Transition of poloidal viscosity by electrode biasing in the Large Helical Device

Abstract

Electrode biasing experiments were carried out in various magnetic configurations on the Large Helical Device (LHD). The transitions of poloidal viscosity, which were accompanied with bifurcation phenomena characterized by a negative resistance in an electrode characteristic, were clearly observed on LHD by the electrode biasing. The critical external driving force required for transition was compared with the local maximum in ion viscosity, and the radial resistivity before the transition also compared with the expected value from a neoclassical theory. The critical driving force increased and the radial resistivity decreased with the major radius of the magnetic axis Rax going outwards. The configuration dependence of the transition condition and the radial resistivity qualitatively agreed with neoclassical theories. The radial electric field and the viscosity were also evaluated by the neoclassical transport code for a non-axisymmetric system, and estimated electrode voltage required for the transition, which was consistent with the experimental results.

Simulation studies of the effect ofE×Brotation on neoclassical toroidal viscosity in tokamaks with small magnetic perturbations

“…The radial electric field amplitude was varied between f e ≡ E r /E r0 = −20 to +20. Note that the poloidal Mach number of the E × B flow, M p = E r /(B p v th ), is at most 0.1, so the nonlinear dependence of neoclassical viscosity on the E × B speed when M p > 1 [25], discussed in the analysis of neoclassical poloidal viscosity in biasing experiments in many heliotron/stellarator devices such as [26][27][28] and references therein, does not matter in this benchmark. The profiles of NTV calculated from FORTEC-3D and Park's combined analytic formula are shown in figure 2.…”

Observation of multi-scale turbulence and non-local transport in LHD plasmas

“…37 Main issue of this electrode biasing experiment itself is the control of the J Â B driving force for …”

Observations of sustained phase shifted magnetic islands from externally imposedm/n = 1/1 RMP in LHD

“…If the poloidal flow can be externally varied, the phase of the magnetic island can be also arbitrarily controlled, which may permit continued utilization of the island divertor concept. One possibility for controlling the poloidal flow is imposing the radial electric field E r , which has been attempted in LHD [14], where the electrode is inserted inside the last closed flux surface. The new finding of the intermediate state brings a distinct paradigm shift in which the magnetic island can be moved and maintained in a partial position of the phase.…”