Betatron emission as a diagnostic for injection and acceleration mechanisms in laser-plasma accelerators

Betatron x-ray emission in laser-plasma accelerators is a promising compact source that may be an alternative to conventional x-ray sources, based on large scale machines. In addition to its potential as a source, precise measurements of betatron emission can reveal crucial information about relativistic laser-plasma interaction. We show that the emission length and the position of the x-ray emission can be obtained by placing an aperture mask close to the source, and by measuring the beam profile of the betatron x-ray radiation far from the aperture mask. The position of the x-ray emission gives information on plasma wave breaking and hence on the laser non-linear propagation. Moreover, the measurement of the longitudinal extension helps one to determine whether the acceleration is limited by pump depletion or dephasing effects. In the case of multiple injections, it is used to retrieve unambiguously the position in the plasma of each injection. This technique is also used to study how, in a capillary discharge, the variations of the delay between the discharge and the laser pulse affect the interaction. The study reveals that, for a delay appropriate for laser guiding, the x-ray emission only occurs in the second half of the capillary: no electrons are injected and accelerated in the first half.Comment: 8 pages, 6 figures. arXiv admin note: text overlap with arXiv:1104.245

Acidity of dibasic carbon acids. Part 5. 1-4 The second acidity constant of 9,10-dihydrodibenz[a,h]anthracene in tetrahydrofuran-geometry, charge distribution of dianion, structure of dimetallic salts

The second equilibrium ion pair acidity constant (pK 2 ) of 9,10-dihydrodibenz[a,h]anthracene (DBDHA, 1H 2 ) with counter ions sodium (pK 2 28.5) and potassium (pK 2 30.4) has been determined in tetrahydrofuran (THF) at 298 K. This value is considerably lower than for other 9,10-dihydroanthracene derivatives. Disodium, dipotassium and dirubidium salts of DBDHA, i.e. 1 2Ϫ , exist as contact ion triplets in THF at 298 K, although the dilithium salt behaves as a solvent-separated ion triplet under the same conditions. In the dianion of DBDHA (1 2Ϫ ) the negative charge is displaced from the deprotonated carbon atoms towards the outer benzene rings. This report deals with the dependence of the second acidity constant (pK 2 ) of 9,10-dihydroanthracene (DHA) derivatives on the structure and size of the π-electron system of the dibasic carbon acid as well as the counter ion. Previously, we measured pK 2 values of DHA, 9-phenyl-and 9,10-diphenyl-9,10-dihydroanthracene (PDHA and DPDHA, respectively) in tetrahydrofuran (THF) with counter ions Na + , K + and Rb It was found that pK 2 values of PDHA and DPDHA are close to those of DHA and they are very strongly dependent on the counter ion. Analysis of the structure of PDHA and DPDHA dianions shows that the angle between the planes of the phenyl substituent and the anthracene ring is 45Њ. 2 It seems that the contribution of phenyl substituents to stabilization of the negative charge of anions is small and is mainly connected with the inductive effect. It is therefore interesting to estimate the contribution of the conjugation effect of annelated phenyl groups to the stabilization of the negative charge of the dianion of DHA. In view of the possible effect of extending the π-electrons array we studied dihydrodibenz[a,h]anthracene (DBDHA) (1H 2 ) and the corresponding dianion, i.e. 1 2Ϫ . According to our calculations (see below) the dianion of DBDHA (1 2Ϫ ) is planar and it follows that the two outer annelated phenyl ring extend the conjugation and thus the stabilization of the negative charge of the dianion of 9,10-dihydroanthracene by conjugation. Here we report results of the calculations of the dianion geometry and the charge distribution, the structure of ion triplets of dimetallic salts and the pK 2 values of DBDHA (1H 2 ) (counter ions are Na + and K + ). Geometry and electron structure were calculated by the AM-1 method. 5 Ion formation of dimetallic salts of 1H 2 is in the form of an ion triplet as it consists of one anion and two cations. The structure of the ion triplet (vida supra) was determined by UV-VIS spectroscopy and pK 2 was found by measuring the equilibrium constant of transmetallation reaction of dimetallic salts of DBDHA (1H 2 ) with several indicator carbon acids with known pK values. Results Preparation of salts and UV-VIS spectroscopic measurements Dimetallic salts were obtained by consecutive two-electron transfer to DBDHA (1) with alkali metals in THF. The first step is the formation of an anion radical and the second is the formation of the dimetallic salt [eqn. (1)], where M is an alkali metal

Mapping the X-Ray Emission Region in a Laser-Plasma Accelerator

The x-ray emission in laser-plasma accelerators can be a powerful tool to understand the physics of relativistic laser-plasma interaction. It is shown here that the mapping of betatron x-ray radiation can be obtained from the x-ray beam profile when an aperture mask is positioned just beyond the end of the emission region. The influence of the plasma density on the position and the longitudinal profile of the x-ray emission is investigated and compared to particle-in-cell simulations. The measurement of the x-ray emission position and length provides insight on the dynamics of the interaction, including the electron self-injection region, possible multiple injection, and the role of the electron beam driven wakefield.Comment: 5 pages, 4 figure

Persistence of magnetic field driven by relativistic electrons in a plasma

The onset and evolution of magnetic fields in laboratory and astrophysical plasmas is determined by several mechanisms, including instabilities, dynamo effects and ultra-high energy particle flows through gas, plasma and interstellar-media. These processes are relevant over a wide range of conditions, from cosmic ray acceleration and gamma ray bursts to nuclear fusion in stars. The disparate temporal and spatial scales where each operates can be reconciled by scaling parameters that enable to recreate astrophysical conditions in the laboratory. Here we unveil a new mechanism by which the flow of ultra-energetic particles can strongly magnetize the boundary between the plasma and the non-ionized gas to magnetic fields up to 10-100 Tesla (micro Tesla in astrophysical conditions). The physics is observed from the first time-resolved large scale magnetic field measurements obtained in a laser wakefield accelerator. Particle-in-cell simulations capturing the global plasma and field dynamics over the full plasma length confirm the experimental measurements. These results open new paths for the exploration and modelling of ultra high energy particle driven magnetic field generation in the laboratory

Shock assisted ionization injection in laser-plasma accelerators

International audienceIonization injection is a simple and efficient method to trap an electron beam in a laser plasma accelerator. Yet, because of a long injection length, this injection technique leads generally to the production of large energy spread electron beams. Here, we propose to use a shock front transition to localize the injection. Experimental results show that the energy spread can be reduced down to 10 MeV and that the beam energy can be tuned by varying the position of the shock. This simple technique leads to very stable and reliable injection even for modest laser energy. It should therefore become a unique tool for the development of laser-plasma accelerators

Effects of laser prepulses on laser-induced proton generation

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

Activation and detoxification of cassava cyanogenic glucosides by the whitefly Bemisia tabaci

Abstract Two-component plant defenses such as cyanogenic glucosides are produced by many plant species, but phloem-feeding herbivores have long been thought not to activate these defenses due to their mode of feeding, which causes only minimal tissue damage. Here, however, we report that cyanogenic glycoside defenses from cassava (Manihot esculenta), a major staple crop in Africa, are activated during feeding by a pest insect, the whitefly Bemisia tabaci, and the resulting hydrogen cyanide is detoxified by conversion to beta-cyanoalanine. Additionally, B. tabaci was found to utilize two metabolic mechanisms to detoxify cyanogenic glucosides by conversion to non-activatable derivatives. First, the cyanogenic glycoside linamarin was glucosylated 1–4 times in succession in a reaction catalyzed by two B. tabaci glycoside hydrolase family 13 enzymes in vitro utilizing sucrose as a co-substrate. Second, both linamarin and the glucosylated linamarin derivatives were phosphorylated. Both phosphorylation and glucosidation of linamarin render this plant pro-toxin inert to the activating plant enzyme linamarase, and thus these metabolic transformations can be considered pre-emptive detoxification strategies to avoid cyanogenesis

Temporary Acceleration of Electrons While Inside an Intense Electromagnetic Pulse

A free electron can temporarily gain a very significant amount of energy if it is overrun by an intense electromagnetic wave. In principle, this process would permit large enhancements in the center-of-mass energy of electron-electron, electron-positron and electron-photon interactions if these take place in the presence of an intense laser beam. Practical considerations severely limit the utility of this concept for contemporary lasers incident on relativistic electrons. A more accessible laboratory phenomenon is electron-positron production via an intense laser beam incident on a gas. Intense electromagnetic pulses of astrophysical origin can lead to very energetic photons via bremsstrahlung of temporarily accelerated electrons

Generalization of predators and nonpredators by juvenile rainbow trout: learning what is and is not a threat

Learned recognition of novel predators allows prey to respond to ecologically relevant threats. Prey could minimize the costs associated with learning the identity of both predators and nonpredators by making educated guesses on the identity of a novel species based on their similarities with known predators and nonpredators, a process known as generalization. Here, we tested whether juvenile rainbow trout, Oncorhynchus mykiss, have the ability to generalize information from a known predator (experiment 1) or a known harmless species (experiment 2) to closely related but novel species. In experiment 1, we taught juvenile trout to recognize a predatory pumpkinseed sunfish, Lepomis gibbosus, by pairing pumpkinseed odour with conspecific alarm cues or a distilled water control. We then tested the trout for a response to pumpkinseeds and to novel longear sunfish, Lepomis megalotis (same genus as pumpkinseed), rock bass, Ambloplites rupestris (same family as pumpkinseed) or yellow perch, Perca flavescens (different family). Trout showed strong learned recognition of pumpkinseed and longear sunfish odour and a weak learned response to rock bass odour but no recognition of yellow perch. In experiment 2, we used latent inhibition to teach juvenile trout that pumpkinseeds were harmless. During subsequent predator learning trials, trout did not learn to recognize pumpkinseed or longear sunfish odour as potential threats, but they did learn that rock bass and yellow perch were threatening. Taken together, these results demonstrate that juvenile rainbow trout can generalize learned recognition of both predator and nonpredator odours based on the phylogenetic relatedness of predators