The Light-ion Pulsed Power Induction Accelerator for the Laboratory Microfusion Facility (LMF)

In order to initiate ignition and substantial energy yield from an inertial confinement fusion target (ICF), a light-ion pulse of ~700 TW peak power and 15-20 ns duration is required. The preconceptual design presented provides this power. The HERMES-III technology of linear inductive voltage addition in a self-magnetically insulated transmission line (MITL) is utilized to generate the 25-36 MV peak voltage needed for lithium ion beams. The 15-20 MA ion current is achieved by utilizing many accelerating modules in parallel. The lithium ion beams are produced in two-stage extraction diodes. To provide the two separate voltage pulses required by the diode, a triaxial adder system is incorporated in each module. The accelerating modules are arranged symmetrically around the fusion chamber in order to provide uniform irradiation onto the ICF target. In addition, the modules are fired in a preprogrammed sequence in order to generate the optimum power pulse shape onto the target. In this paper we present an outline of the LMF accelerator conceptual design with emphasis on the architecture of the accelerating modules

Acoustic Emission from Paper Fracture

We report tensile failure experiments on paper sheets. The acoustic emission energy and the waiting times between acoustic events follow power-law distributions. This remains true while the strain rate is varied by more than two orders of magnitude. The energy statistics has the exponent $\beta \sim 1.25 \pm 0.10$ and the waiting times the exponent $\tau \sim 1.0 \pm 0.1$, in particular for the energy roughly independent of the strain rate. These results do not compare well with fracture models, for (brittle) disordered media, which as such exhibit criticality. One reason may be residual stresses, neglected in most theories.Comment: 4 pages, 5 figure

Rupture by damage accumulation in rocks

The deformation of rocks is associated with microcracks nucleation and propagation, i.e. damage. The accumulation of damage and its spatial localization lead to the creation of a macroscale discontinuity, so-called "fault" in geological terms, and to the failure of the material, i.e. a dramatic decrease of the mechanical properties as strength and modulus. The damage process can be studied both statically by direct observation of thin sections and dynamically by recording acoustic waves emitted by crack propagation (acoustic emission). Here we first review such observations concerning geological objects over scales ranging from the laboratory sample scale (dm) to seismically active faults (km), including cliffs and rock masses (Dm, hm). These observations reveal complex patterns in both space (fractal properties of damage structures as roughness and gouge), time (clustering, particular trends when the failure approaches) and energy domains (power-law distributions of energy release bursts). We use a numerical model based on progressive damage within an elastic interaction framework which allows us to simulate these observations. This study shows that the failure in rocks can be the result of damage accumulation

Time-dependent brittle creep in Darley Dale sandstone

The characterization of time-dependent brittle rock deformation is fundamental to understanding the long-term evolution and dynamics of the Earth's crust. The chemical influence of pore water promotes time-dependent deformation through stress corrosion cracking that allows rocks to deform at stresses far below their short-term failure strength. Here, we report results from a study of time-dependent brittle creep in water-saturated samples of Darley Dale sandstone (initial porosity, 13%) under triaxial stress conditions. Results from conventional creep experiments show that axial strain rate is heavily dependent on the applied differential stress. A reduction of only 10% in differential stress results in a decrease in strain rate of more than two orders of magnitude. However, natural sample variability means that multiple experiments must be performed to yield consistent results. Hence we also demonstrate that the use of stress-stepping creep experiments can successfully overcome this issue. We have used the stress-stepping technique to investigate the influence of confining pressure at effective confining pressures of 10, 30, and 50 MPa (while maintaining a constant 20 MPa pore fluid pressure). Our results demonstrate that the stress corrosion process appears to be significantly inhibited at higher effective pressures, with the creep strain rate reduced by multiple orders of magnitude. The influence of doubling the pore fluid pressure, however, while maintaining a constant effective confining pressure, is shown to influence the rate of stress corrosion within the range expected from sample variability. We discuss these results in the context of microstructural analysis, acoustic emission hypocenter locations, and fits to proposed macroscopic creep laws

Effects of crystallographic anisotropy on fracture development and acoustic emission in quartz

Transgranular microcracking is fundamental for the initiation and propagation of all fractures in rocks. The geometry of these microcracks is primarily controlled by the interaction of the imposed stress field with the mineral elastic properties. However, the effects of anisotropic elastic properties of minerals on brittle fracture are not well understood. This study examines the effects of elastic anisotropy of quartz on the geometry of brittle fracture and related acoustic emissions (AE) developed during indentation experiments on single crystals at ambient pressure and temperature. A Hertzian cone crack developed during blunt indentation of a single crystal of flawless Brazilian quartz parallel to the c axis shows geometric deviation away from predictions based on the isotropic case, consistent with trigonal symmetry. The visible cone crack penetration depth varies from 3 to 5 mm and apical angle from 53 to 40. Electron backscatter diffraction (EBSD) mapping of the crack tip shows that fracturing initiates along a ~40 μm wide process zone, comprising damage along overlapping en echelon high-index crystallographic planes, shown by discrete bands of reduced electron backscatter pattern (EBSP) quality (band contrast).Coalescence of these surfaces results in a stepped fracture morphology. Monitoring of AE during indentation reveals that the elastic anisotropy of quartz has a significant effect on AE location and focal mechanisms. Ninety-four AE events were recorded during indentation and show an increasing frequency with increasing load. They correspond to the development of subsidiary concentric cracks peripheral to the main cone crack. The strong and complex anisotropy in seismic velocity (~28% Vp, ~43% Vs with trigonal symmetry) resulted in inaccurate and high uncertainty in AE locations using Geiger location routine with an isotropic velocity model. This problem was overcome by using a relative (master event) location algorithm that only requires a priori knowledge of the velocity structure within the source volume. The AE location results correlate reasonably well to the extent of the observed cone crack. Decomposition of AE source mechanisms of the Geiger relocated events shows dominantly end-member behavior between tensile and compressive vector dipole events, with some double-couple-dominated events and no purely tensile or compressive events. The same events located by the master event algorithm yield greater percentage of vector dipole components and no double-couple events, indicating that AE source mechanism solutions can depend on AE location accuracy, and therefore, relocation routine that is utilized. Calculations show that the crystallographic anisotropy of quartz causes apparent deviation of the moment tensors away from double-couple and pure tensile/compressive sources consistent with the observations. Preliminary modeling of calcite anisotropy shows a response distinct from quartz, indicating that the effects of anisotropy on interpreting AE are complex and require detailed further study

Enhanced deformation of limestone and sandstone in the presence of high pCO2 fluids

International audienceGeological repositories subject to the injection of large amounts of anthropogenic carbon dioxide will undergo chemical and mechanical instabilities for which there are currently little experimental data. This study reports on experiments where low and high pCO2 (8~MPa) aqueous fluids where injected into natural rock samples. The experiments were performed in flow-through triaxial cells, where the vertical and confining stresses, temperature, and pressure and composition of the fluid were separately controlled and monitored. The axial vertical strains of two limestones and one sandstone were continuously measured during separate experiments for several months, with a strain rate resolution of 10^-11 s-1. Fluids exiting the triaxial cells where continuously collected and their compositions analysed. The high pCO2 fluids induced an increase in strain rates of the limestones by up to a factor of 5, compared to the low pCO2 fluids. Injection of high pCO2 fluids into the sandstone resulted in deformation rates one order of magnitude smaller than the limestones. The creep accelerating effect of high pCO2 fluids with respect to the limestones was mainly due to the acidification of the injected fluids, resulting in a significant increase in solubility and reaction kinetics of calcite. Compared to the limestones, the much weaker response of the sandstone was due to the much lower solubility and reactivity of quartz in high pCO2 fluids. In general, all samples showed a positive correlation between fluid flow rate and strain rate. X-ray tomography results revealed significant increases in porosity at the inlet portion of each core; the porosity increases were dependent on the original lithological structure and composition. The overall deformation of the samples is interpreted in terms of simultaneous dissolution reactions in pore spaces and intergranular pressure solution creep