Multi-pin Langmuir probe measurement for identification of blob propagation characteristics in the Large Helical Device

Abstract In order to investigate the blobby plasma transport in the Large Helical Device, we have measured electrostatic fluctuations around the divertor leg by using a newly-designed multi-pin reciprocating Langmuir probe. Near the low-field side edge of the divertor leg, positive spikes of ion saturation current fluctuation were observed. In addition, the electric field which correlates with the ion saturation current fluctuation was firstly evaluated with the neighboring floating potential measurement. Considering the positional relationship with the magnetic geometry, the identified direction of the electric field inside the blobs is consistent with the theoretically predicted E × B motion. By applying the conditional averaging method, a quantitative speed of the blobs was preliminary estimated.