High Power Laser - Plasma Interactions for Homeland Security Applications

Laser-Plasma Interactions;Homeland Security;Nuclear Engineering and Radiological Sciences;Engineering;Nuclear Engineering & Radiological Sciences

Hah, JungmooNees, John A ;

University of Michigan

关键词: Laser-Plasma Interactions;    Homeland Security;    Nuclear Engineering and Radiological Sciences;    Engineering;    Nuclear Engineering & Radiological Sciences;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/145909/jmhah_1.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Advance in laser technology over the last few decades have allowed progress in intense laser-plasma interaction research. The relativistic plasma generated by intense laser pulses can generate many different forms of radiation. This radiation, including X-rays, has been studied intensively due to the numerous potential applications of these sources. For example, for Homeland Security, radiation sources are already utilized to detect dangerous materials and hidden items that threaten civil safety. Neutrons and THz radiation have been studied as candidates for next generation screening, which may complement typical X-ray techniques. This thesis contains three experimental studies of high-power laser-plasma interactions as sources of radiation for Homeland Security applications, especially at kilohertz repetition-rates using few- millijoule pulses.First, a neutron generation experiment was conducted using a high repetition-rate laser system (1⁄2 kHz) at the University of Michigan. A heavy water (D2O) stream was irradiated by 40 fs pulses, each containing a few millijoules of energy. Acceleration of deuterons (to E < 1 MeV) was achieved through plasma sheath acceleration. Ensuing DD nuclear fusion reactions, in turn, generated neutron fluxes of up to 10^5 s^−1 into 4π steradians. In order to understand the neutron source characteristics, deuteron spectra were measured with CR39 detectors and compared to particle-in-cell (PIC) relativistic plasma dynamics simulations. The neutron source characteristics were analyzed using various neutron detection techniques, including Time-of-Flight measurements, bubble detectors, and neutron-capture gamma-ray measurements.Second, THz generation from laser filamentation in air was investigated. For security applications, THz can complement X-ray scanning, because THz can detect non-metallic materials and dangerous chemicals while not ionizing the sample. Even though there have been extensive studies on THz generation from laser filamentation processes, the exact generation mechanisms are yet to be determined. In this thesis, optimization of THz radiation using an adaptive optic with active feedback was demonstrated. Using a genetic algorithm, the THz radiation was improved six-fold without the need for detailed knowledge of the mechanisms. In particular, the use of a high repetition-rate laser system accelerated the optimization of the THz signal. Another strength of this optimization system is that it can enhance certain THz generation mechanisms depending on the experimental circumstances.Lastly, using a nanosecond pulsed high-power laser system (10 Hz), a long-range detection technique was developed for detection of special nuclear materials. Although direct detection of radiation from nuclear materials can be defeated by radiation shielding, leakage of radiation-ionized gases can provide an alternative indicator of the existence of nuclear materials. For instance, in the presence of ionizing radiation, the ratio of ionized nitrogen to neutral nitrogen would be higher than in no-source air-plasma conditions. By inducing optical breakdown (plasma) near a sample’s position, the ionization levels of the surrounding air were analyzed. To enhance the detection efficiency, an adaptive-optic feedback system was introduced with this ratio as a figure-of-merit. This resulted in a 50 % enhancement in the spectral ratio of the nitrogen lines. In addition, aerosol-initiated plasma spectra were distinguished from the original air-breakdown plasma, as a step toward practical deployment.