Theory of mean E × B shear in a stochastic magnetic field: ambipolarity breaking and radial current

Abstract

The mean E×B shear in a stochastic magnetic field is calculated, using the radial force balance relation and transport equations. This analysis is relevant to the L→H transition with resonant magnetic perturbations (RMP), and special focus is placed upon the physics of non-ambipolar transport and radial current. The key physical process is the flow of fluctuating current along wandering magnetic fields. The increments in poloidal and toroidal rotation, density and ion pressure are calculated. The radial envelope of the magnetic perturbations inside the plasma defines a new scale l_env, which is the characteristic scale of the magnetic fluctuation intensity profile. The net particle outflow due to stochastic magnetic fields is calculated and is determined by the net radial current through the separatrix. Implications for the L→H transition are discussed.

Gas puff imaging measurements during resonant magnetic perturbations on the HL-2A tokamak

“…Since the electrons thermal velocities are much faster than massive ions, the loss of electrons along the disturbed magnetic field lines is much more than ions, which charges the plasma edge positively. Recent comprehensive theoretical investigation gives a detailed physical process in which the stochastic RMP driven magnetic fields produce a radial electron current density, which drives particle transport [23]. We now try to find experimental evidence to verify this explanation.…”

Quasi-mode evolution in a stochastic magnetic field

“…(i) One may notice that zonal flow, a critical player in L-H transition, is missing from our model. In fact, it has been found that the stochastic magnetic field can indeed affect the zonal flow and the radial electric field [8,55]. Many phenomena in RMP experiments, such the increase in the L-H transition power threshold, can be attributed to the weakening of the shear flow.…”

“…However, as turbulence evolution and transport bifurcation now happen in a background stochastic field, an increase in the L-H transition power threshold has been observed on multiple machines [4][5][6]. To get insight into the tripartite trade-off among confinement, heating power, and boundary control, models of turbulence dynamics [7], zonal flow evolution [8], and L-H transition [9] have been reformulated in the presence of extrinsic stochasticity. All these theories are either based on or closely relevant to a fundamental question: how does an ambient stochastic magnetic field modify plasma turbulence and the underlying instability process?…”