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SIMULTANEOUS INJECTION OF STABLE AND RADIOACTIVE IONS INTO UPGRADED MULTI-USER ATLAS
Title
SIMULTANEOUS INJECTION OF STABLE AND RADIOACTIVE IONS INTO UPGRADED MULTI-USER ATLAS
Creator
Perry, Amichay
Date
2015, 2015-12
Description
Argonne Tandem Linac Accelerator System (ATLAS) is a Department of Energy (DOE) national user research facility, located at Argonne National...
Show moreArgonne Tandem Linac Accelerator System (ATLAS) is a Department of Energy (DOE) national user research facility, located at Argonne National Laboratory (ANL). Presently, Radioactive Ion Beams (RIBs) produced in the CAlifornium Rare Isotope Breeder Upgrade (CARIBU) facility are charge bred in an Electron Cyclotron Resonance (ECR) charge breeder prior to post acceleration in ATLAS. A new state of the art Electron Beam Ion Source charge breeder, the CARIBU-EBIS charge breeder, has been developed (not in the scope of the work presented here) at ANL to replace the existing ECR for charge breeding RIBs generated in CARIBU. The CARIBU-EBIS charge breeder is now in the final stages of offline commissioning at the Accelerator Development Test Facility (ADTF). A significant part of the commissioning effort has been devoted to testing the source by breeding singlycharged cesium ions injected from a surface ionization source. Characterization of the CARIBU-EBIS performance has been accomplished through a comparison between the measured properties of extracted beams and simulation results. Following its offline commissioning, CARIBU-EBIS will be relocated to its permanent location in ATLAS. An electrostatic transport line has been designed to transport RIBs from CARIBU and inject them into CARIBU-EBIS. In addition, modifications to the existing ATLAS Low Energy Beam Transport (LEBT) were also required in order to transport the charge bred RIBs from CARIBU-EBIS to ATLAS. A proposal for upgrading ATLAS to a multi-user facility has been explored as well. In this context, beam dynamics simulations show that further modifications to the ATLAS LEBT will enable the simultaneous injection and acceleration of RIBs and stablebeams in ATLAS. Furthermore, a novel technique proposed by Ostroumov et al. will allow for the acceleration of multiple charge states from CARIBU-EBIS, thereby increasing the intensity of available RIBs by up to 60%.
Ph.D. in Physics, December 2015
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Numerical and Experimental Investigation to Improve Radio Frequency Performance of Photonic Band Gap Accelerating Structure
Title
Numerical and Experimental Investigation to Improve Radio Frequency Performance of Photonic Band Gap Accelerating Structure
Creator
Zhou, Ning
Date
2019
Description
In this thesis, the design and experimental work of a Photonic Band Gap (PBG) accelerator cavity with star-shape array is presented. Photonic...
Show moreIn this thesis, the design and experimental work of a Photonic Band Gap (PBG) accelerator cavity with star-shape array is presented. Photonic band gap structures (metallic and/ or dielectric) have been proposed for accelerator applications. These structures act like filters, allowing electromagnetic waves propagating at some frequencies to be transmitted through the lattice, while rejecting the RF fields in some (unwanted) frequency range. Additionally PBG structures are used to support selective field patterns (modes) in a resonator or waveguide by a defect region within the lattice; while damping unwanted higher- or lower-order modes without impacting the supported mode. The unwanted modes affect beam propagation or even distort the beam. Thus, suppression of unwanted modes is important. In this thesis work, a star shape structure is obtained from removing elements in a PBG structure with triangular lattice and employed for integration with a metallic cavity resonator for accelerator applications. Impedance matching is accomplished by adjustment of positions of some elements in the array. The design was fabricated and measured to have an input return loss of over 30 dB at the targeted frequency of 11.4GHz. The measured results are in an excellent agreement with the computer simulation.
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IRREGULAR GROWTH AND INTERFACIAL EFFECT IN THIN FILM MULTILAYER STRUCTURES FOR USES IN PHOTOCATHODE APPLICATIONS
Title
IRREGULAR GROWTH AND INTERFACIAL EFFECT IN THIN FILM MULTILAYER STRUCTURES FOR USES IN PHOTOCATHODE APPLICATIONS
Creator
Lee, ZhengRong
Date
2021
Description
Improving photocathode performance by increasing the electron density while lowering the angular spread of emitted electrons can improve...
Show moreImproving photocathode performance by increasing the electron density while lowering the angular spread of emitted electrons can improve particle accelerator performance, expanding the reach of both fundamental and applied science. Materials science expertise is needed to design new photocathodes with these desired properties. Nemeth, et al, determined that a multilayered photocathode structure consisting of MgO/Ag/MgO could be engineered for higher brightness and lower dispersion [Nemeth, et al, Phys. Rev. Lett. 104, 046801 (2010)]. The dispersion of the surface bands impacts the angular spread of the emitted beam, and the model predicted that the bands could be tuned by precisely controlling the layer thicknesses of the multilayer structure. We synthesized and probed this MgO/Ag/MgO system experimentally. We measured the work function, emittance, and quantum efficiency of multilayer photocathodes with different MgO layer thicknesses to compare with theoretical predictions. We observed that although the general trend was as predicted, the measurements and the model were not in exact agreement [Velasquez, et al, Appl. Surf. Sci. 360, 762 (2016)]. In this work, we have undertaken a study of the electronic structure of the interfaces to explore how these observed deviations may have originated. It is possible that the fabrication process leads to non-ideal interfaces compared to those constructed in the simulations. To study how the fabrication affects the interfaces, hard X-ray photoemission spectroscopy(HAXPES) was used to probe the chemistry of the buried interfaces within the thin film multilayer structure of Ag and MgO. In these multilayer structures, we observed that the silver layers were predominantly metallic. A small high binding energy (ΔE = 0.69 eV) peak was also observed in the Ag 3d core level in the samples. This peak is shifted in the opposite direction of the binding energy shift in silver oxides, suggesting that this peak is not due to the formation of silver oxides at the interfaces with the MgO. Two possible explanations for the origin of this peak then are charge transfer at the interface from the Ag to the oxide monolayer or the formation of silver nanoparticles during the growth process. Based upon simple depth profiling analysis, we postulate the former is the more likely explanation. In addition, the O 1s and Mg 1s core level indicated the presence of Mg(OH)2. The MgO layers react with H2O in the vacuum chamber or ideal gas used as a buffer during sample transfer. Since the theory predicts a strong dependence upon the number of MgO layers surrounding the Ag, the formation of Mg(OH)2 likely contributes to the non- ideal behavior, even given the similarity in the electronic structure to MgO (large band gap insulator) and Mg(OH)2. The speed at which this reaction occurs would significantly limit the lifetime and the utility of the MgO/Ag multilayer photocathodes. In order to custom engineer multilayer photocathodes, complete control over the growth process will be needed to ensure that the ideal surfaces are formed. Using non-reactive materials would greatly increase the lifetime of the engineered photocathodes.
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