124,278 research outputs found
Fast Calculation of the Radiative Opacity of Plasma
Plasma opacity calculations play an important role in solar modelling and many plasma physics and inertial
confinement fusion experiments. This thesis is focussed on the fast calculation of opacity from first principles.
The existing average atom (AA) opacity code IMP [1] is used alongside experimental data and detailed
atomic physics to develop new models; the results show that simple models can give an excellent description
of plasma spectra for a large range of conditions. The results are significant for the development of fast
opacity codes which necessarily use the AA approach.
The application of fast models to very large scale calculations is considered and an efficient approach to
these developed; this allows the fast description of experimental data that would not have otherwise been
possible [2]. Analysis of this data then allows the accuracy of the IMP model to be further discussed. The
atomic model is also considered, and an improved approach implemented. These improvements makes little
difference to the description of experiment provided electron exchange is included. The range of applicability
of the IMP model is then extended to higher density by adding a fast description of line broadening by
electrons. This gives an excellent agreement with both experiment and more advanced opacity codes.
The treatment of atomic term structure can represent a significant portion of code runtime. A good
compromise between detail and efficiency is the unresolved transition array (UTA) formulation; a consistent
theory of UTAs is developed, and various models introduced. The accuracy of these is systematically tested.
It is found that within the validity range of the UTA approach, a good description of the opacity can be
gained using a simple model provided that the linewidth is correct. Various simplified calculations of this
width are tested, and found to be inaccurate [3]
Afghan Narcotrafficking Post-2014 Scenarios
As Afghanistan now faces an uncertain political and security environment following the drawdown of ISAF troops at the end of 2014, the potential for a worsening narcotrafficking threat is great. The report states that the potential for deterioration "underscores the imperative need for Russian and U.S. policymakers to find the political will to resume and perhaps even increase cooperation despite ongoing differences on other issues. Together with regional partners and international organizations, renewed Russian-U.S. cooperation presents the best hope for a brighter future.
The Doomsday Argument in Many Worlds
You and I are highly unlikely to exist in a civilization that has produced
only 70 billion people, yet we find ourselves in just such a civilization. Our
circumstance, which seems difficult to explain, is easily accounted for if (1)
many other civilizations exist and if (2) nearly all of these civilizations
(including our own) die out sooner than usually thought, i.e., before trillions
of people are produced. Because the combination of (1) and (2) make our
situation likely and alternatives do not, we should drastically increase our
belief that (1) and (2) are true. These results follow immediately when
considering a many worlds version of the "Doomsday Argument" and are immune to
the main criticism of the original Doomsday Argument.Comment: 18 page
The Role of Indexing in Subject Retrieval
On first reading the list of speakers proposed for this institute, I
became aware of being rather the "odd man out" for two reasons. Firstly, I
was asked to present a paper on PRECIS which is very much a verbal
indexing system-at a conference dominated by contributions on classification
schemes with a natural bias, as the centenary year approaches, toward the
Dewey Decimal Classification (DDC). Secondly, I feared (quite wrongly, as it
happens) that I might be at variance with one or two of my fellow speakers,
who would possibly like to assure us, in an age when we can no longer ignore
the computer, that traditional library schemes such as DDC and Library of
Congress Classification (LCC) are capable of maintaining their original
function of organizing collections of documents, and at the same time are also
well suited to the retrieval of relevant citations from machine-held files. In
this context, I am reminded of a review of a general collection of essays on
classification schemes which appeared in the Journal of Documentation in
1972. Norman Roberts, reviewing the papers which dealt specifically with the
well established schemes, deduced that "all the writers project their particular
schemes into the future with an optimism that springs, perhaps, as much from
a sense of emotional involvement as from concrete evidence." Since I do not
believe that these general schemes can play any significant part in the retrieval
of items from mechanized files, it appeared that I had been cast in the role of
devil's advocate.published or submitted for publicatio
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