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Efficient Neutron Production from a Novel Configuration of Deuterium Gas-Puff Z-Pinch

A novel configuration of a deuterium z pinch has been used to generate fusion neutrons. Injecting an outer hollow cylindrical plasma shell around an inner deuterium gas puff, neutron yields from DD reactions reached Y-n = (2.9 +/- 0.3) x 10(12) at 700 ns implosion time and 2.7 MA current. Such a neutron yield means a tenfold increase in comparison with previous deuterium gas puff experiments at the same current generator. The increase of beam-target yields was obtained by a larger amount of current assembled on the z-pinch axis, and subsequently by higher induced voltage and higher energies of deuterons. A stack of CR-39 track detectors on the z-pinch axis showed hydrogen ions up to 38 MeV. Maximum neutron energies of 15 and 22 MeV were observed by radial and axial time-of-flight detectors, respectively. The number of DD neutrons per one joule of stored plasma energy approached 5 x 10(7). This implies that deuterium gas puff z pinches belong to the most efficient plasma-based sources of DD neutrons

Simulating trends in soil organic carbon in long-term experiments using the NCSOIL and NCSWAP models

NCSOIL simulates C and N transformations in 4 soil organic pools: Pool I labile, Pool I resistant, Pool II, and Pool III, with half-lives of 2, 17, 115 days and about 150 years, respectively. Pool I labile and Pool I resistant represent the microbial biomass. Pool I and Pool II represent the potentially mineralizable N, or the biologically active soil organic matter. The sum of Pools I, Pool II, and Pool III - the soil organic matter - corresponds to the total organic matter minus residues. Each residue is described by 2 pools. NCSOIL is a stand-alone model. It is also a module of NCSWAP, a larger model which encompasses the soil-water-air-plant system. A number of systems and treatments, including the Rothamsted nitrate treatment and the Calhoun tracer C data were simulated. The initial level of Pool II and the decay rate constant of Pool III were calibrated on the basis of measured total soil organic matter and above-ground production. Simulated data were sensitive to above-ground production as it controlled residues input to soil. Model performance, based on total soil organic matter only, is discussed elsewhere in this issue. Most decay rate constants for Pool III ranged from 1.0E - 5 to 3.0E - 5 d-1. Rate constants outside this range were associated with peculiarities of the soil or agronomic practices. Levels of biologically active organic matter (Pool I plus Pool II) in the top soil layers ranged from 4 to 108 μg N g-1. They were consistent with those reported for the potentially mineralizable nitrogen and reflected the agronomic practice and soil fertility level better than did the total soil organic matter. The simulated biologically active organic matter presented a I year periodic cycle. In the future, a major challenge in modelling studies will be to free simulations from the calibration process and to devise experimental methods which will provide initial values relevant to the dynamic requirements of the model.</p

Simulating trends in soil organic carbon in long-term experiments using the CANDY model

CANDY (CArbon and Nitrogen DYnamics) is a simulation system based on long-term experiments of organic matter turnover and nitrogen dynamics at Bad Lauchstadt, Germany. Key driving variables are soil physical properties, meteorological data and management information. The main application of the CANDY model is the calculation of short-term dynamics of nitrogen transformation and long-term dynamics of organic matter turnover in arable soils. This paper concentrates on the evaluation of the model in simulating carbon dynamics in long-term experiments representing different land uses and very different geographical sites. The experimental data came from data sets that were made available to modellers at a workshop held at Rothamsted in 1995. In this paper we describe how the different data sets were modelled and provide a qualitative assessment of model performance. The performance of several models, including CANDY, are compared quantitatively in Smith et al. (1997). Our results show that the mathematical basis of the model, its consideration of a biological time base and its calculation of the 'reproducing carbon' are applicable over a wide range of sites and land-use scenarios. Most of the standard parameters can be used for other sites and land-use systems.</p

Simulating trends in soil organic carbon in long-term experiments using RothC-26.3

As part of a model evaluation exercise, RothC-26.3, a model for the turnover of organic carbon in non-waterlogged soils, was fitted to measurements of organic carbon from 18 different experimental treatments on 6 long-term experimental sites in Germany, England, the USA, the Czech Republic and Australia. In the fitting process, the model was first run with an annual return of plant C that had been selected iteratively to give the carbon content of the soil at the start of each experiment. This was done for the soil and climate of each site. If the radiocarbon content of the soil organic matter was known, the inert organic carbon (IOM) content of the soil was also calculated for the start of the experiment. Using these carbon and radiocarbon contents as a starting point, the model was then run for each of the experimental treatments to be fitted, using iteratively selected values for the annual return of plant materials to the soil. The value used for each treatment was selected to optimise the fit between modelled and measured data over the whole experimental period: fitting was done by eye. Thus fitted, RothC-26.3 gave an acceptable approximation to the measurements for 14 of the treatments, bearing in mind the experimental errors in measuring soil organic carbon on a per hectare basis. With four of the treatments (Highfield Bare Fallow, Park Grass plot 13d, Ruzyne farmyard manure plot and Tamworth rotation 5), the fit was less satisfactory.</p

Simulating trends in soil organic carbon in long-term experiments using the century model

This paper describes Century Soil Organic Matter (SOM) Model simulations of seven long-term data sets that are the subject of this special issue. We found that Century successfully simulates SOM C across a variety of land use and climate types. Simulations of SOM were most successful in grass and crop systems. This exercise highlights a structural limitation of Century in simulating SOM in a forest with a developed litter layer. Simulations of tree biomass distributions, however, were generally successful. The model failed to capture extreme values of yield and N offtake, although annual averages were quite similar between observations and simulations, leading to reasonable estimates of SOM C. Average yields and SOM were generally higher in amended treatments or rotations including an N-fixing component such as alfalfa. The model successfully predicted SOM dynamics across climates, land use types, and treatments. This suggests that Century is a useful tool for ecosystem studies, particularly those focused on SOM dynamics.</p

Simulating trends in soil organic carbon in long-term experiments using the soil-plant-atmosphere model DAISY

DAISY, a mechanistic model for the simulation of crop production, soil water and N dynamics in agro-ecosystems, was used for simulating trends in total soil carbon (C) for various long-term experiments as part of a model evaluation exercise. Of the seven core datasets in the global SOMNET database, DAISY could be applied to the four arable sites and, with certain critical assumptions, to the single grassland site. The model was, however, not applicable to the two forest sites, because currently DAISY cannot simulate soil water and temperature dynamics in a forest system and does not incorporate leaching of dissolved organic substances, faunal litter incorporation or the effect of low pH on decomposition. The DAISY crop submodel produced some overestimation of crop yields, especially at low N input levels, and simulated root C input to the soil may thus represent a slight overestimation. This may have been balanced, however, by the fact that no account is taken of rhizodeposition or root turnover in the current version of the DAISY model. For this reason too, plant-derived C input in perennial crops (pastures) had to be estimated and input separately. Under these critical assumptions, the model was capable of simulating long-term trends in total soil C quite well for all included sites and treatments (overall correlation, R2= 0.88), except for the Bad Lauchstadt arable site. It was evident that sufficient experimental data on belowground, plant-derived C inputs was not available, especially with perennial crops. Future research in this area should thus have high priority, because such data are a prerequisite if soil organic matter models are to incorporate and improve the simulation of this very important mechanism of soil C input. The partitioning and parameterisation of C between pools in the DAISY model has been discussed and indications were found that the parameterisation of the microbial biomass pools in the model are somewhat inadequate.</p

Energy transformation in Plasma Focus discharge with wire and liner as a load

We present the results of experiments focused on energy transformations during the implosion of the hydrogen current sheath towards an Al wire (120 ěm in diameter) positioned on the top of the inner electrode of the PF-1000 plasma focus facility at the IPPLM in Warsaw. A wire corona is formed at the current sheath impact and ~60 ns after the impact a soft X-ray pulse is emitted. Its spectrum contains AlVI-XII lines accompanied by their satellites. The amount of emitted energy is recorded by two filtered PIN diodes and thermoluminescent dosimeters and depends on symmetry of the current sheath and quality of the current sheath focus. The mean value of energy of keV photons emitted in the runs done without the wire (~1 J) is higher than the energy obtained with the wire (~0.4 J). A time delay between the impact of the current sheath and X-ray pulse, the plasma focus dynamics and soft X-ray emission are interpreted by an axial magnetic field generation and transformations

Stabilizing of Z-pinch and Plasma Focus discharges due to thick wires

This paper presents the results of the diagnostics observing the impact of the plasma sheath imploded in megaamper Z-pinch or plasma focus devices onto the Al or C wires of 30-300 mi m in diameter fixed in the center axis. The discharges were operated with a maximum current of 1.5-3 MA at two devices, PF 1000 at IPFLM in Warsaw, and Stand 300 at RRC Kurchatov Institute in Moscow. At the plasma impact the 10-50 ns XUV pulse was emitted in K-shell lines from the ring or helical-like forms on the surface of the wire corona. The scenario of the production of the non-equilibrium intensities of the C and Al H-like and He-like lines is discussed considering fast transformation of magnetic field, electron beams acceleration, total ionization of the ions and fast recombination in dense and magnetized wire corona