Unique measurements of the basic plasma-flow characteristics in a low pressure ͑р53 mPa H 2 ͒ spherically convergent ion focus are obtained using high-voltage ͑р5 kV͒ emissive and double probes. The radial plasma potential distribution agrees with a collisionless, recirculating, space-charge-limited current model. Flow convergence increases with voltage and neutral pressure and decreases with cathode grid wire spacing and current. Core radii within 4-5 times the ideal geometric limit are measured, and the observed core sizes are consistent with predictions from a multipass orbit model which includes asymmetries in the accelerating potential well. A virtual anode is observed in the converged core region, and no evidence for multiple potential well structures in the core is found. Measurements of the core ion density ͑n ic ϳ10 15 m Ϫ3 ͒ are consistent with simple flow convergence models.

ABSTRACT. The deuterium-deuterium (D-D) fusion reaction rate in a spherically convergent ion focus is observed to significantly exceed the rate predicted by a collisionless flow model. However, a careful consideration of ion-neutral collisions and the trapped neutral density in the cathode account for the extra reactivity without invoking anomalous ion trapping in the converged core region. This conclusion is supported by proton collimation measurements, which indicate that the bulk of the observed reactivity originates outside the core region. In addition, a classical flow model, where charge exchange collisional effects on the ion and fast neutral distributions are included, provides fusion rate estimates that are quantitatively consistent with the observed D-D fusion neutron production rate.

Equilibrium reconstructions of low-aspect ratio (A < 1.3) discharges in the Pegasus toroidal experiment have been performed. Magnetic diagnostics are used for equilibrium constraint and a filament code is used to estimate the significant currents flowing in the vacuum vessel walls. This technique is able to fit the global plasma parameters of plasma current and major radius to within 5%, internal inductance to within 10% and plasma pressure to within 15% as determined by Monte Carlo estimation of the uncertainties in the fit parameters. Determination of the equilibrium properties of the plasma allows an understanding of the dynamics of internal tearing modes and external ideal kink modes that limit plasma performance. Internal tearing modes were found to degrade plasma confinement when rational surfaces are located in regions of low magnetic shear early in the discharge when temperature is lower and resistivity is higher. This confinement degradation limits the maximum achievable plasma current and pressure. Disruptions with precursors growing with a time of ∼90 µs have been found to be consistent with ideal external kink modes with a hybrid growth time.

An efficient ZnTe: O x-ray powder phosphor was prepared by a dry synthesis process using gaseous doping and etching medias. The x-ray luminescent properties were evaluated and compared to standard commercial phosphors exhibited an x-ray luminescent efficiency equivalent to 76% of Gd 2 O 2 S : Tb and an equal resolution of 2.5 lines/ mm. In addition, the fast decay time, low afterglow, and superior spectral match to conventional charge-coupled devices-indicate that ZnTe: O is a very promising phosphor candidate for x-ray imaging applications.

ZnTe:O powder phosphors were successfully prepared by a dry synthesis process using gaseous doping and etching media. It was found that dry doping by O 2 through ball-milling was an effective way to synthesize ZnTe:O powder phosphors and produced a red emission centered at 680 nm with a decay time of 1.1 ms. The emission intensity of dry O 2 -doped samples was three times more intense than from ZnO-doped samples, possibly due to a more uniform distribution of oxygen substitution on tellurium sites. The samples annealed in a 95% N 2 /5% H 2 forming gas atmosphere exhibited a x-ray luminescent efficiency five times higher than did powders annealed in vacuum or N 2 atmosphere. This enhancement was attributed to the removal of surface tellurium oxides. ZnTe:O phosphor screens were prepared with x-ray luminescence efficiencies equivalent to 56% of ZnSe:Cu,Ce,Cl and 76% of Gd 2 O 2 S:Tb screens under 17-keV radiation. An x-ray imaging resolution of 2.5 lines/mm was resolved, the same as that measured for commercial ZnSe:Cu,Ce,Cl and Gd 2 O 2 S:Tb screens. These results indicate that ZnTe:O is a promising phosphor candidate for synchrotron x-ray imaging applications.