Millimeter-wavelength Range Research Articles

The electrodynamic analysis of propagation constants of electromagnetic waves in 3D magnetic nanowire lattices under the magnetic resonance conditions at microwaves

Periodic 3G lattices made of oriented magnetic nanowires are considered under the magnetic resonance in the millimeter-wavelength range. The propagation of electromagnetic waves in anisotropic nanostructured materials based on these lattices is mathematically modeled using the method of autonomous blocks with virtual Floquet channels. The real and imaginary parts of propagation coefficients of longitudinal (clockwiseand counterclockwise polarized) and transverse (ordinary and extraordinary) waves for the zero spatial harmonic of 26 GHz are electrodynamically calculated. These waves propagate in 3D lattices of magnetic (Co80Ni20, Fe) nanowires. The obtained results depend on the intensity of the external constant magnetic field and the lattice period. The methods of control of the frequency dispersion in the millimeter-wavelength range under the influence of the external magnetic field and under the change of the geometry of lattices are determined. It is considered that the direction and intensity of the magnetic bias, its orientation with respect to the nanowire axis, and the mutual orientation of the constant and high-frequency magnetic fields vary.

Large directional optical anisotropy in multiferroic ferroborate

Many technological applications are based on electric or magnetic order of materials, for instance magnetic memory. Multiferroics are materials which exhibit electric and magnetic order simultaneously. Due to the coupling of electric and magnetic effects, these materials show a strong potential to control electricity and magnetism and, more generally, the properties and propagation of light. One of the most fascinating and counter-intuitive recent results in multiferroics is directional anisotropy, the asymmetry of light propagation with respect to the direction of propagation. The absorption in the material can be different for forward and backward propagation of light, which in extreme case may lead to complete suppression of absorption in one direction. Another remarkable effect in multiferroics is directional birefringence, i.e. different velocities of light for different directions of propagation. In this paper, we demonstrate giant directional birefringence in a multiferroic samarium ferroborate. The effect is easily observed for linear polarization of light in the range of millimeter-wavelengths, and survives down to very low frequencies. The dispersion and absorption close to the electromagnon resonance can be controlled and fully suppressed in one direction. Therefore, samarium ferroborate is a universal tool for optical control: with a magnetic field as an external parameter it allows switching between two functionalities: polarization rotation and directional anisotropy.

Millimetre wave attenuation of prototype diagnostic components for the ITER bolometers

Bolometers in current and future fusion devices, in particular those in ITER, are vulnerable to stray radiation from electron cyclotron resonance heating (ECRH) which results in measurement errors for plasma radiation detection. To protect the detectors from this stray radiation in the millimetre wavelength range, dedicated diagnostic components have been designed and tested. One option is to place a top plate which contains a microwave-reflecting grid onto the collimators. Another option investigated is the coating of the collimator channels using a microwave absorbing ceramic. Measurements of the mm-wave attenuation of the collimator in front of the bolometer detectors with and without top plate or coated collimator channels have been performed in the frequency range of 125–420GHz. The attenuation factor of the collimator channels at 170GHz (the ECRH frequency for ITER) with neither microwave grid nor coating is typically 10dB. The coating enhances this to 40dB and including the microwave grid yields at least an attenuation factor of 70dB, which is sufficient to reduce the residual ECRH induced signal significantly below the one due to plasma radiation. Placing a bolometer camera (collimator connected to detector housing) inside the isotropic microwave field of the test facility MISTRAL, the attenuation factor of the full diagnostic set-up using a top plate was determined to be in the order of 45dB. This degraded attenuation implies that particular attention has to be paid to design and quality control of the joints of diagnostic components to prevent microwave leakage.

A magnetized torus for modeling Sagittarius A∗millimeter images and spectra

Context. The supermassive black hole, Sagittarius (Sgr) A*, in the centre of our Galaxy has the largest angular size in the sky among all astrophysical black holes. Its shadow, assuming no rotation, spans ~ 50 microarcsec. Resolving such dimensions has long been out of reach for astronomical instruments until a new generation of interferometers being operational during this decade. Of particular interest is the Event Horizon Telescope (EHT) with resolution ~ 20 microarcsec in the millimeter-wavelength range 0.87 mm - 1.3 mm. Aims. We investigate the ability of the fully general relativistic Komissarov (2006) analytical magnetized torus model to account for observable constraints at Sgr A* in the centimeter and millimeter domains. The impact of the magnetic field geometry on the observables is also studied. Methods. We calculate ray-traced centimeter- and millimeter-wavelength synchrotron spectra and images of a magnetized accretion torus surrounding the central black hole in Sgr A*. We assume stationarity, axial symmetry, constant specific angular momentum and polytropic equation of state. A hybrid population of thermal and non-thermal electrons is considered. Results. We show that the torus model is capable of reproducing spectral constraints in the millimeter domain, and in particular in the observable domain of the EHT. However, the torus model is not yet able to fit the centimeter spectrum. 1.3 mm images at high inclinations are in agreement with observable constraints. Conclusions. The ability of the torus model to account for observations of Sgr A* in the millimeter domain is interesting in the perspective of the future EHT. Such an analytical model allows very fast computations. It will thus be a suitable test bed for investigating large domains of physical parameters, as well as non-black-hole compact object candidates and alternative theories of gravity.

The centimeter-wavelength opacity of ammonia under deep jovian conditions

Extensive measurements of the centimeter-wavelength opacity of ammonia have been conducted under simulated deep jovian conditions using an ultra-high-pressure measurement system built at Georgia Tech. Over 1000 measurements of the opacity of ammonia have been conducted in the 5–20cm wavelength range under simulated jovian conditions (pressures ranging from 0.05 to 99bars, temperatures from 330 to 500K) in a hydrogen–helium atmosphere. These and previous measurements conducted by Hanley et al. (Hanley et al. [2009]. Icarus, 202, 316–355) and Devaraj et al. (Devaraj et al. [2011]. Icarus, 212, 224–235) have been used to empirically derive a consistent ammonia opacity model for the 2mm–20cm wavelength range under the pressure and temperature conditions characteristic of the middle and deep jovian atmosphere. This model can be used reliably in the millimeter-wavelength range up to 3bars of pressure and temperatures up to 300K and in the centimeter-wavelength range up to 100bars of pressure and temperatures up to 500K. In addition, over 800 measurements of the 5–20cm wavelength opacity of ammonia pressure-broadened by water vapor, hydrogen and helium have been conducted under simulated deep jovian conditions for the first time to study the influence of water vapor on the ammonia absorption spectrum. Based on these measurements, a new model has been developed. These laboratory data and the model show that water vapor has a measurable effect on the opacity of ammonia under jovian conditions. The laboratory study and the models will help to improve our understanding of centimeter-wavelength absorption by ammonia in the jovian planets in general, and specifically, will improve retrievals of ammonia and water vapor from the Juno microwave radiometer (MWR) at Jupiter.

Aspherical single-lens objective for radio-vision systems of the millimeter-wavelength range

The characteristics of a system for video imaging of objects in the 3-mm-wavelength range are presented. A calculation procedure and characteristics of an aspherical single-lens objective that is used for imaging (point scattering function, resolution, and field of vision) are presented. An 8 × 8 array of antenna elements that are loaded with detectors is used for the detection of signals in the image plane. A digital processing unit is used for the detection, control, and input-output of electric signals that form the image. The processing and data-transfer rates of the unit are presented. Test experiments on imaging of simple objects (one or several metal spheres with different diameters) are performed in the active radar regime using the proposed system and monochromatic signal.

High-frequency properties of Fe/Cr superlattices with thin Cr layers in the millimeter-wavelength range

Microwave properties of Fe/Cr multilayer nanostructures with thin chromium layers (with thickness tCr < 1 nm) are analyzed. Experiments are performed by the method of penetration of microwaves in the frequency range from 26 to 38 GHz. The dependence of the transmission coefficient for microwaves on the constant magnetic field strength exhibits the microwave magnetoresistive effect and magnetic resonance. The resonance spectrum is reconstructed from measurements at various frequencies. The results of microwave measurements are analyzed together with the results of magnetic and magnetoresistive measurements.

Study of the spectrum of the Kr-CO weakly bound molecular complex in the millimeter-wavelength range

The rotational spectrum of the weakly bound Kr-CO molecular complex formed in a pulsed molecular jet has been measured in the frequency range 112–150 GHz on an intracavity orotron-based spectrometer. The measured b-type transitions include the R branch of the band K = 1-0 with the rotational numbers J from 16 to 25 and the P branch of the band K = 2-1 with the rotational numbers J from 13 to 17. For the K = 1-0 band, we succeeded in recording transitions in complexes that consist of all stable Kr isotopes, i.e., 84Kr-CO, 86Kr-CO, 82Kr-CO, 83Kr-CO, and 80Kr-CO. For the K = 2-1 band, we observed transitions in the two most abundant isotopologues, 84Kr-CO and 86Kr-CO. The obtained data were used to determine the rotational and centrifugal constants of the Kr-CO complex.

The Georgia Tech millimeter-wavelength measurement system and some applications to the study of planetary atmospheres

[1] With the advent of new millimeter-wavelength arrays such as the Atacama Large Millimeter Array capable of providing very precise maps of planetary emission, it becomes very critical to have an accurate knowledge of the millimeter-wavelength properties of gases under those planetary conditions. A millimeter-wavelength planetary atmospheric simulator and measurement system have been developed at the Georgia Tech Planetary Atmospheres Laboratory to accurately measure the propagation properties of gases under simulated planetary atmospheric conditions. The measurement system operates in the 2–4 millimeter-wavelength range and withstands up to 3 bars of pressure. It currently operates in the 190–300 K temperature range, and with minor modifications can operate in the 300–550 K temperature range for measurements of certain highly millimeter-wavelength-opaque gases. The measurements from this system can be used for developing accurate models of the millimeter-wavelength properties of gases under various planetary conditions which can then be used for retrieving the abundances of those gases through various remote sensing techniques.

The PULSAR Radar Instrument and Its Use for Comprehensive Examination of Health Status

Medical application of the PULSAR Doppler radar millimeter-wavelength range measuring system is discussed. The possibility for remote measurement and analysis of heart rate and equilibrium function parameters is demonstrated. Various physiological parameters are measured synchronously in the single beam of the radar. To estimate the person’s functional condition, use of fractal parameters defined on phase plane of normal and pathological zones is suggested. An adaptable method for processing signals reflected from the patient is offered. The method reduces measurement errors.

Single scattering from frozen hydrometeors at microwave frequencies

Scattering of electromagnetic waves from homogeneous or coated spheres can be computed in a mathematically exact way using the Mie theory. Therefore, for many approaches in remote sensing, frozen hydrometeors are parameterized as ice spheres. However, many frozen hydrometeors have non-spherical overall shapes and lack a spherically symmetric internal structure. They exist in a huge variety of shapes and exhibit different mixtures of ice, water and air. Therefore it is desirable to accurately compute scattering from non-spherical particles in order to clearly understand the effect the shape of a hydrometeor has on its scattering pattern. In this study, single scattering parameters like scattering cross section, absorption cross section, and asymmetry factor were calculated for frozen hydrometeors using the Discrete Dipole Approximation (DDA). The particles were modeled as hexagonal plates, columns, needles and dendrites by applying known dimensional relationships. The calculations were carried out over a wide range of centimeter and millimeter-wavelengths (1 GHz to 300 GHz), since millimeter-wave radiometers are highly sensitive to scattering by frozen hydrometeors in the atmosphere. The study results show that for size parameters < 1 (a ratio between wavelength and particle size) the scattering cross section of randomly orientated ice crystals is close to that of an equal volume ice sphere. Absorption cross section and asymmetry factor of non-spherical particles however are up to twice as high as that of equal volume ice spheres. Further the influence of the assumed model for the refractive index of ice at microwave wavelengths on the scattering parameters is investigated.

Propagation of millimeter waves in GaAs and Si double-layer dielectric waveguides

We study the propagation parameters (attenuation and phase shift) of electromagnetic waves in the frequency range 75–110 GHz in dielectric waveguides made of high-resistivity gallium arsenide and silicon, as well as in double-layer n-i and p-n structures based on them. The appearance of new modes is revealed in the case where the vector of the electric field is perpendicular to the interface between the layers, which is related to the presence of the potential barrier and the corresponding charge capacitance. The mode transformation can be used to generate high harmonics (due to the varactor effect) and to monitor the uniformity and the influence of external factors on the properties of dielectric waveguides in the integrated circuits of the millimeter-wavelength range.

Characterization of the atmospheric temperature and moisture conditions above Dome C (Antarctica) during austral summer and fall months

Two sets of radiosounding measurements were taken at Dome C (Antarctica) in December 2003 and January 2003 and 2004, using RS80‐A, RS80‐H, and RS90 Vaisala radiosondes, and from March to May 2005, employing the RS92 model. They were examined following accurate correction procedures to remove the main relative humidity dry bias and the temperature and humidity lag errors. The results showed that a strong cooling usually characterizes the thermal conditions of the whole troposphere from December/January to April/May, with an average temperature decrease from 245 to 220 K at the ground, of around 10 K at upper tropospheric levels, and of more than 15 K in the lower stratosphere. The relative humidity data were found to be affected by dry bias of 5–10%, on average, for the RS80‐A and RS80‐H Humicap sensors and by smaller percentages for the other sensors. The mean monthly vertical profiles of absolute humidity were found to decrease sharply throughout the troposphere, especially within the first 3 km, and to diminish considerably passing from December/January to March/April/May, with average values of precipitable water decreasing from 0.75 to 0.28 mm, median values from 0.69 to 0.25 mm, and first and third quartiles from 0.60 to 0.22 mm and from 0.87 to 0.34 mm, respectively. The results demonstrate that Dome C (where a permanent scientific station has been open for winter operations since austral winter 2005) is a site of comparable quality to the South Pole for both validation of satellite radiance measurements and astronomic observations in the infrared, submillimetric, and millimetric wavelength range, performed with large telescopes that cannot be carried on satellites.

Gasdynamic ECR source of multicharged ions based on a cusp magnetic trap

One of the modern trends in the development of electron cyclotron resonance (ECR) sources of multicharged ions is enhancement of the power and frequency of microwave pumping. Therefore, gyrotrons—powerful sources of radiation in the millimetre wavelength range—are now used for the creation and heating of plasma. These generators of microwave radiation are capable of producing and confining plasma of very high density (1013 cm−3 and higher), thus providing conditions for a substantial increase in the extracted ion beam current. Most ECR sources use for plasma confinement mirror magnetic traps with ‘min B’ configuration, which suppress MHD plasma instabilities. However, it is extremely difficult to construct such systems for pumping frequencies higher than 30 GHz because strong magnetic fields of complicated configuration are needed. All this makes the search for simpler axisymmetric MHD-stable systems for plasma confinement very topical.A cusp trap produced by two coils with opposite currents is the simplest MHD-stable magnetic trap. Magnetic lines at any point of plasma in such a trap have a curvature that suppresses MHD perturbation in the plasma. In the present work the possibility of creation of an effective ECR source of multicharged ions based on a cusp magnetic trap was investigated numerically and in experiments. The pioneer result is the realization of the quasi-gasdynamic regime of plasma confinement in the trap of the multicharged ion ECR source of multicharged ions with cusp configuration of the magnetic field. Experiments were made with a small cusp trap which was designed for plasma creation by 37.5 GHz gyrotron radiation under ECR conditions. The total current and ion charge state distribution in the extracted ion beam were studied. The results of experiments with the simplest kind of cusp trap have demonstrated good correspondence with theoretical calculations, and therefore the adequacy of the developed approach and the possibility to build more effective source on this basis. Two ways of possible evolution of ECR ion sources based on a cusp magnetic trap are proposed.

A waveguide Bragg structure for the millimeter range of wavelengths

The paper presents a theoretical and experimental investigation of a periodic dielectric structure in a rectangular waveguide for the8-mmrange of wavelengths. The structure represents an analog of a single-dimensional photonic crystal in the optical range of wavelengths. The experiment has shown the shape, size, and location of the forbidden gap to be coincident with calculation results. The calculation is performed based on the coupled wave theory adapted to peculiarities of the waveguide propagation of radiation in a resonant structure of the microwave range.

Monte Carlo calculations of hot-electron transport and diffusion noise in GaN and InN

Monte Carlo simulations of high-field transport are used to calculate the spectral density of velocity fluctuations and the diffusion coefficients of hot electrons in GaN and InN. The results of the simulations are complemented by the investigation of the velocity relaxation time through the balance equation method. It is found that the low-frequency noise spectrum remains white up to 100–300 GHz, so that the steady-state diffusion coefficient can be considered as constant in practically the whole millimetre wavelength range. It is shown that the main features of the field dependence of the longitudinal and transverse diffusion coefficients such as initial decrease of the diffusion coefficient followed by a peak near the threshold field for the intervalley transfer, etc are described well both qualitatively and quantitatively by simplified approaches based on velocity relaxation times.

Nature of millimeter wave losses in low loss CVD diamonds

The CVD diamonds dielectric loss mechanisms at the millimeter range of wavelengths are researched. The loss tangent tan δ at frequencies f≅50–200 GHz and temperatures t≅20–500 °C was measured. The loss tangent frequency dependence (at room temperature) is ∼1/ f at frequencies <140 GHz, due to electric conductance in non-diamond inclusions, and deviates upwards at f>140 GHz. From the comparison to the theory it follows that this deviation can be interpreted as exhibition of the nonintrinsic lattice loss connected with the lattice disorder induced one-phonon excitation of acoustic vibrations. The loss increase at temperatures >400 °C is sharper compared to the theoretical dependence for two-phonon intrinsic lattice loss (ILL). It can be connected with the loss due to dipole relaxation (of Debye type) with activation energy ∼0.56 eV. It was confirmed that the theoretical lower loss limit (determined by the ILL) in diamond is extremely low (particularly, at ambient temperatures, at least one order of magnitude lower than the lowest observed loss).

Synthesis of improved-quality images in the millimetre wavelength range

The possibilities of quasi-optical imaging of concealed objects illuminated by millimetre wavelength range radiation are demonstrated both theoretically and experimentally. The influence of the spatial Gibbs effect and of other destructive factors on the quality of the obtained images is studied. A method is proposed for synthesising improved-quality images with enhanced information content in the millimetre wavelength range. It is shown that the theoretical and experimental results are in good agreement.

New method for measurement of complex magnetic permeability in the millimeter-wave range, part II: hexaferrites

Magneto-optical methods were applied for the first time in millimeter wavelength range for characterization of anisotropic ferrites. The principles of the free-space magneto-optical method are presented and the new experimental procedures leading to the determination of millimeter-wave permeability and permittivity in anisotropic ferrite materials are described. The measurements were performed with a computer-controlled W-band (70-120 GHz) quasi-optical-waveguide bridge. A backward-wave oscillator was used as a source of tunable millimeter wave radiation. The oriented Sr-hexaferrite ceramic was selected for the verification of millimeter-wave magneto-optical method. The magneto optical measurements in transverse configuration revealed strong anomalous dispersion in the millimeter-wave refractive index spectrum for Sr hexaferrite, mainly due to the frequency variation of magnetic permeability. Computer simulations revealed a good agreement of measured parameters with known data for Sr hexaferrite. It was shown that the free-carrier absorption (/spl sigma//spl sim/0.05 /spl Omega//sup -1/ cm/sup -1/) and magnetic permeability contribute to the relatively high millimeter-wave losses in hexaferrite ceramics. The frequency dependence of Faraday rotation in the millimeter-wave range was measured, and the experimental results are discussed. The results presented in this paper demonstrate that this new magneto-optical method is capable of providing accurate dielectric and magnetic data in the millimeter-wavelength range.

A wideband variable waveguide coupler for millimeter applications

We present a variable waveguide coupler for the short millimeter-wavelength range. The coupler has low ohmic and insertion losses in all paths, flat coupling characteristic, and high directivity over a large bandwidth. The construction of the coupler allows a variable independent adjustment of power transferred into several paths when several couplers are used in series. The coupler is therefore useful for multichannel receivers and especially for superconductor-insulator-superconductor (SIS) mixer arrays where individual local oscillator (LO) power adjustment might be necessary. We present measurements for a coupler working at 230 GHz and compare them with a simple analytical model.