The Distribution of Ion Energies Incident on AN Icrh Antenna Faraday Shield.

The Faraday shield of an ion cyclotron resonant heating (ICRH) antenna has been identified as a local source of impurities during high-power ICRH experiments on confinement devices. The understanding of the plasma sheath formation at the shield surface and its effect on the distribution of ion energies bombarding the shield is essential in determining the usefulness of ICRH as a way to provide auxiliary heating for the next generation of fusion devices. In the thesis, plasma properties and ion energies were measured in the near field of an ICRH antenna to determine the effects of rf fields in a magnetized plasma sheath on the energy of ions incident on the surface of the Faraday shield. A resonant loop antenna with a two-tier Faraday shield was used on the RF Test Facility at Oak Ridge National Laboratory. The time-varying floating potential was measured with a capacitive probe and the time-averaged floating potential, electron temperature, and density were measured with a Langmuir probe. Both probes were scanned in front of the antenna in the poloidal direction. A gridded energy analyzer located just below the antenna was used to measure the ion energy distribution and surface collector probes were used to measure ion energies on the shield surface. The measurements indicate that the plasma potential and electron temperature in front of the antenna clearly increase with antenna current and near field strength, and follow the rf magnetic field pattern of the antenna, indicating that the electromagnetic field is responsible for the potential formation. A computational model of a magnetized sheath with a time-varying sheath potential was developed to predict the distribution of ion energies at the shield surface, using measured data as input. The model results indicate that there will be a broadening in the distribution when the rf driving frequency approaches the local ion cyclotron frequency near the antenna. The broadening results in an increased population of ions with energies close to the maximum plasma potential. The model results agree with measured ion energy distributions from the energy analyzer. The experimental and computational results of this thesis strongly indicate that the impurity generation seen with high-power ICRH is a result of a change in the sheath potential and an increased electron temperature due to the plasma interaction with the antenna near field.