Direct Two-Dimensional Measurements of the Field-Aligned Current Associated with Plasma Blobs

Abstract

In simple magnetized toroidal plasmas, field-aligned blobs originate from ideal interchange waves and propagate radially outward under the effect of ∇B and curvature induced E × B drifts. We report on first experimental two-dimensional measurements of the field-aligned current associated with blobs, whose ends terminate on a conducting limiter. A dipolar structure of the current density is measured, which originates from ∇B and curvature induced polarization of the blob and is consistent with sheath boundary conditions. The dipole is strongly asymmetric due to the nonlinear dependence of the current density at the sheath edge upon the floating potential. Furthermore, we directly demonstrate the existence of two regimes, in which parallel currents to the sheath do or do not significantly damp charge separation and thus blob radial velocity.PACS numbers: 52.35. Ra, 52.35.Kt, 52.25.Fi Filamentary current structures associated with intermittent blobs and filaments, which propagate across magnetic fields, are observed in natural and laboratory plasmas. For example, satellites reveal current filaments aligned with the Earth magnetic field [1], which are transported with ionospheric blobs. On RFX, a reversed field pinch, insertable magnetic probes measured field aligned currents associated with plasma blobs [2]. In tokamaks, current structures associated with plasma filaments during edge localized mode (ELMs) were measured using fast imaging combined with external magnetic data on MAST [3], reciprocating magnetic probe data in the Scrape Off Layer (SOL) of ASDEX Upgrade [4] and JET tokamaks [5], and filament modeling and comparison with external coil data on DIII-D [6]. Although the field-aligned nature of these current filaments is a common feature, detailed measurements of the two-dimensional (2D) structure in the plane perpendicular to the confining field are missing, thus preventing a fuller understanding of the origin of the current itself. Furthermore, to date, the importance of parallel currents on blob propagation is only indirectly inferred by comparing experimental blob size-versus-speed scalings [7,8] with theory predictions [9].In this Letter, we report on first two-dimensional measurements of the parallel current density associated with radially propagating blobs in the open field line geometry of a simple magnetized toroidal plasma. The experiments are performed on the TORPEX device [10]. Blobs are generated from ideal interchange waves [11] and are propelled by the effective gravity force associated with ∇B and magnetic field curvature causing charge separation, polarizing the blob and a corresponding radially outwards E × B drift [7,12]. Blobs propagate in a region where both of their ends are connected to a conducting limiter, where a plasma sheath is formed. Time-resolved 2D profiles of parallel current density to the limiter are obtained using data from a single-sided Langmuir probe (LP) and from a specially designed current probe, based on an array of magnetic pick-up coils, which are conditional...

Electromagnetic turbulent structures: A ubiquitous feature of the edge region of toroidal plasma configurations

“…More recently a direct observation of current density filaments associated to turbulent structures has been performed in the edge region of the RFX-mod reversed field pinch (RFP) device 8 , where drift kinetic Alfvén structures have been identified 20 . Similar EM filaments have been observed also in the TORPEX experiment 21 , simple magnetized toroidal plasma, where for the first time the parallel current density cross-field map in the filaments, originated by ideal interchange waves, was experimentally revealed.…”

“…The U-probe head, suitable tilted, has been used for the investigation of the same parameters in the tokamak operation also, providing the first direct measurement of parallel current density associated to filaments in this configuration. In the TORPEX device, a further simplified probe has been realized and the first data obtained have been already presented in 21 . This probe, named L-shaped current probe, consists of triaxial coils located at the vertices of a triangle and laying in the perpendicular plane housed in small quartz-tubes.…”

“…This probe, named L-shaped current probe, consists of triaxial coils located at the vertices of a triangle and laying in the perpendicular plane housed in small quartz-tubes. In the specific case of limiter proximity in TORPEX, J measurements were carried out also by a Single Sided Langmuir Probe (SSLP) biased at the limiter potential 21 . In TORPEX, further information has been obtained using other diagnostics, which can be combined together thanks also to the excellent reproducibility of the discharges.…”

Nonclassical Transport and Particle-Field Coupling: from Laboratory Plasmas to the Solar Wind

“…Here, a scenario with B t = 74 mT and B v = 2 mT is used, resulting in a SMT with vertical magnetic field line return distance D ≈ 17 cm, which is dominated by an ideal interchange mode with wavenumbers k ≈ 0, and, k ⊥ ≈ 37 m −1 . This scenario, similar to those extensively studied in Furno et al (2008aFurno et al ( , 2008b, Müller et al (2007), Theiler et al (2008), , Podestà et al (2008), Labit et al (2011), Furno et al (2011 using electrostatic probes, is characterized by the presence of a region on the lowfield side where coherent structures are observed to propagate radially outward resulting in intermittent non-Gaussian transport of particles, heat, momentum and current.…”

Properties of convective cells generated in magnetized toroidal plasmas

“…These blobs are structures of enhanced density that are elongated along the magnetic field but localized in the perpendicular plane. [23][24][25] Their formation, [26][27][28][29] subsequent propagation, [30][31][32] and associated transport, 27,33,34 driven by rB and curvature induced polarization, have been extensively studied in this scenario.…”