The heating neutral beam injectors (HNBs) of ITER are designed to deliver 16.7 MW of 1 MeV D0 or 0.87 MeV H0 to the ITER plasma for up to 3600 s. They will be the most powerful neutral beam (NB) ...injectors ever, delivering higher energy NBs to the plasma in a tokamak for longer than any previous systems have done. The design of the HNBs is based on the acceleration and neutralisation of negative ions as the efficiency of conversion of accelerated positive ions is so low at the required energy that a realistic design is not possible, whereas the neutralisation of H− and D− remains acceptable ( 56%). The design of a long pulse negative ion based injector is inherently more complicated than that of short pulse positive ion based injectors because: negative ions are harder to create so that they can be extracted and accelerated from the ion source; electrons can be co-extracted from the ion source along with the negative ions, and their acceleration must be minimised to maintain an acceptable overall accelerator efficiency; negative ions are easily lost by collisions with the background gas in the accelerator; electrons created in the extractor and accelerator can impinge on the extraction and acceleration grids, leading to high power loads on the grids; positive ions are created in the accelerator by ionisation of the background gas by the accelerated negative ions and the positive ions are back-accelerated into the ion source creating a massive power load to the ion source; electrons that are co-accelerated with the negative ions can exit the accelerator and deposit power on various downstream beamline components. The design of the ITER HNBs is further complicated because ITER is a nuclear installation which will generate very large fluxes of neutrons and gamma rays. Consequently all the injector components have to survive in that harsh environment. Additionally the beamline components and the NB cell, where the beams are housed, will be activated and all maintenance will have to be performed remotely. This paper describes the design of the HNB injectors, but not the associated power supplies, cooling system, cryogenic system etc, or the high voltage bushing which separates the vacuum of the beamline from the high pressure SF6 of the high voltage (1 MV) transmission line, through which the power, gas and cooling water are supplied to the beam source. Also the magnetic field reduction system is not described.
This work presents the first observation of a multi‐stroke positive cloud‐to‐ground lightning flash sharing the same channel to ground mapped with a very high frequency broadband interferometer and a ...Lightning Mapping Array. This type of lightning flash is very rarely observed, and it is currently unclear how frequent it is and even under what conditions it occurs. Our observations indicate a scenario where the first downward positive leader initiates from a decayed negative channel. After the first return stroke, some of the main negative channel branches stop propagating and are likely cut off. A fast recoil leader and/or a fast breakdown play a crucial role in reconnecting these previously decayed leader channels and initiating the subsequent positive stroke. The mechanism we propose to describe the phenomenon allows us to explain its rarity and the discrete positive charge transfer to the ground.
Plain Language Summary
In the same lightning flash, whose usual duration is less than a second, there can be multiple negative cloud‐to‐ground strokes with different terminations or following a preexisting channel to the ground. In contrast, it is not common to have multiple positive cloud‐to‐ground (+CG) strokes, and especially multi‐stroke +CG flashes sharing the same channel to ground are very rarely observed. This polarity asymmetry is not well understood and many aspects are debated. In this letter, we present for the first time a very high frequency (VHF) radio band observation of a multi‐stroke +CG flash along the same channel, observed simultaneously by a VHF broadband interferometer and a Lightning Mapping Array in north‐central Colombia. These combined observations have unprecedented temporal and spatial resolution and allowed us to observe in detail the development of the flash and especially to understand the initiation mechanism of the subsequent positive stroke.
Key Points
First time observation of positive cloud‐to‐ground strokes sharing the same channel to ground with a very high frequency (VHF) broadband interferometer
A fast recoil leader and/or a fast breakdown connecting decayed leader channels play a crucial role in triggering the subsequent positive stroke
The high‐resolution comparison between the first and subsequent stroke shows different VHF signatures related to the channel conductivity
Terrestrial Gamma‐ray Flashes (TGFs) are ten‐to‐hundreds of microsecond bursts of gamma‐rays produced when electrons in strong electric fields in thunderclouds are accelerated to relativistic ...energies. Space instruments have observed TGFs with source photon brightness down to ∼1017–1016. Based on space and aircraft observations, TGFs have been considered rare phenomena produced in association with very few lightning discharges. Space observations associated with lightning ground observations in the radio band have indicated that there exists a population of dimmer TGFs. Here we show observations of TGFs from aircraft altitude that were not detected by a space instrument viewing the same area. The TGFs were found through Monte Carlo modeling to be associated with 1015–1012 photons at source, which is several orders of magnitude below what can be seen from space. Our results suggest that there exists a significant population of TGFs that are too weak to be observed from space.
Plain Language Summary
Terrestrial Gamma‐ray Flashes (TGFs) are short bursts of gamma‐rays produced in the strong electric fields in thunderclouds. Based on space and aircraft observations, TGFs have been considered a rare phenomena. In this paper, we present observations of TGFs from an aircraft campaign that were not detected by a space instrument viewing the same area. Our results reveal that these TGFs were too weak to be observed from space, indicating a significant population of TGFs that are undetectable by space instruments.
Key Points
The ALOFT flight campaign detected six TGFs within a few minutes during an ISS overpass, and none were detected by the ASIM instrument
We show that there must be a population of TGFs that are too weak to be observed from space
The fluence at 15 km of this population is 2–5 orders of magnitude lower than the TGFs observed from space
Most forage cultivars released for the genus
Paspalum
belong to a section named Plicatula. The species of Plicatula are mostly apomictic and consequently the genetic diversity is locked for their ...genetic improvement. The objectives were to evaluate the crossability, hybrid fertility, heterosis, and genetic distances between apomictic accessions and a sexual genotype of species of Plicatula group of
Paspalum
. Crosses were made using 22 apomictic tetraploid accessions belonging to 12 different species as pollen donors, and a sexual tetraploid genotype induced by colchicine from a sexual diploid accession of
P. plicatulum
. Crossability varied between 0 and 16% among crosses. Viable hybrid offspring were recovered from 15 out of 22 crosses. The most successful crosses involved
P. guenoarum
,
P. plicatulum
,
P. chaseanum
, and
P. oteroi
. Fertility of the sampled hybrids varied between 1.6% for the cross involving
P. lenticulare
, and 40.1% for an intraspecific cross (
P. plicatulum
, accession Hojs388). The genetic distance between parents was estimated using amplified fragment-length polymorphism, and it varied between 0.34 and 0.53. There was no correlation between genetic distances and crossability or fertility of the hybrids. Hybrids from the most numerous families were classified for mode of reproduction using flow cytometric seed analysis. The ratio between sexual and apomictic hybrids varied between 0.6:1 and 1.6:1. A selected group of apomictic hybrids were evaluated for several agronomic traits in the field. Heterosis was observed for frost tolerance and cattle preference. The results indicated that gene transfer via hybridization is possible among several species of Plicatula. Superior hybrids for specific traits can be generated and fixed by apomixis.
A comprehensive range of gas and particle phase pollutants were sampled at 1-hour time resolution in urban background Milan during summer 2012. Measurements include several soluble inorganic aerosols ...(Cl−,NO2−,NO3−,SO42−,Ca2+,K+,Mg2+,Na+,NH4+) and gases (HCl, HNO2,HNO3, NH3, NO, NO2,O3, SO2), organic, elemental and black carbon and meteorological parameters. Analysis methods used include mean diurnal pattern on weekdays and Sundays, pollution roses, bivariate polar plots and statistical models using backtrajectories. Results show how nitrous acid (HONO) was mainly formed heterogeneously at nighttime, with a dependence of its formation rate on NO2 consistent with observations during the last HONO campaign in Milan in summer 1998, although since 1998 a drop in HONO levels occurred following to the decrease of its precursors. Nitrate showed two main formation mechanisms: one occurring through N2O5 at nighttime and leading to nitrate formation onto existing particles; another occurring both daytime and nighttime following the homogeneous reaction of ammonia gas with nitric acid gas. Air masses reaching Milan influenced nitrate formation depending on their content in ammonia and the timing of arrival. Notwithstanding the low level of SO2 in Milan, its peaks were associated to point source emissions in the Po valley or shipping and power plant emissions SW of Milan, beyond the Apennines. A distinctive pattern for HCl was observed, featured by an afternoon peak and a morning minimum, and best correlated to atmospheric temperature, although it was not possible to identify any specific source. The ratio of primary-dominated organic carbon and elemental carbon on hourly PM2.5 resulted 1.7. Black carbon was highly correlated to elemental carbon and the average mass absorption coefficient resulted MAC=13.8 ± 0.2 m2 g−1. It is noteworthy how air quality for a large metropolitan area, in a confined valley and under enduring atmospheric stability, is nonetheless influenced by sources within and outside the valley.
•Diurnal pattern of several aerosol pollutant and of their precursors.•Influence of meteorology and synoptic conditions on inorganic PM2.5 and precursors.•Analysis of potential sources of peak levels in a large metropolitan area.•Implications for air quality.
The ITER Neutral Beam Test Facility, in an advanced stage of construction in Padova, includes the installation, tests, and optimization of the full prototype of the ITER Heating Neutral Beams ...injector (HNBs), named MITICA.
The MITICA Neutral Beam Injector will host its main components in a SS304L vacuum vessel composed of two modules, connected between them on site: the Beam Source Vessel (cubic shape of 5 m side and 67 tons weight) containing the Beam Source and the Beam Line Vessel (section 4.5 m x 4.5 m, length 11 m and 76 tons weight) containing the Beam Line Components and the Cryopumps.
The manufacturing is described, going through the FE analyses performed to assess the structural integrity, the materials selection, the welding qualifications, the implementation of the double barrier sealings and the control of the deformations. The Factory Acceptance Tests of the individual vessels are presented, including their Helium Leak Tests.
The two vessels have been assembled on-site inside the MITICA bio-shield. The main outcomes of on-site final assembly and Site Acceptance Tests are described.
Both the vessels have been detail designed, manufactured, installed and tested by De Pretto Industried, from Schio, (VI) Italy, supporting fusion technology’s applications from more than 50 years. The design and the technical support were provided by Consorzio RFXa, while the procurement was managed by Fusion For Energye.
It is generally accepted that polyploids have downsized basic genomes rather than additive values with respect to their related diploids. Changes in genome size have been reported in correlation with ...several biological characteristics. About 75 % of around 350 species recognized for
Paspalum
(Poaceae) are polyploid and most polyploids are apomictic. Multiploid species are common with most of them bearing sexual diploid and apomictic tetraploid or other ploidy levels. DNA content in the embryo and the endosperm was measured by flow cytometry in a seed-by-seed analysis of 47 species including 77 different entities. The relative DNA content of the embryo informed the genome size of the accession while the embryo:endosperm ratio of DNA content revealed its reproductive mode. The genome sizes (2C-value) varied from 0.5 to 6.5 pg and for 29 species were measured for the first time. Flow cytometry provided new information on the reproductive mode for 12 species and one botanical variety and supplied new data for 10 species concerning cytotypes reported for the first time. There was no significant difference between the mean basic genome sizes (1Cx-values) of 32 sexual and 45 apomictic entities. Seventeen entities were diploid and 60 were polyploids with different degrees. There were no clear patterns of changes in 1Cx-values due to polyploidy or reproductive systems, and the existing variations are in concordance with subgeneric taxonomical grouping.
Recent work has raised interest in the use of a LIX®63/Versatic 10/tributylphosphate (TBP) synergistic system to separate nickel and cobalt from gangue metals. However, accelerated and undesirable ...degradation of LIX®63 oxime has previously been reported in the presence of strong organic acids such as di-2-ethylhexyl phosphoric acid (D2EHPA). Although not a strong organic acid, no reliable information regarding the effect of Versatic 10 on LIX®63 stability was available. The present work assessed the effect of organic composition on the stability of LIX®63 oxime when mixed for 6 weeks under harsh strip conditions (20 g/L sulfuric acid, 60 °C). The organic solutions were (i) LIX®63 in Shellsol D70; (ii) LIX®63 and D2EHPA in Shellsol D70; (iii) LIX®63 and Versatic 10 in Shellsol D70; and (iv) LIX®63, Versatic 10 and TBP in Shellsol D70.
The adverse effect of organic acids on the rate of LIX®63 oxime degradation was found to increase in the order:
No organic acid
≪
Versatic 10
≪
D2EHPA.
Oxime decomposition was determined to be a first order reaction. The major degradation product was isolated, characterised and identified as 5,8-diethyl-6,7-dodecanedione (diketone).
Loss of oxime from the LIX®63-only system occurred predominantly by ongoing precipitation of a nickel–oxime complex rather than by oxime degradation. Diketone formation accounted for less than 1% of total oxime loss.
Oxime quickly decomposed in the presence of D2EHPA, with a half life of 6.5 days. Diketone, which in turn slowly decomposed, accounted for essentially all oxime loss. In contrast, only gradual loss of oxime from the LIX®63/Versatic 10/TBP (half life 34 weeks) system was observed, with diketone accounting for 61% of total oxime loss. Similar results were observed in the LIX®63/Versatic 10 system, suggesting the presence of TBP does not materially affect oxime stability or the degradation mechanism.
The presence of diketone in the LIX®63/Versatic 10/TBP system was found to have no adverse effect on metal selectivity, although other degradation products could potentially have a small, adverse effect. The degradation products did not have any observable effect on interfacial tension or phase disengagement time.
Additional studies to better identify factors affecting oxime decomposition (such as aqueous phase composition and temperature) in these synergistic systems are underway.
This paper reports on a laboratory experiment to study the effect of vehicle net charge on the inception of a positive leader from an aircraft exposed to high atmospheric electric fields. The ...experiment models the first stage of aircraft‐triggered lightning in which a positive leader typically develops from the vehicle and is shortly afterwards followed by a negative leader. This mechanism of lightning initiation amounts to around 90% of strikes to aircraft. Aircraft can acquire net charge levels of the order of a millicoulomb from a number of sources including corona emission, charged particles in the engine exhaust, and charge transfer by collisions with particles in the atmosphere. In addition, aircraft could potentially be artificially charged through controlled charge emission from the surface. Experiments were performed on a model aircraft with a 1m wingspan, which was suspended between two parallel electrodes in a 1.45m gap with voltage difference of a few hundred kilovolts applied across it. In this configuration, it is found that the breakdown field can vary by as much as 30% for the range of charging levels tested. The experimental results show agreement with an electrostatic model of leader initiation from aircraft, and the model indicates that the effect can be substantially stronger if additional negative charge is added to the aircraft. The results from this work suggest that flying uncharged is not optimal in terms of lightning avoidance and open up the possibility of developing risk‐reduction strategies based on net charge control.
Plain Language Summary
Commercial aircraft are typically struck by lightning around once per year, and the vast majority of these events are triggered by the aircraft itself. The lightning discharge originates on the surface of the aircraft in areas with sharp edges. Whether a discharge develops is in part due to the net electric charge of the aircraft, which can be acquired both naturally or artificially. Previous work has shown that it is theoretically possible to reduce the likelihood of a lightning strike occurring by manipulating the net charge of the aircraft. In this paper, the authors perform laboratory experiments to validate this hypothesis. These experiments demonstrate that the threshold for lightning could be increased by 30% by charging the aircraft negatively, which means that an aircraft could fly safely through ambient electric fields that are around 30% higher than those of an uncharged baseline. Theoretical estimates suggest that further improvement may be possible if the aircraft were charged to a more negative state than those tested. This work gives laboratory scale experimental evidence that it is possible to reduce the frequency of lightning strikes on aircraft by manipulating their charge and encourages further investigation of the proposed lightning strike risk reduction strategy.
Key Points
Aircraft‐triggered lightning is influenced by the net charge of the floating body
Positive leader inception from an aircraft requires higher amplitude external fields when negatively biasing the vehicle
Experimental results validate a proposed method of aircraft‐triggered lightning strike risk reduction
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