Particle-in-cell simulations of parallel dynamics of a blob in the scrape-off-layer plasma of a generic medium-size tokamak

Abstract

Abstract In this work we investigate, in a fully-kinetic particle approach, the evolution of a blob inside a flux tube in the scrape-off-layer (SOL) plasma of a generic diverted medium-size tokamak and its contribution to the recorded particle and energy flux densities at the divertor. Initially, a steady-state background plasma is reached, with realistic plasma profiles in the parallel direction. On top of it, a blob source with given intensity and temperature is superimposed at the outer midplane. The blob source is active for 10 μ s in order to emulate a generic blob with 1 cm diameter crossing the magnetic flux tube with a typical 1 km s−1 radial drift velocity. We have observed that most of the blob’s hot electrons are screened off by the cold electrons of the SOL plasma and can travel towards the two divertors free of the initial ambipolar forces. Performing a density scan and having a blob-to-background density ratio n b l o b / n b k g , S O L ≈ 5 in the source region, we observe that due to Coulomb collisions these hot electrons can reach the outer divertor or also the inner one, depending on the density of the background SOL plasma. Consequently, the midplane–divertor potential difference increases and the sheath expands leading to a significant increase in the background ion energy flux density onto the divertor. We also observe that when the blob ions reach the divertor the electron flux density on the divertor increases due to ambipolar forces. Through Coulomb collisions, a part of the thermal energy of the hot blob ions is transferred to the main (background) plasma, depending on the plasma density. Most of the energy deposited into the divertor is brought by the ions and the blob-induced background ion energy flux density becomes comparable (low-density case) or much higher (high-density case) than the blob ion energy flux density.