Output power variations in a plasma relativistic microwave amplifier during a 500-ns relativistic electron beam current pulse

A relativistic plasma microwave amplifier with a gain of about 30 dB and an output power of about 60–100 MW in the frequency range from 2.4 to 3.2 GHz is studied experimentally. The total duration of the output microwave pulse is equal to the duration of the current pulse of the driving relativistic electron beam (500 ns); however, the maximum output power is observed only within 200 ns. It is shown that variations in the output microwave power during the current pulse of the annular relativistic electron beam are caused by variations in the beam radius and thickness. Analysis of the experimental data and results of numerical simulations show that the thickness of the electron beam is determined by the density of the cathode emission current.     

Tunable plasma relativistic microwave amplifier

A stable regime of the amplification of a slow plasma wave in a plasma waveguide during the injection of a high-current relativistic electron beam is obtained. For an input-signal frequency of 9.1 GHz, there exists a range of plasma densities in which the spectrum of the output microwave radiation lies in a 0.5-GHz-wide band. For a 40-kW input power at a frequency of 9.1 GHz, the maximum output power is 8 MW. It is shown experimentally for the first time that the beam-plasma amplifier can operate at frequencies of 9.1 GHz and 12.9 GHz. The range of plasma densities in which the regime of amplification is observed agrees with the results of calculations based on linear theory.     

Fine structure of the emission spectra of a relativistic Cherenkov plasma maser

The evolution of the emission spectrum of a relativistic Cherenkov plasma maser is studied both experimentally and numerically. The frequency range of emission is 1.5–6 GHz at a power level of 50 MW and pulse duration of up to 500 ns. It is shown that the relativistic Cherenkov plasma maser is capable of producing both broadband (with a spectrum width of ～1 GHz) and narrowband (≈ 40 MHz) microwave pulses with a tunable mean frequency. Calculations by linear theory and numerical simulations provide a satisfactory explanation of the specific features and the time evolution of the spectra observed. It is suggested that the plasma nonlinearity is responsible for the experimentally observed shortening of the microwave pulses and the broadening of the emission spectrum.     

A 50-MW broadband plasma microwave amplifier

Plasma Physics Reports - A pure amplification regime (without accompanying generation) at two frequencies of 9.1 and 13 GHz is achieved in a plasma relativistic microwave amplifier. It is shown...     

Efficiency of 2.45 and 5.80 GHz microwave irradiation for a hydrolysis reaction by thermostable β-Glucosidase HT1

Microwave irradiation at different frequencies gave unique results for the hydrolyses of glycosyl bonds by β-Glucosidase HT1. With the observed relative complex permittivity data for the reaction buffer, 2.45?GHz microwave radiation affected both waters and ions, while 5.80?GHz only affected waters. We, here, propose that would be one of the unique “microwave nonthermal effects”.     

Emission spectra of a cherenkov plasma relativistic maser

The spectra of a plasma relativistic maser are measured. It is shown that the microwave frequency can be varied from 4 to 28 GHz by varying the plasma density from 4×10¹² to 7×10¹³ cm^?3 at a power of 30–50 MW. The relative width of the emission spectrum is within 50–80% for low plasma densities and 15–30% for high densities. Experimental results are compared with calculations.     

The effects of fat layer on temperature distribution during microwave atrial fibrillation catheter ablation

Abstract

To investigate the effects of fat layer on the temperature distribution during microwave atrial fibrillation catheter ablation in the conditions of different ablation time; 3D finite element models (fat layer and no fat layer) were built, and temperature distribution was obtained based on coupled electromagnetic-thermal analysis at 2.45?GHz and 30?W of microwave power. Results shown: in the endocardial ablation, the existence of the fat layer did not affect the shape of the 50?°C contour before 30?s. The increase speed of depth became quite slowly in the model with fat layer after 30?s. When ablation depth needed fixed, there are no significant effect on effectively ablation depth whether fat layer over or not. However, the existence of fat layer makes the temperature lower in the myocardium, and maximum temperature point closer to the myocardium surface. What is more, in the model with fat layer, effective ablation reach lower maximum temperature and the shallower depth of 50?°C contour. But there are larger ablation axial length and transverse width. In this case, doctor should ensure safety of normal cardiac tissue around the target tissue. In the epicardial ablation, the existence of fat layer seriously affects result of the microwave ablation. The epicardial ablation needs more heating time to create lesion. But epicardial ablation can be better controlled in the shape of effective ablation area because of the slowly increase of target variables after the appearing of 50?°C contour. Doctor can choose endocardial or epicardial ablation in different case of clinic requirement.     

Effects of millimeter-wave radiation on monolayer cell cultures. III. A search for frequency-specific athermal biological effects on protein synthesis

A method recently developed in this laboratory has been used to directly expose BHK-21/C13 cells to high levels of microwave radiation without significant microwave-induced heating (? 0.1 °C). Monolayer cultures were grown on microwave-transparent polystyrene coverslips, placed on the open end of a wave guide, and maintained at 37.2 °C during irradiation at frequencies in both the E- and U-bands (average power densities 292 and 177 mW/cm², respectively). Effects of microwave radiation were assessed at 0.1 GHz increments in the ranges of 38–48 GHz and 65–75 GHz. Protein synthesis was measured in quadruplicate cultures that were allowed to incorporate labeled methionine during the 15-minute period of microwave irradiation. Autoradiographs of each monolayer culture were scanned along the region corresponding to the longer axis of the wave guide aperture using a microdensitometer to quantify incorporation. Since microwave power incident on the cells was previously shown to vary along this axis according to a cosine² relationship from zero at each edge of the wave guide to twice the average power density at the center of the wave guide, this technique should reveal biological effects that might only be manifested in narrow amplitude domains or “power windows.” Observations of protein synthesis in monolayer cultures irradiated at 202 closely spaced frequencies in the E- and U-bands failed to reveal changes associated with microwave exposure. Thus no evidence was obtained in support of the existence of frequency-specific athermal biological effects of microwaves. In addition, no support was found for the existence of amplitude-specific “power windows”.     

Effect of the dc field on the near-surface plasma of a highly nonuniform microwave discharge

The effect of the dc electric field on the near-surface plasma of an electrode microwave discharge at pressures of 1?C5 Torr was studied by the emission spectroscopy method. It is shown that the dc field weakly affects the vibrational distribution of nitrogen molecules in the C₃??_u state, but changes the structure of the near-surface plasma (shifting the intensity maxima of the emission bands) and the strength of the microwave field near the electrode surface. It is also found that the ratio between the intensities of bands of different sequences of the second positive system of nitrogen radiated from the same state depends on the position along the discharge axis.     

Generation of dense multicharged ion flows from an ECR plasma confined in a quasi-gas-dynamic magnetic cusp trap

The possibility of generating dense multicharged ion beams with a current density as high as ～1 A/cm² from an ECR plasma confined in a quasi-gas-dynamic cusp trap is studied both theoretically and experimentally. The most important advantages of this type of ion source are that the plasma in the cusp is stabile against MHD perturbations and that a trap intended to operate at fairly high pump-field frequencies (above 30 GHz) is relatively inexpensive. A theoretical model of confinement of a high-density nonequilibrium ECR plasma (T _e ? T _i ) in an open magnetic trap is proposed and results are presented from model experiments with an ～30-cm-long cusp trap (here, by the cusp length is meant the volume of a paraxial magnetic tube divided by the area of its cross sections in magnetic mirrors) pumped by a pulsed microwave field with a frequency of 37.5 GHz and power of 100 kW. The possibility of achieving a quasi-gas-dynamic regime of plasma confinement of an ECR plasma in a cusp trap is demonstrated. Ion beams with a average ion charge number of 2–4 (depending on the sort of working gas) and current densities unprecedented for ECR sources are obtained. Good agreement between theoretical and experimental results makes it possible to reliably predict the ion beam parameters that can be achieved at even higher microwave frequencies.     

Generation of solar microwave bursts at the second harmonic of the plasma frequency

The nonlinear interaction between upper hybrid waves is simulated numerically for conditions corresponding to the sources of solar microwave bursts (at a frequency of about 5.7 GHz). The source of plasma waves is considered to be an electron beam with a loss-cone-type distribution. It is shown that, for a symmetric double-sided loss cone, the degree of polarization of the radiation generated in a direction transverse to the magnetic field can reach 100% and corresponds to an extraordinary wave. For a one-sided loss cone, in accordance with the previous results, the degree of polarization is found to be low. The efficiency of the plasma mechanism for the radiation generation is estimated.     

Temperature dependence of the microwave properties of aqueous solutions of ethylene glycol between +15 degrees C and -70 degrees C.

The values of the dielectric constant and of the loss tangent for samples of 0 m (distilled H₂O), 1 m, 2 m, 3 m, 4 m, 5 m, 10 m, and 100% ethylene glycol were determined over the temperature range of +15 °C to ?70 °C. An operating frequency of 1.40 GHz to 1.55 GHz was used, allowing the results to be applied directly to both 0.915 GHz and 2.450 GHz studies. Strong temperature and concentration dependencies were found; low concentration solutions tended to behave similarly to water, while higher concentration solutions were more independent. Peak values and discontinuities occurred at different temperatures, depending on concentration. Five molar ethylene glycol was indicated as a near optimum concentration based on toxicity, cryoprotection, and microwave power absorption considerations. A study of blends of cryoprotectants is suggested.     

Influence of refraction on plasma density measurements

The plasma density profile in the T-11M tokamak is measured with a multichannel Cotton-Mouton polarimeter and also with a time-of-flight refractometer. Both instruments operate at frequencies in the range of 140 GHz. Since, in this case, the trajectory of the probing microwave beam deviates from a straight line, the maximum refraction angle being as large as ～0.25 rad, the measurement results should be corrected for refraction. In order to solve this problem, a computer code has been developed for calculating the trajectories of both O-and X-mode probing beams. In calculations, it is assumed that the plasma is cold and that the inequality ω ? 2πν_p (where ω is the probing frequency and ν_p is the collision frequency) is satisfied. The wavelength λ is assumed to be much less than the plasma inhomogeneity scale length, so the calculations are performed in the geometric optics approximation. The dependences of the phase difference ΔΦ (for the Cotton-Mouton polarimeter) and the propagation time of a microwave pulse (for the time-of-flight refractometer) on the linear plasma density 〈nl〉 are calculated. For comparison, similar dependences are calculated for a conventional interferometer operating at the same frequency. It is shown that, in the latter case, the influence of refraction on the measurement results is about three times less. The dependences obtained are presently being used in measurements of 〈nl〉 by means of the Cotton-Mouton polarimeter and the time-of-flight refractometer with allowance for the density profiles recovered by abelianizing the data from the multichannel polarimeter (a series of iterative Abel inversion procedures are performed for each instant of time). The code can also be used to find corrections for refraction when performing interferometric measurements in other devices.     

Structure of a discharge induced by a coaxial microwave plasmatron with a gas-supply channel in the inner electrode

The structure of a discharge induced by a coaxial microwave plasmatron with a gas-supply channel in the inner electrode of a coaxial waveguide is investigated. A plasmatron with a power of up to 10 W operates at a frequency of 10 GHz. Depending on the operation regime, the discharge takes either a filament or torch form. A plasma filament arises at low flow rates of the working gas (argon) and occurs at the border of the potential core of the gas jet. A torch discharge occurs at high flow rates and has the form of a hollow cone. In both cases, the discharge arises in the potential core of the gas jet and does not spread beyond it. The distribution of the microwave field in the discharge plasma is determined.     

Normal Doppler effect in experiments on the interaction of relativistic electron beams with plasma: Plasma relativistic microwave amplifier

The Cherenkov interaction of a high-current relativistic electron beam with a spatially bounded plasma was studied experimentally. In the generation of electromagnetic radiation, an important role is played by the counterpropagating plasma wave produced due to the reflection from the end of the plasma column. It is shown that, at the resonant value of the magnetic field, the normal Doppler effect occurs and the amplitude of the counterpropagating wave decreases. This effect was used to design and create a plasma relativistic microwave amplifier in which 10% of the beam energy is converted into radiation. The radiation frequency is 9.1 GHz, and the radiation spectrum width (±0.17%) is determined by the microwave-pulse duration. The maximum radiation power is 100 MW, the gain factor being 32 dB.     

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.     

Microwave processes in the Octupole Galathea

Results are presented from experimental studies of electromagnetic emission and plasma oscillations in the plasma-frequency range in the Octupole Galathea confinement system. Experiments are performed in the electric-discharge mode at low magnetic fields (the barrier field is 0.002–0.01 T); the working gas is argon or hydrogen. It is found that the most intense microwave oscillations at frequencies of 1–5 GHz are excited near the plasma axis and in the magnetic-barrier region. The oscillations are excited by the discharge current and decay after the voltage is switched off. The experiments show that microwave oscillations excited in the magnetic-barrier region are responsible for the small value of the energy confinement time in the system.     

Microwave-stimulated drug release from liposomes

Microwaves (2450 MHz) are shown to stimulate the release of an aqueous chemotherapeutic drug from phospholipid vesicles. This effect occurs at temperatures below the membrane phase transition temperature of 41 degrees C where these liposomes are normally not leaky. In buffered saline, microwave exposure (60 mW/g) triggers the onset of drug release at 33 degrees C, whereas in plasma a near maximal release is observed as low as 27 degrees C. Significantly, this drug release is enhanced by oxygen and is attenuated by antioxidants. These results demonstrate that phospholipids in artificial membranes devoid of protein are influenced by nonionizing electromagnetic radiation, and that this interaction can be modulated by two physiologically important factors, plasma and oxygen. Such a permeability effect may provide a means for investigating microwave interactions with ordered membrane bilayers.     

Effect of DMSO on the dielectric properties of canine kidney tissue

The dielectric permittivity and conductivity of canine kidney tissue samples were measured at R_f frequencies between ?20 °C and +20 °C. Some of the kidneys had been perfused with DMSO (10%) in canine plasma, others with physiological saline alone. The DMSO greatly increases the conductivity of frozen tissue above that of tissue not treated with this cryoprotectant. Apparently, the chief reason for nonuniform heating of a partially frozen organ in a microwave field is the great change in tissue conductivity as it thaws. We suggest that the effect on the conductivity of tissue should be considered in the choice of a cryoprotectant for tissues which are to be thawed by microwave or radiofrequency irradiation.     

Plasma relativistic microwave electronics

The principles of plasma relativistic microwave electronics based on the stimulated Cherenkov emission of electromagnetic waves during the interaction of a relativistic electron beam with a plasma are formulated. A theory of relativistic Cherenkov plasma microwave oscillators and amplifiers is developed, and model experimental devices are elaborated and investigated. The emission mechanisms are studied theoretically. The efficiencies and frequency spectra of relativistic Cherenkov plasma microwave oscillators and ampli-fiers are calculated. The theoretical predictions are confirmed by the experimental data: the power of the devices attains 500 MW, the microwave frequency can be continuously tuned over a wide band with an upper-to-lower boundary frequency ratio of 7 (from 4 to 28 GHz), and the emission frequency bandwidth can be varied from several percent to 100 percent. These microwave sources have no analogs in vacuum microwave electronics.