RF field measurements in the vicinity of an ICRF antenna

Measurements of the RF fields near an ICRF antenna installed in the central cell of the Phaedrus-B tandem mirror have been made, both in vacuum and in the presence of plasma (n_e < 10¹³ cm^-3, T_e < 100 eV, T_i < 100 eV). The antenna is a Faraday shielded partial (60°) turn loop. The front surface of the Faraday shield is composed of cylindrical elements in an arrangement similar to the Faraday shield design used on the Tokamak Fusion Test Reactor. The test is run at relatively low power levels (<40 kW), in the 3.5-10-MHz frequency range. Two other ICRF systems in the Phaedrus-B central cell sustain and heat the plasma at the 400-kW level. The vacuum field measurements have been compared with the predictions of the ARGUS code, which models details of the Faraday shield structure. Fields in the plasma have been modeled by the ANTENA code of Brian McVey. Particle currents collected by the Faraday shield during plasma operation have also been observed.