Plasma pressure and flows during divertor detachment

Abstract

MHD theory applied to tokamak plasma scrape-off layer (SOL) equilibria requires Pfirsch–Schlüter current, which, because the magnetic lines are open, normally closes through electrically conducting divertor or limiter components. During detached divertor operation the Pfirsch–Schlüter current path to the divertor target is sometimes blocked, in which case theory predicts that the plasma develops a poloidal pressure gradient around the upstream SOL and a corresponding parallel flow, in order to satisfy all the conditions of MHD equilibrium. This paper reports the only known examples of detached diverted plasma in the DIII–D tokamak with blocked Pfirsch–Schlüter current, and they show no clear SOL poloidal pressure differences. However, the predicted pressure differences are small, near the limit of detectability with the available diagnostics. In the more usual DIII–D `partially detached divertor' operation mode, the Pfirsch–Schlüter current appears to never be blocked, and no unusual poloidal pressure differences are observed, as expected. Finally, a local overpressure is observed just inside the magnetic separatrix near the X-point in both attached and detached Ohmically heated plasmas.