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Although there are excellent estimates of ages of terrains on Mars from crater counting, even a few absolute ages would serve to validate the calibration. Results with uncertainties, although much larger than those that could be achieved in labs on Earth, would be extremely valuable. While there are other possibilities for in situ geochronology instruments, we describe here two alternative technologies, being developed in JPL. There are two common features of both. The first is analysis by means of miniature mass spectrometer. The second is use of laser sampling to reduce or avoid sample handling, preparation and pre-treatment and equally importantly, to allow analysis of individual, texturally resolved minerals in coarse-grained rocks. This textural resolution will aid in selection of grains more or less enriched in the relevant elements and allow construction of isochrons for more precise dating. Either of these instruments could enable missions to Mars and other planetary bodies.

The hollow cathode discharge plasma in ion engines is highly non-uniform, geometrically complex, and confined by a magnetic field; elucidating the mechanisms responsible for producing ions with anomalously high energies observed in the downstream regions of hollow cathodes will contribute to understanding the behavior of complex plasmas. Ions with energies in excess of ten times greater than the energy associated with electrostatic acceleration from the largest steady-state potential difference in the plasma discharge of ion engines have been detected.

Distributions of ion energy-per-charge were collected with an electrostatic energy analyzer directed along various lines-of-sight in the near-plume region of a hollow cathode operating in a laboratory discharge chamber. While the analyzer was stationary, the cathode and anode assembly were mounted on a rotary and a linear stage, allowing measurements along the cathode axis and at off-axis angles up to 90°; the linear stage enabled translational motion which allowed for measurements parallel to the cathode axis. At angles below 45°, three distinct ion populations were identified: two low-energy narrow peaks and a broad high-energy “tail” with energies-per-charge in excess of 5 times the discharge voltage. Detection of low-energy ions above 45° was not possible since they were confined by the anode voltage. At large angles, the intensity of the high-energy ion signal decreased to become indistinguishable from noise in the data system; however, based on previous results, the failure to detect high-energy ions at large angles has been attributed to the large distance between the ESA line-of-sight and the cathode axis increased from 0° to 16°, the location of the low-energy ion peaks shift but there is no apparent difference in the high-energy ion tail. Increasing the separation angle further yields a decrease in intensity at all energies. For low energies-per-charge, the shape and intensity of the unidirectional ion-energy distribution does not change when the cathode is translated up to 2.5cm; however, as the distance from the cathode centerline increases, the signal intensity of the high-energy ions decreases in a manner similar to the decrease in high-energy ions seen with increasing angles above 30°. These results indicate that magneto-gas dynamic forced did not accelerate the ions observed in this experiment; additional experiments will elucidate the mass and energy dependence of the mechanisms responsible for high-energy ions.

The effect of tungsten erosion, transport and redeposition on the operation of dispenser hollow cathodes was investigated in detailed examinations of the discharge cathode inserts from an 8200 hour and a 30,352 hour ion engine wear test. Erosion and subsequent redeposition of tungsten in the electron emission zone at the downstream end of the insert reduces the porosity of the tungsten matrix, preventing the flow of barium from the interior. This inhibits the interfacial reactions of the barium-calcium-aluminate impregnant with the tungsten in the pores. A numerical model of barium transport in the internal xenon discharge plasma shows that the barium required to reduce the work function in the emission zone can be supplied from upstream through the gas phase. Barium that flows out of the pores of the tungsten insert is rapidly ionized in the xenon discharge and pushed back to the emitter surface by the electric field and drag from the xenon ion flow. This barium ion flux is sufficient to maintain a barium surface coverage at the downstream end greater than 0.6, even if local barium production at that point is inhibited by tungsten deposits. The model also shows that the neutral barium pressure exceeds the equilibrium vapor pressure of the impregnant decomposition reaction over much of the insert length, so the reactions are suppressed. Only a small region upstream of the zone blocked by tungsten deposits is active and supplies the required barium. These results indicate that hollow cathode failure models based on barium depletion rates in vacuum dispenser cathodes are very conservative.

Ion energy measurements from a ¼ inch cathode were made with krypton and xenon expellants at a range of flow rates and discharge currents using an electrostatic energy analyzer mounted on axis. High energy ion content scaled strongly with discharge current and inversely with mass flow rate and molecular mass. At high current conditions for krypton and xenon peak ion energies exceeded 130eV (>17kms⁻¹) and 75eV (>10kms⁻¹) with high energy tails extending well beyond 140eV and 200eV respectively. No conclusive correlation was round with discharge noise in the form of high frequency voltage oscillations and high energy ion content.

Hollow cathodes that operate with impurities in the xenon propellant exhibit poisoning of the emitter surface, resulting in a higher surface work function and increased operating temperatures. The goal of this paper is to provide insight Into the physical processes responsible for cathode degradation during short-duration oxygen poisoning over a range of discharge currents and surface temperatures. The experimental results presented here show that the cathode is more susceptible to poisoning at higher oxygen concentration levels, lower discharge currents, and lower initial insert temperatures. The threshold current at which the emitter surface becomes susceptible to oxygen poisoning was determined to be between 8.24 A and 10.17 A for an oxygen concentration level of 10 ppm, and between 10.17 A and 12.35 A for oxygen concentration levels of 50 and 100 ppm. To determine the threshold surface temperature, further work must be conducted to isolate the effects of xenon ion sputtering and surface temperature on cathode operation during poisoning.

Low energy sputtering yields at grazing incidence have been investigated experimentally using a quartz crystal microbalance (QCM) technique. This method involved precoating the QCM with a thin film of the desired target material and relating the resonance frequency shift directly to mass loss during ion bombardment. A highly focused, low divergence ion beam provided a well defined incidence angle. Focusing most of the ion current on the center of the target allowed for higher sensitivity by taking into account the radial mass sensitivity of the QCM. Measurements of Mo, Cu, and W sputtering yields were taken for low energy (80–1000 eV) Xe+ and Ar+ to validate this experimental method. The target films ranged from 3.5 to 8.0 µm in thickness and were deposited so that their crystal structure and density would match those of the bulk material as closely as possible. These properties were characterized using a combination of scanning electron microscope imagery, profilometry, and x-ray diffraction. At normal incidence, the sputtering yields demonstrated satisfactory agreement with previously published work. At angles of incidence up to 40° off normal, the data agreed well with predictions from existing theoretical models. Sputtering yields were found to increase by a factor of 1.6 over this range. The optimum angle for sputtering occurred at 55°, after which the yields rapidly decreased. Measurements were taken up to 80° from the surface normal.

Life-limiting processes in hollow cathodes are determined largely by the temperature of the electron emitter. To support cathode life assessment, a noncontact temperature measurement technique which employs a stepper motor-driven fiber optic probe was developed. The probe is driven inside the hollow cathode and collects light radiated by the hot interior surface of the emitter. Ratio pyrometry is used to determine the axial temperature profile. Thermocouples on the orifice plate provide measurements of the external temperature during cathode operation and are used to calibrate the pyrometer system in situ with a small oven enclosing the externally heated cathode. The diagnostic method and initial measurements of the temperature distribution in a hollow cathode are discussed.

Magnetoplasmadynamic thrusters (MPDTs) are coaxial devices that utilize Lorentz forces to accelerate a plasma discharge. Such devices are capable of processing high power (from 100s or kW to multi-MW per device) and are or interest for future spacecraft propulsion applications. However, the performance of these thrusters is limited by the "onset" instability condition, in which radial pinch forces deplete the anode region plasma, resulting In high anode fall voltages and excessive erosion beyond a characteristic current level. A new MPDT was developed utilizing a multi-channel hollow cathode with an extended anode and multi-channel hollow cathode. The thruster was tested in the "quasi-steady" mode with argon propellant at 1.5, 1.0, and 0.6 g/s mass now rates. The thruster was operated initially in the self-generated magnetic field mode, and data are presented on the voltage-current response characteristics to identify early indicators or transition to the MPD "onset" instability in this geometry. This work serves to prepare for later experiments to compare results with various externally-applied magnetic fields to investigate the influence of the magnetic field on the characteristic current for the onset condition.

Few long-term multicenter investigations have evaluated the relationships between aggregate antimicrobial drug use in hospitals and bacterial resistance. We measured fluoroquinolone use from 1999 through 2003 in a network of US hospitals. The percentages of fluoroquinolone-resistant Pseudomonas aeruginosa and methicillin-resistant Staphylococcus aureus (MRSA) were obtained from yearly antibiograms at each hospital. Univariate linear regression showed significant associations between a hospital's volume of fluoroquinolone use and percent resistance in most individual study years (1999-2001 for P. aeruginosa, 1999-2002 for S. aureus). When the method of generalized estimating equations was used, a population-averaged longitudinal model incorporating total fluoroquinolone use and the previous year's resistance (to account for autocorrelation) did not show a significant effect of fluoroquinolone use on percent resistance for most drug-organism combinations, except for the relationship between levofloxacin use and percent MRSA. The ecologic relationship between fluoroquinolone use and resistance is complex and requires further study.