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Magnetization and critical currents of NbTi wires with fine filaments

Description: In high energy accelerators such as the SSC, the magnetization of the superconductor is an important component in determining the harmonic fields at injection (approx.0.3T). In an effort to reduce these residual fields, interest has focused on NbTi conductors with fine filaments which are expected to have a reduced magnetization as dictated by the critical state model. With this in view, the magnetization and critical currents were measured at 4.3K for a set of NbTi wires with filament diameters, d, ranging from 1.0 to 5.0 microns. The data show that, although the magnetization scales linearly with d, it does not do so with the product J/sub c/d for d less than 3 ..mu..m. However, at these d values, the critical transport current density, J/sub c/ of NbTi was observed to decrease rapidly as a function of d. The origin of this J/sub c/ degradation and its effect on the scaling of magnetization within the framework of the critical state model is explored. We also examine the question of the observed asymmetry of the hysteretic magnetization.
Date: January 1, 1985
Creator: Ghosh, A.K. & Sampson, W.B.
Partner: UNT Libraries Government Documents Department

Design and expected performance of a fast scintillator hadron calorimeter

Description: A typical pulse from the 807 calorimeter is shown. This was generated by 4 GeV electrons but the pulses from hadrons and at different energies are not significantly different. The width and shape of this pulse comes from the convolution of a number of sources: (a) The time spread of energy deposition by a shower including time of flight of slow protons and neutrons, (b) scintillator phosphor rise and decay times, (c) shifter rise and decay times, (d) phototube response, (e) time delays in the light collection from different parts of the calorimeter and time dispersion in transmission. The objective of the first phase of this study was to isolate these spearate contributions, estimate how they could be speeded up and find what costs are involved. In the second phase we constructed an extremely crude calorimeter whose pulses should have the same characteristic as in a real device. With this we have observed signals whose mean width was 7 nsec and whose width at 10% of maximum height was 15 nsec. Clipping could reduce these widths to 6 and 12 nsec respectively. We conclude that gate times of less than 20 nsec would be appropriate for such a calorimeter.
Date: January 1, 1983
Creator: Palmer, R.B. & Ghosh, A.K.
Partner: UNT Libraries Government Documents Department

Sensitized photoelectrolysis of water with sunlight. Progress report, June 1--August 31, 1977. [119 references]

Description: The subject of photoelectrolysis of water at illuminated semiconductor electrodes to produce hydrogen is reviewed from 1839 up to the present. 119 literature references are cited. Critical interpretations of important concepts in this area of investigation are given. Techniques for improving the response of cells are discussed, and the expected maximum efficiency is calculated. Additionally, some preliminary experimental results with impurity sensitization of TiO/sub 2/ electrodes and heterostructure formation are presented.
Date: September 1, 1977
Creator: Ghosh, A.K. & Maruska, H.P.
Partner: UNT Libraries Government Documents Department

Sensitized photoelectrolysis of water with sunlight. Final report, June 1, 1977-December 31, 1978

Description: A study was made of solar driven water photoelectrolysis cells employing semiconductor electrodes. An extensive review of the literature was undertaken, and the three major problem areas for these devices were identified: corrosion, poor sunlight absorption, and external bias requirement. Although many semiconductors had been tested, none had proven free of all three defects. Two approaches were thus followed for the experimental studies: impurity sensitization of wide band gap stable oxides, and heterostructure formation between unstable sunlight absorbers and corrosion resistant oxides. Water decomposition was achieved with visible light excitation of Cr-doped TiO/sub 2/. Transport properties were studies for TiO/sub 2/ and SrTiO/sub 3/ electrodes doped with V, Cr, Mn, Fe, Co, and Ni. The correlation between bias requirement and electron affinity of oxides was identified. Performance of heterostructure electrodes was shown to be limited either by pin hole problems or by potential barriers between the valence bands.
Date: December 1, 1978
Creator: Ghosh, A.K. & Maruska, H.P.
Partner: UNT Libraries Government Documents Department

Heterostructure single-crystal silicon photovoltaic cell. Type A, semiconductor heterojunction silicon devices. Annual report, September 28, 1976-November 30, 1977

Description: Important electrical properties for the SnO/sub 2//Si heterostructure solar cell are summarized. The maximum theoretical efficiency and practically achievable efficiency are calculated. The method of fabricating the solar cell is described, and cost estimates and recommendations are given. A paper entitled SnO/sub 2//Si Solar Cells - Heterostructure or Schottky Barrier or MIS Type Device is appended which was previously abstracted for EDB. (LEW)
Date: January 1, 1977
Creator: Ghosh, A.K.; Feng, T. & Fishman, C.
Partner: UNT Libraries Government Documents Department

Quench propagation and training in simulated superconducting magnet windings

Description: Training behavior similar to that which occurs in full scale superconducting accelerator magnets has been observed in small test windings. The test coils are formed from approximately 20 meters of conductor wound non-inductively, in Bifilar fashion. The resulting racetrack shaped coil is molded at elevated temperature to simulate the construction techniques used for the ISABELLE dipoles. The quench current of such windings has been measured as a function of applied field and the effect of parameters such as mechanical loading and porosity have been investigated. The velocity of propagation of the normal front has been measured both along and transverse to the direction of current flow for several test windings. The minimum energy required to produce a self propagating normal zone has also been determined in an attempt to quantify the relative stability of the coils.
Date: January 1, 1981
Creator: Sampson, W.B.; Garber, M. & Ghosh, A.
Partner: UNT Libraries Government Documents Department


Description: High current-density Nb{sub 3}Sn strands made by internal-tin routes are not stable against flux jumps at low fields. Since flux jumps release heat, they can initiate quenching if thermal conductivity to the liquid helium is poor. To make matters worse, tin is a potent contaminant of copper, and reaction of strands to maximize performance leads to the loss of thermal conductivity. We discuss how the root of a solution of this problem lies in optimizing two parameters, RRR and J{sub c}, instead of J{sub c} alone. An important workaround for magnet designers is controlling the balance between performance and stability by reducing the temperature or time of the final heat treatment step. This provides ample J{sub c} while also keeping RRR high. Under these conditions, the instability current density threshold J{sub s} is higher than J{sub c}. Additional factors are also available to improve the management of instabilities, including new strand designs with smaller subelements or divided subelements.
Date: April 3, 2006
Creator: COOLEY, L. & GHOSH, A.
Partner: UNT Libraries Government Documents Department

Thin film polycrystalline silicon solar cells. Quarterly report No. 1, January 1, 1979-March 31, 1979

Description: A theory capable of predicting the performance of polycrystalline silicon solar cells is formulated. It relates grain size to mobility, lifetime, diffusion length, reverse saturation current, open circuit photovoltage and fill factor. Only the diffusion lengths measured by the surface photovoltage technique for grains less than or equal to 5 ..mu..m do not agree with our theory. The reason for this discrepancy is presently being investigated. We conclude that grains greater than or equal to 100 ..mu..m are necessary to achieve efficiencies greater than or equal to 10 percent at AM1 irradiance. The calculations were performed for the case of no grain boundary passivation. At present we are investigating the improvements to be expected from grain boundary passivation. We have determined that the parameters that best fit the available data are as follows: (1) Number of surface states at grain boundaries acting as recombination centers - 1.6 x 10/sup 13//cm/sup 2/. (2) Capture cross section - 2 x 10/sup -16/ cm/sup 2/. (3) Surface recombination velocity at grain boundary - 3.2 x 10/sup 4/ cm/sec. The following types of solar cells are considered in the model: SnO/sub 2//Si Heterostructure, MIS, and p/n junction. In all types of solar cells considered, grain boundary recombination plays a dominant role, especially for small grains. Though the calculations were originally expected to yield only order of magnitude results, they have proven to be accurate for most parameters within 10 percent.
Date: January 1, 1979
Creator: Ghosh, A.K.; Feng, T.; Maruska, H.P. & Fishman, C.
Partner: UNT Libraries Government Documents Department

Magnetization, critical current, and injection field harmonics in superconducting accelerator magnets

Description: The very large energy ratio of machines such as the SSC dictates rather low injection field (for 6T, 20 TeV it is approximately 0.3T). Since the harmonic content at such low fields is largely determined by magnetization currents in the superconductor, the random errors depend on the uniformity of the superconducting wire. In principle the magnitude of the residual fields can be reduced indefinitely by using finer filaments, but in practice there is a lower limit of a few microns. We have compared the injection field harmonics for a number of accelerator dipoles with magnetization measurements made on samples of the conductor used to wind the coils. In addition both the magnetization and harmonics have been compared with short sample critical current measurements made at 5T. The results indicated that an accurate estimate of the variation in injection field harmonics can only be obtained from direct measurements of the magnetization of the cable. It appears feasible to use such measurements to ''shuffle'' magnets for a large accelerator by predicting the low field properties of a magnet before actually winding the coils. 10 refs., 4 figs., 2 tabs.
Date: January 1, 1985
Creator: Ghosh, A.K.; Sampson, W.B. & Wanderer, P.
Partner: UNT Libraries Government Documents Department

Magnetization and critical currents of tin-core multifilamentary Nb sub 3 Sn conductors

Description: This paper presents critical current and magnetization data for some multifilamentary Nb{sub 3}Sn wires that have been produced by the internal-tin method. A comparison of magnetization and transport critical current measurements show that filament bridging during heat treatment is a common occurrence leading to effective filament diameters that are sometimes an order of magnitude larger than the geometrical filament size. At present, J{sub c}'s (in the non-copper region) greater than 1300 A/mm{sup 2} at 10T have been achieved in some conductors, which also exhibit high losses. Low losses have only been seen in conductors with a high local ratio of niobium to copper. Also the use of (Nb-1%Ti) alloy instead of pure Nb helps to reduce low field loss and increase high field J{sub c}. Measurements of the temperature dependence of hysteretic loss to 5T indicate that loss decreases linearly with increasing temperature. 22 refs., 6 figs., 2 tabs.
Date: January 1, 1990
Creator: Ghosh, A.K. & Suenaga, M.
Partner: UNT Libraries Government Documents Department

Normal state resistance and low temperature magnetoresistance of superconducting cables for accelerator magnets

Description: The normal state resistivity of the superconducting NbTi cable used in accelerator magnets is usually specified by the resistance per unit length at room temperature (295 K) and the residual resistance ratio (RRR). Using these resistance parameters, the amount of copper in the multifilamentary wire can be calculated. This method is consistent with the traditional etch and weigh technique, and as such is a alternative and convenient way of specifying the copper to superconductor ratio. In principle the magnetoresistance can be calculated from the RRR and the ''Kohler Plot'', for copper. In practice however, measurements of magnetoresistance for a wide variety of SSC inner cables show considerable disagreement with calculation. In this paper the magnetoresistance data on cables with RRR ranging from 50 to 175 are analyzed taking into account the conductor geometry and the effect of the small interfilamentary spacing on the resistivity of copper. 8 refs., 5 figs., 1 tab.
Date: January 1, 1988
Creator: Sampson, W.B.; Garber, M. & Ghosh, A.K.
Partner: UNT Libraries Government Documents Department

The effect of self field on the critical current determination of multifilamentary superconductors

Description: In determining the short sample critical current of conductors of large cross section or high current density the self field produced by the transport current must be taken account in order to obtain a ''true value'' for the critical current. A simple model calculation for determining this effect is described. Measurements on wires, cables, and monoliths show the validity and self consistency of the procedures. 9 refs., 13 figs., 1 tab.
Date: January 1, 1988
Creator: Garber, M.; Ghosh, A.K. & Sampson, W.B.
Partner: UNT Libraries Government Documents Department

Magnetization measurements on multifilamentary Nb/sub 3/Sn and NbTi conductors

Description: The effective filament size has been determined for a number of high current Nb/sub 3/Sn multifilamentary composites. In most cases it is much larger than the nominal filament size. For the smallest filaments (approx. 1 micron) the effective size can be as much as a factor of forty times the nominal size. Samples made by the internal tin, bronze route, and jelly roll methods have been examined with filaments in the range one to ten microns. Rate dependent magnetization and flux jumping have been observed in some cases. NbTi composites ranging in filament size from nine to two hundred microns and with copper to super-conductor ratios between 1.6:1 and 7:1 have been examined in the same apparatus. Low field flux jumping was only observed in conductors with very large filaments and relatively little stabilizing copper. 9 references, 6 figures, 3 tables.
Date: January 1, 1984
Creator: Ghosh, A.K.; Robins, K.E. & Sampson, W.B.
Partner: UNT Libraries Government Documents Department

The location of the quench origin in a superconducting accelerator magnet

Description: A method of calculating the initial rate of rise of the resistive voltage in a quenching superconducting magnet is described. Comparison of such calculations with data from spontaneously occurring quenches gives the location of the quench origin since the normal state resistance of the conductor is determined by its position in the windings due to the magnetoresistance of the copper matrix. The characteristics of the voltage buildup is used to separate quenches occurring in low field regions, such as the magnet ends, from those starting in the two-dimensional straight section of the coil. The magnitude of V dot is a measure of performance and can be used to determine if the magnet is reaching the maximum current permitted by the conductor parameters.
Date: January 1, 1987
Creator: Ghosh, A.K.; Robins, K.E. & Sampson, W.B.
Partner: UNT Libraries Government Documents Department

Quench propagation across the copper wedges in SSC dipoles

Description: The effect of copper wedges on quench propagation in SSC windings has been studied. The results indicate that the turn-to-turn quench transit time for conductors separated by an insulated copper wedge can be predicted with reasonable accuracy from the bulk quench properties and the mean wedge thickness.
Date: January 1, 1986
Creator: Ghosh, A.K.; Robins, K.E. & Sampson, W.B.
Partner: UNT Libraries Government Documents Department

Induced axial oscillations in superconducting dipole windings

Description: When superconducting accelerator magnets wound from multi-stranded conductor are energized a periodic variation appears in the magnetic field along the axis. This oscillation is present in al components of the field and has a period that is equal to the transposition pitch of the superconducting cable. Such axial variations have been observed even in windings which are not carrying any transport current. A magnetic field was applied to a portion of a dipole winding using a second magnet. Axial oscillations were induced along the total length of the windings including the portion not in the applied field. The amplitude of these oscillations varied with the amount of inert winding inside the energizing magnet and with t;he angle of the applied field. These field variations could be completely applied field. These field variations could be completely eliminated in the external portion of the coil by heating a small section of the winding above the transition temperature.
Date: December 31, 1994
Creator: Sampson, W. B. & Ghosh, A. K.
Partner: UNT Libraries Government Documents Department

The effect of magnetic impurities and barriers on the magnetization and critical current of fine filament NbTi composites

Description: It has been shown that the highest critical currents in fine filament NbTi conductors are obtained when the filaments are closely spaced. However, the filaments in a composite with a high purity copper matrix tend to be coupled via the ''proximity effect'' as the filament spacing is reduced leading to large magnetization at low fields. To reduce the ''proximity'' coupling, trial billets of NbTi have been fabricated with Cu-30% Ni matrix, Cu-0.5% Mn matrix and one with a Ni barrier around the filaments. Wires drawn from these billets have shown that this interfilament coupling can be suppressed for very small filament separations. A full sized commercial billet with Cu-0.5% Mn matrix demonstrates that high J/sub c/ conductors with filament size approx.1 to 5 ..mu..m can be fabricated which show no interfilament coupling at low fields.
Date: January 1, 1987
Creator: Ghosh, A.K.; Sampson, W.B.; Gregory, E.; Kreilick, S. & Wong, J.
Partner: UNT Libraries Government Documents Department

Cold flow modeling of pulverized coal combustors for magnetohydrodynamic channel applications

Description: This report describes an experimental program and techniques for studying the internal aerodynamics of pulverized coal combustors of the type used in magnetohydrodynamic test trains at The University of Tennessee Space Institute. The combustors are modeled with small scale, cold flow models that permit both flow visualization and velocity field surveys to be performed. Water was selected as the working fluid so that the model flow fields had the same Reynolds number as the actual reactive combustors, and also to facilitate flow visualization. The systems used for flow visualization and velocity field surveying are described in detail. The velocity field survey equipment is based on a vector-velocity, laser doppler velocimeter coupled to a controllable field scanning device and a microprocessor for on-line data reduction. Results are presented that were obtained from a laser velocimeter study of recirculating flows in a combustor model. The results show that, even for exceedingly simple geometrical arrangements of oxidant injector configurations, complex three dimensional highly turbulent flow fields exist in the combustor. A brief discussion of the impact of the results on fuel injector positioning is presented.
Date: March 1, 1984
Creator: Schulz, R.J.; Giel, T.V.; Ghosh, A. & Morris, R.D.
Partner: UNT Libraries Government Documents Department

Seismic response of rock joints and jointed rock mass

Description: Long-term stability of emplacement drifts and potential near-field fluid flow resulting from coupled effects are among the concerns for safe disposal of high-level nuclear waste (HLW). A number of factors can induce drift instability or change the near-field flow patterns. Repetitive seismic loads from earthquakes and thermal loads generated by the decay of emplaced waste are two significant factors. One of two key technical uncertainties (KTU) that can potentially pose a high risk of noncompliance with the performance objectives of 10 CFR Part 60 is the prediction of thermal-mechanical (including repetitive seismic load) effects on stability of emplacement drifts and the engineered barrier system. The second KTU of concern is the prediction of thermal-mechanical-hydrological (including repetitive seismic load) effects on the host rock surrounding the engineered barrier system. The Rock Mechanics research project being conducted at the Center for Nuclear Waste Regulatory Analyses (CNWRA) is intended to address certain specific technical issues associated with these two KTUs. This research project has two major components: (i) seismic response of rock joints and a jointed rock mass and (ii) coupled thermal-mechanical-hydrological (TMH) response of a jointed rock mass surrounding the engineered barrier system (EBS). This final report summarizes the research activities concerned with the repetitive seismic load aspect of both these KTUs.
Date: June 1, 1996
Creator: Ghosh, A.; Hsiung, S.M. & Chowdhury, A.H.
Partner: UNT Libraries Government Documents Department


Description: As one of the activities of the US-LHC Accelerator Project, BNL is testing short samples of superconducting cables that will be used in the main LHC dipoles and quadrupoles. The purpose of these tests is to verify that the reels of superconducting cables as supplied by the vendors meet the required critical current specifications. The short-sample testing facility and the computer-assisted testing techniques for acquiring the data will be described. We also describe the data analysis, data storage, and data transmission methods.
Date: March 29, 1999
Partner: UNT Libraries Government Documents Department

Thin film polycrystalline silicon solar cells

Description: During the present quarter efficiency of heterostructure solar cells has been increased from 13 to 13.7% for single crystal and from 10.3 to 11.2% for polysilicon. For polysilicon the improvements can be attributed to reductions in grid-area coverage and in reflection losses and for single crystal to a combination of reduction in grid-area coverage and increase in fill factor. The heterostructure cells in both cases were IT0/n-Si solar cells. Degradation in Sn0/sub 2//n-Si solar cells can be greatly reduced to negligible proportions by proper encapsulation. The cells used in stability tests have an average initial efficiency of 11% which reduces to a value of about 10.5% after 6 months of exposure to sunlight and ambient conditions. This small degradation occurs within the first month, and the efficiency remains constant subsequently. The reduction in efficiency is due to a decrease in the open-circuit voltage only, while the short-circuit current and fill factor remain constant. The effects of grain-size on the Hall measurements in polysilicon have been analyzed and interpreted, with some modifications, using a model proposed by Bube. This modified model predicts that the measured effective Hall voltage is composed of components originating from the bulk and space-charge region. For materials with large grains, the carrier concentration is independent of the inter-grain boundary barrier, whereas the mobility is dependent on it. However, for small rains, both the carrier density and mobility depend on the barrier. These predictions are consistant with experimental results of mm-size Wacker polysilicon and ..mu..m-size NTD polysilicon.
Date: January 1, 1980
Creator: Ghosh, A. K.; Feng, T.; Eustace, D. J. & Maruska, H. P.
Partner: UNT Libraries Government Documents Department

Flux creep in multifilamentary conductors of NbTi and Nb sub 3 Sn

Description: Long term decay of the sextupole field has been observed in accelerator dipole magnets, and this effect has been linked to flux creep'' in the superconductor. To study this problem, the decay of the magnetization of multifilamentary conductors of NbTi and Nb{sub 3}Sn have been measured as a function of time. Measurements show that as a function of ln{ital t} this decay cannot always be characterized by a single decay rate. Long time decay rates are sometimes approximately half that which is observed at short times < 1000 secs. Creep rates are found to: (1) scale with filament diameter, (2) change slowly with field, (3) change rapidly as H is backed off from the critical state and (4) is fairly insensitive to temperature for 0.3T{sub c} < T < 0.8T{sub c}. Results of this investigation are compared to sextupole field decay observed in magnets. 13 refs., 5 figs., 3 tabs.
Date: January 1, 1989
Creator: Ghosh, A.K.; Xu, Youwen & Suenaga, M.
Partner: UNT Libraries Government Documents Department