Observation of Toroidal Flow Antiparallel to the〈Er×Bθ〉Drift Direction in the Hot Electron Mode Plasmas in the Compact

Abstract

A toroidal flow antiparallel to the ͗E r 3 B u ͘ drift direction is observed in the hot electron mode plasmas when a large positive electric field and a sharp electron temperature gradient are sustained inside the internal transport barrier in the Compact Helical System. This toroidal flow reaches up to 5 3 10 4 m͞s at the plasma center, and it is large enough to reverse the toroidal flow driven by a tangentially injected neutral beam. These observations clearly show the plasma favors flow in the minimum =B direction at the transport barrier. DOI: 10.1103/PhysRevLett.86.3040 PACS numbers: 52.55.HcA spontaneous poloidal flow has been recognized to be important in the confinement of toroidal plasmas, since poloidal flow velocity and radial electric field shear were found to contribute an improvement of plasma confinement in H-mode plasmas [1,2]. The turbulence in the plasma is predicted to be suppressed by a E 3 B velocity shear through the mechanism of a nonlinear decorrelation [3,4]. It has been confirmed experimentally that the E 3 B shearing rate exceeds the growth rate of the turbulence at the transport barrier [5][6][7]. The toroidal flow has been considered to be determined by the radial transport of momentum driven by neutral beam injection (NBI) with anomalous shear viscosity [8,9]. Although the E 3 B velocity shear due to the toroidal flow can be important in the plasma core [10], there have been few experiments that demonstrate a spontaneous toroidal flow not driven by NBI in tokamaks [11][12][13][14]. It is generally the case in tokamaks that the plasma rotates parallel (antiparallel) to the plasma current for the coinjected (counterinjected) NBI, which corresponds to a positive (negative) radial electric field ͑E r ͒ in L-mode plasmas [15]. However, when a negative E r is produced at the transport barrier, both localized toroidal flow in the counterdirection and localized poloidal flow in the electron diamagnetic direction are observed even for plasmas with coinjected NBI [5,16,17]. These results show the need for detailed measurements of both toroidal and poloidal flow at the transport barrier, where the toroidal flow profile is not determined by the transport of momentum driven by NBI alone. Yet, despite the importance of the coupling between toroidal and poloidal flow, no studies until now have discussed the toroidal flow at the internal transport, where there is a large radial electric field.In a Heliotron/Torsatron device, there is no toroidal symmetry, and the minimum =B trace has a helical structure and helical flow along the minimum =B can be expected. The significant difference between tokamaks and Heliotron devices is the relationship between the poloidal field direction and the direction of dominant symmetry.The pitch angle of dominant symmetry is even larger than that of the averaged poloidal field in a Heliotron device, while it is zero due to the toroidal symmetry in tokamaks. This helical flow should change sign depending on the sign of E r or on the direction of the magnetic field. H...

Observation of Toroidal Flow on LHD

“…The counter flow (negative) increases when the radial electric field becomes more positive. These results are consistent with an experiment in CHS, where the spontaneous toroidal flow in the counter direction appears associated with the transition from the ion root with small negative E r to the electron root with large positive E r [9]. It should be noted that the direction of the spontaneous toroidal flow is anti-parallel to the direction of <E r × B θ > drift.…”

Observations of spontaneous toroidal flow in the LHD

“…Figure 3 shows the dependence of the toroidal flow on the radial electric field at the plasma edge (R=4.4m). The positive E r drives the toroidal flow into the counter-direction, and this is consistent with the experiment in CHS where the spontaneous toroidal flow in the counter direction appears associated with the transition from the ion root with small negative E r to the electron root with large positive E r [9]. It should be noted that the direction of the spontaneous flow driven by the radial electric field is opposite to that in tokamaks.…”

3-D effects on viscosity and generation of toroidal and poloidal flows in LHD

“…In the open field region outside of the plasma boundary, flow in the counter-direction has been observed in both plasmas. These counter-flows with positive radial electric field seem to be consistent with the observations of radial electric field driven flow in the nested flux surface region in CHS 30 and LHD, 17 although the effects of the stochastic magnetic field should be taken into account. The opposite sign of toroidal flow shear between two plasmas shown in Fig.…”