Superconducting resonator in strong microwave fields: Thermal domain, nonlinear thermal effects

The thermal nonlinearity of superconducting films in an electromagnetic field is investigated theoretically. It is shown that the transition of the superconductor from the superconducting to the normal state as a result of Joule heating exhibits hysteresis, corresponding to the onset of a bistable state in the film. It is shown that as a result of the nonuniformity of the amplitude of the electromagnetic field along the surface of the superconducting film, above some threshold a normal-phase domain forms in the superconductor. It is ascertained that the temporal evolution of the domain walls exhibits the character of thermal autowaves of heating or cooling. The nonlinear radio-frequency characteristics (reflection and transmission coefficients and frequency responses) of a stripline resonator under conditions of thermal nonlinearity are calculated. Zh. Tekh. Fiz. 67, 81–89 (October 1997)     

Estimation of detectability of a horn antenna with nonlinear junction in the case of observation by “nonlinear” radar

Electromagnetic wave scattering by a horn antenna with a nonlinear Al−Al₂O₃−Al junction is studied experimentally and theoretically at the third harmonic of the irradiating field for decimeter wavelengths. The influence of junction nonlinearity on horn antenna detectability is estimated when the latter is observed by a “nonlinear” radar. The 5th Central Research Institute of the Ministry of Defense of the Russian Federation, Voronezh, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 43, No. 2, pp. 126–129, February 2000.     

Highly nonideal classical thermal plasmas: Experimental study of ordered macroparticle structures

The formation of spatially ordered CeO₂ particle structures in a thermal plasma at atmospheric pressure and temperatures of 1700–2200 K is studied. The spatial structure of the particles in the plasma is analyzed using laser time-of-flight counting of individual particles. Probe and optical diagnostics are used to determine the parameters of the thermal plasma. The CeO₂ particles were positively charged (about 10³ electronic charges). The resulting Coulomb interaction parameter for the particles is γ _p>120, which corresponds to a highly nonideal plasma. Zh. éksp. Teor. Fiz. 111, 467–477 (February 1997)     

Thermal conductivity of nonlinear waves in disordered chains

We present computational data on the thermal conductivity of nonlinear waves in disordered chains. Disorder induces Anderson localization for linear waves and results in a vanishing conductivity. Cubic nonlinearity restores normal conductivity, but with a strongly temperature-dependent conductivity κ(T). We find indications for an asymptotic low-temperature κ ∼ T ⁴ and intermediate temperature κ ∼ T ² laws. These findings are in accord with theoretical studies of wave packet spreading, where a regime of strong chaos is found to be intermediate, followed by an asymptotic regime of weak chaos (Laptyeva et al, Europhys. Lett. 91, 30001 (2010)).     

Change of radiation pattern in a plasma monopole antenna

Abstract

In the present work, we have numerically solved the dispersion equation of the surface wave propagating on a uniform collisional plasma column. The electric field and surface current distributions have been computed in different situations. We have investigated the effect of plasma frequency variation on the spatial distribution of the surface current. Results show that varying the electron density of the plasma column enables the plasma column to work as a plasma monopole antenna with a fixed geometrical structure and excited frequency which is able to create different radiation patterns. Our numerical analysis also shows that a little change in the radius of the plasma column has a strong influence on the current distribution at the excited frequency in RF region. This effect can be ignored in the usual (metallic) antenna while it is very important in designing of the plasma monopole antenna.     

Experimental observation of Coulomb ordered structure in sprays of thermal dusty plasmas

A macroscopic Coulomb-ordered structure of polydisperse CeO₂ particles is observed experimentally in a laminar spray of weakly ionized thermal plasma under atmospheric pressure and temperature of about 1700 K. Diagnostic instruments are used to measure plasma parameters. The particles are charged positively and carry about 10³ electron charges. The calculated values of Coulomb coupling parameter γ_p is > 120, corresponding to a strongly coupled plasma. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 3, 176–180 (10 February 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Suppression of nonlocal thermal conductivity in a turbulent plasma

It is shown that anomalous suppression of the electronic thermal conductivity occurs in a weakly collisional plasma as a result of electron scattering not only by charged particles but also by low-frequency turbulent fluctuations. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 579–582 (25 April 1996)     

Stimulated brillouin scattering of electromagnetic ion cyclotron waves in a plasma

Using the fluid model for the nonlinear response of ions, we have studied the nonlinear scattering of an electromagnetic ion cyclotron wave off the ion acoustic wave in a plasma. The low frequency nonlinearity arises through the parallel ponderomotive force on ions and the high frequency nonlinearity arises through the nonlinear current density of ions. For a typical nonisothermal plasma (T _e/T _i∼10) the threshold for this instability in a uniform plasma is ∼1mW/cm². At power densities ≳10² W/cm², the growth rate for backscatter turns out to be ∼10⁴s⁻¹.     

Millimeter microwave study of the superconducting properties of the Cu-C system

A study has been made of a cylindrical Cu-C powder sample operated as a microwave antenna at 35 GHz in the 80–300-K temperature interval. The efficiency of such an antenna is shown to increase nearly twofold from room temperature to liquid nitrogen temperature, which is apparently associated with a superconducting transition close to 80 K. Fiz. Tverd. Tela (St. Petersburg) 39, 816–817 (May 1997)     

Powerful thermonuclear neutron source based on laser excitation of hydrothermal dissipation in a volume-structured medium

An efficient method is proposed for generating thermonuclear neutrons by irradiating with a laser pulse a volume-structured material of subcritical density, consisting of a series of thin layers of condensed matter separated by interlayers of low-density matter (or a vacuum gap). The plasma ions are heated up to thermonuclear temperatures much higher than the electron temperature by hydrothermal dissipation of the energy of the laser radiation, as a wave of thermal explosions of the layers propagates along the laser beam axis, followed by collisions of plasma counterflows with conversion of the kinetic energy into thermal energy of ions. Different variants of the targets and experimental conditions are discussed in order to demonstrate the proposed method of neutron generation. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 8, 521–526 (25 October 1997)     

Surface waves of the potential type at the interface between a metal and an inhomogeneous medium

A study is made of surface waves of the potential type propagating along the interface between a metal and a plasma of nonuniform density, with the thermal motion of the electrons taken into account. Dispersion relation for these waves are derived and solved for a linear plasma density profile. The influence of the nonuniformity of the plasma density on the dispersion properties of the waves is studied. Cases of negative and positive gradients are considered. Zh. Tekh. Fiz. 69, 80–83 (July 1999)     

Electrothermal automodulation in incipient ferroelectric whispering-gallery microwave resonators at 4.2 K

Threshold conditions for electrothermal automodulation instability in high-Q ferroelectric microwave cryogenic resonators operating in the two-mode regime are investigated. The dependence of the electrothermal automodulation frequency on the numbers of interacting modes for different combinations of thermal modes and surface electromagnetic whispering-gallery modes is presented. The threshold power exciting the electrothermal automodulation of the oscillation of the partial mode electromagnetic amplitudes is compared with the threshold power of strictional parametric excitation of acoustic oscillations in the resonator. It is shown that the electrothermal automodulation in the two-mode regime may take place at an excitation power from 10 to 120 μW depending on the combination of interacting thermal and electromagnetic surface modes. Calculated threshold powers are low, which makes it possible to apply the electrothermal automodulation for improving the sensitivity of resonance bolometers and distributed microwave antennas with basic elements built around nonlinear microwave resonators. In addition, the electrothermal automodulation could be applied in developing novel microwave metamaterials. Nonlinear microwave whispering-gallery cryogenic resonators can be used as elements increasing the sensitivity of EPR spectroscopy methods.     

On phase-modulational bifurcation during passive mode locking in lasers

We have found that phase-modulation instability of a single pulse due to nonlinearity of the refractive index of an in-cavity medium can cause the pulse to break up into several stable identical pulses of smaller amplitude. We show that if the refractive-index nonlinearity is greater than a critical value, then stabilization of the regime with several stationary pulses does not occur, and pulse multiplication accompanied by chaotic variation of the intensity continues until the cavity is completely filled with radiation. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 4, 261–264 (25 February 1998)     

Generation and interaction of large-amplitude solitons

The results of a numerical simulation of the interaction and generation of solitons in nonlinear integrable systems which admit the existence of very-large-amplitude solitons are reported. The nonlinear integrable system chosen for study is the Gardner equation, particular examples of which are the Korteweg-de Vries equation (for quadratic nonlinearity) and a modified Korteweg-de Vries equation (for cubic nonlinearity).It is shown that during the evolution process solitons of opposite polarity appear on the crest of the maximum soliton. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 628–633 (10 May 1998)     

Observation of stochastic resonance in a monostable magnetic system

A stochastic resonance on mixed harmonics is observed experimentally in a monostable magnetic system. The curve of the stochastic resonance at higher-order harmonics in systems with a weak nonlinearity is calculated. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 10, 788–792 (25 May 1997)     

Squeezed states of light pulses in the presence of a self-effect in an inertial nonlinear medium

A systematic theory of the formation of squeezed states during the propagation of coherent light pulses in a medium with an inertial Kerr nonlinearity is developed. It is established that the region of the spectrum where the quadrature fluctuations are weaker than the shot noise depends on both the relaxation time of the nonlinearity and the magnitude of the nonlinear phase shift. It is also shown that the frequency at which suppression of the fluctuations is greatest can be controlled by adjusting the phase of the pulse. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 7, 481–485 (10 April 1999)     

Characteristics of electron cyclotron resonance plasma formed by lower hybrid current drive grill antenna

A 3.7 GHz system, which is meant for LHCD experiments on ADITYA tokamak, is used for producing ECR discharge. The ECR discharge is produced by setting the appropriate resonance magnetic field of 0.13 T, with hydrogen at a fill pressure of about 5 × 10⁻⁵ Torr. The RF power, up to 10 kW (of which ∼50% is reflected back), with a typical pulse length of 50 ms, is injected into the vacuum chamber of the ADITYA tokamak by a LHCD grill antenna and is used for plasma formation. The average coupled RF power density (the RF power/a typical volume of the plasma) is estimated to be ∼5 kW/m³. When the ECR appears inside the tokamak chamber for the given pumping frequency (f = 3.7 GHz) a plasma with a density (n _e) ∼ 4 × 10¹⁶ m⁻³ and electron temperature ∼8 eV is produced. The density and temperature during the RF pulse are measured by sets of Langmuir probes, located toroidally, on either side of the antenna. H_α signals are also monitored to detect ionization. An estimate of density and temperature based on simple theoretical calculation agrees well with our experimental measurements. The plasma produced by the above mechanism is further used to characterize the ECR-assisted low voltage Ohmic start-up discharges. During this part of the experiments, Ohmic plasma is formed using capacitor banks. The plasma loop voltage is gradually decreased, till the discharge ceases to form. The same is repeated in the presence of ECR-formed plasma (RF pre-ionization), formed 10 ms prior to the loop voltage. We have observed that (with LHCD-induced) ECR-assisted Ohmic start-up discharges is reliably and repeatedly obtained with reduced loop voltage requirement and breakdown time decreases substantially. The current ramp-up rates also decrease with reduced loop voltage operation. These studies established that ECR plasma formed with LHCD system exhibits similar characteristics as reported earlier by dedicated ECR systems. This experiment also addresses the issue of whether ECR plasma formed with grill antenna exhibits similar behavior as that formed by single waveguide ECR antenna. Our experimental observations suggest that the characteristics of (LHCD system-induced) ECR-assisted Ohmic start-up discharges show similar properties, reported earlier with normal ECR-assisted Ohmic start-up discharges and hence LHCD system may be used as ECR system at reduced toroidal magnetic field for other applications like wall conditioning.      

Dynamics of formation of ordered structures in a thermal plasma with macroparticles

The molecular dynamics method is used to model the formation of ordered structures of charged macroparticles in a thermal plasma at atmospheric pressure. The results of the numerical calculations are compared with the experimental data. It is shown that the ordered structure of macroparticles detected experimentally is far from steady state, since the existence time of the plasma in the experiment is less than the characteristic time of formation of the structure. Zh. éksp. Teor. Fiz. 115, 837–845 (March 1999)     

Generation of a continuum and stimulated Raman scattering harmonics during scattering of electromagnetic radiation of relativistic intensity

A theory of the propagation instability of plane, monochromatic, circularly polarized electromagnetic waves of relativistic intensity in matter is developed for a spatially three-dimensional geometry including arbitrary polarization of the scattered radiation. Harmonic generation owing to striction and relativistic nonlinearity is examined, as well as scattering owing to electron recoil, the decay instability of the harmonics with formation of scattered electromagnetic waves (Stokes components of the stimulated Raman scattering and plasmons), the interaction of electromagnetic waves in the plasma (antistokes stimulated Raman scattering), and the generation of a radiative continuum. The transition of the three-dimensional theory to a one-dimensional problem in the nonrelativistic limit is discussed. Zh. éksp. Teor. Fiz. 113, 2034–2046 (June 1998)     

Determination of the thermal conductivity of polycrystalline diamond films by means of the photoacoustic effect

A new method of determining the heat-conducting properties of diamond films is proposed, based on the photoacoustic effect. This method is used to study diamond polycrystalline films grown on silicon by chemical vapor deposition in a microwave discharge plasma. The thermal conductivity obtained was approximately half that for single-crystal diamond. Zh. Tekh. Fiz. 69, 97–101 (April 1999)