Observation of Plasma Rotation Driven by Static Nonaxisymmetric Magnetic Fields in a Tokamak

We present the first evidence for the existence of a neoclassical toroidal rotation driven in a direction counter to the plasma current by nonaxisymmetric, nonresonant magnetic fields. At high beta and with large injected neutral beam momentum, the nonresonant field torque slows down the plasma toward the neoclassical “offset” rotation rate. With small injected neutral beam momentum, the toroidal rotation is accelerated toward the offset rotation, with resulting improvement in the global energy confinement time. The observed magnitude, direction, and radial profile of the offset rotation are consistent with neoclassical theory predictions [A. J. Cole et al., Phys. Rev. Lett. 99, 065001 (2007)].

Figure 1

(a) Amplitude of $n=3$ $I$ coil current for plasmas with (b) different initial toroidal rotation. (c) Torque density ($=dL/dt$, where $L$ is the angular momentum density) evaluated at $\rho =0.8$ at the $I$ coil current turn-on time for a larger number of discharges. Note that the $y$ axis in (b) is rotation in $\mathrm{km}/\mathrm{s}$ at fixed spatial CER view ($\rho \sim 0.8$, major radius $R\sim 2.2\mathrm{m}$), while in (c) it is rotation in $\mathrm{krad}/\mathrm{s}$ at fixed $\rho$ ($\rho =0.8$). Nearly zero torque is measured for initial rotation approximately $-50\mathrm{km}/\mathrm{s}$, indicating the offset rotation at $\rho \sim 0.8$.

Figure 2

Time histories of (a) $n=3$ $I$ coil current amplitude, (b) plasma toroidal rotation at midradius, and (c) ${\beta }_N$ (suppressed-zero $y$ axis) with constant injected power and momentum for a plasma (discharge 131861) with near-zero initial rotation at midradius. (d) Radial profiles of ${\omega }_E$, and toroidal rotation of carbon ($\Omega$) and deuterium (${\Omega }_D$), at the times indicated by vertical dashed lines in (a),(b),(c) (color coded).

Figure 3

(a) Radial profiles of ${\omega }_E$ and ${\nu }_i/\epsilon$ for discharge 131408 at $t=2.25\mathrm{s}$. (b) Radial profiles of $k_c$, indicating how the experimental offset rotation profile in the deuterium flow compares to the collisionality-dependent theoretical limits of the offset rotation.

Figure 4

Time histories of the plasma toroidal rotation at midradius, for two discharges with ${\beta }_N=1.1$ (131863, gray) and ${\beta }_N=1.9$ (131400, black), and same initial rotation. Vertical dashed line marks the $n=3$ field turn-on.