Evaluation of primary water stress corrosion cracking growth rates by using the extended finite element method

AbstractBackgroundMitigation of primary water stress corrosion cracking (PWSCC) is a significant issue in the nuclear industry. Advanced nickel-based alloys with lower susceptibility have been adopted, although they do not seem to be entirely immune from PWSCC during normal operation. With regard to structural integrity assessments of the relevant components, an accurate evaluation of crack growth rate (CGR) is important.MethodsFor the present study, the extended finite element method was adopted from among diverse meshless methods because of its advantages in arbitrary crack analysis. A user-subroutine based on the strain rate damage model was developed and incorporated into the crack growth evaluation.ResultsThe proposed method was verified by using the well-known Alloy 600 material with a reference CGR curve. The analyzed CGR curve of the alternative Alloy 690 material was then newly estimated by applying the proven method over a practical range of stress intensity factors.ConclusionReliable CGR curves were obtained without complex environmental facilities or a high degree of experimental effort. The proposed method may be used to assess the PWSCC resistance of nuclear components subjected to high residual stresses such as those resulting from dissimilar metal welding parts

XB-MAML: Learning Expandable Basis Parameters for Effective Meta-Learning with Wide Task Coverage

Meta-learning, which pursues an effective initialization model, has emerged as a promising approach to handling unseen tasks. However, a limitation remains to be evident when a meta-learner tries to encompass a wide range of task distribution, e.g., learning across distinctive datasets or domains. Recently, a group of works has attempted to employ multiple model initializations to cover widely-ranging tasks, but they are limited in adaptively expanding initializations. We introduce XB-MAML, which learns expandable basis parameters, where they are linearly combined to form an effective initialization to a given task. XB-MAML observes the discrepancy between the vector space spanned by the basis and fine-tuned parameters to decide whether to expand the basis. Our method surpasses the existing works in the multi-domain meta-learning benchmarks and opens up new chances of meta-learning for obtaining the diverse inductive bias that can be combined to stretch toward the effective initialization for diverse unseen tasks.Comment: In Proceedings of the International Conference on Artificial Intelligence and Statistics (AISTATS) 2024, Valencia, Spai

Quasi-phasematched acceleration of electrons in a density modulated plasma waveguide

Two quasi-phasematching schemes are proposed for efficient acceleration of electrons to relativistic energies using moderate intensity laser pulses. In the first scheme, Direct Laser Acceleration (DLA) in a corrugated plasma waveguide is proposed for acceleration of relativistic electrons with sub-terawatt laser systems, using the laser field directly as the accelerating field. The second scheme uses the fact that a plasma wakefield generated by an intense guided pulse in a corrugated plasma waveguide can accelerate relativistic electrons significantly beyond the well-known dephasing limit. In each case, particle-in-cell (PIC) simulations are used to validate the acceleration concept, demonstrating linear acceleration by either the phase matched laser field or phase-matched wakefield. In the phase matched wakefield case, theory and PIC simulations demonstrate a significant increase in energy gain compared to the standard laser wakefield acceleration (LWFA) scheme. Corrugated plasma waveguides can be generated by the interaction between an ionizing laser pulse and an atomic cluster flow interrupted by an array of thin wires,. When the collisional mean free path of the clusters is greater than the wire diameter, shadows of the periodically located wires are imparted on the cluster flow, leading to the production of axially modulated plasma waveguides after laser heating of the flow. This occurs when the population ratio of clusters to monomers in the gas is high. At other limit, dominated by gas monomer flow, shock waves generated off the wires by the supersonic gas flow disrupts modulated waveguide generation. Lastly, we experimentally demonstrate LWFA with ionization injection in a N5+ plasma waveguide. It is first shown that the plasma waveguide is almost completely composed of He-like nitrogen (N5+). It is then shown that intense pulse channeling in the plasma waveguide drives stronger wakefields, while the ionization injection process is critical to lowering the laser intensity threshold for self-trapping

ATOMIC LUMINESCENCE INDUCED BY COULOMB EXPLOSION IN A SILICON METAL-OXIDE-SEMICONDUCTOR STRUCTURE

The Coulomb fragmentation phenomenon has been known to occur in matter at various different length scales, such as nuclei and atomic or molecular clusters, and microscopic droplets. In many cases, the Coulomb explosion is triggered by sudden ionization with high-intensity (>10²14 W/cm¹2) femto-second laser pulses. At this intensity level, the valence electrons are quickly ripped off and the ionized metal clusters fragment before thermalization occurs. In this thesis, we report Coulomb explosion of Ag atoms induced by electron impact ionization in a Si metal-oxide-semiconductor (MOS) structure (Ag/SiO2/Si). Under positive voltage pulses applied to the Ag gate, kinetic electrons are injected onto the gate through leakage channels formed in oxide and impact-ionize the metal atoms at the gate/dielectric interface. When the Coulomb repulsion among the ions becomes stronger than the binding force of metal atoms, the ions accumulated at the interface explode, atomizing the metal and also adjacent dielectrics. This explosive fragmentation results in atomic luminescence from neutral silver at 328 nm, 338 nm, 521 nm, 547 nm, 769 nm, and 827 nm. The mechanisms of oxide breakdown, localized injection of kinetic electrons, and atomization/luminescence are discussed