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Observations of impurity behavior are presented from ICRF heating experiments at 180 MHz performed over a variety of conditions on the Alcator C tokamak, using graphite limiters and stainless steel antenna Faraday shields. Spectroscopic observations revealed significant increases in metal impurity concentrations during the RF pulse, with iron levels increasing by as much as a factor of 12 at the highest RF powers (-350-400 kW). Analysis of the inferred iron source rates shows an approximately linear dependence on RF power up to 400 kW. with no clear dependence on resonance conditions or bulk plasma parameters.However, a sharp increase in the temperature in the limiter shadow region was observed during the ICRF pulse, which was well correlated with the iron influx rate. It is concluded from this and other evidence that physical sputtering of the Faraday shield due to an elevated sheath potential is the primary source of metal impurities during ICRF heating on Alcator C. The same process, occurring at the graphite limiter, is believed to be the dominant source of carbon and oxygen. Calculated sputtering yields obtained from an edge erosion code demonstrate the plausibility of this model.

The use of characteristic-line radiation from rare-earth targets bombarded by high-energy (up to 1 MeV) electron beams has been evaluated as an x-ray source for dual energy K-edge subtraction imaging of the human coronary arteries. Two characteristic-line x-ray sources, one using the split K alpha 1 and K alpha 2 lines of lanthanum excited by a high-energy electron beam and the other using the K alpha lines of barium and cerium, were studied. A Monte Carlo electron-photon simulation was used to calculate x-ray spectra and energy deposition profiles from targets of these elements bombarded by electrons in the energy range 140 keV to 1 MeV. A general dual-energy imaging model was developed that used these calculated source spectra to numerically investigate the dependence of the subtraction image signal-to-noise ratio on such factors as the ratio of K-line to x-ray continuum yield, continuum spectral shape, x-ray filtering, and detector response. A signal averaging technique for enhancing the signal-to-noise ratio was also evaluated. The results of these calculations were used to identify an optimum electron beam, target, filter, and detector configuration. A compact electron accelerator capable of providing the required electron beam parameters was designed. Calculations indicate that under ideal conditions the optimized system would be capable of imaging 2 mg/cm2 of iodine contrast agent in 20 g/cm2 of tissue with a signal-to-noise ratio of 5, a detector pixel size of 0.25 mm2, and a total image acquisition time of 10 ms. These parameters are consistent with those needed to image the human coronary arteries after an intravenous injection of iodine contrast agent. These capabilities, along with the relatively modest hardware requirements of this system, make it attractive as an x-ray source for dual energy transvenous coronary angiography.

Impurity source mechanisms which appear during the heating of the Alcator C tokamak by injection of lower hybrid frequency waves are described. Silicon is a dominant impurity in these experiments where SiC was the limiter surface material. At low values of injected power, the silicon source rate is dominated by physical sputtering. As the amount of injected power is raised, evaporation caused by the tail electron heat flux to the limiter becomes the primary source for impurities entering the plasma. Measurements of silicon line emission from the plasma as well as other central and edge parameters are presented as functions of injected radiofrequency power.