4,534 research outputs found
On steady poloidal and toroidal flows in tokamak plasmas
The effects of poloidal and toroidalflows on tokamakplasma equilibria are examined in the magnetohydrodynamic limit. âTransonicâ poloidal flows of the order of the sound speed multiplied by the ratio of poloidal magnetic field to total field Bâ/B can cause the (normally elliptic) GradâShafranov (GS) equation to become hyperbolic in part of the solution domain. It is pointed out that the range of poloidal flows for which the GS equation is hyperbolic increases with plasma beta and Bâ/B, thereby complicating the problem of determining spherical tokamakplasma equilibria with transonic poloidal flows. It is demonstrated that the calculation of the hyperbolicity criterion can be easily modified when the assumption of isentropic flux surfaces is replaced with the more tokamak-relevant one of isothermal flux surfaces. On the basis of the latter assumption, a simple expression is obtained for the variation of density on a flux surface when poloidal and toroidalflows are simultaneously present. Combined with Thomson scattering measurements of density and temperature, this expression could be used to infer information on poloidal and toroidalflows on the high field side of a tokamakplasma, where direct measurements of flows are not generally possible. It is demonstrated that there are four possible solutions of the Bernoulli relation for the plasma density when the flux surfaces are assumed to be isothermal, corresponding to four distinct poloidal flow regimes. Finally, observations and first principles-based theoretical modeling of poloidal flows in tokamakplasmas are briefly reviewed and it is concluded that there is no clear evidence for the occurrence of supersonic poloidal flows.This work was jointly funded by the Australian Government
through International Science Linkages Grant No.
CG130047, the Australian National University, the United
Kingdom Engineering and Physical Sciences Research
Council, and by the European Communities under the contract
of Association between EURATOM and CCFE
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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
Toroidal ripple transport of beam ions in the mega-ampeÌre spherical tokamak
The transport of injected beam ions due to toroidalmagnetic field ripple in the mega-ampĂšre spherical tokamak (MAST) is quantified using a full orbit particle tracking code, with collisional slowing-down and pitch-angle scattering by electrons and bulk ions taken into account. It is shown that the level of ripple losses is generally rather low, although it depends sensitively on the major radius of the outer midplane plasma edge; for typical values of this parameter in MAST plasmas, the reduction in beam heating power due specifically to ripple transport is less than 1%, and the ripple contribution to beam ion diffusivity is of the order of 0.1 mÂČ sâ»Âč or less. It is concluded that ripple effects make only a small contribution to anomalous transport rates that have been invoked to account for measured neutron rates and plasma stored energies in some MAST discharges. Delayed (non-prompt) losses are shown to occur close to the outer midplane, suggesting that banana-drift diffusion is the most likely cause of the ripple-induced losses.This work was funded by the RCUK Energy Programme
under Grant EP/I501045, by the Australian Research Council,
and by the European Communities under the Contract of
Association between EURATOM and CCFE
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
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
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
Numerical simulations of chromospheric hard X-ray source sizes in solar flares
X-ray observations are a powerful diagnostic tool for transport,
acceleration, and heating of electrons in solar flares. Height and size
measurements of X-ray footpoints sources can be used to determine the
chromospheric density and constrain the parameters of magnetic field
convergence and electron pitch-angle evolution. We investigate the influence of
the chromospheric density, magnetic mirroring and collisional pitch-angle
scattering on the size of X-ray sources. The time-independent Fokker-Planck
equation for electron transport is solved numerically and analytically to find
the electron distribution as a function of height above the photosphere. From
this distribution, the expected X-ray flux as a function of height, its peak
height and full width at half maximum are calculated and compared with RHESSI
observations. A purely instrumental explanation for the observed source size
was ruled out by using simulated RHESSI images. We find that magnetic mirroring
and collisional pitch-angle scattering tend to change the electron flux such
that electrons are stopped higher in the atmosphere compared with the simple
case with collisional energy loss only. However, the resulting X-ray flux is
dominated by the density structure in the chromosphere and only marginal
increases in source width are found. Very high loop densities (>10^{11}
cm^{-3}) could explain the observed sizes at higher energies, but are
unrealistic and would result in no footpoint emission below about 40 keV,
contrary to observations. We conclude that within a monolithic density model
the vertical sizes are given mostly by the density scale-height and are
predicted smaller than the RHESSI results show.Comment: 19 pages, 9 figures, accepted for publication in Ap
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
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