Ion Loss as an Intrinsic Momentum Source in Tokamaks

A series of coupled experiments in DIII-D and simulations provide strong support for the kinetic loss of thermal ions from the edge as the mechanism for toroidal momentum generation in tokamaks. Measurements of the near-separatrix parallel velocity of D$^+$ with Mach probes show a 1-2 cm wide D$^+$ parallel velocity peak at the separatrix reaching 40-60 km/s, up to half the thermal velocity, always in the direction of the plasma current. The magnitude and width of the velocity layer are in excellent agreement with a first-principle, collissionless, kinetic computation of selective particle loss due to the loss cone [1] including for the first time the measured radial electric field, $E_r$ in steady state. C$^{6+}$ rotation in the core, measured with charge exchange recombination (CER) spectroscopy is correlated with the edge D$^+$ velocity. XGC0 computations [2], which include collisions and kinetic ions and electrons, show results that agree with the measurements, and indicate that two mechanisms are relevant: 1) ion orbit loss and 2) a growing influence of the Pfirsch-Schluter mechanism in H-mode gradients. The inclusion of the measured $E_r$ in the loss-cone model [1] drastically affects the width and magnitude of the velocity profile and improves agreement with the Mach probe measurements. A fine structure in $E_r$ is found, still of unknown origin, featuring large (10-20 kV/m) positive peaks in the SOL and at, or slightly inside, the separatrix of low power L- or H-mode conditions. This high resolution probe measurement of Er agrees with CER measurements where the techniques overlap. The flow is attenuated in higher collisionality conditions, consistent with a depleted loss-cone mechanism. \vskip6pt \noindent [1] J.S.\ deGrassie et al., Nucl.\ Fusion {\bf 52}, 013010 (2011).\par \noindent [2] C.S.\ Chang et al., Phys.\ Plasmas {\bf 11}, 5626 (2004).