Experimental study of the bifurcation nature of the electrostatic potential of a toroidal helical plasma

Abstract

The bifurcation nature of the electrostatic structure is studied in the toroidal helical plasma of the Compact Helical System ͑CHS͒ ͓K. Matsuoka et al., Proceedings of the 12th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Nice, 1988 ͑International Atomic Energy Agency, Vienna, 1989͒, Vol. 2, p. 411͔. Observation of bifurcation-related phenomena is introduced, such as characteristic patterns of discrete potential profiles, and various patterns of self-sustained oscillations termed electric pulsation. Some patterns of the electrostatic structure are found to be quite important for fusion application owing to their association with transport barrier formation. It is confirmed, as is shown in several tokamak experiments, that the thermal transport barrier is linked with electrostatic structure through the radial electric field shear that can reduce the fluctuation resulting in anomalous transport. This article describes in detail spatio-temporal evolution during self-sustained oscillation, together with correlation between the radial electric field and other plasma parameters. An experimental survey to find dependence of the temporal and spatial patterns on plasma parameters is performed in order to understand systematically the bifurcation property of the toroidal helical plasma. The experimental results are compared with the neoclassical bifurcation property that is believed to explain the observed bifurcation property of the CHS plasmas. The present results show that the electrostatic property plays an essential role in the structural formation of toroidal helical plasmas, and demonstrate that toroidal plasma is an open system with a strong nonlinearity to provide a new attractive problem to be studied.

Observation of Plasma Flow at the Magnetic Island in the Large Helical Device

“…This width is determined by the viscous coefficient m and the ratio of radial electric field to radial current and is given by D p m´0´ЌE r ͞j r , where´0´Ќ is the dielectric constant and E r , j r are the radial electric field and neoclassical radial current needed to sustain the E r outside the magnetic island, respectively [14]. The sheared width estimated by using this formula is consistent with the experimental value observed in the compact helical system (CHS) [15]. The value of shear width D is estimated to be 1.4 cm using the neoclassical current and radial electric field,´0´ЌE r ͞j r 0.1 ms and m 2 m 2 ͞s at r 0.9.…”

Observations on core turbulence transitions in ASDEX Upgrade using Doppler reflectometry

“…Using ohmic heated discharges also has the advantage that the toroidal fluid rotation is expected to be small -compared to neutral beam injection (NBI) heated scenarios for example -so that the E × B velocity should be comparable in magnitude to expected phase velocities -which in turn should help to maximise the observation of any transitions in the u ⊥ . Measurements indeed show the core u ⊥ reversing direction with increasing collisionallity which is reminiscent of the E r reversals observed in the CHS [5] and LHD stellarators [6]. However, the stellarator results were attributed to purely neoclassical effects.…”

Bifurcation-Like Behavior of Electrostatic Potential in LHD

“…These are shorter than the confinement time (∼20 ms), therefore, this rapid change is considered to be related to bifurcation phenomenon of the radial electric field. A similar bifurcation phenomenon was observed in CHS [7,8], but the time constant of the potential change is 30 ∼ 70 μs, which is shorter than that of the LHD. The radial profile of the potential was measured with the HIBP, as shown in Fig.…”