Experimental Evidence of a Zonal Magnetic Field in a Toroidal Plasma

Abstract

A zonal magnetic field is found in a toroidal plasma. The magnetic field has a symmetric bandlike structure, which is uniform in the toroidal and poloidal directions and varies radially with a finite wavelength of mesoscale, which is analogous to zonal flows. A time-dependent bicoherence analysis reveals that the magnetic field should be generated by the background plasma turbulence. The discovery is classified as a new kind of phenomenon of structured magnetic field generation, giving insight into phenomena such as dipole field generation in rotational planets. Nowadays, a number of phenomena analogous to geomagnetism are known to occur ubiquitously in nature: e.g., the planetary dynamo, sun spots, the galactic dynamo, etc. A widely accepted hypothesis is that turbulence should be a main player for the phenomena of magnetic field generation [1][2][3]. Along with accumulating support for this hypothesis from direct nonlinear simulations [4,5], laboratory experiments have made progress in observing the phenomena associated with a dynamo in turbulent materials [6 -13]. For instance, the fundamental processes of a dynamo, such as a self-exciting magnetic field [11] and the generation of a magnetic field by turbulent flows [13], have been demonstrated in experiments using turbulent conducting flows of liquid metals.In toroidal plasma, which aims at realizing a Sun on the Earth, strong turbulence sustained by a steep pressure gradient organizes the thermal structure through selfregulating transport. Recently, a structured electric field driven by microscopic turbulence was identified using heavy ion beam probes (HIBPs) in a toroidal plasma named the compact helical system (CHS) [14]. In a magnetically confined plasma, the electric field is equivalent to the perpendicular flows to the confinement magnetic field through E B drift. The structured electric field dubbed zonal flow or zonal electric field [15] has a coaxial symmetry around the magnetic axis, and it fluctuates with a radial wavelength with a constant phase on a magnetic surface. This field is classified as a mesoscale, since the scale of the radial wavelength is larger than the characteristic scale of the turbulence but smaller than the system size.Current theory suggests the possibility that turbulence should generate a zonal magnetic field in addition to a zonal flow [15][16][17][18]. Similarly to the zonal flow, the zonal magnetic field fluctuates homogeneously on the magnetic surface and alternates direction as a function of plasma radius. This field therefore remains finite even upon averaging over a magnetic surface to be regarded as a mean field. The HIBP has an ability to sense a magnetic field fluctuation simultaneously with an electric field one. Trials to search a zonal field, utilizing the advantage of the diagnostics, have been performed in CHS. In this Letter, we present the discovery of a zonal magnetic field-a new kind of structured magnetic field generated from background turbulence. The present experimental results provide the first clear ...

Physics of Alfvén waves and energetic particles in burning plasmas

“…However, while zonal electric fields and corresponding zonal flows are widely measured in experiments with properties that are consistent with the general theoretical framework (Diamond et al, 2005), zonal magnetic fields and currents, predicted theoretically (Chen et al, 2001;Diamond et al, 2005;Gruzinov et al, 2002;Guzdar et al, 2001b), have been only recently observed in experiments in the compact helical system (CHS) (Fujisawa et al, 2007).…”

Experimental studies of zonal flow and field in compact helical system plasma

“…Consequently, the fluctuation less than ϳ1 kHz should have symmetric structure around the magnetic field axis with a radial structure in mesoscale. 28 A constant phase relation between the signals from two different radial positions allows us to estimate the spatiotemporal characteristics of the zonal magnetic field in the radial direction by evaluating the correlation function, C͑⌬t , ⌬r͒ in exactly the same manner as that of the zonal flow. Figure 5͑b͒ shows the cross-correlation obtained by altering an observed position r 2 , shot by shot with fixing the other at r͑=12 cm͒.…”

“…27 The paper presents a brief review of the experimental achievements, using twin HIBPs in CHS, on the system of zonal flows and turbulence, together with the discovery of the zonal magnetic field. 28 The discovery is, to the authors' knowledge, the first evidence to show that the turbulence really generates the structured magnetic field, which is associated with a historical physics problem; i.e., the dynamo problem.…”

Regulation of electron temperature gradient turbulence by zonal flows driven by trapped electron modes

“…[2][3][4] The gyrokinetic simulations have also revealed that the ITG turbulence is regulated by self-generated zonal flows 5 of which existence is confirmed in a helical plasma experiment. 6 Contrary to the progress in the ion heat transport study, it is still difficult to explain the mechanism of anomalous electron heat transport. In a burning plasma, a large part of energy carried by fast alpha particles is absorbed by electrons.…”