Transport Induced by Large Scale Convective Structures in a Dipole-Confined Plasma B. A. Grierson, 1, * M. E. Mauel, 2 M.W. Worstell, 2 and M. Klassen 2 1 Princeton Plasma Physics Laboratory, Princeton University, Princeton, New Jersey 08543, USA 2 Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027 USA (Received 25 November 2009; published 11 November 2010) Convective structures characterized by E  B motion are observed in a dipole-confined plasma. Particle transport rates are calculated from density dynamics obtained from multipoint measurements and the reconstructed electrostatic potential. The calculated transport rates determined from the large- scale dynamics and local probe measurements agree in magnitude, show intermittency, and indicate that the particle transport is dominated by large-scale convective structures. DOI: 10.1103/PhysRevLett.105.205004 PACS numbers: 52.35.Mw, 52.35.Ra, 52.55.Hc The transport of particles, energy, and momentum in- duced by plasma instabilities determines confinement in magnetic fusion systems. Confinement systems based on a levitated magnetic dipole have been proposed as an alter- nate path to a fusion reactor due to their relative simplicity and macroscopic stability [1,2]. One key question to be answered about the dipole configuration is whether it has adequate confinement. While drift-resonant transport of energetic particles is understood in dipole geometry [3,4], the transport of thermal plasma confined by a dipole magnet has not been explored in detail. Theoretical and numerical investigations predict the formation of large- scale interchange vortices [5], making plasma transport in the dipole geometry convective. Convective cells have been previously identified in a magnetic trap with a purely toroidal field [6], in a multipole [7], and in a dipole- confined plasma driven by strong rotation [8] or by sus- tained microwave heating [9]. In this Letter, we report on the first measurement of plasma transport induced by large-scale convective struc- tures in a dipole-confined plasma. An array of 96 retarding- grid particle collectors [10] is biased to image the polar ion current density, and high-speed records of the polar current distribution are used to compute both the instantaneous and time-averaged radial particle flux. We find instantaneous transport occurs in intermittent bursts caused by modula- tion of density perturbations induced by the time-evolution of the electrostatic potential structure. The large-scale potential appears with closed streamlines that sometimes encircle density perturbations, and cause no transport, and at other times drive bursts of radial transport. Local particle transport measured with Langmuir probes show the same time-averaged flux as the multipoint measurement and also show intermittency, indicating that large-scale convective structures dominate interchange particle transport in a dipole-confined plasma. We have previously reported observations of the struc- tures of interchange turbulence in a dipole-confined plasma [9]. Turbulent interchange-mode fluctuations are domi- nated by long wavelength modes with amplitudes which vary chaotically in time, appear in the lab frame at low-frequencies, f< 10 kHz, and have relatively short turbulent correlation times, 50–100 s. The potential fluc- tuations are dominated by two quasicoherent modes that rotate in the lab frame between 1–2 kHz and have azimu- thal mode numbers m ¼ 1 and 2. The density perturbations rotate in the same direction as the potential and possess dominant azimuthal modes number up to m ¼ 4. Both density and potential perturbations are flute-modes, aligned with a constant phase along magnetic field lines. All measurements were obtained using the Collisionless Terrella Experiment (CTX) device, described previously [8–10]. Quasisteady turbulent plasma are sustained with electron cyclotron heating (ECRH) of a hydrogen plasma confined by a strong dipole electromagnet that is mechani- cally supported inside a 1.4 m diameter vacuum vessel. At the poles, the magnetic field strength exceeds 1.5 T, and all fieldlines terminate on insulating surfaces. Figure 1 is a schematic of the experimental apparatus. Plasma density FIG. 1 (color online). The CTX plasma device. Displayed are the magnetic field lines (solid) and ECRH resonance zone (dashed), as well as locations of the movable probes. The polar imager probe array is located at one pole of the electromagnet. PRL 105, 205004 (2010) PHYSICAL REVIEW LETTERS week ending 12 NOVEMBER 2010 0031-9007= 10=105(20)=205004(4) 205004-1 Ó 2010 The American Physical Society