Measurements and effects of impurities in Tokamak plasmas

Impurities are present in tokamak plasmas and can significantly affect the behavior of the discharge. For example, if present in sufficient concentration, impurity ions can radiate a substantial fraction of the input power to the plasma. Because impurity densities and radiative losses can be derived from spectroscopic measurements, spectroscopy plays an important role in tokamak research. High power auxiliary (neutral beam and RF) heating will be necessary for reactors; thus, it is important to study impurity behavior during auxiliary heating in present tokamaks. As an example, the results of a study of the changes in impurity radiation in the 15-360 A region during ion cyclotron RF (ICRF) heating of Princeton Large Torus tokamak plasmas are presented. The following features are found: 1) At 2.0 MW RF power, emissions from all impurity species increase during the RF pulse; this is due primarily to increased impurity densities in the plasma; 2) Metallic impurity (titanium and iron) densities in the central region of the plasma can be up to 0.3% of the central electron density; 3) During both ICRF heating at 2.0 MW RF power and ohmic heating only, the power radiated by metallic impurities is a small (≅22%) fraction of the total input power to the plasma, indicating that radiation from these elements does not dominate the power balance of the discharge; 4) Impurity densities and the local radiated power during the RF pulse scale approximately linearly with RF power up to 2.8 MW; and 5) The principal source of the metallic impurity influx during ICRF heating and ohmic heating only is the ICRF antenna Faraday shields