Doctor of Philosophy, The Ohio State University, 2017, Physics

In the interaction of high intensity (> 10^18 W/cm^2), ultrashort (< 1 ps) laser pulses with solid targets, the laser couples energy into the target through a population of energetic electrons. An electrostatic sheath field is formed along the target surface due to the induced charge separation between the expanding energetic electron cloud and relatively heavy, slow ions. This electrostatic field is capable of accelerating ions to energies greater than 10 MeV/nucleon through the physical mechanism known as target normal sheath acceleration (TNSA). Since its discovery in 2000, TNSA has been widely studied throughout the High Energy Density Physics community in order to understand and enhance this process for its potential applications ranging from cancer therapy to radiography. However, much of this work has been conducted on thin (1s to 10s of microns thick) metal foils exploring optimization only through critical laser parameters such as intensity, energy, and prepulse. This thesis addresses the development of novel micro-structured targets for the enhancement of TNSA ions. These targets were developed through the use of optical lithography techniques to pattern micron-scale structures onto thin substrates at the OSU Nanosystem Laboratory (NSL) user facility. These targets permit on-demand patterning of designs with 1 µm lateral resolution, heights varying between 0.5-2 µm, on substrates ranging from 0.5-2 µm thick with high throughput. An experiment was conducted at the Scarlet laser facility to characterize the effect of structuring the laser-target interface on the energy spectrum of TNSA protons. A comparison of the proton dose from structured versus at photoresist on silicon nitride substrates was performed. Results obtained using radiochromic film demonstrate an increase in proton dose by a factor of 2.3 through the use of structured photoresist as compared to unstructured photoresist of the same thickness. Two dimensional absorption simulat (open full item for complete abstract)

Committee: Douglass Schumacher (Advisor); Louis DiMauro (Committee Member); Jay Gupta (Committee Member); Ulrich Heinz (Committee Member); Kurt Koelling (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2016, Physics

Over the past two decades, a number of experiments have been performed demonstrating the acceleration of ions from the interaction of an intense laser pulse with a thin, solid density target. These ions are accelerated by quasi-static electric fields generated by energetic electrons produced at the front of the target, resulting in ion energies up to tens of MeV. These ions have been widely studied for a variety of potential applications ranging from treatment of cancer to the production of neutrons for advanced radiography techniques. However, realization of these applications will require further optimization of the maximum energy, spectrum, or species of the accelerated ions, which has been a primary focus of research to date. This thesis presents two experiments designed to optimize several characteristics of the accelerated ion beam. The first of these experiments took place on the GHOST laser system at the University of Texas at Austin, and was designed to demonstrate reliable acceleration of deuterium ions, as needed for the most efficient methods of neutron generation from accelerated ions. This experiment leveraged cryogenically cooled targets coated in D2O ice to suppress the protons which typically dominate the accelerated ions, producing as many as 2 x 10^10 deuterium ions per 1 J laser shot, exceeding the proton yield by an average ratio of 5:1. The second major experiment in this work was performed on the Scarlet laser system at The Ohio State University, and studied the accelerated ion energy, yield, and spatial distribution as a function of the target thickness. In principle, the peak energy increases with decreasing target thickness, with the thinnest targets accessing additional acceleration mechanisms which provide favorable scaling with the laser intensity. However, laser prepulse characteristics provide a lower bound for the target thickness, yielding an optimum target thickness for ion acceleration which is dependent on the las (open full item for complete abstract)

Committee: Linn Van Woerkom (Advisor); Robert Perry (Committee Member); Richard Furnstahl (Committee Member); Gregory Lafyatis (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2015, Physics

This thesis presents the development of simulations modeling ion acceleration using the particle-in-cell code LSP. A new technique was developed to model the Target Normal Sheath Acceleration (TNSA) mechanism. Multiple simulations are performed, each optimized for a certain part of the TNSA process with appropriate information being passed from one to the next. The technique allows for tradeoffs between accuracy and speed. Physical length and timescales are met when necessary and different physical models are employed as needed. This TNSA modeling technique is used to perform a study on the effect front-surface structures have on the resulting ion acceleration. The front-surface structures tested have been shown to either modify the electron kinetic energy spectrum by increasing the maximum energy obtained or by increasing the overall coupling of laser energy to electron energy. Both of these types of front-surface structures are tested for their potential benefits for the accelerated ions. It is shown that optimizing the coupling of laser energy to electron energy is more important than producing extremely energetic electrons in the case of the TNSA ions. Simulations modeling the interaction of an intense laser with very thin (<100 nm thick) liquid crystal targets, modeled for the first time, are presented. Modeling this interaction is difficult and the effect of different simulation design choices is explored in depth. In particular, it is shown that the initial electron temperature used in the simulation has a significant effect on the resulting ion acceleration and light transmitted through the target. This behavior is explored through numerous 1D simulations.

Committee: Douglass Schumacher (Advisor); Richard Furnstahl (Committee Member); Gregory Lafyatis (Committee Member); Fengyuan Yang (Committee Member) Subjects: Physics