ICRF induced edge plasma modification and impurity production in a tokamak

In this work, it is shown that RF voltages are induced along the sides of the Faraday shields of ICRF (ion cyclotron range of frequency) antennas and that these voltages cause modifications of plasma parameters and impurity production in the edge regions of fusion research plasmas. Specifically, in-air measurements with a capacitive probe on a model antenna and a Phaedrus-T two-strap antenna showed that RF voltages were induced along the sides of the Faraday shield when the sides were slotted. RF B field measurements with a B dot probe suggested that these voltages were induced primarily by the currents on the radial feeds of the antennas. RF-induced modifications of edge plasma parameters were found along the field lines that mapped to the Faraday shield in both the Phaedrus-B tandem mirror and the Phaedrus-T tokamak. In Phaedrus-T, when capacitive coupling between the edge plasma and the induced voltage was prevented by the attachment of boron nitride (an electric insulator) plates to the sides of the Faraday shield, both the RF-induced modifications of the edge plasma and the impurity production during ICRF were found to be greatly reduced