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A combined superconducting magnetic system with a uniform field of up to 20 T has been developed. The system contains several solenoidal sections and a magnetic flux concentrator consisting of hydroextruded dysprosium. The sections are wound from NbTi, Nb3Sn wires and Nb3Sn ribbon. A magnetic shield made out of Nb3Sn ribbon is applied in order to increase the uniformity of the magnetic field. The magnetic field achieved in the gap between the ends of the concentrator has a high spatial uniformity ΔН/Н ∼ 5 × 10−5.

We have fabricated aligned carbon nanotubes (CNTs) by disproportionation of CO on a well-ordered Co nanowire array prepared on a porous anodic aluminum oxide (AAO) template using overdeposition and etching techniques, and studied their field emission properties. CNTs fabricated at 600 °C were well graphitized, uniform in diameter and height, and aligned vertically with respect to the plane of the template; they also showed good field emission properties. The electron field emission was turned on at a field as low as 1.1 V/μm and the enhancement factor was 3520 by assuming a CNT work function of 5 eV. Also, very uniform emission was observed by applying 3.1 V/μm. These good field emission properties may be attributed to good graphitization, uniformity in diameter and height, and vertical alignment of the CNTs fabricated on an AAO template by CO disproportionation.

Significant improvement of the thermal stability in FeXN (X=Al, Zr, Ta,…) thin films is reported. Sputtered at the reduced target–substrate spacing of 38 mm, 200-nm-thick thermally stable FeXN thin films are obtained. They have hard-axis coercivity ≈ 0.1–2.0 Oe, easy-axis coercivity ≈1.5–3.0 Oe, Hk≈8–16 Oe, and Bs≈19–20 kG. Results of transverse-field annealing experiments in a uniform field of 700 Oe show no significant change of magnetic properties at 150 °C for 3 h. At 150 °C for 24 h Hk decreases by 2–4 Oe. Their easy/hard axes do not rotate and the coercivity remains almost unchanged. This superior thermal stability of FeXN films is very promising for high-moment write heads.

We report a spin-engineering scheme whereby the sign of the giant magnetoresistance in an antiparallel biased spin valve may be selected by varying only layer thicknesses within the structure. The antiferromagnetic coupling in the biased layers leads to either positive exchange bias or positive magnetoresistance in the response of the structure. Hence, spin valves may be fabricated with either positive or negative sensitivities to applied fields in the same sense. When two spin valves of each type are connected in the appropriate Wheatstone bridge configuration, a sensor with four active legs may be constructed. The bias may be set by either the growth field, or by application of uniform field to the entire sensor structure in a single postprocessing anneal step.

Recently introduced yokeless permanent magnets are very suitable for generating uniform fields. Practical yokeless magnets must be shimmed or compensated for fabrication and magnetization tolerances. The particular shimming approach described exploits the magnetic transparency of yokeless structures. The method is based on the use of active elements of high‐energy‐product material. The logic for determining the distribution of active elements is based on measuring the departure from uniformity of the field sampled on a sphere surrounding the region of interest. The magnetic moments of the elements are determined by solving a system of linear equations representing the field generated by active elements on each sampling point.

We develop a criterion for spark-breakdown in non-uniform field geometries with positive polarity and small electrode separations so that breakdown evolves without the formation of a leader. We arrive at the spark-breakdown criterion by framing it in terms of gain and instability conditions, whose relative importance is established from an analysis of the experimental breakdown characteristics and correlations with streamer behavior in short gaps. Results are presented in the context of two generic geometries having coaxial and point-plane electrodes. For nearly uniform field situations, we re-confirm that the breakdown criterion obtained by the usual extension of either the Townsend or Meek criteria satisfactorily predicts the experimental results. On the other hand, for increasing non-uniformity, the results for the corona and spark branches of the breakdown characteristics are shown inconsistent with a breakdown criterion solely based on either the Townsend or streamer mechanisms. In particular, the avalanche gain factor, the primary component of the Townsend and streamer criteria, does not determine the spark breakdown criterion. Streamers can cross the gap for a significantly wide range of applied voltages without triggering a spark. We find that it is the instability condition, derived from a relation between the minimum Laplacian field in the gap and the local streamer body field (which we relate to the streamer sustaining field), that is sufficient for determining the spark threshold thereby yielding a breakdown criterion.

This paper discusses the selection of the design parameters and the optimum geometry of yokeless permanent magnets that generate a uniform field H0 within an arbitrary closed cavity. The optimum design minimizes the volume of magnetic material and it is determined by the geometry of the cavity, the magnitude of intensity H0, and a characteristic point F that controls the configuration of the magnetostatic potential as well as the flux of the magnetic induction. The optimization procedure is applied to a two‐dimensional magnet with a quadrilateral cross section.

This article analyzes compact laterally coupled distributed feedback (DFB) lasers with three defect regions. These devices are more flexible and smaller than conventional DFB lasers, having typical lengths between 20 and 50μm and a width less than 1μm (lateral gratings are inserted in a single-mode waveguide). We optimize the defect regions to achieve an improved performance. In other words, an adequate choice of phase shifts may lead to single-mode operation, lower threshold optical power, higher quantum differential efficiency, and more uniform field distribution. This device is designed to operate under optical pumping.

This paper discusses a technique for improving the homogeneity of the magnetic field of tube-like permanent magnets designed for magnetostrictive devices. The high homogeneity of the magnetic field along the tube is achieved by gradually changing shape or remanence of combined short rings. The analysis of the field in the magnet is performed using the magnetic charge principle and axial magnetic field equations are deduced. An example of design of a longitudinal uniform field for a magnetostrictive rod with large length to diameter ratio is presented.

A perturbation method was given recently for solving electrostatic problems involving a conducting torus which is freely charged or influenced by a uniform field parallel to its axis. It was forseen that one limitation of the method, the fact that no external influence other than the one just mentioned is allowable, could be removed in the case of a circular torus (anchor ring) by means of the Kelvin transformation and superposition, and the present paper develops this aspect. A number of presumably new results are given for a torus influenced by a given charge located on its axis, and it is shown how the procedure can be extended for a very general axisymmetric situation involving a given charge distribution.

For an Ising model satisfying the Lee–Yang condition, the zeros of the partition function Z and those of the associated functions ZA in the space of imaginary magnetic fields at all lattice sites are determined by a single analytic hypersurface. The sense of motion of the zeros of Z as the interactions are varied can be related to the positions of the zeros of the ZA . Contrary to a plausible conjecture, it is not true that all of the zeros of Z in a uniform field tend towards the point z=1 in the complex fugacity plane as the temperature is lowered, but it is possible that the first zero (that nearest to z=1) has a monotone motion. Various simplicity and intertwining properties of the zeros of Z and ZA which generalize earlier results are proved by a new argument which makes direct use of the Lee–Yang property.

The small‐signal gain was measured in a uv‐sustained CO2 laser amplifier employing uniform‐field electrodes of the type used by Chang. Four different profiles were utilized and the spatial distribution of the gain differed for each profile. For a given electrode shape there was an input energy which produced a region of constant gain. A dip or saturation of the gain occurred at the center of the discharge if the input energy exceeded the optimum value, while a monotonic decrease in gain away from the discharge center was produced below this value. The dip in the gain distribution appears to be caused mainly by thermal heating of the gas. The flatter electrode profiles produced wider uniform gain distributions and less saturation because less heating occurred.

We have constructed an eddy current shielded room from high‐conductivity aluminum sheets, 1.88 cm thick. A series of measurements designed to test the performance of the room has been carried out. The results of the measurements have been compared with theoretical predictions and also with the results from similar rooms reported in the literature. In some respects the agreement is good while in other respects considerable discrepancy exists. The response time of the room to a uniform field was found to be in good agreement with theory, whereas the response to a nonuniform field was found to be much worse. We also found that the room does not shield the vertical field component as well as the horizontal components. We give the reason for this and suggest a possible solution.

The optimal dimensions of thin coil systems of three and four square coils for producing uniform magnetic fields are calculated. We find that for three square coils, of side d and separation s between the outer coils, the most uniform field distribution occurs with s/d=0.821 116 and with I′/I=0.512 797. I′/I is the ratio of the currents in the center coil to that of the outer coils. With four square coils, the best uniformity is obtained when a/d=0.128 106 and b/d=0.505 492, where a is the distance from the center to the inner coils and b is the distance from the center to the outer coils. The ratio of the current in the inner pair of coils to that in the outer pair must be I′/I=0.423 514. We compare the uniformity of the field produced by these coil systems with each other and with Rubens’ five‐coil system, both on and off axis. It is shown that the optimal four‐coil design is superior to the three‐ and five‐coil systems. The sensitivity of the uniformity to the precision of construction is discussed. Di...

Breakdown mechanism of a laser triggered spark gap was studied in a uniform field gap. The laser beam was focused on the electrode with positive and negative polarities, and vaporized metal plasma produced by the laser irradiation was used as a trigger. Our investigation shows that the breakdown characteristics in the uniform field are essentially different from those of a needle‐to‐plane gap, and also shows different characteristics under positive and negative polarities.

The generation of quasi‐static uniform electric fields by a slotted conducting spherical shell is fully analyzed for an arbitrary number of slots pairs, and optimum surface potential and slots position are determined. Numerical results for spherical shells with up to 14 slots are discussed.

A calculation has been made of both the first‐ and second‐order optics of a uniform‐field circular‐pole magnet, using the impulse approximation for the fringe‐field effects. From this, the first‐ and second‐order resolution has been derived over the entire focal plane of the magnet as a function of the angle of deflection and the momentum of the particle.

Asymmetric‐top molecules can be deflected in an inhomogeneous electric field, and the deflected molecules can be oriented by allowing them to pass adiabatically into a uniform field. It is possible to account quantitatively for the transmission in a hexapole field if only first order and intermediate (or fast) Stark effects are considered. Rigid rotors with the dipole moment parallel to the a axis and internal rotors with low barriers to rotation have many levels which so interact and these types of molecules are easily deflected. Rigid rotors with the dipole parallel to the b axis have far fewer levels which interact; for these molecules the deflection is weak and may be undetectable. For rigid rotors and free internal rotors it is possible to obtain dipole moments by matching theoretical curves with experimental points.

A low‐frequency NMR spectrometer has been built that will accommodate sample volumes of 0.2 and 1.25 liters. A shielded solenoid provides a uniform field of about 10 G. Signals are detected by a symmetrical rf bridge whose coils are bank wound of litz wire with Q∼300 to 400 at 40 kHz. In the smaller coil the homogeneity is sufficient to distinguish signals of protons in protein from those in fat and water. Spectra have been obtained from gelatin, whole eggs, egg whites, egg yolks, and dead mice. In addition, what is believed to be the first NMR signal ever obtained from a whole living animal has been taken on an anesthetized rat.

The design of a magnet for producing a uniform field is described. The magnet model used in the calculations is that of cylindrical cores surrounded by rims. The positions of the central pole faces and rims can be adjusted independently. A calculation, based on the assumption that the pole pieces are uniformly magnetized, shows that, for a certain positioning of the cores and rims, the z2 and z4 correction terms to Hz can be simultaneously canceled. Practical limitations to uniformity that result from a slight asymmetry in the geometry and from the effect of nonuniformity in magnetization because of domain structure in the iron also are discussed.

A microwave exposure system for use in the study of the bioeffects of cw microwave radiation has been designed, built, and calibrated. The specifications to which the system was designed are as follows: single frequency of 2450 MHz; power density range at the biological sample holder of 0.01–200 mW/cm2; uniform field at the sample holder as defined by a 10% variation in power density over a 15 cm diameter and half‐power variation over a 30 cm diameter; minimum cost consistent with safety, reasonable convenience of operation, and meeting technical criteria; and total space requirement not to exceed a floor space of 4.1×4.9 m. The assembled system costs approximately $13 000 and provides better field characteristics than those specified above.

The experiments of R. C. Fletcher on impulse breakdown are discussed and a simple theory for these experiments is developed by assuming that the breakdown consists of ionization and charge separation in a uniform field. The theory accounts reasonably well for the time lags and also explains the shapes of the breakdown voltage transients. It indicates also that measurement of the mean ionizing time for electrons in the gap should be possible.

The first‐ and second‐order matrix equations for the magnetic optics of the midplane of uniform‐field wedge magnets have been derived. The calculations include rotated input and output wedges and curved surfaces on the input and output faces. A second‐order matrix algebra has been developed to handle beam transport optics for multiple element systems in a manner similar to that which has been used in the past for the first‐order beam transport optics.

Some high‐power millimeter‐wave devices need magnetic fields only near the periphery of a cylindrical region in order to focus an electron beam. Some novel periphery of a cylindrical region in order to focus an electron beam. Some novel permanent‐magnet configurations are described which can bring about weight savings by providing a cross‐sectionally uniform field in an annular region rather than in an entire cyclinder. This is accomplished by an arrangement of axially and radially oriented magnets that allow almost no flux leakage to the environment.

A design is reported for a compact and simply constructed CO2 TEA laser giving pulses of up to 50 J with 0.1‐μs duration from a discharge of dimensions 45×6×6 cm3. The gas is preionized by VUV irradiation from two lines of 34 series arcs to either side of the uniform field electrodes to which potentials of +50 kV and −50 kV are applied. The mechanism of preionization and the conditions for a uniform discharge are considered.

Results are presented of an investigation into the flashover of glass and high‐density alumina cylinders 1.25 cm in length between uniform field electrodes in vacuum. Photographs taken with an image‐converter camera with a framing rate of 2×107 frames/sec are shown. At approximately 80% of the voltage required to produce a flashover, partial discharges occur. When the voltage applied to the gap is increased to breakdown level, the partial discharges persist as the first stage of a two‐stage arc formation. The development of the discharge is shown to be consistent with a flashover mechanism involving charging of the solid surface by electrons emitted at the cathode.

Data on dielectric constants of electrolyte solutions are used to evaluate directly the r^(−4) term in the asymptotic expression for the free energy of interaction of two ions in solution for large separation distance r. Use is made of the fact that for large separations each ion is in a uniform field due to the other, and that information about ions in uniform fields is obtainable from measurements of dielectric constants. For a Z:Z electrolyte, for example, the r^(−4) term is found to be Z^2e^2δ/8πϵ_0r^4, assuming the effect of overlapping solvent structures to be of shorter range; ϵ_0 is the dielectric constant of the solvent and δ is the measured decrement in dielectric constant per unit concentration of added electrolyte. A similar result obtained when one of the particles is uncharged, δ now referring to the decrement observed when the neutral is added to solution. Typical values of the term are given for various substances using the data on δ's. This determination of the r^(−4) term permits some evaluation of ion—image force theories.

A simple instrument is described which can be utilized to check the optical properties of uniform field magnetic spectrometers. It can also be used as an aid to determine the boundaries of the magnetic field required to achieve desired optical properties.

Double focusing of charged particles by a magnet with uniform field and circular poles can be obtained for any single deflection angle by directing the beam at an appropriate point away from the axis of the poles. For a bend of 119.25° the aberration arising from radially divergent ions is eliminated.

Axisymmetric cusp end cells have application for the stabilization and plugging of a tandem‐mirror‐type reactor. Experiments have been performed on the RFC‐XX double cusp in Nagoya, Japan to measure the stability of the hollow plasma in the adiabatically confined region of the device. Experiments were performed with both a uniform field central section and with a mirror central section connecting the two cusps. The plasma is produced by an rf discharge of gas puffed into the vessel. Plasma fluctuations were measured by Langmuir probes and magnetic probes. Stable regions of operation with a hollow plasma in the cusp are found for both magnetic configurations. Various types of instabilities are observed: electrostatic drift type waves with m≥1, lower‐frequency waves with a magnetic perturbation associated with the density perturbation, an m=0 ion‐acoustic mode, and a large amplitude relaxation oscillation.

Recently, Oxtoby and Gelbart applied continuum electrostatic theory on an atomic scale to calculate collisional polarizabilities. In this paper we show how such a description can be derived from first order quantum mechanical perturbation theory, and obtain explicit expressions for the local susceptibilities of atoms and molecules. We apply the theory to a calculation of the pair polarizability of interacting hydrogen atoms, for which the model can be solved in closed form. We also show that the dipole moment induced by an arbitrary (weak) external field may be obtained from the wave function in a uniform field.

Simple analytic formulas that can be used to design a finite width uniform‐field electrode for any desired aspect ratio and field uniformity are derived. The resulting profile is superior to those of Rogowski, Bruce, and Harrison in its smoothness, compactness, and field uniformity. A procedure for designing a three‐dimensional profile is also given.