F.G. Tomasel

Prediction of functional characteristics of ecosystems: a comparison of artificial neural networks and regression models

Abstract We tested the potential of artificial neural networks (ANNs) as predictive tools in ecology. We compared the performance of ANNs and regression models (RM) in predicting ecosystems attributes, with special emphasis on temporal (interannual) predictions of functional attributes of the ecosystem at regional scales. We tested the predictive power of ANNs and RMs using simulated data for six functional traits derived from the seasonal course of the normalized difference vegetation index (NDVI): the annual integral of the NDVI curve (NDVI-I), the maximum (MAX) and minimum (MIN) NDVI, the date of the MAX NDVI (DM) and the date of start (SGS) and end (EGS) of the growing season. For one of these traits (NDVI-I), we also generated a set of data that incorporated the effects of the state of the system in previous years (inertial effects). Even simple non-linearities in the actual functional form of the relationship between environmental variables and ecosystem attributes preclude a precise prediction of these attributes when the rules are not explicit. That was evident for predictions based on both ANNs and RMs under absolutely deterministic conditions (error-free scenario). Non-linearities in the simulated traits of the NDVI curve derive from multiplicative terms in the models. Under the presence of these non-linear terms, a different aggregation of the driving variables (monthly vs. annual or quarterly climatic data) reduce substantially the ability of both RMs and ANNs to predict the independent variable. For the six traits analyzed, the ANNs were able to make better predictions than RMs. The correlation between observed and predicted values of each of the six traits considered was higher for the ANNs than for the RMs. ANNs showed clear advantages to capture inertial effects. The ANN used was able to use previous year information on climate to estimate current year NDVI-I much better than the RM that used the same input information.

Discharge‐pumped soft‐x‐ray laser in neon‐like argon

Starting with the discovery of x‐ray lasers in 1984, laser‐created plasmas remained for almost a decade, the only medium in which large amplification of soft‐x‐ray radiation could be obtained. In this paper the recent first demonstration of large soft‐x‐ray amplification in a discharge‐created plasma column, realized utilizing a fast capillary discharge to collisionally excite the 46.9 nm transition of Ne‐like, Ar is reviewed. Results of the parametrization of the Ar IX discharge‐pumped amplifier, the study of the dynamics of its plasma column, and the measurement of the time history of the laser pulse are reported. Prospects for laser operation at shorter wavelengths are also discussed.

Discharge-driven 46.9-nm amplifier with gain-length approaching saturation

Gain length products up to gl/spl ap/14 for the J=0-1 line of Ne-like Ar at 46.9 nm have been achieved in 15-cm-long plasma columns generated by a fast capillary discharge. Amplification in plasma columns up to 20 cm in length was investigated. The laser line intensity is observed to increase exponentially for plasma lengths of up to 15 cm, above which it is observed to saturate. The saturation behavior is discussed. >

Modeling of fast capillary discharge for collisionally excited soft x-ray lasers: comparison with experiments

In this paper we report results of a model of a fast capillary discharge (FCD) and discuss them in comparison with experiments. The overall good coincidence between theory and experiment and the observation of stable reproducible compression are beneficial properties of FCD which open the possibility for achieving X-ray laser action in a compact discharge device. The required discharge parameters for lasing in different atomic elements have been calculated.

Demonstration of a discharge pumped table-top soft X-ray laser

We report the first observation of large soft X-ray amplification (g*1=7.2) in a discharge created plasma. A fast, /spl sim/40 kA current pulse from a compact discharge was used to excite plasma columns up to 12 cm in length in 4 mm channels, producing population inversion in the J=0-1 line of Ne-like Ar and resulting in a gain of 0.6 cm/sup -1/ at 46.9 nm. The capillary discharge excitation scheme has the potential for increasing the wall-plug efficiency of ultrashort wavelength lasers by two orders of magnitude.

Self‐similar model for tamped ablation driven by thermal radiation

A self‐similar model for the ablation driven by thermal radiation is developed for the case in which the ablated flow cannot freely expand and it is optically thick to the impinging radiation. The results show that the motion is governed by a dimensionless parameter, which determines the importance of the radiation heat conduction relative to the convective energy flux. The model can be applied to the situation arising in ion beam fusion targets, where a cold pusher is ablated by blackbody radiation emitted by the beam absorber layer that surrounds it and tamps the ablated flow. The scaling laws for the magnitudes of the flow are found and the main differences with free flow ablation are discussed.

Fast imaging of transient electron injection in planar magnetron discharges

We describe an example of the use of a high-speed camera in the study of transient phenomena in planar magnetron discharges. Low-current, spontaneous arcs are used to inject a stream of electrons into a magnetron plasma. Fast imaging of the arcs shows how the perturbation produced by the injected electrons travels in the E?B direction with minimum average speeds of approximately 7.4?105?m?s?1.

Influence of process conditions on the optical properties of HfO2/SiO2 coatings for high-power laser coatings

We investigate the variations that occur with changes in the number of layers and with the use of the assist beam main and assist beam energy on the morphology of HfO2/SiO2 quarter wave stacks deposited by dual ion beam sputtering. We show how the addition of sequential HfO2/SiO2 bilayers, up to eight, affects the surface roughness and micro-crystallinity of the top HfO2 layer. We also show that use of the assist source significantly smooths the surface while simultaneously reducing microcrystallinity. The HfO2/SiO2 structures are very robust and can withstand fluences in excess of 3 J/cm2 generated by 1ps pulses from a chirped amplified Ti:Sapphire laser.

Toward saturation of a discharge-pumped soft x-ray amplifier

Operation of a discharge-pumped 46.9 nm Ne-like Ar amplifier at gain length products up to gl approximately equal to 14 is reported. The laser line intensity is observed to increase exponentially for plasma column lengths up to 15 cm, above which it is observed to saturate. The saturation behavior is discussed. Results of the parameterization of the discharge pumped amplifier and the measurement of the soft x-ray laser pulsewidth also are reported. The use of an axial magnetic field, that limits the maximum plasma compression and decreases the density gradients, is observed to cause a moderate increase of the soft x-ray laser intensity and a beam profile that is more Gaussian.

Lasing in Ne-like S and other new developments in capillary discharge ultrashort wavelength lasers

We report our most recent progress in the development of capillary discharge soft x-ray lasers. This includes the first observation of discharge-pumped ultrashort wavelength lasing in a material that is solid at room temperature (S), and preliminary results of discharges in Ca. Excitation by a capillary discharge of S vapor generated by discharge ablation of a solid target resulted in amplification in Ne-like S at 60.8 nm with a gain-length product of 7.5. Overheating of the electron temperature respect to steady-state ionization conditions and transient population effects significantly increased the gain above the steady state-value. The results of two-dimensional near-field and far-field imaging of a saturated table-top Ne-like Ar laser and the measurement of its spatial coherence as a function of amplifier length are also reported and compared with model calculations. The generation of a capillary discharge plasma waveguide is to be used in combination with ultrashort pulse laser excitation for the generation of a new kind of efficient collisional soft x- ray laser is discussed.