Abstract
Measurements of the relaxation of a zonal electrostatic potential perturbation in a nonaxisymmetric magnetically confined plasma are presented. A sudden perturbation of the plasma equilibrium is induced by the injection of a cryogenic hydrogen pellet in the TJ-II stellarator, which is observed to be followed by a damped oscillation in the electrostatic potential. The waveform of the relaxation is consistent with theoretical calculations of zonal potential relaxation in a nonaxisymmetric magnetic geometry. The turbulent transport properties of a magnetic confinement configuration are expected to depend on the features of the collisionless damping of zonal flows, of which the present Letter is the first direct observation.
Highlights
Measurements of the relaxation of a zonal electrostatic potential perturbation in a nonaxisymmetric magnetically confined plasma are presented
A sudden perturbation of the plasma equilibrium is induced by the injection of a cryogenic hydrogen pellet in the TJ-II stellarator, which is observed to be followed by a damped oscillation in the electrostatic potential
That structure consists of a strong magnetic field that is everywhere tangent to a family of nested tori, the so-called flux surfaces
Summary
The nonlinear generation of zonal flows by drift-wavetype turbulence is known to be robust [2], but the question of whether those flows were strongly damped in toroidal geometry and unable to tame turbulent transport troubled the fusion community for some time [3]. In their seminal work [4], Rosenbluth and Hinton showed that long-wavelength zonal flows were not completely suppressed by collisionless Landau-type damping in axissymmetric toroidal devices like tokamaks. Their collisionless time evolution is different from that in tokamaks, in that they exhibit a low-frequency oscillation—different from the geodesic acoustic mode observed in tokamaks—that
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