Generation of DC currents by ICRF near fields in the Scrape-off Layer

Abstract DC currents flowing along magnetic field lines connected to powered ICRF antennas have been observed experimentally. Negative current is collected on the powered ICRF antenna structure, while positive current is collected by magnetically connected Langmuir probes. Models based upon a double probe configuration are tested with the 2D electrostatic PIC code OOPIC and the 2D fluid code SEM. These models can be excited in a symmetrical (two electrode in antiphase) or an asymmetrical way (one electrode). Transverse RF currents can be exchanged with surrounding flux tubes. Simulations show that only “asymmetric” drive can draw net DC currents. As in experiments, they are negative on the “driven” electrode and positive on the grounded one. An electrostatic and low frequency analytical model explains how DC current is produced. The perpendicular width of current structures L⊥ depends on the parallel length L//.