On the mechanisms governing gas penetration into a tokamak plasma during a massive gas injection

A new 1D radial fluid code, IMAGINE, is used to simulate the penetration of gas into a tokamak plasma during a massive gas injection (MGI). The main result is that the gas is in general strongly braked as it reaches the plasma, due to mechanisms related to charge exchange and (to a smaller extent) recombination. As a result, only a fraction of the gas penetrates into the plasma. Also, a shock wave is created in the gas which propagates away from the plasma, braking and compressing the incoming gas. Simulation results are quantitatively consistent, at least in terms of orders of magnitude, with experimental data for a D 2 MGI into a JET Ohmic plasma. Simulations of MGI into the background plasma surrounding a runaway electron beam show that if the background electron density is too high, the gas may not penetrate, suggesting a possible explanation for the recent results of Reux et al in JET (2015 Nucl. Fusion 55 093013)

Velocity-space sensitivity of the time-of-flight neutron spectrometer at JET

The velocity-space sensitivities of fast-ion diagnostics are often described by so-called weight functions. Recently, we formulated weight functions showing the velocity-space sensitivity of the often dominant beam-target part of neutron energy spectra. These weight functions for neutron emission spectrometry (NES) are independent of the particular NES diagnostic. Here we apply these NES weight functions to the time-of-flight spectrometer TOFOR at JET. By taking the instrumental response function of TOFOR into account, we calculate time-of-flight NES weight functions that enable us to directly determine the velocity-space sensitivity of a given part of a measured time-of-flight spectrum from TOFOR

Relationship of edge localized mode burst times with divertor flux loop signal phase in JET

A phase relationship is identified between sequential edge localized modes (ELMs) occurrence times in a set of H-mode tokamak plasmas to the voltage measured in full flux azimuthal loops in the divertor region. We focus on plasmas in the Joint European Torus where a steady H-mode is sustained over several seconds, during which ELMs are observed in the Be II emission at the divertor. The ELMs analysed arise from intrinsic ELMing, in that there is no deliberate intent to control the ELMing process by external means. We use ELM timings derived from the Be II signal to perform direct time domain analysis of the full flux loop VLD2 and VLD3 signals, which provide a high cadence global measurement proportional to the voltage induced by changes in poloidal magnetic flux. Specifically, we examine how the time interval between pairs of successive ELMs is linked to the time-evolving phase of the full flux loop signals. Each ELM produces a clear early pulse in the full flux loop signals, whose peak time is used to condition our analysis. The arrival time of the following ELM, relative to this pulse, is found to fall into one of two categories: (i) prompt ELMs, which are directly paced by the initial response seen in the flux loop signals; and (ii) all other ELMs, which occur after the initial response of the full flux loop signals has decayed in amplitude. The times at which ELMs in category (ii) occur, relative to the first ELM of the pair, are clustered at times when the instantaneous phase of the full flux loop signal is close to its value at the time of the first ELM

Dry season water use patterns under Guiera senegalensis L. shrubs in a tropical savanna

The extent of rooting and the water use of Guiera senegalensis bushes at a fallow savanna site in south-west Niger were investigated. Low root length densities were found up to the maximum sampling distance (8 m horizontally and 2 m vertically) from the base of the bushes. Changes in soil water content of the soil profile were monitored using a neutron probe at seven locations over a 20 m transect between two G. senegalensis bushes. During the 1993/94 dry season, a progressive drying front was observed moving away from a G. senegalensis bush both horizontally (up to 10 m) and vertically (up to 4 m). Mean water loss from the top 4 m of the soil profile over the 8-month dry season was 151 mm. This was attributed to water use by G. senegalensis, and comprised 28% of the 1993 total annual rainfall. Clearing fallow savanna land for millet production will result in increased deep drainage partly through reduced dry season water use by the deep rooted bushes

Measurement of surface redistribution of rainfall and modelling its effect on water balance calculations for a millet field on sandy soil in Niger.

During rain there can be substantial redistribution of water at the surface of sandy soils in the Sudano-Sahelian zone, because of localised runoff and runon. This results in variable infiltration over a field. Measurements of spatial variability in infiltration and crop growth were made in a millet field at the southern supersite of the HAPEX-Sahel experiment in Niger. Infiltration was calculated from the change in soil water storage measured using a neutron probe at up to 33 locations, before and after rain storms exceeding 10 min. Data were obtained for five storms in 1993 and 1994. Infiltration varied from 0.3 to 3.4 times the recorded rainfall, though more than 80% of the locations had infiltration between 0.6 and 1.2 times the recorded rainfall. There was some consistency between storms, with locations at the extremes of infiltration having consistently high or low infiltration. The amount of infiltration had little discernable influence on crop growth, other than possibly at the very dry and very wet sites, where growth was reduced. The soil water balance model, SWIM, was used to assess the consequences of variable infiltration and crop growth on the partitioning of water losses between evaporation and drainage in 1992. Simulation of variable infiltration suggested that it has relatively little effect on evaporation, but considerable effect on point drainage. Once there was sufficient infiltration to cause drainage (which was achieved in all but extreme runoff areas), there was a linear relationship between any further cumulative infiltration and the annual loss through drainage, with typically more than 70% of any further input being lost as drainage. This linear relationship meant that on a field scale, variable infiltration had minimal effect on drainage, with increased drainage from runon areas tending to be at the expense of reduced drainage from runoff areas

The role of surface water redistribution in an area of patterned vegetation in a semi-arid environment, south-west Niger

The surface hydrology of a semi-arid area of patterned vegetation in south-west Niger is described. In this region alternating bands of vegetation and bare ground aligned along the contours of a gently sloping terrain give rise to a phenomenon known as "brousse tigrée" (tiger bush). At the selected study site the vegetation bands are 10-30 m wide, separated by 50-100 m-wide bands of bare ground. Five species of shrub dominate, #Guiera senegalensis, #Combretum micranthum, #C. nigricans, #Acacia ataxacantha and #A. macrostachya$. Herbaceous vegetation is generally limited to the upslope edges of vegetation bands. A comprehensive field programme was undertaken to investigate the hydrology. Topographic, vegetation and surface feature surveys were carried out in conjunction with the measurement of rainfall, surface and subsurface hydraulic conductivity, particle size and soil moisture content. Four types of vegetation class are recognised, each tending to occupy a constant position relative to the others and to the regional slope. In a downslope direction the classes are : bare ground, grassy open bush, closed bush, bare open bush, bare ground etc. The nature of the ground surface is closely linked to the vegetation class. Over the bare, bare open and grassy open classes various types of surface crust are present with each type of crust tending to occupy a constant position on the regional slope relative to the vegetation class and other crust types. Below closed bush crusts are generally absent. The typical downslope sequence from the downslope boundary of a vegetation band is : structural (sieving) crust -> erosion crust -> (gravel crust) -> sedimentation crust -> microphytic sedimentation crust -> no crust -> sieving crust, etc. It is also shown that these crust types are dynamic and evolve from one to the other as hydrological conditions change... (D'après résumé d'auteur

Estimation of rainfall inputs and direct recharge to the deep unsaturated zone of southern Niger using the chloride profile method

An estimate of direct groundwater recharge below a region of natural woodland (tiger bush) has been made in south-west Niger using the solute profile technique. Data has been collected from a 77 m deep well drug within the study area covered by HAPEX-Sahel (Hydrological and Atmospheric Pilot Experiment), an international large-scale energy, water and carbon balance experiment carried out during the summer of 1992. During well construction samples were taken from the unsaturated zone at the following intervals: every 25 cm from 0–10 m, every 50 cm from 10–62.5 m, then every metre to the bottom of the well. Pore water was extracted from each sample either by centrifugation or elutriation and analysed for chloride; moisture contents of samples were obtained gravimetrically. These data have been used to produce depth profiles of pore water chloride concentration and moisture content throughout the unsaturated zone. From these profiles it has been possible to derive an estimate of historic direct recharge at the site. The chloride concentration of rainfall, which is required to make the estimate, was determined from the analysis of 123 rainfall samples collected from five EPSAT (vers une Estimation des Précipitation par Satellite au sahel) rain gauges in 1992. A mean recharge rate of 13 mm year−1 (range 10–19 mm) is estimated for the upper 70m of the profile, with a total residence time of 790 years (range 520–990 years). This is considered to be a representative estimate of the magnitude of direct recharge taking place below tiger bush areas

HAPEX-Sahel

An estimate of direct groundwater recharge below a region of natural woodland (tiger bush) has been made in south-west Niger using the solute profile technique. Data has been collected from a 77 m deep well dug within the study area covered by HAPEX-Sahel (Hydrological and Atmospheric Pilot Experiment), an international large-scale energy, water and carbon balance experiment carried out during the summer of 1992. During well construction samples were taken from the unsatured zone at the following intervals : every 25 cm from 0-10 m, every 50 cm from 10-62.5 m, then every metre to the bottom of the well. Pore water was extracted from each sample either by centrifugation or elutriation and analysed for chloride ; moisture contents of samples were obtained gravimetrically. These data have been used to produce depth profiles of pore water chloride concentration and moisture content throughout the unsaturated zone. From these profiles it has been possible to derive an estimate of historic direct recharge at the site. The chloride concentration of rainfall, which is required to make the estimate, was determined from the analysis of 123 rainfall samples collected from five EPSAT (vers une Estimation des Précipitations par Satellite au Sahel) rain gauges in 1992. A mean recharge rate of 13 mm/year (range 10-19 mm) is estimated for the upper 70 m of the profile, with a total residence time of 790 years (range 520-990 years). This is considered to be a representative estimate of the magnitude of direct recharge taking place below tiger bush areas. (Résumé d'auteur

Soil measurements during HAPEX-Sahel intensive observation period.

This article describes measurements made at each site and for each vegetation cover as part of the soils program for the HAPEX-Sahel regional scale experiment. The measurements were based on an initial sampling scheme and included profile soil water content, surface soil water content, soil water potential, infiltration rates, additional measurements on core samples, and grain size analysis. The measurements were used to categorize the state of the surface and profile soil water regimes during the experiment and to derive functional relationships for the soil water characteristic curve, unsaturated hydraulic conductivity function, and infiltration function. Sample results for different supersites and different vegetation covers are presented showing soil water profiles and total soil water storage on days corresponding to the experimental 'Golden Days'. Sample results are also presented for spatial and temporal distribution of surface moisture content and infiltration tests. The results demonstrate that the major experimental objective of monitoring the supersites during the most rapid vegetative growth stage with the largest change of the surface energy balance following the rainy season was very nearly achieved. Separation of the effects of probable root activity and drainage of the soil profile is possible. The potential for localized advection between the bare soil and vegetation strips of the tiger bush sites is demonstrated