Abstract
Unstable fluctuations develop in the initially quiescent plasmain the improved confinement mode of the H-1 heliac when theradial electric field (Er) shear exceeds some criticalvalue. These unstable Er shear-driven modes are shown togenerate zonal-flow-like poloidally symmetric potentialstructures, similar to those generated in the low confinementmode (Shats M G and Solomon W M 2002 Phys. Rev. Lett. 88 045001). The structures modulate their parent waves, thebackground Er shear and the fluctuation-driven radialtransport. The onset of zonal flows is observed as a precursorto the plasma confinement bifurcation to an even higherconfinement regime.
Highlights
The generation of zonal flows (ZFs) in toroidal plasma has been a focus of active research in recent years
In this paper we present experimental results on the generation of ZF-like structures in the improved confinement mode (H-mode) and on the role of these structures in the confinement bifurcations
As this f ≈ 0.8 kHz feature develops at t > 50 ms, it starts modulating the higher-frequency (∼11 kHz) fluctuations in the poloidal electric field shown in figure 6(d)
Summary
The generation of zonal flows (ZFs) in toroidal plasma has been a focus of active research in recent years. It has been demonstrated that these structures, seen as low-frequency fluctuations in the plasma potential, have zero poloidal wavenumbers kθ, finite radial wavenumbers kr, (kr kθ ∼ 0) and are generated through a three-wave interaction process in various modes of confinement in the H-1 heliac. Several different types of confinement bifurcations have been observed in the lowtemperature plasma in the H-1 heliac It has been shown, to other toroidal experiments, that the radial electric field plays a very important role in these bifurcations [6, 8]. Fluctuations in H-1 can linearly generate a mean radial electric field (through the radial non-ambipolar transport) and can nonlinearly generate time-varying Er. Here we briefly discuss these confinement bifurcations observed in H-1.
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