Multipactor discharge on a dielectric at different inclination angles of the microwave electric field relative to the dielectric surface

Multipactor discharge on a dielectric is studied numerically and analytically for different inclination angles α of the microwave electric field with respect to the dielectric surface. The power absorbed in the discharge is calculated, and analytic estimates for the average current density of secondary electrons and the average energy of electrons bombarding the dielectric surface are obtained as functions of the angle α and the electron oscillation energy in the microwave field. It is found that the dependence of the absorbed power on the inclination angle of the external microwave field has a minimum at α ～20°–30°.     

Particle-in-cell simulation of multipactor discharge on a dielectric in a parallel-plate waveguide

An original 2D3V (two-dimensional in coordinate space and three-dimensional in velocity space) particle-in-cell code has been developed for simulation of multipactor discharge on a dielectric in a parallelplate metal waveguide with allowance for secondary electron emission (SEE) from the dielectric surface and waveguide walls, finite temperature of secondary electrons, electron space charge, and elastic and inelastic scattering of electrons from the dielectric and metal surfaces. The code allows one to simulate all stages of the multipactor discharge, from the onset of the electron avalanche to saturation. It is shown that the threshold for the excitation of a single-surface multipactor on a dielectric placed in a low-profile waveguide with absorbing walls increases as compared to that in the case of an unbounded dielectric surface due to escape of electrons onto the waveguide walls. It is found that, depending on the microwave field amplitude and the SEE characteristics of the waveguide walls, the multipactor may operate in two modes. In the first mode, which takes place at relatively low microwave amplitudes, a single-surface multipactor develops only on the dielectric, the surface of which acquires a positively potential with respect to the waveguide walls. In the second mode, which occurs at sufficiently high microwave intensities, a single-surface multipactor on the dielectric and a two-surface multipactor between the waveguide walls operate simultaneously. In this case, both the dielectric surface and the interwall space acquire a negative potential. It is shown that electron scattering from the dielectric surface and waveguide walls results in the appearance of high-energy tails in the electron distribution function.     

Coupled plasmon-waveguide resonators: a new spectroscopic tool for probing proteolipid film structure and properties.

A variant of surface plasmon resonance (SPR) spectroscopy has been developed that involves a coupling of plasmon resonances in a thin metal film and waveguide modes in a dielectric overcoating. This new technique is referred to as coupled plasmon-waveguide resonance (CPWR) spectroscopy. It combines a greatly enhanced sensitivity (due to increased electromagnetic field intensities at the dielectric surface) and spectral resolution (due to decreased resonance linewidths), with the ability to directly measure anisotropies in refractive index and optical absorption coefficient in a dielectric film adsorbed onto the surface of the overcoating. Experimental data obtained with an egg phosphatidylcholine bilayer are presented to document these properties.     

Simple method to measure power density entering a plane biological sample at millimeter wavelengths

A simple method for measuring microwave power density is described. It is applicable to situations where exposure of samples in the near field of a horn is necessary. A transmitted power method is used to calibrate the power density entering the surface of the sample. Once the calibration is available, the power density is known in terms of the incident and reflected powers within the waveguide. The calibration has been carried out for liquid samples in a quartz cell. Formulas for calculating specific absorption rate (SAR) are derived in terms of the power density and the complex dielectric constant of the sample. An error analysis is also given.     

Millimeter wave dosimetry at exposure of cell monolayers

The specific absorption rate, the amplitude of the electric field and the power flux density of millimeter waves in a cell monolayer within a well of a multi-well plate or a Petri dish are calculated. The radiation power absorption decrease in the cell layer compared to the solution is shown. The presence of the cells causes a slight increase of the electric field amplitude in the medium. The dielectric of the bottom of the well or Petri dish plays the role of a coupling layer that results in complex frequency dependences of the power reflection coefficient and the specific absorption rate.     

Wall plasma in a wideband dielectric cherenkov maser

Results are reported of experimental investigations that have revealed the presence of a plasma in the interaction region of a model wideband relativistic microwave amplifier—a dielectric Cherenkov maser. The electrodynamic properties of a hybrid system—a waveguide with an annular dielectric liner and a plasma layer adjacent to its inner wall—are analyzed. Experiments with a high-current accelerator have revealed that the power of the emitted microwaves at the output of the system increases strongly when an external microwave source at different frequencies in the X-band is switched on. However, this effect was found to be hard to reproduce. Indirect evidence is obtained of the fact that, during the transport of an electron beam and under the action of the signal from a high-power pulsed magnetron, the plasma in the system is created at the surface of the dielectric. In the model of a cold magnetized plasma, a dispersion relation is derived for axisymmetric waves in a system with a wall plasma layer. The spectra of the waveguide and plasma modes in the system and the transverse structure of their electromagnetic fields are investigated thoroughly as functions of the plasma density and layer thickness. It is shown that even a very thin layer of a high-density plasma results in a large frequency shift of the dispersion curve of the waveguide mode, in which case the coupling impedance at a fixed frequency decreases sharply. On the other hand, a layer of a moderately dense plasma increases the coupling impedance for the waveguide mode. It is established that, in a configuration with a wall plasma layer, the longitudinal component of the electric field of a plasma mode whose power flux in the dielectric is of a volumetric nature reverses direction across the layer.     

Surface microwave discharges in air

A microwave discharge excited on the outer surface of a dielectric antenna has been investigated. The transverse and longitudinal dimensions and propagation velocities of the discharge have been measured as functions of the air pressure and the power and duration of the exciting microwave pulse. The spatial distributions and time evolution of the gas temperature, electron density, and radiation intensity of the discharge have been determined. It is shown that the degree of ionization of the discharge plasma can exceed 10%. The spatial distribution of the electron density is found to depend strongly on the air pressure.     

The effect of a variable dielectric coefficient and finite ion size on Poisson-Boltzmann calculations of DNA-electrolyte systems.

The results of variable dielectric coefficient Poisson-Boltzmann calculations of the counter-ion concentration in the vicinity of an all-atom model of the B-form of DNA are presented with an emphasis on the importance of spatial variations in the dielectric properties of the solvent, particularly at the macro-ion-solvent interface. Calculations of the distribution of hard-sphere electrolyte ions of various dimensions are reported. The presence of a dielectric boundary significantly increases the magnitude of the electrostatic potential with a concomitant increase in the accumulation of small counter-ions in the groove regions of DNA. Because ions with radii greater than 2 A have restricted access to the minor groove, the effect there is less significant than it is within the major groove. Changes in the dielectric coefficient for the electrolyte solution, allowing variation from 10 to 25, 40, 60, and 78.5 within the first 7.4 A of the surface of DNA, substantially increases the calculated surface concentration of counter-ions of all sizes. A lower dielectric coefficient near the macro-ion surface also tends to increase the counter-ion density in regions where the electrostatic potential is more negative than -kT. Regardless of the choice of dielectric coefficient, the number of ions in regions where the electrostatic potential is less than -kT remains the same for 0.153 M added 1-1 monovalent electrolyte as for the case without added salt.(ABSTRACT TRUNCATED AT 250 WORDS)     

Microwave-induced pressure waves in a model of muscle tissue

Microwave-induced mechanical stress waves were studied in simulated muscle tissue. Pulsed microwave energy at 5.655 GH_z induced pressure waves that were recorded with a hydrophone transducer. Each pulse produced a peak power density greater than 1.5 kW/cm². Microwave absorption measurements within the model showed energy deposition to be mostly confined to a region within 2 cm of the irradiated surface. The average specific absorption rate (SAR) at the surface of the sample was about 100 W/kg. The microwave-induced stress wave propagated at a velocity of 1,600 m/sec with peak pressures of approximately 300 pascals and was detectable after having traveled a total distance of 0.61 m on a path that included two reflections at model-container interfaces.     

Theoretical evaluation of dielectric absorption of microwave energy at the scale of nucleic acids

A theoretical model is proposed for the evaluation of dielectric properties of the cell nucleus between 0.3 and 3 GHz, as a function of its nucleic acids (NA) concentration (CNA). It is based on literature data on dielectric properties of DNA solutions and nucleoplasm. In skeletal muscle cells, the specific absorption rate (SAR) ratio between nucleoplasm and cytoplasm is found to be larger than one for CNA above 30 mg/ml. A nearly linear relationship is found between CNA and this nucleocytoplasmic SAR ratio. Considering the nanoscale of the layer of condensed counterions and bound water molecules at the NA-solution interface, the power absorption per unit volume is evaluated at this precise location. It is found to be between one and two orders of magnitude above that in muscle tissue as a whole. Under realistic microwave (MW) exposure conditions, however, these SAR inhomogeneities do not generate any significant thermal gradient at the scale considered here. Nevertheless, the question arises of a possible biological relevance of nonnegligible and preferential heat production at the location of the cell nucleus and of the NA molecules.     

Experimental study of a multipactor discharge on a dielectrics surface in a high-Q microwave cavity

Results from experimental studies of multipactor discharges on the surfaces of various dielectrics placed in a high-Q cylindrical microwave cavity excited at the TE₀₁₃ mode in the X-band are presented. The thresholds for the onset and maintenance of a multipactor discharge on quartz, polycrystalline diamond, lithium fluoride, and Teflon surfaces possessing different roughness are determined. It is shown that, in such a resonance system, a steady multipactor discharge can operate without transition into the stage of microwave breakdown of the desorbed gas. It is found that, due to long-term action of the discharge, a thin carbon-containing film is deposited on the dielectric surface, which leads to an increase in the breakdown threshold.     

Microwave dielectric absorption of DNA in aqueous solution

The dielectric properties of aqueous solutions of DNA were measured at frequencies ranging from 0.1 to 12 GHz. The results are analyzed using the Maxwell mixture theory and yield a value for the hydration of the DNA of about 0.4 g/g, which is in the range observed in other investigations. No evidence was found for an additional absorption effect at microwave frequencies, which has been predicted to occur in certain DNA analogs due to the vibrational excitation of the double helix by the applied microwave field.     

Transient changes during microwave ablation simulation : a comparative shape analysis

Microwave ablation therapy is a hyperthermic treatment for killing cancerous tumours whereby microwave energy is dispersed into a target tissue region. Modelling can provide a prediction for the outcome of ablation, this paper explores changes in size and shape of temperature and Specific absorption rate fields throughout the course of simulated treatment with different probe concepts. Here, an axisymmetric geometry of a probe embedded within a tissue material is created, solving coupled electromagnetic and bioheat equations using the finite element method, utilizing hp discretisation with the NGSolve library. Results show dynamic changes across all metrics, with different responses from different probe concepts. The sleeve probe yielded the most circular specific absorption rate pattern with circularity of 0.81 initially but suffered the largest reduction throughout ablation. Similarly, reflection coefficients differ drastically from their initial values, with the sleeve probe again experiencing the largest change, suggesting that it is the most sensitive the changes in the tissue dielectric properties in these select probe designs. These collective characteristic observations highlight the need to consider dielectric property changes and probe specific responses during the design cycle.

     

Experimental study of the plasma effect on the generation of microwave radiation in systems with a virtual cathode

Results are presented from experimental studies of the plasma effect on the generation of microwave radiation in systems with a virtual cathode. Using a triode with a virtual cathode as an example, it is shown that the cathode and anode plasmas reduce the generation efficiency; in particular, the power of the generated microwave radiation decreases and the radiation frequency and the microwave pulse duration change appreciably. It is demonstrated that, at high microwave powers, the power radiated into free space can be reduced by the plasma generated at the surface of the output window. This plasma appears due to discharges developing on the window surface under the combined action of bremsstrahlung, UV radiation, electrons and ions arriving from the beam formation zone, and the microwave electric field.     

Some considerations when using a microwave oven as a laboratory research tool

Microwave ovens can be used in laboratories for the rapid heating of material – either to dry them completely or to subject a workpiece to sudden thermal stress or electric field stress. Determinations of the moisture content levels in soil or leaf tissue samples, for example, can be made within tens of seconds rather than hours. It is often assumed that placing a load within a microwave oven will result in it being heated evenly as well as quickly, but this is not always the case. This paper describes how a microwave oven works and illustrates how the heating effect within a workpiece can vary. The size and shape of a sample as well as its physical properties determine the power absorption. Equal volumes of water in different shaped containers attain different final temperatures and a tall, cylindrical water load is shown to have different temperatures at different levels. Most microwave ovens do not have a true, variable power capability, but rely on an on/off timing ratio to vary mean power. If this is not appreciated, then erroneous conclusions might be drawn from a set of experiments involving different power levels. Changes in mains supply voltage can affect the amount of energy dissipated in a load. This may introduce variations in results if experiments are conducted over a period of several hours. Experiments are described which illustrate these effects and some criteria and working practices are suggested to improve the consistency and reliability of results when using a microwave oven as a research tool.     

Picosecond relaxations in model substances for proteins: A millimeter-wave investigation on crystalline alkyl amides

The dielectric absorption at millimeter-wave (mm-wave) frequencies (50–150 GHz) of N-methylacetamide (NMA), N,N-dimethylacetamide (DiNMA), and N,N-dimethylacrylamide (DiNMAcry) is measured. Measurements are performed using the oversized-cavity technique in the temperature range from liquid helium to room temperature. Additionally, a mm-wave interferometeric measurement at room temperature is made. NMA and DiNMAcry exhibit monotonic increases of the absorption coefficient with temperature as well as with frequency. For DiNMA a monotonic increase of the absorption coefficient with frequency is also found, while the absorption coefficient as a function of temperature shows a pronounced maximum at approximately 30 K. At this maximum the absorption coefficient of DiNMA exceeds those of NMA and DiNMAcry by about two orders of magnitude. The dielectric behavior of the three substances can be described by relaxation processes in asymmetric double-well potentials. For the low-temperature relaxation in DiNMA the double well could be established by two possible positions of the molecule in the crystal that are separated by a rotational movement. Hydrogen bonds and long side chains may hinder these relaxational movements in NMA and DiNMAcry, respectively, and thereby account for their comparatively lower absorption. The results are compared with similar results recently obtained on proteins and synthetic biopolymers.     

Dielectric properties of muscle and liver from 500 MHz–40 GHz

Dielectric properties are the most important parameters determining energy deposition when biological tissues are exposed to radio frequency and microwave fields. Energy absorption is determined by the specific absorption rate (SAR). SAR distributions can be computed accurately only if the complex relative permittivity of the target tissue is known to a sufficiently high accuracy, and currently there is a lack of data on the dielectric properties of biological tissues at high frequencies. In this study, tissue dielectric properties are measured using an open-ended coaxial probe technique from 500 MHz up to 40 GHz. We present dielectric data for ex vivo bovine and porcine muscle and liver tissues at 37 °C. One-pole Cole–Cole model is used to fit the measured data as a function of frequency and the dispersion parameters are presented. This data is supported by an accurate study on reference liquids such as methanol and ethanediol.     

Electrical properties of water: a new insight

New insight into the electrical properties of liquid water, from a standpoint of the physics of electrolytes, is proposed. The dielectric spectrum of water at frequencies 10⁴–10¹¹ Hz is described by a simple diffusional model taking into account the electrophoretic and relaxation effects inherent in electrolytes. The static dielectric permittivity and microwave absorption are derived from diffusion of Coulomb interacting H₃O⁺ and OH^? ions instead of orientational motion of H₂O molecules. The drift component of diffusion provides the proton dc-conductivity. Ion concentration is found to be 7 orders of magnitude higher than commonly accepted (～1% of the total concentration of H₂O molecules). The findings refer to the basic properties of water and therefore can be the key to solving the water-related problems.     

Dual-Band Infrared Near-Perfect Absorption by Fabry-Perot Resonances and Surface Phonons

In this article, we present a simple absorber design which enables dual-band near-perfect absorption at infrared (IR) frequencies. The absorber is an unpatterned hBN/dielectric/hBN triple layer, with a 1150-nm-thick hBN film as the top layer, a 850-nm-thick dielectric film as the middle layer, and a hBN substrate. Unlike the metal/dielectric/metal triple layer, it is found that the high efficiency absorption at specific wavelengths is mainly caused by two mechanisms: Fabry-Perot (FP) resonances and surface phonons. The absorption response is found sensitive to the top and middle layers. The two mechanisms can be coupled to affect the absorption spectra by choosing a proper thickness of the top and middle layers.     

Tuning the Dipolar Plasmon Hybridization of Multishell Metal-Dielectric Nanostructure: Gold Nanosphere in a Gold Nanoshell

Because of the interaction between dipole resonances of the inner gold sphere and the outer gold shell, gold-dielectric-gold multishells with sub-50 nm diameter may at most have three hybridization modes of surface plasmon resonance (SPR). Theoretical calculations based on quasi-static theory indicate that there are blending and splitting of SPR bands in the absorption spectra, which makes the number of absorption peak tunable by changing the radius of inserted gold sphere, thickness of gold shell, dielectric constant of middle dielectric shell or outer environment. The two absorption peaks at longer wavelength, which correspond to the hybridization from the bonding shell plasmon and the sphere plasmon, are usually intense and well tunable. The absorption peak at shorter wavelength, which corresponds to the symmetric coupling between the anti-bonding shell plasmon and the sphere plasmon, is relative weak and only occurs with large dielectric constant of the middle shell, small dielectric constant of the outer surrounding, large inner radius of the gold shell, and small radius of the inner gold sphere. Furthermore, the physical origin of these plasmon hybridizations in gold-dielectric-gold multishells nanostructure has also been illuminated by analyzing the local electric field distributions.