Loop Array Research Articles

The Slotted Coax as a Beam Electrode

The slot-coupled TEM line has been employed at CERN as a pick-up electrode in the GHz range. It is a compact and broad band device and will prove itself commercially attractive once its coupling efficiency is competitive with an array of quarter wave loops. The slotted coupler presented in this paper is comprised of two tubes joined along their length. The bottom tube is the beam pipe and the top tube is the TEM line which picks up the electromagnetic energy from the beam particles. Energy is transferred from the beam to the TEM line through a series of slots which are mutual to the two tubes and which occur where the tubes are joined. In this device the signal voltages add in phase when the beam velocity matches the line velocity. The pick-up impedance can thus be large if the structure is long. It turns out, however, that the signal in the synchronous case is weaker than the signal obtained from an equal length array of loop couplers. In this paper the authors derive the response of the slotted line in a general non-synchronous case and explore the possible advantages of designing the coupler with reduced velocity to increasemore » the coupling efficiency. Other properties as well as a theoretical analysis of the TEM line are presented.« less

Characterisation of defects in InP substrates

Unusual inclusion-like defects (grappes) which occur in many LEC InP ingots have been studied in an {001} Ge-doped wafer by optical microscopy of the etched wafer surface, reflection and transmission X-ray topography, and transmission polarised infra-red microscopy. The defects produced a variety of different contrast features on reflection X-ray topographs. By using all the techniques in combination to examine individual defects it was deduced that the observed contrast effects were consistent with previous postulates regarding the structures of these defects. Essentially, all the defects are similar in structure but vary in diameter from 5–200 μm, and consist of central core regions surrounded by prismatic dislocation loop arrays. It was also established that the majority of defects are situated on line dislocations, and they possess effective Burgers vectors which differ from those of the dislocations on which they are located.

Analysis by electron microscopy of the variable segment in the maxi-circle of kinetoplast DNA from Trypanosoma brucei

The 20.5-kbp maxi-circle from the kinetoplast DNA of Trypanosoma brucei contains a 5-kbp segment which is not cut by most restriction endonucleases and which varies in size in closely-related trypanosome strains (Borst, P., Fase-Fowler, F., Hoeijmakers, J.H.J. and Frasch, A.C.C. (1980) Biochim. Biophys. Acta 610, 197–210). We have now analysed partial denaturation maps of the linearized maxi-circles by electron microscopy and find that the variable segment is not more AT-rich than the remainder of the maxi-circle. Early denaturation begins at two separate regions of the maxi-circle outside the variable region and one of these corresponds with the position of the gene for the large (12 S) ribosomal RNA. Denaturation-renaturation of maxi-circles leads to the formation of partially mismatched duplexes that look like underwound loops in electron micrographs. These loops are only found in the variable region and they vary in size and appearance. Under our renaturation conditions single-stranded maxi-circle DNA is devoid of secondary structure and this suggests that the underwound loops arise by misalignment of straight tandem repeats in the DNA. We have also analysed heteroduplexes between maxi-circles from two closely related T. brucei strains that differ by 1 kbp in the size of their variable segment. Most molecules had no underwound loops and contained mismatched regions in the variable segment only. The appearance of these regions is diverse, varying from fully duplex with two single-stranded loops to molecules with a heterogeneous array of smaller loops. The total size of single-stranded DNA in the heteroduplexes may be as high as 1.2 μm, i.e., a factor 4 higher than the size difference between the heteroduplex partners. We conclude that the variable region consists of imperfect tandem repeats of a sequence that evolves rapidly. This region might contain the origin of maxi-circle replication.

Neutron irradiation damage in MgO, Al 2O 3 and MgAl 2O 4 ceramics

The consequences of point defect aggregation in single-crystal and polycrystalline Al 2O 3 and MgAl 2O 4 and polycrystalline MgO, irradiated to fast-neutron (>0.1 MeV) doses ⩽2–3 × 10 26 n m −2, have now been investigated over a range of temperatures using macroscopic density measurements of swelling and transmission electron microscopy of defect aggregates. Al 2O 3 irradiated between 925 and 1100 K exhibits 2–4 vol% swelling, dense dislocation networks and aligned voids, while single-crystal MgAl 2O 4 spinel irradiated under similar conditions, does not swell and forms only isolated interstitial dislocation loops which do not unfault. Planar arrays of voids form either side of grain boundaries in polycrystalline MgAl 2O 4 spinel, however. MgO irradiated at 430 K swells 2.8% and contains a high density of small elongated loops and exhibits a tendency towards dislocation network formation despite the low irradiation temperature. Al 2O 3 and MgAl 2O 4 irradiated at 430 K exhibits dense arrays of small loops. The nature and origin of the observed aggregate defect structures are described and correlated with swelling behaviour and with previous studies which are reviewed.

Equivalent circuit model for arrays of square loops

Square-loop arrays are of interest as frequency selective surfaces. Experimental results for these arrays are presented, and a simple equivalent circuit model is described which predicts the plane-wave transmission characteristics for normal incidence. These elements are a new member of the group of arrays for which simple equivalent circuits are available.

The identification of faulted prismatic dislocation loops in single crystals of undoped InP

High-voltage transmission electron microscopy has shown that undoped single crystals of indium phosphide, grown by the liquid-encapsulated Czochralski technique, can contain linear arrays of faulted dislocation loops. The plane of the loops is (1 1 0), the fault vector is 1/n [1 1 0] and the Burgers vector of the dislocation loop is 1/n [1 1 0]. A direct correlation has been obtained between these loops and arrays of both ridge and prism features revealed by chemical etching. Sequential use of two etchants has also established a direct link between the faulted dislocation loops and the slip dislocations induced by thermal stresses during crystal growth. A study of a number of crystals grown from differently prepared starting materials suggests that the formation of the loops is associated with departures from stoichiometry and inhibited by the presence of dopant impurities.

Structural properties of MgO AND MgAl 2O 4, after fission neutron irradiation near room temperature

Polycrystalline MgO and MgAl 2O 4 (spinel) samples were irradiated at 430 ±5 K in HFIR to a fast neutron fluence of 2.1 × 10 26 n/m 2, E > 0.2 MeV, and 4.6 × 10 26 thermal n/m 2. Swelling, microstructure, and mechanical strength were evaluated after irradiation and compared to control samples. Both materials swelled substantially, 2.6–3.0% in the case of MgO, and 0.8% in the case of MgAl 2O 4. The substructure of the MgO was found to contain a dense array of dislocation loops while the spinel showed heavy, but unresolved damage. Results of diametral compression testing showed significant strengthening for both materials. This result has important implications for the use of cubic technological ceramics in fusion applications.

Characteristics of Yagi arrays of two concentric loops with loaded elements

An expression for the phase velocity of the wave propagation along two concentric and loaded circular loop arrays is derived. The traveling wave idea is applied to derive a relationship for the wave propagation constant along the structure. The analysis is based on a circuit theory method which gives the dispersion relation in terms of the mutual impedances between a reference and the other elements in the array. In addition, Flouquet's theorem is used to account for the periodicity of the structure. The array operates at two separate passbands, which correspond to the traveling waves along the inner and the outer subarrays. It is found that for a given phase velocity of the traveling wave along the structure, a capacitive loading increases the operating frequency of the array. Furthermore, a capacitive loading of the inner array alone increases both the separation of the passbands and the bandwidth of the second passband. The excitation of finite arrays is also studied, and it is found that the gain and the input impedance characteristics are more superior when only the outer array is excited.

Multiply driven and loaded coaxial circular loop arrays

Characteristics of single- or double-loaded coaxial loop arrays with arbitrary circumferences are studied when the sources or loads are located diagonally on the loops. For finite gaps at the deriving points and finite lengths of the loads, integral equations for the loop currents are obtained. These integral equations are then reduced to a matrix one by expanding the currents and the kernel in Fourier series of the azimuthal coordinate. For the special case of loaded Yagi arrays numerical results for the gain and the input impedance are presented. These results reveal that a capacitive loading of the array generally broadens the gain bandwidth of the array over which the gain remains essentially constant. A resistive loading of the exciter, on the other hand, yields a broadband characteristic for the driving-point admittance.

Intuitive derivation of loop inverses and array algebra

Array algebra forms the general base of fast transforms and multilinear algebra making rigorous solutions of a large number (millions) of parameters computationally feasible. Loop inverses are operators solving the problem of general matrix inverses. Their derivation starts from the inconsistent linear equations\(\mathop A\limits_{m n} \mathop X\limits_{n ,1} \ne \mathop L\limits_{m ,1} \) by a parameter exchangeX→L0, where\(\mathop {L_0 }\limits_{p , 1} = \mathop {A_0 }\limits_{p n} \mathop X\limits_{n , 1} \)X is a set of unknown observables,A0 forming a basis of the so called “problem space”. The resulting full rank design matrix of parameters L0 and its l-inverse reveal properties speeding the computational least squares solution\(\mathop {\hat L_0 }\limits_{p , 1} \) expressed in observed values\(\mathop L\limits_{m , 1} \). The loop inverses are found by the back substitution expressing ∧X in terms ofL through\(\hat L_0 \). Ifp=rank (A) ≤n, this chain operator creates the pseudoinverseA+. The idea of loop inverses and array algebra started in the late60's from the further specialized case,p=n=rank (A), where the loop inverse A0−1(AA0−1)l reduces into the l-inverse Al=(ATA)−1AT. The physical interpretation of the design matrixA A0−1 as an interpolator, associated with the parametersL0, and the consideration of its multidimensional version has resulted in extended rules of matrix and tensor calculus and mathematical statistics called array algebra.

Design data for coaxial Yagi array of circular loops

Numerical data for the design of uniform coaxial Yagi arrays of circular loops are presented. For the given data the electrical dimensions of the exciter and the reflector combination are selected to be optimum for the directive gain in the forward direction.

Properties of coaxial Yagi loop arrays

The resonant properties of circular loops as elements of a coaxial loop array are studied. It is shown that array characteristics can be obtained accurately by retaining only the resonant modes or the few modes around them. Effects of the various array parameters on the array characteristics are also studied. It is found that the reflector, the exciter, and the wire cross section have a small effect on the forward gain and radiation patterns, but the directors have a large effect. All parameters are found to have a large effect on the backward gain and the input admittance.

Data file management in shift-register memories

The paper proposes a shift-register memory, structured as a two-dimensional array of uniform shift-register loops which are linked by flow-steering switches, whose switch control scheme is tailored to perform with great efficiency data management operations on sequentially organized files. The memory operates in a linear input/output mode to perform record insertion, deletion, and relocation on an existing file, and in a sublinear mode for rapid internal file movement to expedite file positioning and record retrieval and update operations. The memory, implemented as a large capacity charge-coupled device or magnetic domain memory, permits efficient data management on very large databases at the level of secondary storage and lends itself to applications as a universal disk replacement, particularly in database computers.

The Yagi array of concentric loops

An analysis of wave propagation on an infinitely long Yagi array of concentric circular loops is outlined. A dispersion relation is solved numerically for the phase velocity of the wave. The array is found to possess two distinct passbands corresponding to the resonances of the inner and the outer loops, respectively. The theoretically determined phase velocities are compared with experimentally measured values.

Lattice Defects in Normal Alcohol Crystals

Single crystals of normal octadecanol were grown by slow evaporation of p-xylene solution at room temperature. The crystals were of the γ-form structure (monoclinic) whose typical shape was a lozenge with major surfaces parallel to (001) surrounded by four {110} planes. X-ray diffraction topographic observation revealed that grown-in dislocations had <100>, <010>, <110> and <11̄0> Burgers vectors and lay on the (001) plane. Stress induced dislocations had the shortest Burgers vector <010>. The easiest glide plane was (001) plane. From inclusions, dislocation bands parallel to <010> were generated. They are considered to be arrays of prismatic dislocation loops induced by the prismatic punching mechanism.

On the description of crystal defects by point-force arrays

A modification to the continuum theory of crystal defects is described in which the interatomic reaction forces developed when a defect is introduced are replaced by point forces in a continuum. The locations of the forces are determined by the atomic positions surrounding the defect, and the magnitudes by the requirement that the force array produce the proper displacement field for the defect. The technique is applied to the construction of primitive prismatic and shear dislocation loops in a hypothetical isotropic simple cubic lattice. A screw dislocation is formed from an array of shear loops, and the displacement field, width, and self-energy are calculated. Applications of the model to other dislocation configurations are suggested.

Storage Management Operations in Linked Uniform Shift-Register Loops

A new storage structure, called a uniform ladder, consists of a linear array of equal shift-register loops, each holding one record and linked by flow-steering switches. Data exchange across a loop boundary is mandatory if the controlling switch is on and forbidden if off. For MRU (Most Recently Used) storage management, the most important operation is the climbing of data to the top of the ladder from a depth of D loops, which takes only (D + 1)/2 record periods in the uniform ladder. Program switching is enhanced by efficient schemes for partial environmental exchanges and also by internal block transfers. A pushdown stack can be efficiently implemented by a change in the record storing technique.

Pile-ups of circular twist disclinations

This paper first examines the regeneration mechanism of disclination loops. An example is then given to illustrate the multiplication and the subsequent pile-up of twist disclination loops at a phase boundary under a simple shear stress. The problem of disclination loop pile-up in a homogeneous medium is formulated by considering the total strain energy change in the formation of an array of n concentric loops, which include the disclination self-energy, interaction energy, and the work done by an externally applied moment. Numerical results of the stress concentration on the outermost loop and the total strain energy change are obtained as functions of the angle of twist per unit length of the specimen and the positions of pile-up loops. It has been concluded that the concept of twist disclination loop pile-up can be applied in a continuous medium where the Burgers vector of each loop can be assumed to be infinitesimally small. The restriction on the magnitude of disclination Burgers vector seems to have cast some doubt on the validity of the proposed disclination regeneration mechanism in crystals. The results of this paper can be readily extended to discuss disclination arrays on the basal planes of hexagonal crystals. Furthermore, when disclination loop arrays are formed parallel to a bimaterial interface, their equilibrium positions can be determined by taking into consideration the contribution of stresses from the image disclinations.

Bubble ladder for information processing

A binary switch has Been proposed to steer two bubble streams to either cross or bypass each other under external electronic control. A linear array of bubble shift register loops can be linked together by these flow-steering switches to form a storage structure which will be called a bubble ladder. It is shown that the ladder can alternate between two data structures - LIFO and FIFO. For storage units based on the ladders, a scheme is developed to rearrange data items according to recency of usage for improving average access time.

Optimum design of Yagi array of loops

The purpose of this study is to determine the optimum structure of a Yagi array of circular loops subject to constraints on directivity, bandwidth, or the size of the array. It is shown that an array configuration is completely determined when any two of these constraints are specified. It is also shown that the optimized Yagi array of loops may have either a directional beam in the endfire direction or a conical-shell beam.