Dynamic Homogenization of Acoustic Metamaterials with Coupled Field Response

AbstractAcoustic metamaterials (AMM) are heterogeneous materials with dynamic subwavelength structures that can generate useful effective responses of interest to ultrasonic imaging applications such as negative refraction and zero index. Traditional effective medium models fail to capture details of frequency dependent AMM response and can give non-causal properties. This work derives non-local expressions for effective properties for an infinite periodic lattice of heterogeneities in an isotropic fluid using conservation of mass and momentum and the equation of state. The resulting model correctly predicts a causal effective material response by considering coupling between the ensemble-averaged volume strain and momentum fields

High Energy Electron Confinement in a Magnetic Cusp Configuration

We report experimental results validating the concept that plasma confinement is enhanced in a magnetic cusp configuration when beta (plasma pressure/magnetic field pressure) is order of unity. This enhancement is required for a fusion power reactor based on cusp confinement to be feasible. The magnetic cusp configuration possesses a critical advantage: the plasma is stable to large scale perturbations. However, early work indicated that plasma loss rates in a reactor based on a cusp configuration were too large for net power production. Grad and others theorized that at high beta a sharp boundary would form between the plasma and the magnetic field, leading to substantially smaller loss rates. The current experiment validates this theoretical conjecture for the first time and represents critical progress toward the Polywell fusion concept which combines a high beta cusp configuration with an electrostatic fusion for a compact, economical, power-producing nuclear fusion reactor.Comment: 12 pages, figures included. 5 movies in Ancillary file

Moving from parent consultant to parent collaborator : one pediatric research team\u27s experience.

We share our experiences as academic physical therapists and parents of young people with cerebral palsy working together as a research team, describe and critically review how our working relationship has evolved and propose further enhancements to realize our shared vision. This manuscript is informed by a call for “family-centered research,” transcripts of face-to-face meetings held over a period of 11/2 days, the INVOLVE document and our experiences over almost a decade, as well as other related literature. Authentic collaborative research partnerships between academic researchers and parents embodying trust, mutual respect and shared social responsibility take time and effort to develop and sustain. Rehabilitation research is more meaningful and may be more impactful when strong collaborative partnerships between researchers and health service users are in place

Stability of large vacancy clusters in silicon

Using a density-functional-based tight-binding method we investigate the stability of various vacancy clusters up to a size of 17 vacancies. Additionally, we compute the positron lifetimes for the most stable structures to compare them to experimental data. A simple bond-counting model is extended to take into account the formation of new bonds. This yields a very good agreement with the explicitly calculated formation energies of the relaxed structures for V6 to V14. The structures, where the vacancies form closed rings, such as V6 and V10, are especially stable against dissociation. For these structures, the calculated dissociation energies are in agreement with experimentally determined annealing temperatures and the calculated positron lifetimes are consistent with measurements.Peer reviewe