12,931 research outputs found
The static potential in QED with non-minimal coupling
Here we study the effect of the non-minimal coupling j^{\mu}\eps
\partial^{\nu} A^{\alpha} on the static potential in multiflavor QED.
Both cases of four and two components fermions are studied separately at
leading order in the expansion. Although a non-local Chern-Simons term
appears, in the four components case the photon is still massless leading to a
confining logarithmic potential similar to the classical one. In the two
components case, as expected, the parity breaking fermion mass term generates a
traditional Chern-Simons term which makes the photon massive and we have a
screening potential which vanishes at large inter-charge distance. The extra
non-minimal couplings have no important influence on the static potential at
large inter-charge distances. However, interesting effects show up at finite
distances. In particular, for strong enough non-minimal coupling we may have a
new massive pole in the photon propagator while in the opposite limit there may
be no poles at all in the irreducible case. We also found that, in general, the
non-minimal couplings lead to a finite range {\bf repulsive} force between
charges of opposite signs.Comment: 19 pages and 7 figure
A comparison between detailed and configuration-averaged collisional-radiative codes applied to non-local thermal equilibrium plasma
A collisional-radiative model describing nonlocal-thermodynamic-equilibrium
plasmas is developed. It is based on the HULLAC (Hebrew University Lawrence
Livermore Atomic Code) suite for the transitions rates, in the zero-temperature
radiation field hypothesis. Two variants of the model are presented: the first
one is configuration averaged, while the second one is a detailed level
version. Comparisons are made between them in the case of a carbon plasma; they
show that the configuration-averaged code gives correct results for an
electronic temperature Te=10 eV (or higher) but fails at lower temperatures
such as Te=1 eV. The validity of the configuration-averaged approximation is
discussed: the intuitive criterion requiring that the average
configuration-energy dispersion must be less than the electron thermal energy
turns out to be a necessary but far from sufficient condition. Another
condition based on the resolution of a modified rate-equation system is
proposed. Its efficiency is emphasized in the case of low-temperature plasmas.
Finally, it is shown that near-threshold autoionization cascade processes may
induce a severe failure of the configuration-average formalism.Comment: 9
An overview of tea research in Tanzania - with special reference to the Southern Highlands.
The history of tea development in Tanzania from the early part of this century
to the present is summarised. Average yields of made tea from well managed
estates in the Mufindi district have increased from around 600 kg ha-1 in the
late 1950s to 3000 kg ha-1 at the present time: by comparison, yields from
smallholder farms have remained much lower, averaging only 400-500 kg ha-1.
There have been a large number of technical, economic and other changes over the
last 30 to 40 years. The removal of shade trees, the use of herbicides, the
application of NPK compound fertilisers, the introduction of irrigation (on some
estates) and changes in harvesting policy have all contributed to the increases
in yield. Financial and infrastructural problems have contributed to the low
yields from many smallholders and others, and have limited the uptake of new
technology. The contribution of research is reviewed, from the start of the Tea
Research Institute of East Africa in Kenya in 1951, through to the development
of the Marikitanda Tea Research Centre in Amani in 1967; the Ngwazi Tea Research
Unit in Mufindi (1967 to 1970, and from 1986), and lastly the Kifyulilo Tea
Research Station, also in Mufindi in 1986. The yield potential of well
fertilized and irrigated clonal tea, grown at an altitude of 1800 m, is around
6000 kg ha-1. This potential is reduced by drought, lack of fertilizer, bush
vacancies and inefficient harvesting practices. The corresponding potential
yields at high (2200 m) and low (1200 m) altitude sites range from 3000-3500 kg
ha-1 up to 9000-10000 kg ha-1 and are largely a function of temperature. The
opportunities for increasing yields of existing tea, smallholder and estate, are
enormous. Tea production in the Southern Highlands of Tanzania is about to
expand rapidly. Good, appropriate research is needed to sustain this development
over the long term, and suggestions on how best this is done in order to assist
the large scale producers as well as the smallholders, are discussed
Operational experience and design recommendations for teleoperated flight hardware
Teleoperation (remote manipulation) will someday supplement/minimize astronaut extravehicular activity in space to perform such tasks as satellite servicing and repair, and space station construction and servicing. This technology is being investigated by NASA with teleoperation of two space-related tasks having been demonstrated at the Oak Ridge National Lab. The teleoperator experiments are discussed and the results of these experiments are summarized. The related equipment design recommendations are also presented. In addition, a general discussion of equipment design for teleoperation is also presented
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