Metallurgical Analyses of Niobium for Superconducting Radio Frequency Cavities

Superconducting radio frequency cavities have gained use in accelerator systems for particle physics research. Careful production of the cavities has the greatest influence on their efficiencies as uniform interior surfaces are required for high accelerating gradients. Small variations in the surfaces of these cavities, such as inclusions, voids, and cracks, cause large deficiencies in the accelerating gradients. Processes to remove such deficiencies usually include eddy current scanning, buffered chemical polishing, and electropolishing. These methods do not provide a consistent means of producing a uniform interior surface. The effectiveness of tumbling as a mass finishing technique was analyzed. This process completely removed the weld line. The effects of weld line removal on cavity efficiencies will be examined.

Crystallographic Characterization of GaN Nanowires by Raman Spectral Image Mapping

Obtaining structural information of nano structured materials often requires electron microscopy for suficient spatial and crystallographic resolution. This study uses Raman spectral imaging to extract information regarding crystalline orientation and structure by non-invasive means. Seeking a correlation between crystallographic facet and favored Raman mode, Gallium Nitride (GaN) nanowires were imaged by confocal Raman microscopy with a 532nm laser, and scanning electron microscopy. Raman spectral maps containing pixel-by-pixel spectra were acquired. Comparison to scanning electron microscope (SEM) images revealed that for regularly-shaped wires at least 230nm in width, the E2 mode is observed more strongly in the [1 1 2] and [-1 -1 2] "smooth" facets of the wire, while the A1(TO) mode is only observed in the [0 0 1] "rough" facet, suggesting a strong surface-structure dependence of Raman signal that can be exploited for imaging. Further experimentation on irregular and small wires that exhibit only the E2 peak, on other favored modes in GaN, and with other group III/V nitrides is recommended.

Simulation and Optimization of the HINS Ion Source Extraction System

The heart of the High Intensity Neutrino Source (HINS) linear accelerator (linac) is a magnetron-type, circular aperture H􀀀 source, which is currently being tested at Fermilab. Although this prototype already delivers the beam current and emittance required by the HINS project, an exploration of whether or not the performance of the source could be improved was undertaken. To this end, the extraction geometry of the source was simulated with SIMION 8.0 and Finite Element Method Magnetics. The effects of changing the angle of the extraction cone (cone angle), the size of the gap between the extraction cone and the source plate (extraction gap), and the aperture of the extraction cone (extraction aperture) were studied. These parameters were chosen because we thought that they would have the greatest impact on space charge effects, which is a major source of emittance growth in this ion source. Based on the results of these simulations, four different configurations were ultimately tested in the ion source. The simulations indicated that the final emittance of the source should be significantly decreased by utilizing geometry with a 45 degree cone angle, a 4 mm extraction gap with extraction voltage of 25000 V, and a 3 mm extraction aperture. Subsequent emittance measurements on the ion source have confirmed this result. This new geometry also allows the source to output a higher current beam with the same duty factor.

Mechanical Properties of Buckypaper Laminate Composites and Buckypaper Subjected to Microwave Irradiation

Hydrogen has proven itself to be of particular interest as a clean renewable energy storage system. However, modern pressure tanks cannot handle the high pressures required to store the large quantities needed. Carbon nanotubes have already been shown as an effective mechanical reinforcement for composite materials. This research furthered this idea and studied if nanotube composites could be used as an outer reinforcement layer on pressure tanks. Laminate buckypaper composites and buckypaper subjected to microwave irradiation were tested. Single-wall carbon nanotubes were formed into buckypaper by vacuum filtration. Strips were cut from this paper and irradiated with microwaves in an effort to weld the nanotubes. Single and four ply laminate composites consisted of non-irradiated buckypaper strips and were layered with one of two epoxies. Tensile testing was conducted on samples press cut with a microtensile die conforming to ASTM D1708 standards. Samples were strained at 0.5 mm per minute until failure while recording the extension applied and resulting force. Laminate samples were also tested using field emission scanning electron microscopy to determine failure type and buckypaper epoxy impregnation. Microwave powers above 130 W were found to cause excessive damage to the buckypaper strips while 120 W formed only minimal damage. Samples microwaved at 120 W gave a 19 percent increase in average modulus of elasticity and a decrease of 31 percent in average ultimate tensile strength (UTS) in comparison to raw buckypaper. Both single and four ply laminates caused a decrease in both average modulus of elasticity and UTS, with the exception of four ply laminates with a 50 percent weight loading of epoxy where the UTS increased. Samples were determined to fail in a brittle manner and epoxy impregnation was found to be very low. With the low resulting average modulus of elasticities and UTSs, these buckypapers and composites were determined to not have the mechanical properties necessary for pressure tank reinforcement. Further research with larger sample sets is needed to determine this conclusively.