S. N. Vlasov

The absorption investigation in CVD-diamond plates and windows at 50–200 GHz

Abstract Absorption investigations into a number of CVD-diamond plates, as well as absorption investigations into the output windows of megawatt gyrotrons in the range of frequencies 50–200 GHz have been carried out. Significant additional surface absorption was found and its influence was defined. The theory for measurement of this additional absorption was developed. The possible mechanism of microwave absorption in CVD-diamond is briefly discussed.

Open resonators for low dielectric loss measurements

Open resonators are studied based on the scattering theory. In particular, a theory of open resonators with a dielectric plate is constructed. This theory is used to develop a technique for measuring small dielectric loss. Measurements of the loss tangent in diamond plates show that the volume absorption in the best samples is ≈7×10−6 in the high-frequency part of the millimeter-wave range. A significant absorption in thin surface layers that is caused by surface treatment techniques (polishing, vacuum soldering, etc.) is revealed.

Gyrotrons with echelette resonators

We consider gyrotrons with echelette resonators formed by diffraction gratings on the inner surface of a cone with the ribs perpendicular to the cone axis. The Q-factor of the operating cavity mode is controlled by the choice of grating parameters. Selective properties of the resonator are determined by the frequency dependence of the antireflection coefficient. Such resonators have a higher selectivity and more effective coupling with the output waveguide than cylindrical resonators, which allows gyrotron power to be increased. Echelette resonators can be used in gyrotrons at higher cyclotron harmonics owing to their higher selectivity without fear of exciting lower harmonics. Starting currents and efficiency of a gyrotron with an echelette resonator are found. Experimental studies of gyrotrons with echelette resonators confirmed the possibility to rarefy considerably the gyrotron oscillation spectrum.

ON PERMITTIVITY MEASUREMENT BY A RESONANCE METHOD

In a quasioptical approximation we develop a theory of an open cavity with a dielectric plate placed normally to the cavity axis. We find conditions under which some natural frequencies of the cavity are practically independent of the plate position in the cavity. From experimental measurements of one of these frequencies and with variation of the length of the cavity with and without the plate, with the same oscillation- mode structure, we determine the refractive index and plate thickness. Coincidence between the obtained (“design”) thickness and mechanically measured one is a correctness criterion for the results obtained.

Quasi-optical resonator diplexer with universal polarization characteristics

A theory of a diplexer that can operate with an arbitrary polarization of the wave flow is constructed. A model experiment at a frequency of 34 GHz is carried out in good agreement with the theory. Diplexers of the given type are proposed to be used in experiments within the controlled fusion program.

Methods for investigating thin dielectric films in the millimeter range

An original method based on determining the characteristics of open Fabry-Perot resonator modes with different polarizations is proposed for measuring parameters of dielectric plates and films with a thickness smaller than λ/2 in the millimeter and submillimeter wavelength ranges. This method is used for determining the refractive index and tanδ, as well as the thickness of films made of isotropic materials. For anisotropic films of known thickness, the method makes it possible to measure the permittivity tensor components. Popular film materials such as Teflon (polytetrafluoroethylene, PTFE), lavsan (Mylar, polyethyleneterephthalate, PETP), and polyamide with a minimal thickness of ∼5 μm are investigated. Appreciable anisotropy of roll film materials and the dependence of the dielectric properties on the thickness, which is associated with manufacturing technology, are revealed. The dependence of the refractive index and tanδ on the air humidity is investigated

Open Resonators Excited by Wave Beams

We study open resonators on the basis of the scattering theory. In particular, we develop the theory of an open resonator with a dielectric plate. This theory makes it possible to find the eigenfrequencies of such a system and their dependences on various parameters. Dispersion due to the dielectric plate is determined. The theory developed can be used as a basis for the method of measuring extremely small dielectric losses in the short-wavelength part of the millimeter-wave band. We also analyze the properties of a two-mirror resonator as a reflective filter and formulate the quality requirements for the exciting beam.

Total Transformation of an Electromagnetic Wave Incident on a Corrugated Boundary between Transparent Dielectrics to a Transmitting Wave of the (-1)st-Order Harmonic

We present the results of a theoretical and experimental study of a new phenomenon of total transformation of an electromagnetic wave incident from a dielectric on a corrugated boundary between two transparent dielectrics to a transmitting wave of the (-1)st-order harmonic propagating in the other dielectric. The theoretical study is based on numerical solution of the full-wave equations for electromagnetic field. The experimental study is made using a Teflon prism with a corrugated surface located in a two-mirror resonator.

Laboratory study of two-dimensional nonuniform surface currents over a sphere moving in a fluid column

This paper presents the results of laboratory measurements of surface currents over a sphere moving in the fluid column based on the particle tracing velocimetry. A theoretical model of the fluid flowing around the sphere is presented. The experimental data are compared with the results of theoretical calculations. It is shown that, at low tow speeds of the sphere, the experimentally measured current velocities exceed the velocities calculated from the theoretical model. At greater tow velocities when generation of surface waves by the sphere was observed in the experiment, the experimental data agree well with the results of the theoretical calculations.

Mass Transfer of the Liquid Phase in a Partially Saturated Porous Medium Under the Action of Low-Frequency Elastic Vibrations

We study the model of a partially saturated porous medium comprising a system of identical parallel cylindrical capillaries filled up with equal droplets. The basic features of the liquid-phase drift and the accompanying sound attenuation and droplet coalescence effects are analyzed in the case of external compression.