Floating potential measurements in the near field of an ion cyclotron resonance heating antenna

Abstract

Large variations in the plasma potential can lead to large sheath potentials at the surface of the Faraday shield of an ion cyclotron resonance heating (ICRH) antenna. This sheath can accelerate ions to energies where sputtering is significant, resulting in impurity generation. The time-varying floating potential is being measured in the near field of an ICRH antenna by using the Radio Frequency Test Facility (RFTF) at Oak Ridge National Laboratory. The antenna used is a resonant loop antenna that has a two-tier Faraday shield with a layer of graphite on the outer tier. The electron cyclotron heated plasma in RFTF is produced by a 10.6-GHz klystron. The magnetic field at the antenna is ∼2 kG, with a plasma density of ∼1010 cm−3 with an electron temperature of ∼6 eV. Ionization by rf power from the antenna produces a local plasma of ∼1011 with an electron temperature of 12 to 20 eV. The rf floating potential is measured by using a capacitively coupled probe that is scanned in front of the antenna, parallel to the current strap. A stationary Langmuir probe is used to measure the time-averaged floating potential. Measurements indicate that the potential scales with the antenna current and increases with plasma density to values of up to 300 V peak-to-peak. The rf component of the floating potential seems to follow the near field pattern of the antenna, indicating that these fields may be responsible for the potential formation.