7,801 research outputs found
Local and nonlocal parallel heat transport in general magnetic fields
A novel approach that enables the study of parallel transport in magnetized
plasmas is presented. The method applies to general magnetic fields with local
or nonlocal parallel closures. Temperature flattening in magnetic islands is
accurately computed. For a wave number , the fattening time scales as
where is the parallel
diffusivity, and () for non-local (local) transport. The
fractal structure of the devil staircase temperature radial profile in weakly
chaotic fields is resolved. In fully chaotic fields, the temperature exhibits
self-similar evolution of the form , where is a
radial coordinate. In the local case, is Gaussian and the scaling is
sub-diffusive, . In the non-local case, decays algebraically,
, and the scaling is diffusive,
Local and Regional Food Aid Procurement: An Assessment of Experience in Africa and Elements of Good Donor Practice
This Policy Synthesis is a summary of a longer report that discusses the procurement of food aid within the country or region where it is needed. Referred to as local and regional procurement – LRP – this practice has become a major element in multilateral food aid response over the past decade1. The paper examines the relevance and the rationale for using LRP, reviews the efficiency of World Food Program (WFP) LRP activities in Africa relative to inkind food aid and to prices in the markets in which it occurs, and proposes a classification of risks in LRP. It then discusses a range of potential LRP modalities, and proposes a framework of guiding principles, information systems, and operational procedures for responsible and effective LRP. Finally, the paper briefly discusses the implications of this research for expansion of U.S. government (USG) authority to engage in LRP.Crop Production/Industries, Farm Management,
Generalized hydrodynamics of a dilute finite-sized particles suspension: Dynamic viscosity
We present a mesoscopic hydrodynamic description of the dynamics of colloidal
suspensions. We consider the system as a gas of Brownian particles suspended in
a Newtonian heat bath subjected to stationary non-equilibrium conditions
imposed by a velocity field. Using results already obtained in previous studies
in the field by means of a generalized Fokker-Planck equation, we obtain a set
of coupled differential equations for the local diffusion current and the
evolution of the total stress tensor. We find that the dynamic shear viscosity
of the system contains contributions arising from the finite size of the
particles.Comment: To appear in Physical Review
Composite infrared bolometers with Si_3N_4 micromesh absorbers
We report the design and performance of 300-mK composite bolometers that use micromesh absorbers and support structures patterned from thin films of low-stress silicon nitride. The small geometrical filling factor of the micromesh absorber provides 20× reduction in heat capacity and cosmic ray cross section relative to a solid absorber with no loss in IR-absorption efficiency. The support structure is mechanically robust and has a thermal conductance, G < 2 × 10^(−11) W/K, which is four times smaller than previously achieved at 300 mK. The temperature rise of the bolometer is measured with a neutron transmutation doped germanium thermistor attached to the absorbing mesh. The dispersion in electrical and thermal parameters of a sample of 12 bolometers optimized for the Sunyaev–Zel’dovich Infrared Experiment is ±7% in R (T), ±5% in optical efficiency, and ±4% in G
- …