Different Types Of Perturbations Research Articles

Robustness of vectorial laser systems

The polarization robustness of vectorial lasers with respect to different kinds of perturbation is theoretically investigated in the framework of a Landau potential approach. It is shown that usual internal and external perturbations may be classified in two main classes: uniform perturbations which alter the two eigenstates in the same manner, and differential perturbations that favor one eigenstate with respect to the other. It is also demonstrated that the coupling between the two eigenstates completely modifies the robustness of the system to these different types of perturbations. The crucial role of this coupling is experimentally isolated using two original different arrangements, allowing the value of the coupling constant to be tuned around the value one and then to reach very low coupling values. Good agreement is observed between experiment and theory. In particular, it is shown that systems highly robust with respect to any kind of perturbation can be built in two different ranges of the coupling constant, the first one corresponding to strong coupling regimes and the second one to very weak coupling regimes

Robustness analysis of feedback systems with block-diagonal structured uncertainties

The results for magnitude-bounded diagonal perturbation are extended to norm-bounded block-diagonal perturbation. A useful consequence to this extension is that an easily computable upper bound for the structured singular value via suboptimal diagonal similarity scaling is obtained. It is concluded that the results are applicable to a wider class of perturbations, i.e. the norm which bounds the perturbation p in this study need not be the 2-norm (maximum singular value). In the scalar case, this does not give much advantage, as all pth-norms reduce to the magnitude of the element. However, in the matrix case, different norms would characterize different types of perturbations. >

The Concept of FID‐detected Hole‐burning in Pulsed EPR Spectroscopy

AbstractA new class of pulsed EPR experiments, based on FID‐detected hole‐burning, is introduced. A transient spectral hole burnt into an inhomogeneously broadened EPR line by means of a selective microwave pulse is shifted or broadened by different types of perturbations and is subsequently recorded in a single experiment via an FID following a nonselective microwave pulse. The proposed approach often exceeds corresponding electron spin‐echo experiments with respect to sensitivity and resolution. A number of applications of the FID‐detected hole‐burning technique are discussed and the predicted features of the new experiments are verified by model calculations and experiments.

Effects of diacylglycerols on the structure of phosphatidylcholine bilayers: a 2H and 31P NMR study.

The interaction of four diacylglycerols (DAGs) with multilamellar phospholipid bilayers consisting either of dipalmitoylphosphatidylcholine (DPPC) or of a mixture of DPPC and bovine liver phosphatidylcholine (BL-PC) extracts was investigated by a combination of 31P and 2H NMR spectrometry. We found that saturated and unsaturated long-chain DAGs induce different types of perturbations into the bilayer structure. The saturated DAGs dipalmitin and distearin induce lateral phase separation of the lipids into (i) DAG-enriched gellike domains and (ii) relatively DAG-free regions in the liquid-crystalline phase. In the latter regions, the order parameters along the fatty acyl chains of DPPC are practically identical with the control. This phase separation effect was observed in both model systems studied, and its extent is dependent upon DAG concentration and temperature. Only bilayer phases were present upon addition of dipalmitin or distearin at all concentrations and temperatures studied. The unsaturated DAGs diolein and DAG derived from egg PC (egg-DAG) affect PC bilayers in the following two ways: (i) by increasing the order parameters of the side chains, as observed for both DPPC and BL-PC model systems; (ii) by inducing nonbilayer lipid phases, as observed for BL-PC, but not DPPC. At a concentration of 25 mol % of an unsaturated DAG in mixed PC bilayers, a peak corresponding to isotropic lipid conformation appeared and increased in intensity with increase in temperature, while at 32 mol % hexagonal and bilayer phases coexisted.(ABSTRACT TRUNCATED AT 250 WORDS)

Postural responses to changing task conditions.

The experimental goal was to investigate discrepancies in the literature concerning postural adaptation and to determine if the prior presentation of horizontal perturbations affected the amplitude of responses to rotational perturbations. Surface EMG recordings from lower leg muscles (gastrocnemius (GAS) and tibialis anterior (TA)) were recorded in twelve subjects, and the amplitudes of the responses were statistically analyzed. We did not find differences between the responses to rotational perturbations which preceded or followed horizontal perturbations. This finding did not support the hypothesis that differences in the order of presentation of the different types of perturbations accounted for the discrepancies in the literature. Furthermore, our design did not show the progressive elimination of the GAS response within three to five sequential trials. Instead, we found a slow but significant response amplitude reduction over ten trials without yielding a permanent disappearance of the response. When analyzing the GAS responses to the rotational perturbations only, we found two components that contributed to the response reduction: 1) an initial reduction between trials one and subsequent trials, which could be due to habituation of a startle-like response; and 2) a second reduction which was more gradual. Our results also showed an immediate change in the response amplitude on the first trial, when the type of perturbation was changed. This is inconsistent with the view that ankle musculature stretch and joint movement are the primary inputs driving the postural responses. Since small ankle dorsiflexing rotations produced by the platform translations caused large GAS responses while large ankle dorsiflexing rotations produced by direct platform rotations caused small GAS responses, this suggests that multiple sensory inputs contribute to the responses. We propose that an initial compensation to a new perturbation type occurs within the first trial by the integration of these divergent sensory inputs.

Stability and robustness properties of universal adaptive controllers for first-order linear systems

Quite recently, examples of non-linear adaptive controllers have been found that adaptively stabilize any scalar first-order linear system. In this paper we describe two general classes of such ‘universal’ adaptive stabilizers that include the previously proposed controllers as special cases, Different types of perturbations of these controllers are studied and we report on extensive simulation experiments. Our results demonstrate considerable qualitative differences in the dynamical behaviour of various adaptive universal stabilizers. While the previously proposed controllers all exhibit the bursting phenomena, we give an explicit example of a stabilizing controller that does not show bursting behaviour.

Triaxiality in the proton-neutron interacting boson model: Systematic study of perturbations in the SU(3) limit.

In the present article, we study systematically the influence of different types of perturbations on the ${\mathrm{SU}}^{\mathrm{*}}$(3) dynamical symmetry. Both a Majorana perturbation, a d-boson energy perturbation, and the ${\mathrm{SU}}^{\mathrm{*}}$(3)\ensuremath{\rightarrow}O(6) and ${\mathrm{SU}}^{\mathrm{*}}$(3)\ensuremath{\rightarrow}SU(3) transitions are considered. In all cases, we study the variation of the ground and gamma bands as well as some important electromagnetic transitions, in particular the B(M${1;1}_{1}^{+}$\ensuremath{\rightarrow}${0}_{1}^{+}$) value. Even if the present calculations start from a simple Hamiltonian (a consistent Q Hamiltonian), the results can be of interest for the study of the Xe, Ba A\ensuremath{\simeq}130 mass region, the Ru, Pd A\ensuremath{\simeq}100 mass region, and the Pt, Os A\ensuremath{\simeq}190 mass region.

Frontogenesis in a Continuously Varying Potential Vorticity Fluid

Abstract The geostrophic momentum approximation and a Lagrangian formulation are employed to consider the nature of the fronts that result from the action of a stretching deformation field in a continuously varying potential vorticity fluid. In such a fluid, the tropopause is represented by a shallow region over which the potential vorticity changes from a representative tropospheric value to a representative stratospheric one. Decreasing the depth of this zone resulted in an increase in the intensity of the upper-level front. Reduction in the cross-front temperature contrast reduced the intensity of both the surface and upper-level front. Most notably it resulted in the elimination of the deep tropopause fold in the vicinity of the upper-level front. The environment in which the fronts form may have temperature perturbations that are the result of previous or contemporaneous frontogenetic processes. The effects of two different types of perturbations were studied with the continuous model. The inclusion ...

Internal waves arising in the presence of the unsteady motion of a source in a continuously stratified fluid

A general approach to the investigation of linear internal and surface waves in a stably stratified fluid, arising from different types of perturbations, is presented in [1]. The methods of calculation of the internal waves generated by the motion of a mass source are developed for particular cases of steady horizontal motion in a continuously stratified fluid in [2] and arbitrary motion in an unbounded exponentially stratified fluid in [3]. Internal waves generated by other types of perturbations have also been investigated but only for particular cases of motion (see, for example, [4]). This paper presents a method for the calculation of unsteady, linear, internal gravity waves arising in an inviscid incompressible fluid with continuous stable stratification.

Regulation of microsomal enzymes by phospholipids: IX. Production of uniquely modified forms of microsomal UDP-glucuronyltransferase by treatment with phospholipase A and detergents

The kinetic parameters of microsomal UDPglucuronyltransferase (EC 2.4.1.17) were compared after treatment of microsomes with phospholipase A and Triton X-100. Treatments with phospholipase A and Triton had differential effects on activities at V (assayed in both forward and reverse directions), the affinity of the enzyme for UDPglucuronic acid and UDP, the specificity of binding of UDPsugars, and the sensitivity of the enzyme to treatment with UDP- N-acetylglucosamine. Comparisons of lysophosphatide, Triton, and cholate-activated forms indicated that different types of detergents activate by separate mechanisms. These data are taken as evidence that selective types of changes of the composition and structure of microsomal lipids lead to selective types of changes of the properties of UDPglucuronyltransferase. Activation of UDPglucuronyltransferase in response to different types of perturbations of the membrane lipids is thus a specific process. It was found that constraints on the maximal potential activity of the reverse reaction catalyzed by UDPglucuronyltransferase is more extensive in untreated microsomes than constraint on the forward reaction. Also, the “activated forms” of UDPglucuronyltransferase have less activity than the untreated enzyme if assays are carried out under conditions presumed to exist in vivo. These results indicate the complexity of lipid-protein interactions as regulators of membrane-bound enzymes. They also indicate the functional significance of this type of regulation for the activity of UDPglucuronyltransferase.

Mode conversion in periodically disturbed thin-film waveguides

Mode conversion in a periodically perturbed thin-film optical waveguide is studied in detail. Three different types of perturbations are considered: periodic index of refraction of the film, periodic index of refraction of the substrate, and periodic boundary. The applications in filters, mode converters, and distributed feedback lasers are discussed.

Low‐frequency electromagnetic oscillations of the Earth‐ionosphere cavity

The observational and theoretical work on the low‐frequency electromagnetic oscillations of the earth‐ionosphere cavity, known as the Schumann resonances, is reviewed. Additional theoretical considerations are introduced to allow the models to incorporate the asymmetries of the actual cavity. These models are used to explain the observed variations of the power and of the apparent frequency of the resonant modes. The theoretical studies show that the structure of the low D or C region of the ionosphere is critical to these resonances, and the observed frequencies must rule out certain proposed ionospheric models. The perturbations of the cavity are also studied, and comparisons are made between different types of perturbations. The resonance data from July 9, 1962, are used to confirm the tremendous extent of the ionospheric perturbations caused by the Starfish high‐altitude nuclear explosion. The possible effects of natural perturbations are also examined.