Non-stationary Population Research Articles

Nonstationary parametric amplification of polychromatic radiation propagating in an extended absorbing resonant medium

An experimental and theoretical study of the two-wave interaction (of a probe wave and a pumping wave) of wide-band multimode laser pulses with an optically dense extended resonant medium (metastable neon atoms in a positive glow-discharge column) is presented. Depending on the pumping power and characteristics of the medium, various regimes of interaction between the field and the matter, leading to qualitatively different types of amplification spectra, are realized: a cooperative interaction regime at low pumping powers corresponds to amplification spectra consisting of two peaks, which lie on both sides of the absorption line; in the regime of nonstationary Rabi oscillations (Rabi flopping), the amplification appears in the form of an isolated peak at a frequency close to the resonance in the substance; under certain interaction conditions, a dispersive shape of amplification spectra is observed. A significant gain in the probe field (by several times at a length of 10 cm) is obtained as a result of a nonstationary population inversion upon rotation of the Bloch vector of the resonant medium under the action of a strong pumping field in the course of propagation of the two waves.

NITROGEN DEPOSITION AND EXTINCTION RISK IN THE NORTHERN PITCHER PLANT, SARRACENIA PURPUREA

Chronic nitrogen deposition may affect the long-term survival of plant pop- ulations. The northern pitcher plant, Sarracenia purpurea, is a model system for forecasting extinction risk in the face of increasing nitrogen deposition. Uptake of N occurs directly from pitcher leaves, which fill with rainwater and prey, and accumulate nutrients. We monitored the demography of two populations of S. purpurea in ombrotrophic bogs of northern New England (USA) and described population growth with a four-stage matrix model. Growth, survivorship, and reproduction were in close balance, and the model pre- dicted population growth rates close to zero, with long doubling times. In a life-table response experiment, we manipulated in situ concentrations of nitrogen and phosphorus in pitcher plant leaves. At high levels of N and high N:P ratios, population growth rates were significantly depressed. Using local N-deposition records, we forecast different N-deposition scenarios and built a nonstationary population growth model, in which a different transition matrix at each time step reflected the current deposition regime. Autocorrelation in the time series of N deposition rates generated periodic increases in population size superimposed on long-term trends of exponential decline. For a 1% annual increase in N deposition rates, this model predicted a substantial risk of extinction at 100 yr (0.38-0.70), with an estimated population survival time of 160-180 yr. Although slow-growing populations of Sarracenia can buffer substantial environmental change, chronic increases in N deposition rate threaten population persistence.

Transient collisional excitation X-ray lasers with 1-ps tabletop drivers

Recent results are presented for 1-ps driven X-ray laser amplification in Ne-like and Ni-like transient collisional excitation research at the Lawrence Livermore National Laboratory. Plasma formation, ionization and collisional excitation are optimized using two laser pulses of 600- and 1-ps duration at tabletop energies of typically 5 J or less in each beam. Gain of 35 cm/sup -1/ and higher has been measured on the 147 /spl Aring/ 4d/spl rarr/4pJ=0/spl rarr/1 transition of Ni-like Pd and is a direct consequence of the nonstationary population in, version produced by the high intensity picosecond pulse. We report the characterization of the X-ray laser properties including the transient gain lifetime and beam divergence for different Ne-like and Ni-like X-ray lasers.

Effects of interface states on the non-stationary transport properties of Schottky contacts and metal-insulator-semiconductor tunnel diodes

The effects of interface states on the non-stationary transport properties (capacitance and conductance) of Schottky contacts and metal-insulator-semiconductor (MIS) tunnel diodes are studied. Our study is based upon a detailed analysis of the various carrier-exchange mechanisms taking place at a metal-semiconductor contact and incorporates all the effects of the non-equilibrium and non-stationary population of the interface states. The theory offers an unambiguous interpretation of the experimental results in the whole range of biases and frequencies. The relevance of our results for the determination of the density of interface states from capacitance and conductance measurements and for the evaluation of the effects of the interfacial layer thickness in MIS tunnelling diodes has been considered.

Demonstration of X-Ray Amplification in Transient Gain Nickel-like Palladium Scheme

We report experimental results of x-ray amplification of spontaneous emission in a Ni-like transient collisional excitation scheme. The Ni-like plasma formation, ionization, and collisional excitation requires irradiation of a slab target by two laser pulses: a formation beam with 5 J energy of 800 ps duration and a pump beam of 5 J energy in 1.1 ps. A gain of $35{\mathrm{cm}}^{\ensuremath{-}1}$ and a $\mathrm{gL}$ product of 12.5 are measured on the $4d\ensuremath{\rightarrow}4p$ $J\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0\ensuremath{\rightarrow}1$ transition for Ni-like Pd at 147 \AA{} with an 8 mm line focus. The high efficiency of this scheme at ``table-top'' laser energies is a direct consequence of the nonstationary population inversion produced by the high intensity picosecond pulse.

Short Pulse X-Ray Laser at 32.6 nm Based on Transient Gain in Ne-like Titanium

A novel two-step excitation scheme for an efficient table-top x-ray laser has been realized for the first time. A nanosecond pulse creates a plasma of neonlike ions of titanium, followed by a subpicosecond pulse which excites a nonstationary population inversion. With only a few joules of pump energy, a compact x-ray laser at 32.6 nm with a very high gain coefficient of $g\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}19{\mathrm{cm}}^{\ensuremath{-}1}$ and a gain-length product of $gL\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}9.5$ was achieved.

The notion of population economics.

Economic literature has not adequately addressed population problems and growth models have assumed the population growth rate as an exogenous constant. Life expectancy and the quality of documentation over a long period have corrected the problem of nonstationary population however. In addition mathematical and computational possibilities have been increased thereby allowing population issues to be considered endogenous and variable in models. Recognizing that population changes affect supply and demand and should be studied a group of economists have found a society to address such population problems--the European Society for Population Economics. Within the realm of population economics are subfields that should be included in any studies. For example economists must examine household behavior and its influence on having or not having children. Another subfield includes studying the effects of changes in birth rates. For example birth rates affect age distribution and each age distribution has a typical distribution of demand. A young society would demand schooling and child care while an old society would need elderly care and health facilities. To enhance optimum population growth policy makers have several options. For example family allowances and child care influence procreation but they require an increase in state expenditures and taxes. Providing health care for all members of society improves the quality of the population and its size. Implementing a migration policy also increases the population. Education and retraining strengthens the quality of the labor force but some argue that it does not lead to procreation.

Exact sampling distributions from the infinite neutral alleles model

The exact transient sampling distribution is found for the infinite neutral alleles model with mutation, thus extending the stationary distribution found by Ewens (1972). The first eigenfunction in the sampling distribution depends only on the homozygosity. In a large sample the expected number of allele types is close to the expected number in a stationary distribution and depends little on the initial frequencies or the time of sampling. The expected number of types in a sample for a non-stationary population is tabulated.The sampling distribution from a population with no mutation is found. The probability that the population is monomorphic given that the sample contains only one type is tabulated, where initially the population has a large number of alleles with equal frequencies.A study is made of the joint distribution of the allele frequencies in two samples taken timetapart from a stationary population. Of particular interest is the number of allele types in common in two such samples. The distribution of the number of types in common in a population viewed at timetapart is also found and tabulated for different mutation rates and time. The expected frequency of the allele types in common is tabulated.The distribution of allele frequencies in two divergent populations of common ancestry is shown to be the same as the distribution of frequencies in a single population at two time points 2tapart. A sufficient statistic for the time of divergence is shown to be the paired frequencies of the allele types in common. The expected waiting time until the populations have no allele types in common is tabulated.

Exact sampling distributions from the infinite neutral alleles model

The exact transient sampling distribution is found for the infinite neutral alleles model with mutation, thus extending the stationary distribution found by Ewens (1972). The first eigenfunction in the sampling distribution depends only on the homozygosity. In a large sample the expected number of allele types is close to the expected number in a stationary distribution and depends little on the initial frequencies or the time of sampling. The expected number of types in a sample for a non-stationary population is tabulated.The sampling distribution from a population with no mutation is found. The probability that the population is monomorphic given that the sample contains only one type is tabulated, where initially the population has a large number of alleles with equal frequencies.A study is made of the joint distribution of the allele frequencies in two samples taken timetapart from a stationary population. Of particular interest is the number of allele types in common in two such samples. The distribution of the number of types in common in a population viewed at timetapart is also found and tabulated for different mutation rates and time. The expected frequency of the allele types in common is tabulated.The distribution of allele frequencies in two divergent populations of common ancestry is shown to be the same as the distribution of frequencies in a single population at two time points 2tapart. A sufficient statistic for the time of divergence is shown to be the paired frequencies of the allele types in common. The expected waiting time until the populations have no allele types in common is tabulated.