Experimental Evidence of Momentum Transport Induced by an Up-Down Asymmetric Magnetic Equilibrium in Toroidal Plasmas

The first experimental evidence of parallel momentum transport generated by the up-down asymmetry of a toroidal plasma is reported. The experiments, conducted in the Tokamak à Configuration Variable, were motivated by the recent theoretical discovery of ion-scale turbulent momentum transport induced by an up-down asymmetry in the magnetic equilibrium. The toroidal rotation gradient is observed to depend on the asymmetry in the outer part of the plasma leading to a variation of the central rotation by a factor of 1.5–2. The direction of the effect and its magnitude are in agreement with theoretical predictions for the eight possible combinations of plasma asymmetry, current, and magnetic field.

Figure 1

CXRS spectra measured at three radial location (top plots) for the $s^{+}$ (red) and $s^{-}$ (blue) configurations ($b^{-}{j}^{+}$) shown in the bottom left and right plots (poloidal flux contours). The toroidal rotation profiles of the carbon impurity (full line) and main ions (dashed line) are shown in the middle bottom plot. Positive values: clockwise viewed from above.

Figure 2

Left: Electron temperature (top) and density (bottom) from Thomson scattering. Right: Carbon temperature (top) and density (bottom) from CXRS. The profiles are shown for the $s^{+}$ (red) and $s^{-}$ (blue) cases of Fig. 1. The vertical dashed line indicates the sawtooth inversion radius.

Figure 3

Left: Variation of the intrinsic rotation gradient ${\widehat{u}}_i^{\prime }(s^{+})-{\widehat{u}}_i^{\prime }(s^{-})$ of deuterium assessed from linear GKW simulations for ${\rho }_{{\psi }_1}$ (blue), ${\rho }_{{\psi }_2}$ (red), and ${\rho }_{{\psi }_3}$ (black) as a function of the wave vector $k_{\theta }{\rho }_i$. Right: Corresponding mode growth rate $\gamma$ (full line) and real frequency ${\omega }_R$ (dashed line), negative values meaning in the electron diamagnetic drift direction.