Blair Thornton

Spectroscopic Measurements of Solids Immersed in Water at High Pressure Using a Long-Duration Nanosecond Laser Pulse

The effects of pressure on the emission lines of a submerged metallic plate irradiated by a long 150 ns duration laser pulse have been investigated. While spectroscopic measurements using multiple-pulse irradiation of submerged targets are sensitive to pressure, the interactions of a long-pulse occur during a time window where the properties of the laser pulse dominate the transient pressures surrounding the ablated region. The emission lines are not sensitive to external pressures of up to 30 MPa, and the results demonstrate that long-pulse ablation is available as an analytical technique to study solids immersed at high pressure.

Effects of Pressure on the Optical Emissions Observed from Solids Immersed in Water Using a Single Pulse Laser

The effects of pressure on the optical emissions of a laser ablated zinc plate immersed in water have been investigated. Well defined emission spectra were observed from plumes generated directly underwater after excitation using a single laser pulse of duration <10 ns. It was demonstrated that an increase in water pressure from 0.1 to 30 MPa (300 atm) does not have any significant effect on the intensity, broadness, or fluorescence lifetime of the observed spectra. The results suggest that laser-induced breakdown spectroscopy is, in principle, a technique suitable for in situ elemental analysis of deep sea minerals.

Development of a fiber-coupled laser-induced breakdown spectroscopy instrument for analysis of underwater debris in a nuclear reactor core

To inspect the post-accident nuclear core reactor of the TEPCO Fukushima Daiichi nuclear power plant (F1-NPP), a transportable fiber-coupled laser-induced breakdown spectroscopy (LIBS) instrument has been developed. The developed LIBS instrument was designed to analyze underwater samples in a high-radiation field by single-pulse breakdown with gas flow or double-pulse breakdown. To check the feasibility of the assembled fiber-coupled LIBS instrument for the analysis of debris material (mixture of the fuel core, fuel cladding, construction material and so on) in the F1-NPP, we investigated the influence of the radiation dose on the optical transmittance of the laser delivery fiber, compared data quality among various LIBS techniques for an underwater sample and studied the feasibility of the fiber-coupled LIBS system in an analysis of the underwater sample of the simulated debris in F1-NPP. In a feasible study conducted by using simulated debris, which was a mixture of CeO2 (surrogate of UO2), ZrO2 and Fe, we selected atomic lines suitable for the analysis of materials, and prepared calibration curves for the component elements. The feasible study has guaranteed that the developed fiber-coupled LIBS system is applicable for analyzing the debris materials in the F1-NPP.

Cavity Formation and Material Ablation for Single-Pulse Laser-Ablated Solids Immersed in Water at High Pressure

The effects of hydrostatic pressure on cavity formation and material ablation have been investigated for a brass plate immersed in water irradiated by a single laser pulse of duration <10 ns. Shadowgraph imaging and volumetric measurements of the ablated material demonstrate that the density of the material inside the cavity does not vary significantly for hydrostatic pressures between 0.1 and 30 MPa (300 atm) during the early stages, <600 ns after laser irradiation, indicating that the pressures induced by focusing a high-power laser in the confined medium dominate the transient pressure regime over this period.

On-Site Quantitative Elemental Analysis of Metal Ions in Aqueous Solutions by Underwater Laser-Induced Breakdown Spectroscopy Combined with Electrodeposition under Controlled Potential

We propose a technique of on-site quantitative analysis of Zn(2+) in aqueous solution based on the combination of electrodeposition for preconcentration of Zn onto a Cu electrode and successive underwater laser-induced breakdown spectroscopy (underwater LIBS) of the electrode surface under electrochemically controlled potential. Zinc emission lines are observed with the present technique for a Zn(2+) concentration of 5 ppm. It is roughly estimated that the overall sensitivity over 10 000 times higher is achieved by the preconcentration. Although underwater LIBS suffers from the spectral deformation due to the dense plasma confined in water and also from serious shot-to-shot fluctuations, a linear calibration curve with a coefficient of determination R(2) of 0.974 is obtained in the range of 5-50 ppm.

Distribution of local 137Cs anomalies on the seafloor near the Fukushima Dai-ichi Nuclear Power Plant

An estimated 3.5±0.7×10(15) Bq of (137)Cs is thought to have been discharged into the ocean following the melt down at Fukushima Dai-ichi Nuclear Power Plant (F1NPP). While efforts have been made to monitor seafloor radiation levels, the sampling techniques used cannot capture the continuous distribution of radionuclides. In this work, we apply in situ measurement techniques using a towed gamma ray spectrometer to map the continuous distribution of (137)Cs on the seafloor within 20 km of the F1NPP. The results reveal the existence of local (137)Cs anomalies, with levels of (137)Cs an order of magnitude higher than the surrounding seafloors. The sizes of the anomalies mapped in this work range from a few meters to a few hundreds of meters in length, and it is demonstrated that the distribution of these anomalies is strongly influenced by meter scale features of the terrain.

Investigation of Influence of Hydrostatic Pressure on Double-Pulse Laser-Induced Breakdown Spectroscopy for Detection of Cu and Zn in Submerged Solids

The effects of pressure on double-pulse laser-induced breakdown spectroscopy (LIBS) for analysis of the composition of solids submerged in water have been investigated. It has been found that while an increase in water pressure results in an overall reduction in plasma temperature and increased broadness in the observed spectra, analytically useful spectra can be observed up to 5 MPa (50 atm). The results suggest that double-pulse laser-induced breakdown spectroscopy may be suitable for in situ measurement of the chemical composition of solids submerged in lakes, rivers, and shallow seas.

Development and field testing of laser-induced breakdown spectroscopy for in situ multi-element analysis at sea

The application of laser-induced plasmas has been investigated as a mechanism to perform in situ, multi-element chemical analysis of liquids and immersed solids at sea during marine surveys. Analytically useful spectra have been observed from plasmas generated by irradiation of a high power pulsed laser in both bulk liquids and immersed solids using a single pulse at hydrostatic pressures of up to 30MPa. Experiments were performed at sea using the in situ laser-induced breakdown spectroscopy (LIBS) device I-SEA (In situ Seafloor Element Analyser) mounted on-board the remotely operated vehicle (ROV) Hyper-Dolphin of the Japan Agency for Marine Science and Technology (JAMSTEC). During the sea trials real-time, multielement analysis was successfully achieved for the first time for both liquids and immersed solids at a depth of 200 m.

Internal actuation of underwater robots using Control Moment Gyros

A novel control scheme based on internal actuation using Control Moment Gyros (CMGs) is proposed as the basis of an unrestricted attitude control system suitable for application to Autonomous Underwater Vehicles (AUVs). The coupled dynamic equations of a CMG actuated underwater robot are derived, taking into account interactions with the fluid environment. A nonlinear feedback control law is developed based on energy considerations of the coupled system. Singularities, redundancy and null motion are discussed in the context of CMGs and a mathematical escapability condition is developed based on the differential geometry of null motion. A geometric study of a CMG pyramids singularities forms the basis of a global steering law that guarantees real-time singularity avoidance whilst maintaining exactness in some constrained workspace. The practical design considerations of a CMG pyramid developed for a laboratory scale underwater robot are discussed. Finally the results of practical experiments using the CMG pyramid are presented to demonstrate the steering and control performance of the system.

Development of a landing algorithm for autonomous underwater vehicles using laser profiling

The autonomous underwater vehicle has proven to be a useful tool for ocean research. However detailed seafloor observations such as microscopic analysis of sand grain structure or study of microbial colonies require a platform with stable footing on the seafloor, which cannot be provided by a cruising type AUV. In this research the authors propose a new class of AUV, capable of landing to provide a stable, but mobile, platform with which to perform these observations. This paper proposes a sensing system and a software algorithm to enable the AUVs to perform landing. A light sectioning based method is used to scan the seafloor with high resolution. Since the seafloor can change abruptly and at short intervals, the reliability and functioning of such technology requires real-time seafloor classification and detection of suitable landing sites. A landing algorithm has been developed which uses three dimensional bathymetry data and calculates a landing vector coordinate in real-time. A microscope sensor payload developed to obtain magnified images of the seafloor after landing has also been tested. Data from sea experiments are presented, where the algorithm demonstrated real-time generation of landing vector coordinates for an ROV.