Particle pinch in global tokamak edge simulations

Abstract

The inward particle flux, or particle pinch, is routinely observed in magnetically confined fusion experiments, yet its mechanism is not fully understood. We study the particle pinch in the tokamak edge with the flux-driven global turbulence model GDB. Starting from a flat density profile fueled only near the last closed flux surface, the simulation develops a strong inward particle flux that builds a centrally peaked density profile over $O(10)$ ms. The pinch coexists with the usual outward turbulent heat transport; two distinct mechanisms carry it. In the early stage, when density and temperature gradients oppose each other ($\eta_\alpha=L_n/L_{T_\alpha}<0$), drift-wave turbulence drives the inward flux through electron thermal diffusion. In the late stage, once the density profile has flattened, a persistent inward equilibrium $E\times B$ flux, carried by the poloidally asymmetric density and electrostatic potential, drives the central peaking. While the Pfirsch--Schl\"uter neoclassical transport sets the amplitude of the up-down asymmetric density, classical theory predicts no net radial flux from this asymmetry at leading order. The observed flux flows in a separate non-ideal channel, opened by parallel resistivity, electron inertia, and electromagnetic induction in the equilibrium electron force balance. This channel shifts the equilibrium potential poloidally against the density by $\delta_s\simeq-0.08\pi$ and supplies the late-phase density build-up. These results show that a flux-driven global edge simulation can self-consistently produce a centrally peaked density profile without ad hoc assumptions, and bear on longstanding edge questions such as density pedestal formation.