Double ionization of helium is investigated for 60-, 120-, and 420-MeV/u (v/c =0.34-0.72)U +-ion impact. The measured double-to-single ionization ratios indicate that, even for these very high velocities, double ionization of the He target results predominantly from independent interactions of the projectile with both target electrons. It is concluded that the asymptotic high-velocity regime for one-step double ionization (i.e. , "shakeoff") has not yet been reached even for U projectiles at 420 MeV/u, and, in fact, cannot be reached for projectiles with q 7, thereby verifying that the ionic charge is fully as important as the velocity in determining the importance of a given ionization mechanism.PACS number(s): 34.50.Fa

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This effort examines the spectroscopy of the second spectrum of the iodine atom (I II) in order to determine one, or more, useful transitions for laser-induced fluorescence of an accelerated atomic iodine singly charged ion (I + ). While the second spectrum of iodine has been analyzed, it is not particularly well characterized. Nor has it been studied substantially within a plasma such as those of interest to the spacecraft propulsion community. Our goal is to examine the spectral data available in the literature and determine transitions suitable for development into diagnostics tools, such as laserinduced fluorescence (LIF), to examine the plasma acceleration within an electro-static plasma propulsion thruster. While xenon remains the preferred propellant for electrostatic spacecraft propulsion, a number of alternative propellants are being analyzed in various laboratories. Some of the propellants that have been investigated in the recent literature include krypton, bismuth, and iodine. Of these alternative propellant candidates, iodine is the least well investigated. However, due to its close mass (127 versus 131 amu) compared to xenon, it has strong potential for use as an electro-static propulsion propellant. Iodine's benefits include a solid density of 4.9 g/cc, a low boiling point of 183 • C. Compared to xenon storage density of 1.2 g/cc at 2,000 psi, or the bismuth boiling point of 1,564 • C, there appear to be system level advantages to iodine fueled electrostatic spacecraft propulsion. This effort focuses on the development of a laser-induced fluorescence diagnostic tool for the iodine ion.

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