Concurrent optimization technique for a controlled structure

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76192/1/AIAA-1994-1631-489.pd

Analysis of the mutual inductance between two parallel plates for the detection of surface flaws

There has recently been much effort behind the development of NDE methods applicable to the detection of surface/subsurface flaws in thin metallic structures with a rapid scan capability. One such method, an electromagnetic technique using a current-sheet parallel to the surface of a specimen in order to induce eddy current flow shows a high potential for satisfying the rapid scan requirement stated above. The technique is based on the detection of flaw-induced magnetic field components normal to the specimen surface by an appropriate detection mechanism positioned above the current-sheet as shown schematically in Fig. 1. As indicated in this figure, the current-sheet separates the source of the normal magnetic field components from the detector in such a way that the electric and magnetic properties of the current-sheet can be a major factor affecting the strength of the detected signals. The purpose of the present study is, therefore, to perform a detailed investigation on the effect of the material properties of the current-sheet on the detected signal strength and to establish a simple theoretical model for the detection mechanism

Atomic Hydrogen Cleaning of Polarized GaAs Photocathodes

Atomic hydrogen cleaning followed by heat cleaning at 450$^\circ$C was used to prepare negative-electron-affinity GaAs photocathodes. When hydrogen ions were eliminated, quantum efficiencies of 15% were obtained for bulk GaAs cathodes, higher than the results obtained using conventional 600$^\circ$C heat cleaning. The low-temperature cleaning technique was successfully applied to thin, strained GaAs cathodes used for producing highly polarized electrons. No depolarization was observed even when the optimum cleaning time of about 30 seconds was extended by a factor of 100

Flux focusing eddy current probe

A flux-focusing electromagnetic sensor which uses a ferromagnetic flux-focusing lens simplifies inspections and increases detectability of fatigue cracks and material loss in high conductivity material. The unique feature of the device is the ferrous shield isolating a high-turn pick-up coil from an excitation coil. The use of the magnetic shield is shown to produce a null voltage output across the receiving coil in the presence of an unflawed sample. A redistribution of the current flow in the sample caused by the presence of flaws, however, eliminates the shielding condition and a large output voltage is produced, yielding a clear unambiguous flaw signal. The maximum sensor output is obtained when positioned symmetrically above the crack. Hence, by obtaining the position of the maximum sensor output, it is possible to track the fault and locate the area surrounding its tip. The accuracy of tip location is enhanced by two unique features of the sensor; a very high signal-to-noise ratio of the probe's output which results in an extremely smooth signal peak across the fault, and a rapidly decaying sensor output outside a small area surrounding the crack tip which enables the region for searching to be clearly defined. Under low frequency operation, material thinning due to corrosion damage causes an incomplete shielding of the pick-up coil. The low frequency output voltage of the probe is therefore a direct indicator of the thickness of the test sample

PARMELA VS MEASUREMENTS FOR GTF AND DUVFEL

The particle-pushing PARMELA was used to design the photo-injector beamline of the Linac Coherent Light Source (LCLS) to be built at SLAC in 2005. PARMELA predicts that projected emittances smaller than 1.2 mm.mrad and slice emittance smaller than 1.0 mm.mrad will be achievable for 1nC, 10ps electron bunches with an S-band RF gun and an emittance compensating system. To benchmark PARMELA, comparisons between simulations and measurements for two photo-injector test facilities, the Gun Test Facility (GTF) at SLAC and the Deep Ultra Violet FEL (DUVFEL) at BNL, have been performed. Aspects of the modeling of fields and initial distributions are discussed. The agreement between measured and simulated beam parameters (projected and slice emittance, Twiss parameters) is satisfying. Accordingly, it gives credibility to the extrapolation made for studying the LCLS case. PARMELA also indicates possible improvements in the tuning of those facilities to achieve the LCLS required beam properties.

Empirical comparison of high gradient achievement for different metals in DC and pulsed mode

For the SwissFEL project, an advanced high gradient low emittance gun is under development. Reliable operation with an electric field, preferably above 125 MV/m at a 4 mm gap, in the presence of an UV laser beam, has to be achieved in a diode configuration in order to minimize the emittance dilution due to space charge effects. In the first phase, a DC breakdown test stand was used to test different metals with different preparation methods at voltages up to 100 kV. In addition high gradient stability tests were also carried out over several days in order to prove reliable spark-free operation with a minimum dark current. In the second phase, electrodes with selected materials were installed in the 250 ns FWHM, 500 kV electron gun and tested for high gradient breakdown and for quantum efficiency using an ultra-violet laser.Comment: 25 pages, 13 figures, 5 tables. Follow up from FEL 2008 conference (Geyongju Korea 2008) New Title in JVST A (2010) : Vacuum breakdown limit and quantum efficiency obtained for various technical metals using DC and pulsed voltage source

Start-to-end Transport Design and Multi-particle Tracking for the ILC Electron Source

A train of 2-ns micro bunches of longitudinally polarized electrons are generated in a 120-kV DC-gun based injector in the ILC electron source; a bunching system with extremely high bunching efficiency to compress the micro-bunch down to 20 ps FWHM is designed. Complete optics to transport the electron bunch to the entrance of the 5-GeV damping ring injection line is developed. Start-to-end multi-particle tracking through the beamline is performed including the bunching system, pre-acceleration, chicane, 5-GeV superconducting booster linac, spin rotators and energy compressor. It shows that 94% of the electrons from the DC-gun are captured within the damping ring 6-D acceptance--A{sub x} + A{sub y} {le} 0.09 m and {Delta}E x {Delta}z {le} ({+-} 25 MeV) x ({+-} 3.46 cm)--at the entrance of the damping ring injection line. The field and alignment errors and orbit correction are analyzed

Eye care utilization patterns in Tehran population: a population based cross-sectional study

BACKGROUND: The aim of this study is to determine eye care visits which are an indicator of eye care service utilization by Tehran population and its determinants. METHODS: Through a population-based, cross-sectional study, 6497 Tehran citizens were sampled. All participants had complete eye examinations and an interview regarding demographic and socioeconomic status variables, past medical and eye history, and their previous and last eye care visits. RESULTS: Among those sampled, 4565 people participated in the study (response rate of 70.3%). Among these participants, 34.7 % had never visited an ophthalmologist or optometrist (95% confidence interval [CI]: 32.4 to 36.9) and 43.2% had not seen an eye care provider in the last 5 years. Multivariate logistic regression revealed that men (OR = 1.30), younger participants (each year increase in age: OR = 0.98) and the less educated (each year increase in education: OR = 0.93) were more likely, and the visually impaired were less likely (OR = 0.41) to have neglected eye care. CONCLUSION: A large proportion of the population, including those in the high risk group who require eye care, has never utilized any eye care service. These data suggest that efforts have to be made to better understand the causes and to optimize the utilization of the available eye care services in the population