Short-wavelength Electromagnetic Waves Research Articles

Synthesis and characterization of phosphorylated cellulose nanocrystals for enhanced UV stability of epoxy nanocomposite films

Epoxy resins offer excellent chemical resistance, low-temperature curing, and low shrinkage. Despite these excellent properties, they are susceptible to UV degradation, which compromises their durability in outdoor applications. Herein, we fabricated a nanocomposite epoxy film based on phosphorylated cellulose nanocrystal (P-CNC) to improve the UV stability of cured epoxy resin by incorporating liquid ammonia-treated P-CNC into an epoxy matrix using integral blending method. Ammonium phosphate groups ((NH4)2HPO4) were formed by nucleophilic attack of phosphates by ammonia on the P-CNC surface. The functional groups, morphology, crystallinity index, and surface charges of P-CNC were characterized using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD), and electrophoretic light scattering (ELS), respectively. The effect of P-CNC on the UV stability of the fabricated epoxy nanocomposite film was investigated using FTIR and UV–Vis before and after exposure to UV photons. The results showed that the incorporation of 1.0 wt.% of surface-treated P-CNC into the epoxy matrix maintained the UV stability of the nanocomposite film up to 800 h. Finally, a mechanism for UV stability of P-CNC epoxy nanocomposite film was proposed whereby the (NH4)2HPO4 groups grafted onto the P-CNC surface absorb UV photons and convert them into low-energy thermal energy or short wavelength electromagnetic waves, which are not sufficient to initiate photodegradation of the nanocomposite film. Thus, this new film provides resistance against UV degradation for extended service life for outdoor applications of epoxy resin.

Suppression of nonlinear standing wave excitation via the electrical asymmetry effect

The electrical asymmetry effect (EAE) enables separate control of the ion flux and the mean ion energy in capacitively coupled plasmas (CCP). While a variety of plasma processing applications benefit from this, large-area, very-high-frequency CCPs still suffer from lateral nonuniformities caused by electromagnetic standing wave effects (SWE). Many of such plasma sources are geometrically asymmetric and are operated at low pressure so that high frequency nonlinear plasma series resonance (PSR) oscillations of the RF current are self-excited. These PSR oscillations lead to the presence of short wavelength electromagnetic waves and a more pronounced SWE. In this work, we investigate the influence of the EAE on the nonlinear standing wave excitation in a geometrically asymmetric, low pressure capacitively coupled argon plasma driven by two consecutive harmonics (30 MHz and 60 MHz) with an adjustable phase shift, θ. We use a hairpin probe to determine the radial distribution of the electron density in combination with a high-frequency B-dot probe to measure the radial distribution of the harmonic magnetic field, which in turn is used to calculate the harmonic current density based on Ampere’s law. Our experimental results show that the asymmetry of the discharge can be reduced electrically via the EAE. In this way the self-excitation of high frequency PSR oscillations can be attenuated. By tuning θ, it is, therefore, possible to switch on and off the nonlinear standing wave excitation caused by the PSR and, accordingly, the plasma uniformity can be optimized.

Turbulence Scales of the Monin–Obukhov Similarity Theory in the Anisotropic Mountain Boundary Layer

The characteristics of turbulence over mountainous terrain are investigated with an aim to predict propagation of short-wavelength electromagnetic waves (including optical waves). For this purpose, long-term observations of the day- and night-time stratification of atmospheric turbulence are continued at the Sayany solar observatory. A new mobile AMK-03-4 hard-and-software system is used, which is designed for measuring the characteristics of turbulent meteofields. New experimental data are obtained on the turbulent scales of velocity V* (friction velocity) and temperature T*, which are important characteristics in the Monin–Obukhov similarity theory. A comparison of the experimental results with the data obtained by the authors earlier in different periods of time and in different climatic and geographical regions is performed. An experimental validation of the similarity theory postulates in the atmospheric boundary layer is provided. It is shown that the traditional similarity theory can be extended to an arbitrary (e.g., mountainous) boundary layer. This offers a solution to the problem of predicting the characteristics of turbulence in an anisotropic layer, including that in the mountain regions.

Interaction of plasma and electromagnetic waves in warm motional plasma: Local approximation method

The behavior of short-wavelength electromagnetic waves passing through a warm motional plasma is studied. The velocity and the self-magnetic field of the plasma particles are directed transversely to the direction of wave propagation. Using local approximation method, dielectric tensor and dispersion relation of perpendicular waves are derived employing the Fluid-Maxwell model. The role of temperature in propagation of EM waves, and as a result, in the instability caused by interaction between extraordinary EM waves and modified electron plasma waves is demonstrated. Moreover, the effects of self-magnetic field strength on the instability are illustrated.

20 years of research on the Alcator C-Mod tokamak

The object of this review is to summarize the achievements of research on the Alcator C-Mod tokamak [Hutchinson et al., Phys. Plasmas 1, 1511 (1994) and Marmar, Fusion Sci. Technol. 51, 261 (2007)] and to place that research in the context of the quest for practical fusion energy. C-Mod is a compact, high-field tokamak, whose unique design and operating parameters have produced a wealth of new and important results since it began operation in 1993, contributing data that extends tests of critical physical models into new parameter ranges and into new regimes. Using only high-power radio frequency (RF) waves for heating and current drive with innovative launching structures, C-Mod operates routinely at reactor level power densities and achieves plasma pressures higher than any other toroidal confinement device. C-Mod spearheaded the development of the vertical-target divertor and has always operated with high-Z metal plasma facing components—approaches subsequently adopted for ITER. C-Mod has made ground-breaking discoveries in divertor physics and plasma-material interactions at reactor-like power and particle fluxes and elucidated the critical role of cross-field transport in divertor operation, edge flows and the tokamak density limit. C-Mod developed the I-mode and the Enhanced Dα H-mode regimes, which have high performance without large edge localized modes and with pedestal transport self-regulated by short-wavelength electromagnetic waves. C-Mod has carried out pioneering studies of intrinsic rotation and demonstrated that self-generated flow shear can be strong enough in some cases to significantly modify transport. C-Mod made the first quantitative link between the pedestal temperature and the H-mode's performance, showing that the observed self-similar temperature profiles were consistent with critical-gradient-length theories and followed up with quantitative tests of nonlinear gyrokinetic models. RF research highlights include direct experimental observation of ion cyclotron range of frequency (ICRF) mode-conversion, ICRF flow drive, demonstration of lower-hybrid current drive at ITER-like densities and fields and, using a set of novel diagnostics, extensive validation of advanced RF codes. Disruption studies on C-Mod provided the first observation of non-axisymmetric halo currents and non-axisymmetric radiation in mitigated disruptions. A summary of important achievements and discoveries are included.

Multiple diffraction of electromagnetic waves by a wedge of concave shape

Multiple diffraction of short-wavelength electromagnetic waves by a wedge of concave shape is considered. Analysis is performed on the basis of geometrical theory of diffraction (GTD) and uniform theory of diffraction (UTD). The resulting wave field is represented as a superposition of a variety of edge-diffracted and specularly reflected waves, treated as “channels of multiple diffraction”. Algorithms of UTD allow of describing birth and disappearance of diffraction channels under evolution of the wedge shape. Classification of diffraction channels is suggested and dominant channels are selected. The “four/five channels model” of scattering is proposed, which describes effectively diffraction of electromagnetic waves by steep and breaking water waves of mesoscale spectrum and embraces the main interference and polarization phenomena, characteristic of low-grazing angles of radar sounding, including: (a) phenomenon of enhanced backscattering, (b) Brewster phenomenon, responsible for suppression of vertical polarization, and (c) phenomenon of “super events”: gigantic spikes of radar cross section. It is shown that radar cross section for plain individual mesowave of 30 cm height and of 1 m front width may reach as large values as 1–10 m 2.

EC beam tracing in fusion plasmas

The aim of the beam tracing technique is to include in the description of short-wavelength electromagnetic wave beams the effects of diffraction, which are neglected by the standard ray tracing method, but can play a significant role in focused or collimated wave beams. Beam tracing is, from a physical point of view, very close to other similar approaches, such as the parabolic equation and the complex eikonal description. In the beam tracing method, however, the problem is greatly simplified, since the full wave equation is reduced to a set of ordinary differential equations that describe the behaviour of the beam axis, the width of the beam and the curvature of the wave front. The propagation of an electromagnetic wave beam in a simplified (slab) geometry can be studied analytically. These calculations allow investigation of situations in which the role of diffraction becomes significant, and clarification of the basic mathematical features of the formalism. In order to solve the problem for realistic plasma geometries and arbitrary launching conditions for the wave beam, a numerical approach is necessary. A new code is presented, in which the beam tracing equations are integrated. Examples of absorption and current drive profiles are shown for RTO/RC ITER.

Alfvén vortices and chaos in electron–positron dusty magnetoplasma with equilibrium flows

A set of coupled nonlinear differential equations which govern the dynamics of low-frequency short-wavelength electromagnetic waves in a nonuniform magnetized electron–positron (pair) plasma with dust has been derived. In the linear limit, a local dispersion relation has been derived and analyzed. It is shown that in the absence of density gradients and equilibrium sheared plasma flow, can make the dust shear Alfvén waves unstable. In the nonlinear case, the temporal behavior of nonlinear dissipative system can be expressed in the form of well known Lorenz and Stenflo type equations that admit chaotic trajectories. On the other hand, a quasi-stationary solution of the mode coupling equations can be represented in the form of dipolar and vortex-chain solutions. The results of our present investigation should be helpful for understanding plasma transport and wave phenomena in the pulsars magnetosphere.

Order and chaos in ETG-driven drift–dissipative waves with sheared flows

We derive a system of nonlinear equations that govern the dynamics of low-frequency short-wavelength electromagnetic waves in the presence of equilibrium density, temperature, magnetic field and velocity gradients. In the linear limit, a local dispersion relation is obtained and analyzed. New ηe-driven electromagnetic drift modes and instabilities are shown to exist. In the nonlinear case, the temporal behaviour of a nonlinear dissipative system can be written in the form of Lorenz- and Stenflo-type equations that admit chaotic trajectories. On the other hand, the stationary solutions of the nonlinear system can be represented in the form of dipolar and vortex-chain solutions.

Anomalous Heat Transport and Vortex Formation Due to Electron–Temperature–Gradient Driven Drift Waves In a Sheared Flow Plasma

By employing Braginskii transport equations for electrons, we derive a system of nonlinear equations which govern the dynamics of low-frequency short–wavelength electromagnetic waves in the presence of equilibrium density, temperature and magnetic field gradients. New electron-temperature-gradient driven electromagnetic drift–wave instabilities are shown to exist. Anomalous electron energy transport caused by nonthermal electromagnetic fluctuations is also derived. Furthermore, possible stationary solutions of the nonlinear system are obtained in the form of spatially bounded dipolar as well as chain of vortices. The results of our investigations should be helpful to understand the wave phenomena in space and laboratory plasmas.

Radiative transfer theory for inhomogeneous media with random extinction and scattering coefficients

The small-angle scattering approximation of the scalar radiative transfer equation is examined in the case where the extinction and scattering coefficients have a component that is a deterministic function of position along the propagation path and a component that is a random function of position transverse to the propagation direction. It is found that the resulting stochastic radiative transfer equation can be reduced to a system of two stochastic integrodifferential equations for the average and fluctuating components of the radiant intensity. The system is solved to yield two transfer equations: one that describes the average radiant intensity and one that describes the spatial correlation function of the intensity fluctuations. The integrodifferential equation for the average intensity is then solved and applied to a simple propagation scenario; it is found that the fluctuations in the extinction and scattering coefficients reduce the effects due to the average values of these parameters, and also that the effect of these is greater near the point of observation than near the point of transmission of the radiation. An approximate solution is also derived for the equation giving the correlation function. The equations developed here should find application in problems involving short wavelength electromagnetic wave propagation through media possessing variable characteristics of turbulence and turbidity, such as in plasmas, the atmosphere, and the ocean.