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Laboratory scale tests of electrical impedence tomography

Description: Electrical impedance tomographs (magnitude and phase) of known, laboratory-scale targets are reported. Three methods are used to invert electrical impedance data and their tomographs compared. The first method uses an electrical resistance tomography (ERT) algonthm (designed for DC resistivity inversion) to perform impedance magnitude inversion and a linearized perturbation approach (PA) to invert the imaginary part. The second approximate method compares ERT magnitude inversions at two frequencies and uses the frequency effect (FE) to compute phase tomographs. The third approach, electrrcal impedance tomography (EIT), employs fully complex algebra to account for the real and imaginary components of electrical impedance data. The EIT approach provided useful magnitude and phase images for the frequency range of 0.0625 to 64 Hz; images for higher frequencies were not reliable. Comparisons of the � ERT and EIT magnitude images show that both methods provided equivalent results for the water blank, copper rod and PVC rod targets. The EIT magnitude images showed better spatial resolutron for a sand-lead mixture target. Phase images located anomalies of both high and low contrast IP and provided better spatial resolution than the magnitude images. When IP was absent from the data, the EIT algorithm reconstructed phase values consistent with the data noise levels.
Date: December 1, 1998
Creator: Binley, A; Daily, W; LaBredcque, D & Ramirez, A
Partner: UNT Libraries Government Documents Department

Impedance Scaling and Impedance Control

Description: When a machine becomes really large, such as the Very Large Hadron Collider (VLHC), of which the circumference could reach the order of megameters, beam instability could be an essential bottleneck. This paper studies the scaling of the instability threshold vs. machine size when the coupling impedance scales in a ``normal`` way. It is shown that the beam would be intrinsically unstable for the VLHC. As a possible solution to this problem, it is proposed to introduce local impedance inserts for controlling the machine impedance. In the longitudinal plane, this could be done by using a heavily detuned rf cavity (e.g., a biconical structure), which could provide large imaginary impedance with the right sign (i.e., inductive or capacitive) while keeping the real part small. In the transverse direction, a carefully designed variation of the cross section of a beam pipe could generate negative impedance that would partially compensate the transverse impedance in one plane.
Date: June 1, 1997
Creator: Chou, W. & Griffin, J.
Partner: UNT Libraries Government Documents Department

Electrical impedance tomography of the 1995 OGI gasoline release

Description: Electrical impedance tomography (EIT) was used to image the plume resulting from a release of 378 liters (100 gallons) of gasoline into a sandy acquifer. Images were made in 5 planes before and 5 times during the release, to generate a detailed picture of the spatial as well as the temporal development of the plume as it spread at the water table. Information of the electrical impedance (both in phase and out of phase voltages) was used or several different frequencies to produce images. We observed little dispersion in the images either before or after the gasoline entered the acquifer. Likewise, despite some laboratory measurements of impedances, there was no evidence of a change in the reactance in the soil because of the gasoline.
Date: October 1, 1996
Creator: Daily, W. & Ramirez, A.
Partner: UNT Libraries Government Documents Department

Longitudinal coupling impedance of a hole in the accelerator beam pipe

Description: In the design of modern accelerators, an accurate estimate of coupling impedance is very important. The sources which give rise to coupling impedance are the geometric discontinuities in the accelerator beam pipe. In various discontinuities such as RF cavities, bellows, and collimators, the coupling impedance of the holes has not been well understood. Although coupling impedance can be obtained in general from the Fourier transform of the corresponding wake potential which may be obtained numerically, this is time consuming and requires a large amount of computer storage when applied to a small dimension of a discontinuity in a typical beam pipe, often imposing a fundamental limitation of the numerical approach. More fundamentally, however, numerical calculation does not have the predictive power because of limited understanding of how the coupling impedance of a hole should behave over a wide frequency range. This question was studied by developing a theoretical analysis based on a variational method. An analytical formula for the coupling impedance of a hole is developed in this work using a variational method. The result gives good qualitative agreements with the coupling impedances evaluated numerically from the Fourier transform of the wake potential which is obtained from the computer code MAFIA-T3. The author shows that the coupling impedance of a hole behaves quite similar to the impedance of an RLC-resonator circuit. Important parameters used to describe such a resonator circuit are the resonant frequency and bandwidth. The author provides a theoretical insight on how to parameterize properly the numerical impedance of a hole when data exhibit complicated dependence on frequency. This is possible because one can show that the parameters are a function of the dimensionless quantity kd alone, with k the free-space wave number and d the radius of hole.
Date: December 1993
Creator: Chae, Yong-Chul
Partner: UNT Libraries Government Documents Department

Experimental study of coupling impedance: Part I longitudinal impedance measurement techniques

Description: Beam coupling impedances for the 7-GeV APS storage ring have been numerically estimated. In order to confirm these calculations, measurements of the coupling impedance of various vacuum components around the main storage ring were done with a coaxial wire method. In this paper, the procedure of the longitudinal impedance measurement techniques will be described. As an example, sections of the Cu beam chamber, the Cu beam + antechambers, and the Al beam + antechambers were used as a device under test (DUT) to obtain the results. The transverse impedance measurements will be described in a separate paper.
Date: October 22, 1991
Creator: Song, J.J.
Partner: UNT Libraries Government Documents Department

Coupling impedance of a long slot and an array of slots in a circular vacuum chamber

Description: We find the real part of the longitudinal impedance for both a small hole and a long slot in a beam vacuum chamber with a circular cross section. The slot can be arbitrarily long; the only requirement on the dimensions of the slots is that its width be much smaller than c/w. Regular array of N slots periodically distributed along the pipe is also considered.
Date: June 1, 1995
Creator: Stupakov, G.V.
Partner: UNT Libraries Government Documents Department

Impedance study for the PEP-II B-factory

Description: The paper summarizes results of the impedance studies of the components of the B-factory. The prime goal of this activity was to support the design of the vacuum chamber and, at the same time, to get a reasonable model of the machine impedance, which can be used later for detail studies of collective effects.
Date: June 1, 1995
Creator: Heifets, S.; Daly, C.E. & Ko, K.
Partner: UNT Libraries Government Documents Department

Updated Impedance Estimate of the PEP-II RF Cavity

Description: This paper presents an updated estimate of the higher-order mode impedance spectrum of the RF cavities for the PEP-II B-factory. The cavity is designed for continuous operation at 476 MHz with up to 150 kW wall dissipation and heavy beam loading. To reduce the growth rates of coupled-bunch instabilities the cavity higher-order modes are damped by three rectangular waveguides and broad-band loads. The results of detailed measurements on the first high-power cavity with all absorbers in place are presented and the damping effect due to the high-power coupler is discussed. Results are compared with earlier measurements of a cold-test model. Implications for the design of the broad-band bunch-by-bunch feedback systems and high-power HOM loads are discussed.
Date: June 1996
Creator: Rimmer, R. A.; Byrd, J.; Irwin, M. & Goldberg, D. A.
Partner: UNT Libraries Government Documents Department

Coaxial wire impedance measurements of BPM buttons for the PEP-II B- factory

Description: The coaxial wire impedance measurement uses a conducting rod placed along the beam axis in the vacuum chamber, forming the center conductor in a coaxial line system. Tapers at either end of this section allow for smooth impedance transformation from the 50{Omega} lines used in common microwave measurement equipment, to the characteristic impedance of the vacuum chamber and center conductor, typically around 200{Omega}. RF and microwave absorptive material placed in the ends of the vacuum chamber and in the impedance matching tapers minimizes reflections which cause trapped modes within the apparatus, allowing measurements to be made above the traveling-wave cut-off frequency of the vacuum vessel (typically 2.5 - 3.0 GHz for PEP-II). A smooth vessel of the same cross-section as that containing the device under test is used in a reference measurement Resonances within the apparatus are difficult to avoid completely and require careful placing of absorptive material, manufacture of test and reference chambers, and assembly of apparatus.
Date: September 1, 1995
Creator: Corlett, J.N.
Partner: UNT Libraries Government Documents Department

A potpourri of impedance measurements at the advanced photon source storage ring

Description: Machine coupling impedances were determined in the APS storage ring from measurements of the bunch length, synchronous phase, and synchrotron and betatron tunes vs single-bunch current. The transverse measurements were performed for various numbers of small gap insertion device (ID) chambers installed in the ring. The transverse impedance is determined from measurements of the transverse tunes and bunch length as a function of single-bunch current. The shift in the synchrotron tune was measured as a function of bunch current from which the total cavity impedance was extracted. The loss factor was determined by measuring the relative synchronous phase as a function of bunch current. The longitudinal resistive impedance is calculated using the loss factor dependence on the bunch length. From these results, the authors can estimate what the impedance would be for a full set of ID chambers.
Date: August 1, 1997
Creator: Sereno, N.S.; Chae, Y.C.; Harkay, K.C.; Lumpkin, A.H.; Milton, S.V. & Yang, B.X.
Partner: UNT Libraries Government Documents Department

Validation of Electrical-Impedance Tomography for Measurements of Material Distribution in Two-Phase Flows

Description: A series of studies is presented in which an electrical-impedance tomography (EXT) system is validated for two-phase flow measurements. The EIT system, developed at Sandia National Laboratories, is described along with the computer algorithm used for reconstructing phase volume fraction profiles. The algorithm is first tested using numerical data and experimental phantom measurements, with good results. The EIT system is then applied to solid-liquid and gas-liquid flows, and results are compared to an established gamma-densitometry tomography (GDT) system. In the solid-liquid flows, the average solid volume fractions measured by EIT are in good agreement with nominal values; in the gas-liquid flows, average gas volume fractions and radial gas volume fraction profiles from GDT and EIT are also in good agreement.
Date: October 16, 1998
Creator: Ceccio, S.L.; George, D.L.; O'Hern, T.J.; Shollenberger, K.A. & Torczynski, J.R.
Partner: UNT Libraries Government Documents Department

Measurement of rf voltages on the plasma-touching surfaces of ICRF antennas

Description: Measurements of the rf voltages on Faraday shields and protection bumpers have been made for several loop antennas, including the mock-up antenna and Al for JET, the original antenna for Tore Supra, the present ASDEX-U antenna, and the folded waveguide. The loop antennas show voltages that scale to {approx}12 kV for a maximum input voltage of 30 kV with 0/0 phasing. The voltages are dramatically reduced for 0/{pi} phasing. These voltages are significant in that they can substantially increase the rf sheath potential beyond the levels associated with the simple electromagnetic field linkage from the current straps that results in plasma heating. In this paper, we investigate and measure the source of these voltages, their scaling with antenna impedance, and the differences between the loop arrays.
Date: September 1995
Creator: Hoffman, D. J.; Baity, F. W.; Bell, G. L.; Bigelow, T. S.; Caughman, J. B. O.; Goulding, R. H. et al.
Partner: UNT Libraries Government Documents Department

A study of parameters useful for describing plasma-opening switches

Description: Plasma opening switches (POS) have been used continually and studied since their introduction in 1975. During that period they have performed well for prepulse suppression and sharpening the front of the power pulse. Their use for long conduction time and rapid opening to stand off high voltage in the same POS has met with very limited success. There has been a large theoretical effort involving models and particle-in-cell simulations (PICS), but the connection between theory and experiment has been tenuous at best, and convincing agreement with experiment has been minimal. The authors believe progress toward long conduction and rapid opening would be faster if macroscopic physical parameters describing the physics of the switch were used to compare experiment to simulation. One of these parameters (electron flow impedance) has been used to describe the electrical characteristics of the POS. This parameter provides a good description of both the standard POS (SPOS) and the magnetically controlled POS (MCPOS) because its value is sensibly independent of load current. An additional parameter, the effective mass of the plasma, was measured in one MCPOS experiment. In this article they describe other parameters important to operation of the SPOS and the MCPOS, and parameters important in designing PICS used to study these devices.
Date: September 1, 1998
Creator: Mendel, C.W. Jr.; Seidel, D.B. & Rosenthal, S.E.
Partner: UNT Libraries Government Documents Department

Development of an electrical impedance tomography system for an air-water vertical bubble column

Description: Because the components of a multiphase flow often exhibit different electrical properties, a variety of probes have been developed to study such flows by measuring impedance in the region of interest. Researchers are now using electric fields to reconstruct the impedance distribution within a measurement volume via Electrical Impedance Tomography (EIT). EIT systems employ voltage and current measurements on the boundary of a domain to create a representation of the impedance distribution within the domain. The development of the Sandia EIT system (S-EIT) is reviewed The construction of the projection acquisition system is discussed and two specific EIT inversion algorithms are detailed. The first reconstruction algorithm employs boundary element methods, and the second utilizes finite elements. The benefits and limitations of EIT systems are also discussed. Preliminary results are provided.
Date: September 1, 1995
Creator: O`Hern, T.J.; Torczynski, J.R.; Ceccio, S.L.; Tassin, A.L.; Chahine, G.L.; Duraiswami, R. et al.
Partner: UNT Libraries Government Documents Department

Quantitative tomographic measurements of opaque multiphase flows

Description: An electrical-impedance tomography (EIT) system has been developed for quantitative measurements of radial phase distribution profiles in two-phase and three-phase vertical column flows. The EIT system is described along with the computer algorithm used for reconstructing phase volume fraction profiles. EIT measurements were validated by comparison with a gamma-densitometry tomography (GDT) system. The EIT system was used to accurately measure average solid volume fractions up to 0.05 in solid-liquid flows, and radial gas volume fraction profiles in gas-liquid flows with gas volume fractions up to 0.15. In both flows, average phase volume fractions and radial volume fraction profiles from GDT and EIT were in good agreement. A minor modification to the formula used to relate conductivity data to phase volume fractions was found to improve agreement between the methods. GDT and EIT were then applied together to simultaneously measure the solid, liquid, and gas radial distributions within several vertical three-phase flows. For average solid volume fractions up to 0.30, the gas distribution for each gas flow rate was approximately independent of the amount of solids in the column. Measurements made with this EIT system demonstrate that EIT may be used successfully for noninvasive, quantitative measurements of dispersed multiphase flows.
Date: March 1, 2000
Partner: UNT Libraries Government Documents Department

Fault current limiter-predominantly resistive behavior of a BSCCO shielded-core reactor

Description: Tests were conducted to determine the electrical and magnetic characteristics of a superconductor shielded core reactor (SSCR). The results show that a closed-core SSCR is predominantly a resistive device and an open-core SSCR is a hybrid resistive/inductive device. The open-core SSCR appears to dissipate less than the closed-core SSCR. However, the impedance of the open-core SSCR is less than that of the closed-core SSCR. Magnetic and thermal diffusion are believed to be the mechanism that facilitates the penetration of the superconductor tube under fault conditions.
Date: June 30, 2000
Creator: Ennis, M. G.; Tobin, T. J.; Cha, Y. S. & Hull, J. R.
Partner: UNT Libraries Government Documents Department

Calculation of longitudinal and transverse impedances of Ferrite-50 beampipe insert for Cornell B-Factory. Final report

Description: As outlined in M Tigner`s letter to D. Prosnitz (dated 5/16/91), our task was to calculate the longitudinal monopole (m = 0) and the transverse dipole (m = 1) impedances if a simple structure consisting of a beam pipe with a recessed ferrite (Ferrite-50) ring. In support of this goal we also undertook to compute the impedance of a small scale model for which impedance measurements had been made by L. Walling, now of SSCL. To carry out the required calculations we used the AMOS wakefield code together with a recently developed physics package that models dispersive media in the time domain. In conclusion, we found reasonable agreement with Walling`s measurements on the scale model for {ital Z}{sub {parallel}0} and for Re[{ital Z}{sub r1}]. However, there is a qualitative difference between experiment and calculation found for Im[{ital Z}{sub r1}].
Date: December 16, 1991
Creator: DeFord, J.F. & Shang, C.C.
Partner: UNT Libraries Government Documents Department

A study of the failure mechanism of chlorine anodes

Description: Thin coating RuO{sub 2}{minus}TiO{sub 2} electrodes, which mimic the DSA anodes, have been prepared and tested for their activity toward the chlorine evolution reaction and subjected to life time testing. Rutherford Backscattering Spectrometry has been used concurrently with electrochemical measurements to analyze changes in the ruthenium content of the coating. The decrease in electrode activity is found to be closely related to a decrease in Ru content, and the measured profiles indicate that the loss takes place across the thin coating. Failure is observed for electrodes with a Ru content below a critical concentration, but there is no evidence for the build up of a pure TiO{sub 2} layer. AFM imaging of an anode after failure sustained the hypothesis of loss of material.
Date: December 31, 1995
Creator: Vallet, C.E.; Zuhr, R.A.; Tilak, B.V. & Chen, C.P.
Partner: UNT Libraries Government Documents Department

Network asymptotics for high contrast impedance tomography

Description: Fluid contaminant plumes underground are often electrically conducting and, therefore, can be imaged using electrical impedance tomography. The authors introduce an output least-squares method for impedance tomography problems that have regions of high conductivity surrounded by regions of lower conductivity. The high conductivity is modeled on network approximation results from an asymptotic analysis and its recovery is based on this model. The smoothly varying part of the conductivity is recovered by a linearization process as is usual. The authors present the results of several numerical experiments that illustrate the performance of the method.
Date: May 1, 1996
Creator: Borcea, L.; Papanicolaou, G.C. & Berryman, J.G.
Partner: UNT Libraries Government Documents Department

W, WSi{sub x} and Ti/Al low resistance OHMIC contacts to InGaN, InN and InAlN

Description: W, WSi{sub 0.44} and Ti/Al contacts were examined on n{sup +} In{sub 0.65}Ga{sub 0.35}N, InN and In{sub 0.75}Al{sub 0.25}N. W was found to produce low specific contact resistance ({rho}{sub c} {approximately} 10{sup {minus}7} {Omega} {center_dot}cm{sup 2}) ohmic contacts to InGaN, with significant reaction between metal and semiconductor at 900 {degrees}C mainly due to out diffusion of In and N. WSi{sub x} showed an as-deposited {rho}{sub c} of 4{times}10{sup {minus}7} {Omega} {center_dot}cm{sup 2} but this degraded significantly with subsequent annealing. Ti/Al contacts were stable to {approximately} 600 {degrees}C ({rho}{sub c} {approximately} 4{times}10{sup {minus}7} {Omega} {center_dot}cm{sup 2} at {le}600 {degrees}C). The surfaces of these contacts remain smooth to 800 {degrees}C for W and WSi{sub x} and 650 {degrees}C for Ti/Al. InN contacted with W and Ti/Al produced ohmic contacts with {rho}{sub c} {approximately} 10{sup {minus}7} {Omega} {center_dot}cm{sup 2} and for WSi{sub x} {rho}{sub c} {approximately} 10{sup {minus}6} {Omega} {center_dot}cm{sup 2}. All remained smooth to {approximately} 600 {degrees}C, but exhibited significant interdiffusion of In, N, W and Ti respectively at higher temperatures. The contact resistances for all three metalization schemes were {ge} 10{sup {minus}4} {Omega} {center_dot}cm{sup 2} on InAlN, and degrades with subsequent annealing. The Ti/Al was found to react with the InAlN above 400 {degrees}C, causing the contact resistance to increase rapidly. W and WSi{sub x} proved to be more stable with {rho}{sub c} {approximately} 10{sup {minus}2} and 10{sup {minus}3} {Omega} {center_dot}cm{sup 2} up to 650 {degrees}C and 700 {degrees}C respectively.
Date: June 1996
Creator: Vartuli, C. B.; Pearton, S. J.; Abernathy, C. R.; MacKenzie, J. D.; Shul, R. J.; Zolper, J. C. et al.
Partner: UNT Libraries Government Documents Department

Low-power, high-speed InGaAs/InP photoreceiver for highly-parallel optical data links

Description: Low-power photoreceivers based on InGaAs/InP heterojunction bipolar transistors (HBTs) and p-i-n diodes for highly-parallel optical data links have been designed, fabricated and characterized. The receivers and designed to operate from 980 nm to over 1.3 {mu}m and interface directly with 3.3 V CMOS. SPICE was utilized to investigate circuit topographies that minimize power dissipation while maintaining large signal operation required to interface directly with CMOS. Low-power dissipation of {approximately}10 mW/channel has been achieved at bit rates up to 800 Mbits/sec. Performance characteristics of discrete HBTs and of low-power photoreceivers fabricated with p-i-n/HBT circuits are reported.
Date: September 1, 1995
Creator: Lovejoy, M.L.; Patrizi, G.A. & Enquist, P.M.
Partner: UNT Libraries Government Documents Department

Photoconductive semiconductor switches for firing sets and electro-optic modulators

Description: Optically activated GaAs switches operated in their high main mode are being used or tested for pulsed power applications as diverse as low impedance, high current pursers, and high impedance, low current Pockels cell or Q switch drivers. These are important to firing sets in munitions, lasers used in detonation of munitions, and lasers used in large weapons effects simulators (such as Jupiter). For firing, sets we have switched 2.8 kA at 3 kV dc charge in a very compact package. For driving Q switches, the load is the small (30 pF) capacitance of the Q switch which is charged to 6 kV. We have demonstrated that we can modulate a laser beam with a sub ns risetime. Some aspects of the switches that are relevant to most of these applications are lifetime (longevity), leakage resistance, jitter, and trigger energy. This paper will describe the specific project requirements and switch parameters in all of these applications, and emphasize the switch research and development that is being pursued to address the important issues.
Date: July 1, 1995
Creator: Loubriel, G.M.; Zutavern, F.J.; Baca, A.G.; Hjalmarson, H.P.; Helgeson, W.D. & O`Malley, M.W.
Partner: UNT Libraries Government Documents Department

Comparison of gamma-densitometry tomography and electrical-impedance tomography for determining material distribution in liquid-solid flows

Description: The spatial distribution of materials in multiphase flows is of importance to many industrial processes. For example, in indirect coal liquefaction, a reactive gas is bubbled through a catalyst-laden liquid (slurry), and a spatially nonuniform gas distribution can reduce process efficiency by inducing large-scale buoyancy-driven recirculating flows. Gamma-densitometry tomography (GDT) and electrical-impedance tomography (EIT) are techniques with the potential of providing spatially resolved information on material distribution in multiphase flows. GDT and EIT have both been applied to a liquid-solid flow for comparison purposes. The experiment consisted of a cylinder (19 cm diameter) filled with water, in which 80 {micro}m glass spheres were suspended by a mixer to achieve solid volume fractions of 0.01, 0.02, and 0.03. Both GDT and EIT revealed a relatively uniform distribution of solids in the measurement plane, and the average solid volume fractions from both techniques were in good agreement.
Date: March 1, 1997
Creator: Shollenberger, K.A.; Torczynski, J.R.; O`Hern, T.J.; Adkins, D.R.; Ceccio, S.L. & George, D.L.
Partner: UNT Libraries Government Documents Department

Advanced tomographic flow diagnostics for opaque multiphase fluids

Description: This report documents the work performed for the ``Advanced Tomographic Flow Diagnostics for Opaque Multiphase Fluids`` LDRD (Laboratory-Directed Research and Development) project and is presented as the fulfillment of the LDRD reporting requirement. Dispersed multiphase flows, particularly gas-liquid flows, are industrially important to the chemical and applied-energy industries, where bubble-column reactors are employed for chemical synthesis and waste treatment. Due to the large range of length scales (10{sup {minus}6}-10{sup 1}m) inherent in real systems, direct numerical simulation is not possible at present, so computational simulations are forced to use models of subgrid-scale processes, the accuracy of which strongly impacts simulation fidelity. The development and validation of such subgrid-scale models requires data sets at representative conditions. The ideal measurement techniques would provide spatially and temporally resolved full-field measurements of the distributions of all phases, their velocity fields, and additional associated quantities such as pressure and temperature. No technique or set of techniques is known that satisfies this requirement. In this study, efforts are focused on characterizing the spatial distribution of the phases in two-phase gas-liquid flow and in three-phase gas-liquid-solid flow. Due to its industrial importance, the bubble-column geometry is selected for diagnostics development and assessment. Two bubble-column testbeds are utilized: one at laboratory scale and one close to industrial scale. Several techniques for measuring the phase distributions at conditions of industrial interest are examined: level-rise measurements, differential-pressure measurements, bulk electrical impedance measurements, electrical bubble probes, x-ray tomography, gamma-densitometry tomography, and electrical impedance tomography.
Date: May 1, 1997
Creator: Torczynski, J.R.; O`Hern, T.J.; Adkins, D.R.; Jackson, N.B. & Shollenberger, K.A.
Partner: UNT Libraries Government Documents Department