Plasma dynamics of a laser filamentation-guided spark

International audienceWe investigate experimentally the plasma dynamics of a centimeter-scale, laser filamentation-guided spark discharge. Using electrical and optical diagnostics to study monopolar discharges with varying current pulses we show that plasma decay is dominated by free electron recombination if the current decay time is shorter than the recombination characteristic time. In the opposite case, the plasma electron density closely follows the current evolution. We demonstrate that this criterion holds true in the case of damped AC sparks, and that alternative current is the best option to achieve a long plasma lifetime for a given peak current

Absolute Spectral Radiation Measurements From 200-ns 200-kA X-Pinch in 10-eV-10-keV Range With 1-ns Resolution

International audienceA series of seven X-ray diodes and four Si p-i-n detectors with K- and L-filters was employed to measure the absolute time-resolved spectra of 200-ns 200-kA molybdenum and copper X-pinch plasmas. We observe a 10-mum-size 0.4-0.7-ns X-ray source at a total power yield level of 1.5 GW with about 35% in the range above 1 keV. The extreme ultraviolet part of the spectrum can be fitted by a Planckian function with a temperature of 65-75 eV. In the region above 800 eV, the spectrum can be fitted by an exponential distribution with an effective temperature of ~1 keV for Mo X-pinch and ~500 eV for Cu plasma. The X-ray source yields 200-550 mJ in this spectral range. The total XUV and X-ray yield varies in the range 10-30 J

Commutation rapide déclenchée par filamentation laser femtoseconde

International audienceLa revue scientifique de l'Ecole polytechnique (parution annuelle) ISSN : 1775-0385 Le phénomène de filamentation laser dans lair a été décrit dans un précédent numéro (Flash X n° 12) par A. Houard et A. Mysyrowicz, chercheurs du LOA (Laboratoire dOptique appliquée). Rappelons ici quil apparaît spontanément le long dun faisceau laser de durée femtoseconde (fs) quand sa puissance dépasse un seuil critique de quelques GW. un équilibre dynamique entre la diffraction naturelle du faisceau, leffet Kerr, et la défocalisation par lair ionisé, maintient lintensité dans le coeur du faisceau sur une distance qui peut dépasser un mètre, entraînant la formation dun canal de plasma dans le sillage de limpulsion. Le point de démarrage, la longueur des canaux de plasma et leur distribution transverse sont contrôlables en jouant sur les paramètres de limpulsion laser initiale (énergie, durée, diamètre du faisceau)

Interferometric study of low density channels and guided electric discharges induced in air by laser femtosecond filaments

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Principal component analysis of electron beams generated in K-shell aluminum X-pinch plasma produced by a compact LC-generator

International audiencePrincipal component analysis (PCA) method is applied and compared with the line ratios of H-like and He-like transitions, in order to investigate the effects of electron beam on the K-shell Aluminum synthetic spectra. It is also used as a diagnostics to estimate the plasma parameters of K-shell Al X-pinch plasma spectrum. This spectrum is produced by the explosion of two 25-μm Al wires on a compact LC (40 kV, 200 kA) generator. The database for the principal component extraction is created over a previously developed, non-LTE, collisional radiative K-shell Aluminum model. As a result, PCA shows an agreement with the line ratios which are sensitive to plasma electron temperatures, densities and beam fractions. Principal component analysis also illustrates that the addition to the non-LTE model of a fraction f of electrons in an energetic beam, generates the clusters in a three dimensional vector space which are translations of each other and follows reverse v-shaped cascade trajectories, except for the f = 0.0 case. Modeling of a typical shot by PCA gives the plasma electron temperature of Te = 100 eV, density of Ne = 1 × 1020 cm−3 and hot electron fraction of f = 0.2 (with a beam energy centered at 10 keV)

Linear discriminant based prey-predator analysis of hot electron effects on the X-pinch produced K-shell Al plasmas

International audienceLinear Discriminant Analysis (LDA) is applied to investigate the electron beam effects on the X-pinch produced K-shell Aluminum spectra. The radiating plasma is produced by the explosion of two 25-μm Al wires (XP622) on a compact L-C (40kV, 200kA and 200ns) generator and was analyzed using the full set of diagnostics: current signals by close-to-load B-dot probing, fast filtered PCDs, time-integrated x-ray pinhole imaging and time-integrated crystal spectrometry in the keV region. The electron temperature and density, and the hot electron beam fraction in the Al plasma, were extracted using coefficients of linear discriminant components derived from the non-LTE collisionally radiative K-shell Al model and compared with the ones obtained by principal component analysis method [1]. As a result, LDA can also be used for plasma diagnostics and can investigate the electron beam effects on the synthetic spectra, especially in the details of intercombination and dielectronic satellite lines of weak transitions. Linear discriminant vector spectra analysis shows that the weak transitions of Al He Ly and He to move in Langmuir wave type oscillations. A 3-D representation of linear discriminant analysis shows that the addition of a fraction of electrons in an energetic beam, reflects the quantized clusters as well as outward spiral turbulence. These spirals are modeled with logistic growth with predator called Predator-prey model. This modeling suggests that the electron beams and ions represent the preys and predator, respectively. The center region of the spirals has low temperatures (50-100 eV) but more stability in which the low ionization is fixed by the electron beams. Also not only LD [2] but also LD [3] coefficients obtained from LDA are used to estimate the temperature of a given test plasma which makes a significant improvement compare to due to the less information loss. The modeled plasma electron temperatures and densities are about Te = 75 eV and ne = 1x1019 cm-3 in the presence of electron beams with fraction of f = 0.2 and energy centered at 10 keV. oral presentatio

Spectroscopic analysis of X-pinch plasma produced on the compact LC-generator of Ecole Polytechnique using artificial neural networks

International audienceA back-propagation artificial neural network algorithm is applied to a Mo X-pinch to estimate plasma parameters from typical L-shell spectra in the keV region. The spectrum was generated by a very compact LC-generator (40 kV, 200 kA) for driving 25-μm Mo two-wire X-pinches with a current rise-time of 200 ns. The neural network was trained over a set of synthetic spectra generated using a previously developed L-shell non-LTE collisionally radiative model. As a result, electron temperature and density of the Mo plasma were estimated as Te = 1000 eV and ne = 9 × 1020 cm−3. Furthermore, effects of electron beams on plasma parameters have been investigated through the inclusion of hot electrons in the kinetic model. The small fraction of hot electrons resulted in a better fitting of spectrum and decreased parameters Te = 650 eV and ne = 3 × 1020 cm−3

Linear discriminant analysis based predator-prey analysis of hot electron effects on the X-pinch plasma produced K-shell Aluminum spectra

International audienceIn this study, Linear Discriminant Analysis (LDA) is applied to investigate the electron beam effects on the X-pinch produced K-shell Aluminum plasma. The radiating plasma is produced by the explosion of two 25-μm Al wires on a compact L-C (40 kV, 200 kA and 200 ns) generator, and the time integrated spectra are recorded using de Broglie spectrographs. The ion and electron oscillations of K-shell Al plasma are extracted using LDA of spectral database of non-LTE K-shell Al model. A three dimensional representation of LDA shows that the presence of electron beam exhibits outward spirals of Langmuir turbulence and the center region of the spirals recieves lower electron temperatures of 50–100 eV. These spirals then are modeled by logistic growth of predator-prey model. This modeling suggests that the ions (LD1: most dominant eigenvector of LDA) and electrons (LD2: second most dominant eigenvector of LDA) represent the predators and preys, respectively. Besides, addition of electron beams transforms evanescent oscillations to the standing ones

PC spectral analysis of L-shell copper X-pinch plasma produced by the compact generator of Ecole polytechnique

International audienceHigh Energy Density Physics session oral presentation refered article Principal Component Analysis (PCA) is applied and compared with the line ratios of special Ne-like transitions for investigating the electron beam effects on the L-shell Cu synthetic spectra. The Principal Components (PC) of L-shell Cu are extracted over a non-LTE collisional radiative L-shell Cu model with and without presence of hot electrons to discuss the electron beam effects. Furthermore, PC spectra of Ne-like transitions are also studied as an alternative diagnostics to investigate the polarization sensitivity of these transtions. The extracted PCs are used to estimate the plasma electron temperature, density and beam fractions from a representative time-integrated spatially-resolved L-shell Cu X-pinch plasma spectrum. The experimental spectrum is produced by the explosion of 25-µm Cu wires on a compact L-C (40 kV, 200 kA, 200 ns) generator. The modeled plasma electron temperatures are about Te ∼ 125eV and ne = 5×1019 cm−3 in the presence of the fraction of the beams with f ∼ 0.05 and centered energy of ∼10 keV

Evolution of a laser filamentation triggered electric discharge in air

International audienceSummary form only given. Laser filamentation guided electric discharges have many interesting applications, among which are the laser lightning rod, plasma aerodynamic control, high-power closing switches and plasma antennas. The latter cases rely on a precise control of the discharge plasma lifetime, either to adapt the switch or antenna on-state time to the current situation. In this work, we study the link between the temporal evolution of a laser filamentation triggered discharge plasma column and the discharge current waveform. The centimeter long plasma is generated by the discharge of a 2 nF capacitor charged to 15 kV, yielding a constant electric energy of 200 mJ. Various ballast resistances ranging from 400 to 10 Ω allow modifying the current waveform in the monopolar regime. A 29 μH coil can also be used to bring the circuit in the oscillatory regime. Plasma is characterized by means of two-color interferometry, enabling us to extract space and time-resolved electron density and neutral density radial profiles. We found that, in the monopolar regime, as the current pulse amplifies and shortens, electron density decay becomes dominated by electron-ion recombination, while low-current impulses result in a plasma decay closely related to the current waveform. Longer plasma lifetime is achieved using strong and short current pulses. As for the oscillatory regime, we show that in this case the plasma has a very slow decay, resulting in more favorable conditions for the generation of long lived plasma columns