Particle pinch in the tokamak edge*

Particle pinch, a process in which electrons/ions are somehow transported up (rather than down) the plasma pressure gradient, is a fundamental and yet not fully understood process in plasma physics, partially in the tokamak edge. An understanding of this process is vital not only because it would enable the capability to predict density profiles from first-principle theory, but also it is a key step to untangle some of the longstanding problems in tokamak physics, e.g., pedestal formation in the LH transition, density limit, absence of particle transport barrier in I-mode, and so on. Recently, a significant progress on the particle pinch research in the tokamak edge has been made by using GDB model – a drift-reduced Braginskii based 3D electromagnetic turbulence model which can self-consistently evolve both plasma and flow quantities in full annulus (including both closed flux region and the scrape-off-layer). A robust particle pinch has been observed in flux driven 3D global edge simulations when a narrow particle source is located near the last closed flux surface. The inward particle flux, accompanied by outward heat flux, is carried by fluctuating ExB drift in these fully nonlinear simulations. With a simplified 1D linear model, toroidal ITG mode is identified as the possible dominate pinch driver, even in the negative density gradient (hence, negative η_e and η_i) range. Moreover, the neoclassical effect, namely Pfirsch-Schluter ion heat flux is found to have a significant impact on the final steady-state density profile.