| 1. |
- Björklund Svensson, Jonas, et al.
(författare)
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Low-divergence femtosecond X-ray pulses from a passive plasma lens
- 2021
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Ingår i: Nature Physics. - : Springer Science and Business Media LLC. - 1745-2481 .- 1745-2473. ; 17:5, s. 639-645
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Tidskriftsartikel (refereegranskat)abstract
- Electron and X-ray beams originating from compact laser-wakefield accelerators have very small source sizes that are typically on the micrometre scale. Therefore, the beam divergences are relatively high, which makes it difficult to preserve their high quality during transport to applications. To improve on this, tremendous efforts have been invested in controlling the divergence of the electron beams, but no mechanism for generating collimated X-ray beams has yet been demonstrated experimentally. Here we propose and realize a scheme where electron bunches undergoing focusing in a dense, passive plasma lens can emit X-ray pulses with divergences approaching the incoherent limit. Compared with conventional betatron emission, the divergence of this so-called plasma lens radiation is reduced by more than an order of magnitude in solid angle, while maintaining a similar number of emitted photons per electron. This X-ray source offers the possibility of producing brilliant and collimated few-femtosecond X-ray pulses for ultra-fast science, in particular for studies based on X-ray diffraction and absorption spectroscopy. X-ray pulses with low divergences are produced in a laser-wakefield accelerator by focusing electron bunches in a dense passive plasma lens.
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| 2. |
- Cantono, Giada, et al.
(författare)
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Laser-driven proton acceleration from ultrathin foils with nanoholes
- 2021
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322 .- 2045-2322. ; 11:1
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Tidskriftsartikel (refereegranskat)abstract
- Structured solid targets are widely investigated to increase the energy absorption of high-power laser pulses so as to achieve efficient ion acceleration. Here we report the first experimental study of the maximum energy of proton beams accelerated from sub-micrometric foils perforated with holes of nanometric size. By showing the lack of energy enhancement in comparison to standard flat foils, our results suggest that the high contrast routinely achieved with a double plasma mirror does not prevent damaging of the nanostructures prior to the main interaction. Particle-in-cell simulations support that even a short scale length plasma, formed in the last hundreds of femtoseconds before the peak of an ultrashort laser pulse, fills the holes and hinders enhanced electron heating. Our findings reinforce the need for improved laser contrast, as well as for accurate control and diagnostics of on-target plasma formation.
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| 3. |
- Persson, Anders, 1956-, et al.
(författare)
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Vertical distribution of benthic community responses to fish predators, and effects on algae and suspended material
- 2006
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Ingår i: Aquatic Ecology. - Dordrecht, Netherlands : Springer Netherlands. - 1386-2588 .- 1573-5125. ; 40:1, s. 85-95
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Tidskriftsartikel (refereegranskat)abstract
- The vertical positioning of benthic invertebrates should be a trade-off between the risky, but productive, sediment surface and the safer, but physiologically harsher, conditions deeper down in the sediment. This is because the foraging efficiency of benthic fish decreases with sediment depth, whereas the sediment surface is generally better oxygenated and has a higher resource quality than lower layers. We studied how two benthic fish predators, bream (Abramis brama) and ruffe (Gymnocephalus cernuus), affected the community composition and vertical distribution of benthos, and their indirect effects on algae and suspended material, in field enclosures. Whereas bream had significant effects on the density, composition and distribution of the benthos, ruffe had no such effects. The total benthos biomass in bream treatments was an-order of magnitude lower in the upper sediment layer (0-1 cm) and three times lower in the middle layer (1-3 cm) than in the controls, whereas there were no significant effects in the deepest layer (3-10 cm). Bivalves persisted in the deepest layer although their density was reduced in shallow sediment, whereas gastropods faced the risk of local extinction in the presence of bream. As indirect effects, small-bodied cladocerans, phytoplankton, periphyton and both organic and inorganic suspended material were higher in the bream treatments. We conclude that the impact of bream diminished substantially with increasing sediment depth, enabling invertebrates to survive in the sediment and to persist in the presence of bream. However, there were no indications of any group adjusting their vertical position behaviourally as a response to predation threat.
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| 4. |
- Ljungberg, Peter, et al.
(författare)
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Effects of small-scale habitat fragmentation on predator–prey interactions in a temperate sea grass system
- 2013
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Ingår i: Marine Biology. - : Springer Science and Business Media LLC. - 0025-3162 .- 1432-1793. ; 160:3, s. 667-675
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Tidskriftsartikel (refereegranskat)abstract
- During the last decades, fragmentation has become an important issue in ecological research. Habitat fragmentation operates on spatial scales ranging over several magnitudes from patches to landscapes. We focus on small-scale fragmentation effects relevant to animal foraging decision making that could ultimately generate distribution patterns. In a controlled experimental environment, we tested small-scale fragmentation effects in artificial sea grass on the feeding behaviour of juvenile cod (Gadus morhua). Moreover, we examined the influence of fragmentation on the distribution of one of the juvenile cod’s main prey resources, the grass shrimp (Palaemon elegans), in association with three levels of risk provided by cod (no cod, cod chemical cues and actively foraging cod). Time spent by cod within sea grass was lower in fragmented landscapes, but total shrimp consumption was not affected. Shrimp utilised vegetation to a greater extent in fragmented treatments in combination with active predation. We suggest that shrimp choose between sand and vegetation habitats to minimize risk of predation according to cod habitat-specific foraging capacities, while cod aim to maximize prey-dependent foraging rates, generating a habitat-choice game between predator and prey. Moreover, aggregating behaviour in grass shrimp was only found in treatments with active predation. Hence, we argue that both aggregation and vegetation use are anti-predator defence strategies applied by shrimp. We therefore stress the importance of considering small-scale behavioural mechanisms when evaluating consequences from habitat fragmentation on trophic processes in coastal environments.
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| 5. |
- Seton, Ragnar, 1985-, et al.
(författare)
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A structured evaluation of regression models for predicting CO2 concentration from plasma emission spectra
- 2022
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Ingår i: Spectrochimica Acta Part B - Atomic Spectroscopy. - : Elsevier. - 0584-8547 .- 1873-3565. ; 194
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Tidskriftsartikel (refereegranskat)abstract
- This study is an extensive comparison of the predictive performance of a bagging neural network (BANN), partial least squares (PLS) regressor, and a kernel-based, nonlinear PLS (KPLS) regressor, given experimentally obtained CO2-N2-Ar plasma emission spectra. The spectra, 36231 in total, were obtained from controlled gas mixtures with varying CO2 concentrations fed to a stripline split-ring resonator microplasma source and recorded with an UV-NIR, 2 nm resolution spectrometer. The regression methods’ dependence on (i) number of observations used in training, (ii) preprocessing steps, and (iii) feature selection (in this case wavelength ranges) was evaluated by training and testing 60-66 models per method, each with a unique combination of the aforementioned configuration options, and each trained 4 times with random train-test splits. To compare the models a custom metric that compounds R2, Pearson correlation, and a weighted root mean squared relative error was used. The results show that the BANN models outperform both PLS and KPLS, reaching a peak score of 0.873, with the others getting 0.561 and 0.581, respectively, using the (− ∞ , 1] metric. The top performing BANN model was trained without any feature selection or preprocessing, these steps were, however, required for both the best PLS and KPLS models. In a wider perspective, the results show that BANNs are not only suitable as in-place replacements for PLS-based methods, but increase regression model prediction accuracy on low resolution spectra to such an extent that they offer modelling of previously unattainable, nonlinear information contained in emission spectra datasets.
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| 6. |
- Behrens, Jane W., et al.
(författare)
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Personality- and size-related metabolic performance in invasive round goby (Neogobius melanostomus)
- 2020
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Ingår i: Physiology and Behavior. - : Elsevier. - 0031-9384 .- 1873-507X. ; 215, s. 1-6
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Tidskriftsartikel (refereegranskat)abstract
- Differences between individuals in behavioral type (i.e. animal personality) are ecologically and evolutionarily important because they can have significant effects on fitness components such as growth and predation risk. In the present study we are used the invasive round goby (Neogobius melanostomus) from an established population in controlled experiments to examine the relationships among personality, metabolic performance, and growth rate (inferred as size-at-age). Boldness was measured as the time to return to normal behavior after a simulated predator attack, where fish with shorter freezing times were categorized as "bold" and fish with longer times were categorized as "shy." We show that bold fish have significantly higher standard metabolic rate (SMR) than their shy conspecifics, whereas there was no difference between personality types in their maximum metabolic rate (MMR) or aerobic scope (AS). Bold fish furthermore had a smaller size-at-age as compared to shy fish. Together this provides evidence of a metabolic underpinning of personality where the high-SMR bold fish require more resources to sustain basic life functions than their low-SMR shy conspecifics, indicating that bold round goby from established populations with high densities (and high competition for food) pay a price of reduced growth rate.
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| 7. |
- Eilers, Gerriet, et al.
(författare)
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The Radiocarbon Intracavity Optogalvanic Spectroscopy Setup at Uppsala
- 2013
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Ingår i: Radiocarbon. - : Cambridge University Press (CUP). - 0033-8222 .- 1945-5755. ; 55:3-4, s. 237-250
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Tidskriftsartikel (refereegranskat)abstract
- Accelerator mass spectrometry (AMS) is by far the predominant technology deployed for radiocarbon tracer studies. Applications are widespread from archaeology to biological, environmental, and pharmaceutical sciences. In spite of its excellent performance, AMS is expensive and complicated to operate. Consequently, alternative detection techniques for 14C are of great interest, with the vision of a compact, user-friendly, and inexpensive analytical method. Here, we report on the use of intracavity optogalvanic spectroscopy (ICOGS) for measurements of the 14C/12C ratio. This new detection technique was developed by Murnick et al. (2008). In the infrared (IR) region, CO2 molecules have strong absorption coefficients. The IR-absorption lines are narrow in line width and shifted for different carbon isotopes. These properties can potentially be exploited to detect 14CO2, 13CO2, or 12CO2 molecules unambiguously. In ICOGS, the sample is in the form of CO2 gas, eliminating the graphitization step that h is required in most AMS labs. The status of the ICOGS setup in Uppsala is presented. The system is operational but not yet fully developed. Data are presented for initial results that illustrate the dependence of the optogalvanic signal on various parameters, such as background and plasma-induced changes in the sample gas composition.
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| 8. |
- Persson, Anders, et al.
(författare)
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Evaluation of Intracavity Optogalvanic Spectroscopy for Radiocarbon Measurements
- 2013
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Ingår i: Analytical Chemistry. - : American Chemical Society (ACS). - 0003-2700 .- 1520-6882. ; 85ASAP:14, s. 6790-6798
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Tidskriftsartikel (refereegranskat)abstract
- Ever since the first publication of intracavity optogalvanic spectroscopy (ICOGS) in 2008, this novel technique for measuring the 14C/12C ratio in carbon dioxide has rendered considerable attention. As a result, there are currently at least five different research groups pursuing research on ICOGS. With a claimed limit of detection of 10–15 (14C/12C), i.e., in the same order as accelerator mass spectroscopy, achieved with a relatively inexpensive and uncomplicated table-top system, ICOGS has major scientific and commercial implications. However, during the past 5 years, no research group has been able to reproduce these results or present additional proof for ICOGS’s capability of unambiguous 14C detection, including the authors of the original publication. Starting in 2010, our group has set up a state-of-the-art ICOGS laboratory and has investigated the basic methodology of ICOGS in general and tried to reproduce the reported experiments in particular. We have not been able to reproduce the reported results concerning the optogalvanic signals dependence on14C concentration and wavelength and, ultimately, not seen any evidence of the capability of ICOGS to unambiguously detect 14C at all. Instead, we have found indications that the reported results can be products of measurement uncertainties and mistakes. Furthermore, our results strongly indicate that the reported limit of detection is likely to be overestimated by at least 2 orders of magnitude, based on the results presented in the original publication. Hence, we conclude that the original reports on ICOGS cannot be confirmed and therefore must be in error.
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| 9. |
- Axelsson, Karin, 1968-, et al.
(författare)
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Communication Analysis of Public Forms : Discovering Multi-functional Purposes in Citizen and Government Communication
- 2007
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Ingår i: International Journal of Public Information Systems. - Sundsvall : MidSweden University. - 1653-4360. ; :3, s. 161-181
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Tidskriftsartikel (refereegranskat)abstract
- This paper adopts a communication perspective on forms in public e-services. This perspective emphasizes that the main purpose of such forms is to facilitate communication between citizens and government agencies. The form is perceived as a tool for performing communicative actions. A communication analysis (CA) method, originally developed for systems requirements engineering, is applied on a public form; i.e. the medical certificate used in connection with a citizen’s application for a provisional driving license in Sweden. The CA method consists of a set of questions related tothree communicative categories; conditions, actions, and consequences. The CA method is used in order to explore the communicative roles of forms in public e-services. As a result of the communication analysis four multi-functional purposes of citizen and government agency communication are discovered. These purposes contribute to the understanding and evaluation of forms in public e-services. The communicative roles, and their multi-functional purposes, are important design features to focus in the development of e-services and electronic forms. Besides these findings, another outcomeis that the CA method has been tested in an e-government context. The communication perspective, as well as the CA method, contribute with useful insights in this context.
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| 10. |
- Batani, D., et al.
(författare)
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Effects of laser prepulses on laser-induced proton generation
- 2010
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 12
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Tidskriftsartikel (refereegranskat)abstract
- Low-intensity laser prepulses (<10(13) W cm(-2), nanosecond duration) are a major issue in experiments on laser-induced generation of protons, often limiting the performances of proton sources produced by high-intensity lasers (approximate to 10(19) W cm(-2), picosecond or femtosecond duration). Depending on the intensity regime, several effects may be associated with the prepulse, some of which are discussed in this paper: (i) destruction of thin foil targets by the shock generated by the laser prepulse; (ii) creation of preplasma on the target front side affecting laser absorption; (iii) deformation of the target rear side; and (iv) whole displacement of thin foil targets affecting the focusing condition. In particular, we show that under oblique high-intensity irradiation and for low prepulse intensities, the proton beam is directed away from the target normal. Deviation is towards the laser forward direction, with an angle that increases with the level and duration of the ASE pedestal. Also, for a given laser pulse, the beam deviation increases with proton energy. The observations are discussed in terms of target normal sheath acceleration, in combination with a laser-controllable shock wave locally deforming the target surface.
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