Azimuthally symmetric MHD and two-fluid equilibria with arbitrary flows

Magnetohydrodynamic (MHD) and two-fluid quasi-neutral equilibria with azimuthal symmetry, gravity and arbitrary ratios of (nonrelativistic) flow speed to acoustic and Alfven speeds are investigated. In the two-fluid case, the mass ratio of the two species is arbitrary, and the analysis is therefore applicable to electron-positron plasmas. The methods of derivation can be extended in an obvious manner to several charged species. Generalized Grad-Shafranov equations, describing the equilibrium magnetic field, are derived. Flux function equations and Bernoulli relations for each species, together with Poisson's equation for the gravitational potential, complete the set of equations required to determine the equilibrium. These are straightforward to solve numerically. The two-fluid system, unlike the MHD system, is shown to be free of singularities. It is demonstrated analytically that there exists a class of incompressible MHD equilibria with magnetic field-aligned flow. A special sub--class first identified by S. Chandrasekhar, in which the flow speed is everywhere equal to the local Alfven speed, is compatible with virtually any azimuthally symmetric magnetic configuration. Potential applications of this analysis include extragalactic and stellar jets, and accretion disks.Comment: 18 pages, 0 figure

Full orbit simulations of collisional impurity transport in spherical tokamak plasmas with strongly-sheared electric fields

The collisional dynamics of test impurity ions in spherical tokamak plasmas with strongly-sheared radial electric fields is investigated by means of a test particle full orbit simulation code. The strength of the shear is such that the standard drift ordering can no longer be assumed and a full orbit approach is required. The effect of radial electric field shear on neoclassical particle transport is quantified for a range of test particle mass and charge numbers and electric field parameters. It is shown that the effect of a sheared electric field is to enhance the confinement of impurity species above the level observed in the absence of such a field. The effect may be explained in terms of a collisional drag force drift, which is proportional to particle charge number but independent of particle mass. This drift acts inwards for negative radial electric fields and outwards for positive fields, implying strongly enhanced confinement of highly ionized impurity ions in the presence of a negative radial electric field.Comment: 16 pages, 6 figures, submitted to Nuclear Fusio

Field-guided proton acceleration at reconnecting X-points in flares

An explicitly energy-conserving full orbit code CUEBIT, developed originally to describe energetic particle effects in laboratory fusion experiments, has been applied to the problem of proton acceleration in solar flares. The model fields are obtained from solutions of the linearised MHD equations for reconnecting modes at an X-type neutral point, with the additional ingredient of a longitudinal magnetic field component. To accelerate protons to the highest observed energies on flare timescales, it is necessary to invoke anomalous resistivity in the MHD solution. It is shown that the addition of a longitudinal field component greatly increases the efficiency of ion acceleration, essentially because it greatly reduces the magnitude of drift motions away from the vicinity of the X-point, where the accelerating component of the electric field is largest. Using plasma parameters consistent with flare observations, we obtain proton distributions extending up to gamma-ray-emitting energies (>1MeV). In some cases the energy distributions exhibit a bump-on-tail in the MeV range. In general, the shape of the distribution is sensitive to the model parameters.Comment: 14 pages, 4 figures, accepted for publication in Solar Physic

Electron Inertial Effects on Rapid Energy Redistribution at Magnetic X-points

The evolution of non-potential perturbations to a current-free magnetic X-point configuration is studied, taking into account electron inertial effects as well as resistivity. Electron inertia is shown to have a negligible effect on the evolution of the system whenever the collisionless skin depth is less than the resistive scale length. Non-potential magnetic field energy in this resistive MHD limit initially reaches equipartition with flow energy, in accordance with ideal MHD, and is then dissipated extremely rapidly, on an Alfvenic timescale that is essentially independent of Lundquist number. In agreement with resistive MHD results obtained by previous authors, the magnetic field energy and kinetic energy are then observed to decay on a longer timescale and exhibit oscillatory behavior, reflecting the existence of discrete normal modes with finite real frequency. When the collisionless skin depth exceeds the resistive scale length, the system again evolves initially according to ideal MHD. At the end of this ideal phase, the field energy decays typically on an Alfvenic timescale, while the kinetic energy (which is equally partitioned between ions and electrons in this case) is dissipated on the electron collision timescale. The oscillatory decay in the energy observed in the resistive case is absent, but short wavelength structures appear in the field and velocity profiles, suggesting the possibility of particle acceleration in oppositely-directed current channels. The model provides a possible framework for interpreting observations of energy release and particle acceleration on timescales down to less than a second in the impulsive phase of solar flares.Comment: 30 pages, 8 figure

Recent Advances in Encapsulation, Protection, and Oral Delivery of Bioactive Proteins and Peptides using Colloidal Systems

There are many areas in medicine and industry where it would be advantageous to orally deliver bioactive proteins and peptides (BPPs), including ACE inhibitors, antimicrobials, antioxidants, hormones, enzymes, and vaccines. A major challenge in this area is that many BPPs degrade during storage of the product or during passage through the human gut, thereby losing their activity. Moreover, many BPPs have undesirable taste profiles (such as bitterness or astringency), which makes them unpleasant to consume. These challenges can often be overcome by encapsulating them within colloidal particles that protect them from any adverse conditions in their environment, but then release them at the desired site-of-action, which may be inside the gut or body. This article begins with a discussion of BPP characteristics and the hurdles involved in their delivery. It then highlights the characteristics of colloidal particles that can be manipulated to create effective BPP-delivery systems, including particle composition, size, and interfacial properties. The factors impacting the functional performance of colloidal delivery systems are then highlighted, including their loading capacity, encapsulation efficiency, protective properties, retention/release properties, and stability. Different kinds of colloidal delivery systems suitable for encapsulation of BPPs are then reviewed, such as microemulsions, emulsions, solid lipid particles, liposomes, and microgels. Finally, some examples of the use of colloidal delivery systems for delivery of specific BPPs are given, including hormones, enzymes, vaccines, antimicrobials, and ACE inhibitors. An emphasis is on the development of food-grade colloidal delivery systems, which could be used in functional or medical food applications. The knowledge presented should facilitate the design of more effective vehicles for the oral delivery of bioactive proteins and peptides

The quasi-linear relaxation of thick-target electron beams in solar flares

The effects of quasi-linear interactions on thick-target electron beams in the solar corona are investigated. Coulomb collisions produce regions of positive gradient in electron distributions which are initially monotonic decreasing functions of energy. In the resulting two-stream instability, energy and momentum are transferred from electrons to Langmuir waves and the region of positive slope in the electron distribution is replaced by a plateau. In the corona, the timescale for this quasi-linear relaxation is very short compared to the collision time. It is therefore possible to model the effects of quasi-linear relaxation by replacing any region of positive slop in the distribution by a plateau at each time step, in such a way as to conserve particle number. The X-ray bremsstrahlung and collisional heating rate produced by a relaxed beam are evaluated. Although the analysis is strictly steady state, it is relevant to the theoretical interpretation of hard X-ray bursts with durations of the order of a few seconds (i.e., the majority of such bursts)

A critical Mach number for electron injection in collisionless shocks

Electron acceleration in collisionless shocks with arbitrary magnetic field orientations is discussed. It is shown that the injection of thermal electrons into diffusive shock acceleration process is achieved by an electron beam with a loss-cone in velocity space that is reflected back upstream from the shock through shock drift acceleration mechanism. The electron beam is able to excite whistler waves which can scatter the energetic electrons themselves when the Alfven Mach number of the shock is sufficiently high. A critical Mach number for the electron injection is obtained as a function of upstream parameters. The application to supernova remnant shocks is discussed.Comment: 4 pages, 2 figure, accepted for publication in Physical Review Letter

Molecular Gastronomy: A Food Fad or an Interface for Science-based Cooking?

A review is given over the field of molecular gastronomy and its relation to science and cooking. We begin with a brief history of the field of molecular gastronomy, the definition of the term itself, and the current controversy surrounding this term. We then highlight the distinction between molecular gastronomy and science-based cooking, and we discuss both the similarities and the distinctions between science and cooking. In particular, we highlight the fact that the kitchen serves as an ideal place to foster interactions between scientists and chefs that lead to benefits for the general public in the form of novel and high-quality foods. On the one hand, it can facilitate the implementation of new ideas and recipes in restaurants. On the other hand, it challenges scientists to apply their fundamental scientific understanding to the complexities of cooking, and it challenges them to expand the scientific understanding of many chemical and physical mechanisms beyond the common mass-produced food products. In addition, molecular gastronomy forms an ideal base to educate the general public about the basic principles of science and cooking and how they can be utilized to improve the awareness of the role of food and nutrition for the quality of life