We have performed a study of quantum turbulence generated in oscillatory counterflow as a continuation of previous experiments on various channel flows of superfluid helium, in the form of coflow, thermal DC counterflow and pure superflow. We have investigated its development, steady state properties and temporal decay, as well as the effect of the resonant mode used to generate the turbulence at three different temperatures, 1.45 K, 1.65 K and 1.83 K. The attenuation of low amplitude second sound, orientated perpendicularly to the long axis of the resonator, was used to determine the amount of quantized vortices created. One of the main goals of this work was to characterize the critical parameters for the onset of instabilities in oscillatory counterflow and to determine their values. Decay measurements of the vortex line density allowed us to distinguish between Vinen-type and Kolmogorov- type decays of quantum turbulence.

We perform high precision measurements of an isotope shift between the two stable isotopes of helium. We use laser excitation of the 2^3 S_1-2^3 P_0 transition at 1083 nm in a metastable beam of 3He and 4He atoms. A newly developed tunable laser frequency selector along with our previous electro-optic frequency modulation technique provides extremely reliable, adaptable, and precise frequency and intensity control. The intensity control contributes negligibly to overall experimental uncertainty by stabilizing the intensity of the required sideband and eliminating the unwanted frequencies generated during the modulation of 1083 nm laser carrier frequency. The selection technique uses a MEMS based fiber switch and several temperature stabilized narrow band (~3 GHz) fiber gratings. A fiber based optical circulator and an inline fiber amplifier provide the desired isolation and the net gain for the selected frequency. Also rapid (~2 sec.) alternating measurements of the 2^3 S_1-2^3 P_0 interval for both species of helium is achieved with a custom fiber laser for simultaneous optical pumping. A servo-controlled retro-reflected laser beam eliminates residual Doppler effects during the isotope shift measurement. An improved detection design and software control makes negligible subtle potential biases in the data collection. With these advances, combined with new internal and external consistency checks, we are able to obtain results consistent with the best previous measurements, but with substantially improved precision. Our measurement of the 2^3 S_1-2^3 P_0 isotope shift between 3He and 4He is 31 097 535.2 (5) kHz. The most recent theoretic calculation combined with this measurement yields a new determination for nuclear size differences between 3He and 4He: ∆r_c=0.292 6 (1)_exp (8)_th (52)_exp fm, with a precision of less than a part in 〖10〗^4 coming from the experimental uncertainty (first parenthesis), and a part in 〖10〗^3 coming from theory. This value is consistent with electron scattering measurement, but a factor of 10 more precise. It is inconsistent (4 sigma) with a recent isotope shift measurement on another helium transition (2^1 S_0-2^3 S_1). Comparisons with ongoing muonic helium measurements may provide clues to the origin of what is currently called the proton puzzle: electronic and muonic measurements of the proton size do not agree. In the future, the experimental improvements described here can be used for higher precision tests of atomic theory and quantum electrodynamics, as well as an important atomic physics source of the fine structure constant.

Neutron radiation detectors are an integral part of the Department of Homeland Security (DHS) efforts to detect the illicit trafficking of radioactive or special nuclear materials into the U.S. In the past decade, the DHS has deployed a vast network of radiation detection systems at various key positions to prevent or to minimize the risk associated with the malevolent use of these materials. The greatest portion of this detection burden has been borne by systems equipped with 3He because of its highly desirable physical and nuclear properties. However, a dramatic increase in demand and dwindling supply, combined with a lack of oversight for the existing 3He stockpile has produced a critical shortage of this gas which has virtually eliminated its viability for detector applications. A number of research efforts have been undertaken to develop suitable 3He replacements; however, these studies have been solely targeted toward simple detection cases where the overall detection efficiency is the only concern. For these cases, an insertion of additional detectors or materials can produce reaction rates that are sufficient, because the neutron spectral response is essentially irrelevant. However, in applications such as safeguards, non-proliferation efforts, and material control and accountability programs (MC&A), a failure to use detectors that are spectrally matched to 3He can potentially produce dire consequences. This is because these more difficult detection scenarios are associated with fissile material assessments for 239Pu and other actinides and these analyses have almost universally been calibrated to an equivalent 3He response. In these instances, a “simple” detector or material addition approach is neither appropriate nor possible, due to influences resulting from the complex nature of neutron scattering in moderators, cross sections, gas pressure variations, geometries, and surrounding structural interference. These more challenging detection cases require a detailed computational transport analysis be performed for each specific application. A leveraged approach using adjoint transport computations that are validated by forward transport and Monte Carlo computations and laboratory measurements can address these more complex detection cases and this methodology was utilized in the execution of the research. The initial task was to establish the fidelity of a computational approach by executing radiation transport models for existing BF3 and 3He tubes and then comparing the modeling results to laboratory measurements made using these identical devices. Both tubes were 19.6 cm in height, 1-inch in diameter, and operated at 1 and 4 atm pressure respectively. The models were processed using a combination of forward Monte Carlo and forward and adjoint 3-D discrete ordinates (SN) transport methods. The computer codes MCNP5 and PENTRAN were used for all calculations of a nickel-shielded plutonium-beryllium (PuBe) source term that provided a neutron output spectra equivalent to that of weapons-grade plutonium (WGPu). Once the computational design approach was validated, the adjoint SN method was used to iteratively identify six distinct plug-in models that matched the neutron spectral response and reaction rate of a 1-inch diameter 3He tube with a length of 10 cm and operating at 4 atm pressure. The equivalent designs consist of large singular tubes and dual tubes containing BF3 gas, 10B linings, and/or 10B-loaded polyvinyl toluene (PVT). The reaction rate for each plug-in design was also verified using forward PENTRAN and MCNP5 calculations. In addition to the equivalent designs, the adjoint method also yielded various insights into neutron detector design that can lead to additional designs using a combination of different detector materials such as BF3/10B-loaded PVT, 10B-lined tubes/10B-loaded PVT, etc.

Many technological hurdles remain before a viable commercial magnetic fusion energy reactor can be constructed, including the development of plasma-facing components with long lifetimes that can survive the harsh environment inside a reactor. One such component, the divertor, which maintains the purity of the plasma by removing fusion byproducts from the reactor, must be able to accommodate very large incident heat fluxes of at least 10 MW/m^2 during normal operation. Modular helium-cooled tungsten divertors are one of the leading divertor designs for future commercial fusion reactors, and a number of different candidates have been proposed including the modular He-cooled divertor concept with pin array (HEMP), the modular He-cooled divertor concept with multiple-jet-cooling (HEMJ), and the helium-cooled flat plate (HCFP). These three designs typically operate with helium coolant inlet temperatures of 600 °C and inlet pressures of 10 MPa. Performing experiments at these conditions to evaluate the thermal performance of each design is both challenging and expensive. An alternative, more economical approach for evaluating different designs exploits dynamic similarity. Here, geometrically similar mockups of a single divertor module are tested using coolants at lower temperatures and pressures. Dynamically similar experiments were performed on an HEMP-like divertor with helium and argon at inlet temperatures close to room temperature, inlet pressures below 1.4 MPa, and incident heat fluxes up to 2 MW/m^2. The results are used to predict the maximum heat flux that the divertor can accommodate, and the pumping power as a fraction of incident thermal power, for a given maximum tungsten temperature. A new nondimensional parameter, the thermal conductivity ratio, is introduced in the Nusselt number correlations which accounts for variations in the amount of conduction heat transfer through the walls of the divertor module. Numerical simulations of the HCFP divertor are performed to investigate how the thermal conductivity ratio affects predictions for the maximum heat flux obtained in previous studies. Finally, a helium loop is constructed and used to perform dynamically similar experiments on an HEMJ module at inlet temperatures as high as 300 °C, inlet pressures of 10 MPa, and incident heat fluxes as great as 4.9 MW/m^2. The correlations generated from this work can be used in system codes to determine optimal designs and operating conditions for a variety of fusion reactor designs.

We were unable to verify the existence of ion mobility discontinuities in either superfluid helium at 1 K or liquid nitrogen. The velocity-field dependence in helium was described by an increased interaction with the normal fluid, due to an increase in the roton number density close to the ion surface. The mobility results in nitrogen were interpreted as being due to liquid motion, following a theory by Kopylov. The D.C. results showed that the effect of a grid on the transmission of an ion beam could be described by a field dependent grid transmission coefficient, independent of the ion velocity. The vortex ring transmission through a grid was a complex function of vorticity being captured by the grid, the capture and escape probabilities of the bare ions by vorticity, and the onset for vorticity propagating throughout the ion cell.

No description available.

This thesis details the design and construction of an experiment to measure the radial distribution of laser gain in a cylindrical Helium-Neon laser gain tube. This distribution is important as it can effect the transverse mode structure of a running ring laser. Earlier theoretical models of the distribution were not supported by high quality experimental data and fail to take into account some physical processes. A resolution of 8 parts per million in gain and 50 μm in radial position has been achieved. Gain distributions have been measured and are shown to be well modeled by a 0th order Bessel function with first roots at the tube walls and a central dip depending on excitation power; except for the region very near to the tube walls where a very rapid increase in gain has been observed. Hydrogen has been identified by spectroscopic analysis as the primary constituent of gas contamination and cause of the long term reduction in gain of large ring lasers. Additional work has been done to detect a proposed non-classical Lense-Thirring field around a spinning lead superconductor. It was found that any effect is at least 20 times smaller than predicted. Techniques and tools for data acquisition programming have been reviewed focusing on difficulties with coupling of user interface and application logic, monolithicity, difficulties with scripting and algorithm implementation.

No description available.

The 62 million year old lava flows of Baffin Island and West Greenland represent the earliest phase of magmatism in the North Atlantic Igneous Province (NAIP). These picritic lavas are characterised by high magnesium contents owing to their high proportion of olivine crystals. The parental magmas for the picrites are likely to have accumulated olivine crystals on their transit through the lithosphere and crust. Debate over the origin of accumulated crystals in the lavas results in uncertainty in the temperature and composition of the parental magmas for the early NAIP. The magnesium-rich olivine crystals (up to Fo93) in the picrites of this study are shown not to have a xenocrystic origin. The samples, therefore, support the inference of high potential temperatures for the Baffin Island-West Greenland magmas, ~200oC above ambient mantle. The picrites of Baffin Island and West Greenland display the highest terrestrial magmatic 3He/4He (up to 50 Ra, where Ra is the atmospheric value 1.39 x 10-6), values that are considerably higher than the highest 3He/4He in contemporary ocean island basalts, which reach a maximum of ~30 Ra. High 3He/4He in Baffin Island and West Greenland are associated with a wide range of incompatible trace element and lithophile radiogenic isotopic compositions, not dissimilar to the range of compositions displayed by lavas at mid-ocean ridges, and overlapping the range displayed by most northern hemisphere ocean island basalts. Crustal contamination modelling in which high-grade Proterozoic crustal basement rocks are mixed with depleted parents cannot account for the compositional trends displayed by the picrites. Major and trace element compositions were determined on melt inclusions in high- 3He/4He picrites that span a wide range of whole-rock incompatible trace element and radiogenic isotopic compositions. The melt inclusions support the findings from the whole-rock study since melt inclusion compositions reflect the composition of their associated whole-rock, with no anomalous compositions present. In addition, there is no evidence for a contribution of a proportion of depleted melts to the source of the relatively enriched whole-rock samples. Therefore, since all melt inclusions were contained within high-3He/4He samples, it is shown that high 3He/4He is a feature of both depleted and relatively enriched melt compositions. The wide range in whole-rock compositions of the Baffin Island and West Greenland picrites represents that of the sub-lithospheric mantle source region and is inconsistent with derivation of the picrites from residues of ancient mantle depletion. The apparent decoupling of helium from trace elements and radiogenic isotopes is hard to reconcile with simple mixing of a high-helium concentration, high-3He/4He reservoir with various depleted and enriched helium-poor mantle reservoirs. It is possible that primordial helium has diffused into a reservoir with a composition similar to that of the convecting upper mantle. However, this must have occurred after the development of existing mantle heterogeneity. The high-3He/4He picrites require the existence of a deep, primordial helium-rich reservoir. Whether this reservoir is present in the upper or deep mantle, or even the core, remains uncertain.

No description available.