Investigation on the relation between edge radial electric field asymmetries in RFX-mod and density limit

Abstract

In all major confinement devices (tokamaks, stellarators, spheromaks and reversed-field pinches-RFPs), a density limit has been found. Results summarized in a recent work by Puiatti et al (2009 Nucl. Fusion 49 045012) show that in the RFP high density does not cause a disruption, but a sequence of increasingly critical phenomena. First, at intermediate density there is the disappearance of the high-confinement quasisingle helicity/single helical axis regimes. Then, at densities close to the Greenwald limit, toroidally and radially localized density accumulation and radiation condensation are observed, together with a fast resistive decay of the plasma current, which constitutes the real operative limit of the device. In this paper we discuss the effect of the magnetic ripple on test particle motion, showing that the accumulation of electrons in the X-points of the edge m = 0 islands is responsible for a modulation of the radial electric field E r which is at the core of the density limit mechanism. These results can be also relevant for the explanation of X-point multifaceted asymmetric radiation from the edge formation, observed in L-mode density limit discharges of JET.

Edge ambipolar potential in toroidal fusion plasmas

“…7(c) and 7(d) in Ref. 17], and has been interpreted in the past in terms of finite ion Larmor radius effects. 27 But there is a large portion of the toroidal angle where v / > 0 (E r > 0), with two null points that define a huge convective cell (diameter $ 3 m).…”

“…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.…”

Density limit studies in the tokamak and the reversed-field pinch

“…In the RFX RFP [14], data points in the plane (n G , n 0 ), with n 0 the central electron density, seem to follow a Hugill-Greenwald scaling, with scarce data points for n 0 > n G , as published several times in the past [15][16][17][18][19][20][21], but a real disruptive limit when approaching n G has never been found [18,20]. This can be highlighted by grouping data in classes, according to the value of the toroidal loop voltage, as shown in figure 1(a).…”