Experimental evidence of increased electron temperature, plasma potential, and ion energy near an ICRF antenna Faraday shield

Abstract

Plasma properties and ion energies have been measured near the surface of the Faraday shield for an ICRF antenna to determine the effect of RF fields on the energy of ions bombarding the shield surface. A resonant loop antenna with a two-tier Faraday shield was used on the RF Test Facility at Oak Ridge National Laboratory. The ECH plasma was produced by a 10.6 GHz klystron and had a central density of ∼10 11 cm −3 and an electron temperature of 5–10 eV. The static magnetic field at the antenna was ∼ 2 kG. A capacitive probe was used to measure the time-varying floating potential and a Langmuir probe was used to measure the time-averaged electron temperature, density, and DC floating potential. Both probes were scanned in front of the antenna. Ion energies were measured with a gridded energy analyzer located next to the antenna. The time-varying floating potential followed the magnetic field pattern of the antenna, indicating that the electromagnetic field is responsible for the potential formation. Potential values of 300 V p-p have been measured. Electron temperatures increased with RF power and reached values ≥60 eV for an RF power of ∼25 kW. The energy of ions hitting a grounded surface also increased with RF power and exceeded 300 eV at ∼25 kW of RF power (compared with energies of 5–15 eV with no RF power). The increase in ion energy can be at least partially explained by an increase in the sheath potential, which in turn is caused by an increase in the electron temperature.