Simultaneous attainment of high electron and ion temperatures in discharges with internal transport barriers in ASDEX upgrade

Internal transport barriers have been demonstrated to exist also under conditions with T(e) approximately T(i) approximately 10 keV and predominant electron heating of the tokamak core region. Central electron cyclotron heating was added to neutral beam injection-heated ASDEX Upgrade discharges with a preexisting internal transport barrier, established through programmed current ramping leading to shear reversal. Compared to a reference internal transport barrier discharge without electron cyclotron resonance heating, the electron heat conductivity in the barrier region was found not to increase, in spite of a fivefold increase in electron heat flux, and also angular momentum and ion energy transport did not deteriorate.     

Z4-Kerdock Codes, Orthogonal Spreads, and Extremal Euclidean Line-Sets

When m is odd, spreads in an orthogonal vector space of type⁺(2m + 2,2) are related to binary Kerdock codes and extremalline-sets in 2^{m + 1} with prescribed angles. Spreads in a 2m-dimensionalbinary symplectic vector space are related to Kerdock codesover Z₄ and extremal line-sets in with prescribed angles. These connections involve binary, realand complex geometry associated with extraspecial 2-groups.A geometric map from symplectic to orthogonal spreads is shownto induce the Gray map from a corresponding Z₄-Kerdock codeto its binary image. These geometric considerations lead tothe construction, for any odd composite m, of large numbersof Z₄-Kerdock codes. They also produce new Z₄-linear Kerdockand Preparata codes. 1991 Mathematics Subject Classification:primary 94B60; secondary 51M15, 20C99.     

Probing local and long-range structure simultaneously: an in situ study of the high-temperature phase transition of alpha-AlF3

We show in this Communication that the combination of Rietveld and pair distribution function (PDF) analyses allows unique insight into the nature and driving force of the phase transition of alpha-AlF3, which was not available from conventional structural analysis methods alone. The use of image plate technology allows structural changes to be followed in "real time" and reduces the time required to collect high-resolution PDF data from hours (with a conventional solid state detector) to seconds. This methodology produces raw data that can simultaneously be analyzed by both Rietveld and PDF analysis.     

Application of Combinatorial Procedures in the Search for Serine-Protease-Like Activity with Focus on the Acyl Transfer Step

Recursive deconvolution of a 729-membered peptide library has identified three active sequences, in which both Ser and His are present in one of the two tripeptidic chains generated on a steroidal scaffold (see structural formula), for the cleavage of an activated p-nitrophenyl ester. This combinatorial approach aims at searching for serine-protease-like activity.     

Perspectives from above and beyond the proton drip-line

This paper will review the dramatic increase in our knowledge of one and two proton unbound nuclei [1] such as recoil decay tagging [2] are revealing unique insights into the structure of nuclei beyond the proton drip-line. These studies of excited states provide complementary information to proton radioactivity studies, particularly regarding the role of deformation [3]. Radioactive beams are being used to study two-proton unbound resonances and to study explosive nuclear astrophysical reactions in the region of the proton drip-line.     

Trapping guests within a nanoporous metal-organic framework through pressure-induced amorphization

The release of guest species from within a nanoporous metal-organic framework (MOF) has been inhibited by amorphization of the guest-loaded framework structure under applied pressure. Thermogravimetric analyses have shown that by amorphizing ZIF-8 following sorption of molecular I(2), a hazardous radiological byproduct of nuclear energy production, the pore apertures in the framework are sufficiently distorted to kinetically trap I(2) and improve I(2) retention. Pair distribution function (PDF) analysis indicates that the local structure of the captive I(2) remains essentially unchanged upon amorphization of the framework, with the amorphization occurring under the same conditions for the vacant and guest-loaded framework. The low, accessible pressure range needed to effect this change in desorption is much lower than in tradition sorbents such as zeolites, opening the possibility for new molecular capture, interim storage, or controlled release applications.     

An atomistic MD simulation and pair-distribution-function study of disorder and reactivity of alpha-AlF3 nanoparticles

Cubic nanoparticles of alpha-AlF(3) containing 864 and 2048 atoms were investigated by using molecular dynamics simulations. Significant structural rearrangements of these particles occurred, primarily at the edges and corners of the particles, and 3 and 5 membered (Al-F-)(n) ring structures were observed in addition to the 4-membered rings seen in bulk alpha-AlF(3). These 3 and 5 membered ring structures are, however, present in other metastable forms of AlF(3), which are formed at low temperatures from high surface area precursors. The surfaces of the nanoparticles were very dynamic on the timescale of the MD run, Al-F bonds being continually broken and formed, resulting in the movement of the low coordinate Lewis acid Al sites on the surfaces of the particles. The Lewis acid sites, which represent the catalytically active sites for F/Cl exchange reactions, are largely present at the corners and edges of the particles. The particles show larger rhombohedral distortions than present in the bulk phase and do not undergo a rhombohedral to cubic phase transition at elevated temperatures. The results are compared with pair distribution function (PDF) analysis results from fluorinated gamma-Al(2)O(3), nanoparticles of AlF(3) prepared by plasma routes and alpha- and beta-AlF(3). Broad peaks between 3.3 and 4.5 A in the PDF plots of the fluorinated Al(2)O(3) and the nanoparticles indicate a distribution of Al-F distances arising from Al and F atoms in connected AlF(6) octahedra; this is consistent with the presence of ring structures other than those found in alpha-AlF(3).