the reconstruction of the magnetic topology shows that the highly radiating region corresponds to the presence of peripheral m=0 magnetic islands well detached from the wall. The emerging indication is that in RFPs a reduction of the m=0 activity could be a way to overcome the density limit.

Edge ambipolar potential in toroidal fusion plasmas

“…In the RFX, evidence of a causal connection between the convective cell and the MARFE (and thus, the Greenwald limit) is compelling. [15][16][17] To better understand the topology of the E r field, we can map the measured values (as a function of u) onto an edge flux surface calculated with VMEC/V3FIT: the adaptation of VMEC to the reversed field pinch is a recent addition in the framework of a collaboration between Consorzio RFX and the Auburn University. 34 The result is shown in Fig.…”

“…Actually, the equipotential surfaces already define the shape of the convective cell measured by the GPI in RFX, and thẽ E ÂB flow can be defined as the motion of electrons and ions along the equipotential surfaces,ṽ EÂB Á rU ¼ 0, that conserves kinetic energy and can drive the macroscopic transport features which are seen near the Greenwald limit. [15][16][17] The presence of the convective cell is made evident by differentiating Eq. (3), and obtaining the map of (E f , E w p ), shown in Fig.…”

“…In the RFX-mod reversed field pinch 14 (major radius R ¼ 2 m, minor radius a ¼ 0.5 m), there is a large body of evidence that the flow associated with the edge 0/1 island is ultimately responsible for an edge density peaking which drives the Greenwald limit. [15][16][17] In this paper, we will describe an analytical model of the ambipolar potential response to an edge magnetic island, Invited speaker. c) Electronic address: gianluca.spizzo@igi.cnr.it which has been introduced in test-particle simulations with the guiding-center code ORBIT.…”

RFX-mod: A multi-configuration fusion facility for three-dimensional physics studies

“…Spontaneous helical states tend to disappear as the density is increased above a Greenwald fraction n=n G ' 0:3. 32 The phenomenon is not completely understood at the moment, but it is likely related with plasma-wall interaction (PWI) phenomena, which become more severe when the density increases, as discussed in Refs. 32 and 33.…”

How to Face the Complexity of Plasmas?

“…Several papers have been published to provide tentative explanations of the phenomenon. Unfortunately, to the best of the author's knowledge, none of these theories works for the RFP where the same limit is present [106,107], but this has been overlooked by tokamak experts.…”