Precise Field Measurements Research Articles

Development of a precise magnetic field measurement system for fast-changing magnetic fields

Several recent applications for fast ramped magnets have been found that require precise measurement of the time-dependent fields. In one instance, accelerator dipoles will be ramped at 1 T/sec, with measurements needed to the typical level of accuracy for accelerators, /spl Delta/B/B better than 0.01%. To meet this need, we have begun development of a system containing 16 stationary pickup windings that will be sampled at a high rate. It is hoped that harmonics through the decapole can be measured with this system. Precise measurement of the time-dependent harmonics requires that both the pickup windings and the voltmeters be nearly identical. To minimize costs, printed circuit boards are being used for the pickup windings and a combination of amplifiers and ADC's for voltmeters. In addition, new software must be developed for the analysis. The paper will present a status report on this work.

Remote sensing of CO2 and H2O emission rates from Masaya volcano, Nicaragua

Although CO 2 and H 2 O account for more than 90 mol% of volcanic gases, the rates at which these gases are emitted from volcanoes are difficult to determine because of their high atmospheric background levels. We report the first precise field measurements of volcanic CO 2 , and H 2 O, in addition to HCl, HF, and SO 2 , in the plume of Masaya volcano, Nicaragua, a basaltic volcano with a record of Plinian activity. The molar ratios for CO 2 : SO 2 (2.3–2.5) and H 2 O: SO 2 (66–69) observed in February–March 1998 and March 1999 show no significant variation over the 12 month period. The molar composition of the gas is similar to other basaltic arc volcanoes in Central America. Emission rates of SO 2 from the summit crater, determined by correlation spectroscopy, averaged 21 kg s −1 during the study periods, indicating CO 2 , H 2 O, HCl, and HF emission rates of 32–36, 380–420, 7.0–7.8, and 0.86–0.95 kg s −1 , respectively. At these rates it takes only a few years to emit the equivalent volatiles associated with Masaya9s prehistoric Plinian eruptions.

Characterization of a large superconducting split pair magnet for swept field measurements

A Hall probe was inserted into the magnet of an optically detected magnetic resonance (ODMR) spectrometer to study residual field at zero current, homogeneity, and hysteretic effects. Residual fields were on the order of 10 mT at zero current. The magnet was homogeneous to within one part in 8000 or better over a region of 2.5 cm along its vertical axis. Several hysteresis loops were studied for field sweeps. The largest shifts were about 10 mT. Using the information collected, recommendations are made on how to improve precise field measurements for ODMR.

Monitoring of a water-cooled coil of a hybrid magnet by means of precise measurements of field and resistance

It is found that the deterioration of a water-cooled resistive coil was usually preceded by a decrease in electrical resistance accompanied by a decrease in a coil constant (i.e. the ratio of the magnetic field to current). Methods for precise resistance and field measurements are described for use in forecasting coil burnt-out. Plots of coil constant versus current for several magnets verify the technique used. >

Floristic diversity in a model system using experimental microcosms

Most investigations of floristic diversity have involved studies1–15 of natural vegetation. Progress using these approaches has been limited because some potentially important factors are not amenable to precise field measurement or manipulation. Here we describe an alternative research strategy in which communities were allowed to develop in turf microcosms providing factorial combinations of soil heterogeneity, grazing and mycorrhizal infection, all of which are capable in theory6,10,13,16–19 of promoting diversity. Both grazing and mycorrhizas increased diversity markedly by raising the biomass of the subordinate species relative to that of the canopy dominant. The effect of grazing is shown to be due to the differential sensitivity of the canopy dominant to defoliation. Export of assimilate from canopy to subordinate species through a common mycelial network is likely, together with enhancement of mineral nutrient capture, to be involved in the beneficial effect of mycorrhizas. No major effects of soil heterogeneity upon diversity were detected.

Highly precise field measurement of the Topaz magnet at the Tristan collider

The magnetic field distribution of the TOPAZ detector for the 30 × 30 GeV e +e − colliding beam experiment at TRISTAN, has been measured very precisely over a volume of 2.3 m in diameter and 4.2 m in length. This paper will describe the details of the reconstruction procedure of the field distribution from the measured data and the results of a comparison with a POISSON estimation. An accuracy of 10 −4 in the measurement of B ⊥/ B z has been achieved.

Report on the Production Magnet Measurement System for the Fermilab Energy Saver Superconducting Dipoles and Quadrupoles

The measurement system and procedures used to test more than 900 superconducting dipole magnets and more than 275 superconducting quadrupole magnets for the Fermilab Energy Saver are described. The system is designed to measure nearly all parameters relevant to the use of the magnets in the accelerator including maximum field capability and precision field measurements. The performance of the instrumentation with regard to precision, reliability, and operational needs for high volume testing will be described. Previous reports have described the measurement system used during development of the Saver magnets from which this system has evolved.

SPASYN-an electromagnetic relative position and orientation tracking system

Two relatively remote independent body coordinate frames are related in both position and orientation (six degrees of freedom) using precise electromagnetic field measurements. Antenna triads are fixed in each body frame. Variously polarized excitations in one body are correlated with signals detected in the remote body. Near-field and far-field processing strategies are presented with applications.

Effects of drought on grain growth

DROUGHT severely decreases grain yield in many cereal growing regions and the resulting fluctuations in world food supply have serious repercussions1. Crop breeders and physiologists are therefore studying varietal characteristics that may enable cereals to yield more consistently in drought conditions2. Very few precise field measurements show how drought affects grain growth, but procedures have been advocated to provide a more precise description of grain growth and a more rigorous examination of sources of assimilate used for grain growth3–5. We have used these procedures to analyse how the droughts of 1975 and 1976 affected grain growth of wheat crops in England and to define particular physiological traits which govern yield.

Relationships of Labor Costs to Selected Variables in Flue-Cured Tobacco Production

Since 1960, data from “typical” enterprise budgets have been used extensively in several studies dealing with a wide variety of problems. Budgets were compiled in cost-of-production studies by Green, Pierce and Williams, Coutu and Mangum and the North Carolina Agricultural Extension Service. Without surprise, extensive use of these budgets has focused attention on their limitations.More extensive and precise field measurements were needed for several cost items, particularly labor costs. In addition, certain types of cost-input or cost-output relationships were implicit in such constant-type coefficients—relationships which may or may not exist in reality. It may be true, for example, that X hours of priming labor are required to harvest Y pounds of tobacco, but it might require α + 1.8X hours to harvest 2Y pounds.

Education Requirements in Surveying and Geodesy

As a result of the lack of training of scientists and engineers in modern methods of precise field measurements and in elementary geodetic phenomena, numerous engineering projects have experienced expensive errors. Administrators and planners responsible for initiating projects are so lacking in this knowledge that they are unaware of the need for geodetic consultants. The abandonment of the teaching of plane surveying and the substitution of two other courses as required subjects for all engineering and science students would remedy this situation. The courses would be: (1) Principles of precise field measurements and (2) elementary aspects of geodesy. The recommended courses could each be taught in a single quarter. The student generally familiar with these subjects would not make many of the mistakes of the past and would be aware of the occasional need for consultation with a qualified geodesist in the design and planning of new projects.

Superconductivity ofα- andβ-Mercury

Precise critical field measurements and a direct measurement of ${(\frac{\ensuremath{\partial}H}{\ensuremath{\partial}P})}_{T}$ as a function of temperature were made on physically identical samples of $\ensuremath{\alpha}$- and $\ensuremath{\beta}$-Hg. The purpose of these measurements was to obtain data on the effects of crystal structure on the properties of superconductors, and to permit calculation of various thermodynamic quantities difficult to obtain in any other way. The critical fields of the two phases were found to be identical when expressed in terms of the reduced variables $\frac{H}{{H}_{0}}$ and $\frac{T}{{T}_{c}}$. No generalizations of this type could be found to explain the pressure effects. The advantages of an ${H}^{2}$ vs ${T}^{2}$ and ($\frac{\ensuremath{\partial}{H}^{2}}{\ensuremath{\partial}P}$) vs ${T}^{2}$ analysis for extrapolation to absolute zero are stressed. The critical fields of several representative superconductors are compared with the critical field predicted by the Bardeen-Cooper-Schrieffer theory, using a plot that emphasizes the detailed shape of the curves at low temperatures. This plot also can be interpreted in terms of the $\frac{\ensuremath{\theta}}{{T}_{c}}$ dependence of the width of the energy gap. The agreement between calorimetric and critical field determinations of the electronic specific heat in the normal state is shown to be improved by using the ${H}^{2}\ensuremath{-}{T}^{2}$ extrapolation. The volume dependence of the reduced energy gap is shown to be very small for those superconductors for which pressure effect data are available.

Superconducting Critical Field of Tantalum as a Function of Temperature and Pressure

The results of precise critical field measurements on tantalum samples which show soft superconducting behavior are given along with direct measurements of the pressure effect, ${(\frac{\ensuremath{\partial}{H}_{c}}{\ensuremath{\partial}P})}_{T}$, as a function of temperature. The Bardeen-Cooper-Schrieffer theory is used as a guide in the extrapolation of these data to absolute zero from 1.1\ifmmode^\circ\else\textdegree\fi{}K. The advantages of using an ${H}^{2}$ vs ${T}^{2}$ representation for both the critical-field and pressure-effect data are discussed, and it is shown that if both sets of data can be represented in terms of power series [${H}^{2}$ or ${(\frac{\ensuremath{\partial}{{H}_{c}}^{2}}{\ensuremath{\partial}P})}_{T}$ vs ${T}^{2}$] over a limited range of temperature, it is then possible to write down explicit power series expressions for the differences in the thermodynamic functions between the normal and superconducting states over this same temperature range. The electronic contributions to the specific heats and the thermal expansions for tantalum are calculated from the experimental data.