The hot star wind momentum problem eta = M-upsiloninfinity/(L/c) much greater than 1 is revisited, and it is shown that the conventional belief, that it can be solved by a combination of clumping of the wind and multiple scattering of photons, is not self-consistent for optically thick clumps. Clumping does reduce the mass loss rate. M, and hence the momentum supply, required to generate a specified radio emission measure epsilon, while multiple scattering increases the delivery of momentum from a specified stellar luminosity L. However, in the case of thick clumps, when combined the two effects act in opposition rather than in unison since clumping reduces multiple scattering. From basic geometric considerations, it is shown that this reduction in momentum delivery by clumping more than offsets the reduction in momentum required, for a specified epsilon. Thus the ratio of momentum deliverable to momentum required is maximal for a smooth wind and the momentum problem remains for the thick clump case. In the case of thin clumps, all of the benefit of clumping in reducing lies in eta reducing. M for a given epsilon so that extremely small filling factors f approximate to 10(-4) are needed. It is also shown that clumping affects the inference of M from radio epsilon not only by changing the emission measure per unit mass but also by changing the radio optical depth unity radius R-rad, and hence the observed wind volume, at radio wavelengths. In fact, for free-free opacity proportional to n(2), contrary to intuition, R-rad increases with increasing clumpiness.

<p> A high speed recoil transport system is developed making possible the simultaneous study of many decay products of the compound nucleus 221*Ac. Oeterminati ons of the efficiency properties of the transport system, essential for absolute cross section measurements , are discussed in detail. </p> <p> A study of angular momentum effects is made possibly by synthesis of 221*Ac using three heavy ion reactions. Absolute cross sections for (HI,xn) and (HI,pxn) reaction products are determined relative to established cross sections for production of 150Dy. Relative cross sections for francium production are also determined. </p> <p> A sophisticated statistical model calculation is adapted to provide a framework for interpretation of the results. Some insight is gained into the roles played by basic physical concepts in the systematic of compound nucleus decay in this mass region </p> / Thesis / Doctor of Philosophy (PhD)

<p> A number of questions are examined concerning many body correlations in connection with electron gas at metallic densities (2 < = rs <= 5.7) and the annihilation of a positron in simple metals, by means of a technique involving the two particle correlation Green's function. Estimates are made of low temperature contributions to angular correlation data, which describe the momentum distribution of annihilating electron positron pairs, in the form of smearing at the sharp cutoff corresponding to the Fermi momentum from electron- and positron- photon interactions, and in the form of broad tails beyond the cutoff resulting from the high-momentum components introduced into the electron wave function by the presence of a periodic crystal lattice. </p> <p> Phonon effects are introduced into the perturbation expansion of the two particle Green's function describing an electron positron pair. A calculation of the lowest order phonon contribution seems to indicate that such effects do not explain the smearing at the Fermi momentum. </p> <p> A Green's function calculation of the first-order enhancement of the lattice tails, due to the positron-electron correlation, is made by introducing particle-lattice interactions explicitly in a model based on a simple metal such as sodium. It considers a weak potential and treats as zero the lattice components corresponding to other than nearest-neighbours points in reciprocal lattice space. The enhancement for rs =4, which is almost a constant, is very similar to that for the main part of angular correlation data. This indicates that, for simple metals at least, angular correlation data can be interpreted directly from a free-particle model. </p> <p> Short-range correlations among opposite-spin electrons are examined by field-theoretic techniques as a step to obtaining a fundamental understanding of the correlations among electrons at metallic densities. A calculation of the p.d.f. for opposite-spin electrons is positive over a wide range of metallic densities and seems to account for short-range correlations of the Coulomb hole through the multiple scattering of particle-particle ladders. </p> / Thesis / Doctor of Philosophy (PhD)

<p> Preparations for an investigation of interfacial momentum transfer in two-phase two-component critical flows have been completed.</p> <p> The experiments involve the measurement of flow rates, axial pressure profiles, axial and transverse void fraction profiles, and axial wall shear stress profiles of steady-state gas-liquid critical flow in a vertical diverging nozzle. A photographic study is to be initiated to record the flow structure. The results of these experiments will be used to develop constitutive relations for interfacial momentum transfer.</p> <p> An experimental loop capable of circulating a gas-liquid mixture in a vertical test section was modified to suit the requirements of this investigation. The void fraction profiles are measured with a traversing gamma densitometer using a 20 mCi Co57 source. The wall shear stress profiles are obtained using the electrochemical method to measure the mass transfer coefficients of electrodes mounted flush with the test section wall. The liquid phase is an electrolyte and the gaseous phase can be air, nitrogen or freon. The latter is used to better approximate the densities of a steam-water flow.</p> <p> This report describes the required theory, measurement techniques, design and operation of the loop, and the experimental procedures.</p> / Thesis / Master of Engineering (MEngr)

In absence of external fields, vibrational modes of periodic systems are usually considered as linearly polarized and, as such, they do not carry angular momentum. Our work proves that non-adiabatic effects due to the electron-phonon coupling are time-reversal symmetry breaking interactions for the vibrational field in systems with non-collinear magnetism and large spin-orbit coupling. Since in these systems the deformation potential matrix elements are necessarily complex, a nonzero synthetic gauge field (Berry curvature) arises in the dynamic equations of the ionic motion. As a result, phonon modes are elliptically polarized in the non-adiabatic framework and intrinsic vibrational angular momenta occur even for non-degenerate modes and without external probes. These results are validated by performing fully relativistic ab-initio calculations on two insulating platinum clusters and a metallic manganese compound, with non-collinear magnetism. In both cases, non-adiabatic vibrational modes carry sizeable angular momenta comparable to the orbital electronic ones in itinerant ferromagnets.

The growth of data capacity in optical communications links, which form the critical backbone of the modern internet, is facing a slowdown due to fundamental nonlinear limitations, leading to an impending "capacity crunch" on the horizon. Current technology has already exhausted degrees of freedom such as wavelength, amplitude, phase and polarization, leaving spatial multiplexing as the last available dimension to be efficiently exploited. To minimize the significant energy requirements associated with digital signal processing, it is critical to explore the upper limit of unmixed spatial channels in an optical fiber, which necessitates ideally packing spatial channels either in real space or in momentum space. The former strategy is realized by uncoupled multi-core fibers whose channel count has already saturated due to reliability constraint limiting fiber sizes. The later strategy is realized by the unmixed multimode fiber whose high spatial efficiency suggest the possibility of high channel-count scalability but the right subset of mode ought to be selected in order to mitigate mode coupling that is ever-present due to the plethora of perturbations a fiber normally experiences. The azimuthal modes in ring-core fibers turn out to be one of the most spatially efficient in this regard, by exploiting light’s orbital angular momentum (OAM). Unmixed mode counts have reached 12 in a ~1km fiber and 24 in a ~10m fiber. However, there is a fundamental bottleneck for scalability of conventionally bound modes and their relatively high crosstalks restricts their utility to device length applications. In this thesis, we provide a fundamental solution to further fuel the unmixed-channel count in an MMF. We utilize the phenomenon of topological confinement, which is a regime of light guidance beyond conventional cutoff that has, to the best of our knowledge, never been demonstrated till publications based on the subject matter of this thesis. In this regime, light is guided by the centrifugal barrier created by light’s OAM itself rather than conventional total internal reflection arising from the index inhomogeneity of the fiber. The loss of these topologically confined modes (TCMs) decreases down to negligible levels by increasing the OAM of fiber modes, because the centrifugal barrier that keeps photons confined to a fiber core increases with the OAM value of the mode. This leads to low-loss transmission in a km-scale fiber of these cutoff modes. Crucially, the mode-dependent confinement loss of TCMs further lifts the degeneracy of wavevectors in the complex space, leading to frustration of phase-matched coupling. This thus allows further scaling the mode count that was previously hindered by degenerate mode coupling in conventionally bound fiber modes. The frustrated coupling of TCMs thus enables a record amount of unmixed OAM modes in any type of fiber that features a high index contrast, whether specially structured as a ring-core, or simply constructed as a step-index fiber. Using all these favorable attributes, we achieve up to 50 low-loss modes with record low crosstalk (approaching -45 dB/km) over a 130-nm bandwidth in a ~1km-long ring-core fiber. The TCM effect promises to be inherently scalable, suggesting that even higher modes counts can be obtained in the future using this design methodology. Hence, the use of TCMs promises breaking the record spectral efficiency, potentially making it the choice for transmission links in future Space-Division-Multiplexing systems. Apart from their chief attribute of significantly increasing the information content per photon for quantum or classical networks, we expect that this new light guidance may find other applications such as in nonlinear signal processing and light-matter interactions.

The purpose of these studies was to measure the drag coefficient of a small model submarine to add data to a Reynolds number study. First, a laser Doppler velocimeter (LDV) was used to measure the flow characteristics of the Mississippi State University water tunnel. The velocity and turbulence intensity profiles were measured for a range of freestream velocities of 8.6 m/s to 10.7 m/s. Several wake velocity profiles were taken for a model submarine at downstream distances of x/d = 10 to x/d = 28, with a freestream velocity of 8.6 m/s. A formula for the drag coefficient that uses only mean velocity measurements in the wake was derived for an axisymmetric body using the assumptions of a self-similar wake and a power law behavior of the wake scales. The experimental drag coefficient results are compared to computational fluid dynamic (CFD) solutions.

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The main issues discussed in this paper are related to the refinement of the on-ground casualty risk computation for the specific case of INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL). The current approach, unable to predict the spacecraft attitude motion, assumes random tumbling motion as initial condition to simulate the fragmentation process. The wide experience in break-up analysis, acquired after years of practice with simulation, identified attitude to be one of the major drivers of uncertainty. The Space Debris Office (SDO) demanded a specific research in the field of the long-term propagation applied to the attitude motion and INTEGRAL offered the perfect test bench to conduct a preliminary study in this direction. In particular, observing whether environmental torques were able to trigger stable attitude motion, maintainable till re-entry, was considered to be the major challenge. The propagation of coupled orbital􀀀attitude motion for a random attitude configuration represents only one side of the coin. Indeed, chaos theory analysis constituted the other. The use of the Poincaré map in a non-canonical way managed to bring evidence for constrained motion in the angular rate motion of INTEGRAL, under gravity perturbations. Such results allowed to conduct further investigation on the overall attitude motion and estimate that the attitude configuration at the re-entry appears as precession about the maximum axis of inertia, in the majority of the cases. / De aspekter som behandlas i detta examensarbete är relaterade till skaderisken när rymdskrot som passerar atmosfären och landar på marken. Detta illustreras för en specifik satellit: INTEGRAL. Den nuvarande strategin som används i rymdindustrin är oförmögen att tillräckligt noggrant prediktera satellitens at-titydförändring vid atmosfärsinträdet och antar därför en stokastisk tumlande rörelse som initialvillkor för en analys av sönderdelningen av farkosten när den passerar atmosfären. Den erfarenhet som finns i rymdindustrin kring sönderdelningssimuleringar har identifierat att attityden är den faktor som genererar störst osäkerhet i resultaten. För att bättre förstå attitydens betydelse användes INTEGRAL i den fallstudie som presenteras i denna rapport. Specifikt studerades hur externa kraftmoment från rymdmiljön kan skapa en stabil attitydrörelse, som behålls ända till farkostens inträde i atmosfären. Propageringen av den kopplade rörelsen bana-attityd för en stokastisk attitydkonfiguration representerar endast en del av denna analys, där kaosteori representerar den andra delen. Med hjälp av Poincaré-mappning har simuleringar som indikerar en begränsad vinkelhastighet för INTEGRAL-satelliten när den utsätts för gravitationsstörning. I majoriteten av de analyserade fallen representerades attityden av en precession kring den största huvudtröghetsaxeln.