| 1. |
- Bundschuh, J., et al.
(författare)
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Groundwater arsenic in the Chaco-Pampean Plain, Argentina : Case study from Robles County, Santiago del Estero Province
- 2004
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Ingår i: Applied Geochemistry. - : Elsevier BV. - 0883-2927 .- 1872-9134. ; 19:2, s. 231-243
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Tidskriftsartikel (refereegranskat)abstract
- In large parts of rural Argentina people depend on groundwater whose As content exceeds the Argentine drinking water standards (0.05 mg l(-1)). The most affected areas are located in the Chaco-Pampean Plain, where aquifers comprise Tertiary loess deposits (in the Pampean Plain) and Tertiary and Quaternary fluvial and aeolian sediments (in the Chaco Plain). Robles county is located in the alluvial cone of the Dulce River consisting of loess (aeolian), and gravel, silt, sand and clay (alluvial) deposits. In the shallow aquifers, more than 48% of the 63 studied wells show As at toxic levels (maximum 4.8 mg l(-1)), while in the deep groundwater the concentration is below 0.05 mg l(-1). The pH of the shallow groundwaters range between 6.5 and 9 and generally have high electrical conductivity with mean values of 2072 and 1693 muS/cm(-1) in the years 1998 and 1999, respectively. Arsenic concentrations are high in the alkaline Na-HCO3 type groundwaters, where As correlates positively with Na+ and HCO3-. Moreover, As correlates positively with Mo, U, and V, while a negative correlation was observed with Ca2+ and Mg2+. The potential sources of groundwater As are: (i) layers of volcanic ash with 90% of rhyolitic glass; (ii) volcanic glass dispersed in the sediments; and (iii) clastic sediments of metamorphic and igneous origin. Great lateral variability in the concentration of groundwater As is caused by several hydrogeological and hydrogeochemical factors.
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| 2. |
- Hawkes, N. C., et al.
(författare)
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Observation of zero current density in the core of JET discharges with lower hybrid heating and current drive
- 2001
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 8711:11, s. art. no.-115001
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Tidskriftsartikel (refereegranskat)abstract
- Simultaneous current ramping and application of lower hybrid heating and current drive (LHCD) have produced a region with zero current density within measurement errors in the core (r/a less than or equal to 0.2) of JET tokamak optimized shear discharges. The reduction of core current density is consistent with a simple physical explanation and numerical simulations of radial current diffusion including the effects of LHCD. However, the core current density is clamped at zero, indicating the existence of a physical mechanism which prevents it from becoming negative.
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| 3. |
- Hawkes, N. C., et al.
(författare)
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The formation and evolution of extreme shear reversal in JET and its influence on local thermal transport
- 2002
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 0741-3335 .- 1361-6587. ; 44:7, s. 1105-1125
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Tidskriftsartikel (refereegranskat)abstract
- In JET discharges where lower hybrid heating and current drive (LHCD) is applied early during the current ramp, a region of the plasma with zero current density is formed near the axis. At the boundary of this region the current density is large and B-theta increases rapidly over a small distance. In the central region the safety factor, q, is effectively infinite, but this falls steeply in the boundary region. Outside the boundary region q reaches a minimum, where the magnetic shears equivalent to r/q (dq/dr) becomes zero. The formation of this region of zero current is dependent on both the heating and the current drive effects of the LHCD. When LHCD is switched off the current profile begins to relax towards the resistive peaked current distribution of fully inductive tokamak operation. If LHCD is not used in the current rise then these current profiles are not established. Although the physical mechanism exists to drive the central plasma current below zero, in most cases it appears to be prevented from going negative. At least one MHD mechanism has been identified which could be responsible for this. The presence of the zero central current is closely linked to the periodic relaxation events seen in these discharges. In these discharges, internal transport barriers have been observed with additional heating powers substantially below the values required to obtain barriers in monotonic q profile cases.
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