Observation of Oscillatory Radial Electric Field Relaxation in a Helical Plasma

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.

Figure 1

Schematic view of the TJ-II magnetic geometry and experimental setup. The positions of the pellet injection line and the two HIBP systems in equal cross sections of two consecutive periods are shown. The upper plot shows a top view of one-half of the toroidal magnetic structure of the TJ-II stellarator. The magnetic axis is shown in red, and a section of the last closed flux surface is shown in gray. The inset shows the cross section of several flux surfaces at the poloidal location of the pellet injector. These and the cross sections at the location of the HIBPs are shown in a front view in the lower plot.

Figure 2

Evolution of plasma parameters following a pellet injection into the TJ-II stellarator. Top: Line-averaged electron density and $H_{\alpha }$ pellet monitor. Center: Plasma electrostatic potential at two radial locations from the HIBP diagnostic. Bottom: Radial electric field in two peripheral radial locations from Doppler reflectometry [18]. In the central plots, the smoothed HIBP signals are shown in lighter blue and fits to a model damped oscillation are shown in red.

Figure 3

Time traces of the $H_{\alpha }$ ablation monitor in a pellet injection (top) and of the electrostatic potential in a Rosenbluth-Hinton gyrokinetic numerical simulation and in a pellet-induced transient in the TJ-II stellarator (bottom).

Figure 4

Comparison of measured and simulated oscillation frequencies and damping rate for several discharges and radial positions. Measurements obtained with the HIBP I and II are labeled with circles and squares, respectively. The gray rectangle encompasses the values obtained from the simulated zonal potential relaxation for several radii in the range $0.3\le \rho \le 0.9$ and five different experimental profile sets representative of the discharges.