Small Changes In Path Length Research Articles

How Do Reaction and Reactor Conditions Affect Photoinduced Electron/Energy Transfer Reversible Addition–Fragmentation Transfer Polymerization?

The impact of conditions was investigated on a model photoinduced electron/energy transfer reversible addition–fragmentation chain transfer (PET-RAFT) polymerization. Within the cylindrical geometries studied, with relatively small changes in path length, the impact of reaction vessel dimensions and dilution was relatively small on the polymerization kinetics and control of the polymerization. This suggests that PET-RAFT can be relatively insensitive to small changes in reactor geometry and reaction volume when cylindrical systems are used. The intensity of the photoreactor was a key factor in determining reaction rate, with an approximate 1/2 order scaling of the apparent rate with intensity. Reactant concentration ratios were also important, with an approximate 1/2 order of the apparent rate with the photocatalyst loading and an approximate −1/2 order scaling apparent polymerization rate coefficient with the RAFT agent concentration. However, there is a limit to rate increases with higher Ir catalyst lo...

Beamforming Tradeoffs for Initial UE Discovery in Millimeter-Wave MIMO Systems

Millimeter-wave MIMO systems have gained increasing traction towards the goal of meeting the high data-rate requirements in next-generation wireless systems. The focus of this work is on low-complexity beamforming approaches for initial UE discovery in such systems. Towards this goal, we first note the structure of the optimal beamformer with per-antenna gain and phase control and the structure of good beamformers with per-antenna phase-only control. Learning these beamforming structures in mmW systems is fraught with considerable complexities such as the need for a non-broadcast system design, the sensitivity of the beamformer approximants to small path length changes, etc. To overcome these issues, we establish a physical interpretation between these beamformer structures and the angles of departure/arrival of the dominant path(s). This physical interpretation provides a theoretical underpinning to the emerging interest on directional beamforming approaches that are less sensitive to small path length changes. While classical approaches for direction learning such as MUSIC have been well-understood, they suffer from many practical difficulties in a mmW context such as a non-broadcast system design and high computational complexity. A simpler broadcast solution for mmW systems is the adaptation of directional codebooks for beamforming at the two ends. We establish fundamental limits for the best beam broadening codebooks and propose a construction motivated by a virtual subarray architecture that is within a couple of dB of the best tradeoff curve at all useful beam broadening factors. We finally provide the received SNR loss-UE discovery latency tradeoff with the proposed constructions. Our results show that users with a reasonable link margin can be quickly discovered by the proposed design with a smooth roll-off in performance as the link margin deteriorates.

Measurement of texture in steel by laser-ultrasonic surface waves

A laser-ultrasonic approach based on the propagation of surface skimming longitudinal wave (P-wave) and Rayleigh wave (SAW) is investigated for the measurement of texture in moderately thick steel plates. The angular dependence of the P-wave and SAW velocities presents different patterns, both of amplitude of a few % attributed to texture. Moreover, the difference of the P-wave and SAW profiles is shown to be very robust against small path length changes or other detrimental effects. For measurements on large plates, a solution based on the rotation of a generation line using an axicon lens is tested. Texture variations across the width of steel plates and the influence of residual stress are discussed.

Electrostriction measurements on low permittivity dielectric materials

To measure the electrostrictive effects in low permittivity materials, extremely sensitive instrumentation is required. A modified compressometer for resolving fractional changes in capacitance on the order of 10 −6 is used in this work, along with a modified single beam interferometer capable of sub-angstrom resolution in displacement. For the compressometric method, a high sensitivity capacitance bridge, GenRad 1615, is coupled with two lock-in amplifiers to detect attofarad (10 −18F) level capacitance changes caused by in-phase cyclic uniaxial stresses on samples. The interferometer is a Michelson –Morley type instrument modified to detect changes in interference fringe intensity for very small changes in path length. The measurements confirmed by both techniques are used to establish a set of reliable and accurate data of electrostriction coefficients for low permittivity materials. Using these recent data, along with widely accepted data on ferroelectric materials and soft polymers, the linear relationship between electrostriction coefficient (Q) and the ratio of elastic compliance over dielectric permittivity ( s/ε 0ε r) is obtained. This leads to an effective way to predict the electrostriction coefficient in dielectric materials.

Skeletal correlates of coral density banding and an evaluation of radiography as used in sclerochronology

The skeletal correlates of density band patterns seen on radiographs of coral skeletons is investigated by theoretical analysis of radiography, comparison of radiographs and skeleton, and modelling of skeletal slices of the massive Indo-Pacific coral Pontes lutea and the branching Caribbean coral Pontes porites. Radiography resolves finer detail of the coral skeleton than has previously been recognised. Small changes in path length of individual skeletal elements attenuating an X-ray beam, including dissepiments as thin as 3 to 10 pm, can generate contrast on radiographs as well as variations in skeletal bulk density. The annual high/low density band pattern was found to be correlated with areas of relatively thick/thin skeletal elements respectively. Secondary density variations seen on radiographs, 'fine' bands, were found to be correlated with the position of corallite walls oriented perpendicular to the the long axis of the skeletal slice or dissepiments. Modelling of skeletal slices showed that errors in the density band record may be introduced by (1) skeletal slices not following the growth axis of the colony, and (2) changes in corallite orientation which generate density variations that are not part of any growth record. The use of radiography as a tool for demonstrating growth records in coral skeletons is critically reviewed. It is proposed that the skeletal growth record consists of patterns of thickeninglthinning and spacing of skeletal elements. The illustrations of these patterns by analysis of the actual skeleton, rather than by radiography, may prove to be a very accurate method of determining growth records in corals.

Two-frequency fiber-optic sensor system using high-birefringence optical fiber

A fiber sensor system based on high-birefringence optical fiber and a frequency-stabilized He-Ne Zeeman laser (lambda = 0.6328 microm) has been demonstrated. The small path-length changes of interest are extracted from the amplitude or phase of the laser beat frequency by using either the phase-sensitive detection or the phase-demodulation technique. A minimum detectable optical phase delay of 1.5 x 10(-6) rad/Hz((1/2)) or 17 x 10(-6) rad/Hz((1/2)) has been achieved.

Path length variations due to changes in tropospheric refraction

An experiment is described in which microwave Doppler is used to determine very small changes in path length to spacecraft tracked by the Deep Space Tracking Network (DSN). The experiment was carried out to test the detection capabilities of the DSN system to gravitational radiation of very low frequency (10−2–10−4 Hz). In this work spectral analysis of Doppler variations were performed for periods over 4 hours and more.These results indicated that one of major sources of noise was due to rapid variations in tropospheric refraction. The results obtained a differential path length variation, ΔL/L, of 1 part in 1014 for periods between 100 and 1 000 seconds.Doppler spectra are shown and a general discussion of the experiment is given.