Radio‐frequency‐sheath‐driven edge plasma convection and interaction with the H mode

Abstract

It is shown that radio‐frequency (rf) antenna sheaths can bias the edge plasma potential and drive steady‐state convective cells in the scrape‐off layer (SOL). The resulting E×B convective flow opposes the direction of the sheared flow in the SOL induced by the radially decaying Bohm sheath potential. A two‐dimensional fluid simulation shows that the interaction of the opposing poloidal flows produces secondary vortices, which connect the edge of the confined plasma to the antenna limiters, when the antenna–plasma separation is typically of order a few times the local electron skin depth at the antenna. Estimates for typical tokamak edge parameters suggest that the transit time of particles and energy across these vortices is rapid enough to cause the broadening of SOL density and temperature profiles observed during high‐power heating with ion cyclotron range of frequency (ICRF) antennas in monopole phasing. Radio‐frequency‐sheath‐driven convection is also a good candidate to explain the phasing dependen...