Browsing by Subject "Acceleration"
Now showing 1 - 3 of 3
- Results Per Page
- Sort Options
Item Generation, measurement and application of x-rays from laser-plasma electron accelerators(2021-12-03) Hannasch, Andrea Jeanne; Downer, Michael Coffin; Ditmire, Todd; Paban, Sonia; Ketcham, Richard; Cowan, TomThis dissertation presents a comprehensive study of the generation mechanisms, diagnostic techniques and possible applications of few keV to 100 MeV x-rays generated by laser wakefield electron accelerators. Chapters 1-3 review the principles of x-ray science and laser wakefield acceleration, and 3 mechanisms by which laser wakefield accelerators produce x-rays: 1) betatron oscillations of the electrons while still accelerating; 2) inverse Compton scatter (ICS) x-rays involving electron oscillations induced when electrons collide with a counter-propagating laser pulse after exiting the accelerator; 3) bremsstrahlung from the impact of accelerated electrons with a solid target. Chapters 4-6 then present original, recently published work, starting in chapter 4 with experiments that characterized secondary x-rays from a laser wakefield accelerator at Helmholtz-Zentrum Dresden Rossendorf. In this work, a laser wakefield accelerator was driven by the 150 TW DRACO laser system and produced electrons tunable in energy from 250 to 350 MeV. I co-designed and built a compact calorimeter consisting of a stack of x-ray absorbers alternating with imaging plates. This single device enabled me to unfold spectra of all three major types of x-rays, both individually and in mixtures: 1) few-keV betatron x-rays, 2) ICS x-rays that were spectrally peaked at ~1 MeV photon energy, and 3) broadband bremsstrahlung with an average energy of ~30 MeV and a high energy tail extending beyond 100 MeV photon energy. Chapter 5 presents results obtained at The University of Texas in which I extended the work in chapter 4 and used a redesigned compact calorimeter to characterize secondary x-rays generated from a GeV-class accelerator. In this work, the accelerator was driven by the 1 PW Texas Petawatt Laser (TPW) which accelerated electrons to energies ranging from 500 MeV to 2 GeV. The compact calorimeter was redesigned for improved sensitivity to photons from 1 MeV to >100 MeV and enabled me to unfold ICS x-rays that were peaked at ~10 MeV photon energy, and broadband bremsstrahlung with average energies ~80 MeV. Chapter 6 then presents additional results obtained on the DRACO laser system in which I characterized the capabilities of a LPA and plasma mirror to generate ICS x-rays in both a linear and nonlinear regime. I used a CsI(Tl) scintillator to characterize the strength and divergence of ICS x-rays generated by retro-reflecting the accelerator’s spent drive laser pulse back onto the accelerated electrons using a plasma mirror. These measurements showed that the laser-electron interaction ranged from sub-relativistic to relativistic, depending on the plasma mirror distance from the accelerator exit. Finally, chapter 7 presents unpublished results from the TPW and presents unfolded spectra from a bremsstrahlung target scan in which a series of targets ranging from 25 μm-thick Kapton to 7.6 mm-thick Pb were used to produce Bremsstrahlung with average energies ranging from 60 MeV to >100 MeV. Chapter 7 also presents preliminary results from the application of bremsstrahlung x-rays to nuclear activation of copper. This dissertation concludes with a summary of the presented results and a discussion of future directions for laser plasma produced x-ray science.Item QoS-aware mechanisms for improving cost-efficiency of datacenters(2018-05) Zhu, Haishan; Erez, Mattan; Pingali, Keshav; Chang, Jichuan; de Veciana, Gustavo; Tiwari, MohitWarehouse Scale Computers (WSCs) promise high cost-efficiency by amortizing power, cooling, and management overheads. WSCs today host a large variety of jobs with two broad performance requirements categories: latency-critical (LC) and best-effort (BE). Ideally, to fully utilize all hardware resources, WSC operators can simply fill all the nodes with computing jobs. Unfortunately, because colocated jobs contend for shared resources, systems with high loads often experience performance degradation, which negatively impacts the Quality of Service (QoS) for LC jobs. In fact, service providers usually over-provision resources to avoid any interference with LC jobs, leading to significant resource inefficiencies. In this dissertation, I explore opportunities across different system-abstraction layers to improve the cost-efficiency of dataceters by increasing resource utilization of WSCs with little or no impact on the performance of LC jobs. The dissertation has three main components. First, I explore opportunities to improve the throughput of multicore systems by reducing the performance variation of LC jobs. The main insight is that by reshaping the latency distribution curve, performance headroom of LC jobs can be effectively converted to improved BE throughput. I develop, implement, and evaluate a runtime system that achieves this goal with existing hardware. I leverage the cache partitioning, per-core frequency scaling, and thread masking of server processors. Evaluation results show the proposed solution enables 30% higher system throughput compared to solutions proposed in prior works while maintaining at least as good QoS for LC jobs. Second, I study resource contention in near-future heterogeneous memory architectures (HMA). This study is motivated by recent developments in non-volatile memory (NVM) technologies, which enable higher storage density at the cost of same performance. To understand the performance and QoS impact of HMAs, I design and implement a performance emulator in the Linux kernel that runs unmodified workloads with high accuracy, low overhead, and complete transparency. I further propose and evaluate multiple data and resource management QoS mechanisms, such as locality-aware page admission, occupancy management, and write buffer jailing. Third, I focus on accelerated machine learning (ML) systems. By profiling the performance of production workloads and accelerators, I show that accelerated ML tasks are highly sensitive to main memory interference due to fine-grained interaction between CPU and accelerator tasks. As a result, memory resource contention can significantly decreases the performance and efficiency gains of accelerators. I propose a runtime system that leverages existing hardware capabilities and show 17% higher system efficiency compared to previous approaches. This study further exposes opportunities for future processor architecturesItem Simulation and experimental analyses of human movement : application to post-stroke hemiparetic gait(2010-08) Peterson, Carrie Lynn, 1981-; Neptune, Richard R.; Abraham, Lawrence D.; Barr, Ronald E.; Kautz, Steven A.; Longoria, Raul G.Stroke is the leading cause of long term disability with improved walking being an important goal following stroke. Understanding deficits that result in reduced walking performance by hemiparetic subjects is important for the design of effective rehabilitation strategies. The goal of this research was to investigate muscle coordination and mechanical work in hemiparetic walking and mechanisms of acceleration and deceleration in nondisabled walking as a framework for investigating non-steady state walking in hemiparetic subjects. Musculoskeletal modeling and simulation analyses were used to compare individual muscle contributions to important walking subtasks and muscle mechanical work by representative hemiparetic subjects (limited community and community walkers) during pre-swing with a representative speed and age-matched control. Simulation analyses identified decreased paretic soleus and gastrocnemius contributions to forward propulsion and power generation as the primary impairment in the limited community walker compared to the control. Comparison of mechanical work showed that total paretic and non-paretic fiber work was increased in the limited community walker, which was primarily related to decreased fiber and tendon work by paretic soleus and gastrocnemius. The decreased output by the ankle plantar flexors required compensatory work by other muscles. Also, the experimental analyses of accelerated and decelerated walking showed that the ankle plantar flexor moment was positively related to braking and propulsive impulses, which increased with speed. Thus, deficits of the paretic plantar flexors limit forward propulsion and increase mechanical work during pre-swing, and would limit the ability of hemiparetic walkers to accelerate and decelerate, which are essential tasks in daily living activities. For the community walker, simulation analyses showed that deficits in paretic swing initiation are a primary impairment. Specifically, the paretic gastrocnemius and hip flexors contributed less to swing initiation in the community walker compared to the control subject. Total paretic and non-paretic fiber work was increased in the community walker, primarily due to increased work by the hip abductors and adductors. Because step length and step frequency were positively related to walking speed in accelerated and decelerated walking, impaired paretic swing initiation would likely limit the community walker’s ability to accelerate and decelerate.