Experimental study of efficiency optimization in a three-cavitygyroklystron amplifier

An experimental study quantifying the effects of both penultimate cavity tuning and magnetic field profiling on the saturated efficiency of a three-cavity gyroklystron amplifier has been performed at a frequency of 4.5 GHz. As predicted by theory, it is observed that the penultimate cavity detuning in the gyroklystron is in the opposite direction from a conventional klystron for enhanced efficiency operation. This is a result of the opposite energy dependence of the bunching mechanism for the two interactions. Magnetic field profiling has also been shown to have a very strong effect on the amplifier efficiency, both through beam loading effects in detuning the penultimate cavity and through enhanced energy extraction in the output cavity. The measured efficiency of the gyroklystron is observed to improve from 5% to 21% by profiling the magnetic field along the device, and then from 21% to 29% by penultimate cavity detuning with an optimized magnetic field profile     

Threshold Frequency of Helical Electron–Hole Plasma Instability

Experimental evidence on the dependence of the threshold frequency of silicon oscillistors on the threshold electric field strength, magnetic induction, temperature, and injecting-contact separation is presented. In the temperature interval, where the weak magnetic field criterion is roughly satisfied, the experimental results are shown to be adequately explained by the classical theory of the bulk helical instability of an extrinsic plasma. The threshold frequency in this temperature interval is determined by the sum of two components. One component is due to the ambipolar drift of helical plasma perturbations, and the other results from the presence of the charge-carrier concentration gradient in a direction normal to the vectors of the electric field strength and magnetic induction. In short oscillistors (0.85·10^–3, 2.38·10^–3 m) at 77 K, a semiconductor plasma, wherein the helical instability is excited, approximates an intrinsic plasma, and the threshold frequency is determined by the rotation rate of helical perturbations.     

Theory of gyro-backward wave oscillators with tapered magneticfield and waveguide cross section

A theory is developed which describes gyro-backward wave oscillators (gyro-BWO's) with arbitrarily relativistic electron beams and tapered external magnetic field and waveguide wall radius. For the case of linear tapering the optimal parameters are found that correspond to highly efficient operation. The frequency tunability as the function of magnetic field and voltage deviations has been analyzed. It is shown how to use the results given in terms of normalized parameters for designing concrete gyro-BWO's. For one concrete design the effect of electron velocity spread on the efficiency has been studied     

Ac magnetic field screening by high-Tc superconductor films and single-crystals

A method is proposed to measure the screening efficiency of an ac magnetic field by superconducting films with the use of a strip line as an ac magnetic field source capable of operating within a broad frequency range. The dependence of the efficiency of ac magnetic field screening by YBCO films and single-crystal BiSrCaCuO on temperature and the frequency of magnetic field oriented perpendicular to the sample plane has been studied in the 10⁴–10⁹-Hz range. This relation is shown to depend substantially on the technique used in the film fabrication. Fiz. Tverd. Tela (St. Petersburg) 39, 228–230 (February 1997)     

Parametric excitation of helicon and acoustic waves in piezoelectric semiconductor-plasma

The parametric excitation of a helicon and an acoustic wave in a piezoelectric semiconductor-plasma in the presence of a strong magnetic field has been investigated using the coupled mode theory. The expressions for the threshold value of the electric field required for the onset of instability and for the growth rate well above the threshold have been obtained. It is observed that an acoustic wave of higher frequency and higher phase velocity than that of the pump wave cannot be excited. The analysis has been applied to the case of n type InSb sample where the threshold value of the electric field is found to be of the order 5.2 × 10³ Vm^?1 and the growth rate at an electric field 5.2 × 10⁴ Vm^?1 is of the order of 8.7 × 10¹⁰ sec^?1.     

NMR phase noise in bitter magnets

We have studied the temporal instability of a high field resistive Bitter magnet through nuclear magnetic resonance (NMR). This instability leads to transverse spin decoherence in repeated and accumulated NMR experiments as is normally performed during signal averaging. We demonstrate this effect via Hahn echo and Carr--Purcell--Meiboom--Gill (CPMG) transverse relaxation experiments in a 23-T resistive magnet. Quantitative analysis was found to be consistent with separate measurements of the magnetic field frequency fluctuation spectrum, as well as with independent NMR experiments performed in a magnetic field with a controlled instability. Finally, the CPMG sequence with short pulse delays is shown to be successful in recovering the intrinsic spin--spin relaxation even in the presence of magnetic field temporal instability.     

On the role of strong turbulence in low-pressure rf discharges in a magnetic field

The results of an experimental and theoretical investigation of strong turbulence produced by an external rf field in the plasma of a beam-plasma discharge are presented. The modulation instability of a rf electric field, perpendicular to a constant magnetic field, in the frequency range between the lower-hybrid and ionic Langmuir frequencies, has been investigated. It is shown that the low-frequency fields can be used to control beam relaxation. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 9, 608–612 (10 November 1996)     

Large-signal theory of gyro traveling wave tubes at cyclotronharmonics

A nonlinear theory of gyrotron traveling wave tubes (gyro-TWTs) at cyclotron harmonics has been developed taking into account the electron velocity spread and the possibility of operating with significant Doppler frequency up-shift (CARM operation). It is shown that the orbital efficiency of the relativistic gyro-TWT operating at the second cyclotron harmonic with large frequency up-conversion may exceed 60%. It is also shown that the influence of the axial inhomogeneity of the wave field on the relation between amplitudes of electric and magnetic fields of the wave causes small changes in the efficiency of gyro-TWTs. The results obtained demonstrate the sensitivity of the harmonic gyro-TWT efficiency with respect to electron velocity spread at different axial wave numbers. The expressions for the gain are derived and discussed,     

Frequency dependence of the coefficient of microwave radiation reflection from magnetic fluid at liquid nitrogen temperatures

The coefficient of microwave radiation reflection from a plane layer of a magnetic fluid magnetized by a magnetic field of 11.5 kOe is measured in the temperature range 253–293 K. The concentration of the magnetic phase (magnetite) is 15 vol %. In this temperature range, the dynamic susceptibility of nanoparticles exhibits strong dispersion (ferromagnetic resonance). Experimental data are treated by invoking the theory of high-frequency magnetization of an ensemble of interacting isotropic superparamagnetic particles. It is shown that, when the temperature drops below the room value, the trends observed at elevated temperatures (a decrease in the fraction of the power reflected from the magnetic fluid and an increase in the frequency of a minimum in the frequency dependence of the power) are retained.     

Influence of magnetism on the structural stability of cubic L2<Subscript>1</Subscript>Ni<Subscript>2</Subscript>MnGa

We present calculations of magnetic field-dependent properties of magnetic shape memory (MSM) Heusler alloys by means of density functional theory calculations. The effects of an external magnetic field on structural properties are simulated by fixing the magnetic moments within the framework of the fixed spin moment (FSM) method. We calculate the binding surface as a function of the magnetic moment and the tetragonal distortion. For magnetizations of 10% below the equilibrium value, the energy of the martensitic L1₀ phase steeply increases leading to a relative stabilization of the L2₁ phase in a confined magnetization range. Calculations of the phonon dispersion in the direction [ξξ0]2π/a suggest that the instability at ξ≈1/3 disappears with decreasing magnetization, allowing a nearly stable spectrum in a small magnetization interval.     

Correlation of electrical, magnetic, and thermal properties of the La0.85Ag0.15MnO3 manganite near the phase transition temperature

The correlation between electrical, magnetic, and lattice properties of manganites has been studied by analyzing the temperature dependence of the electrical resistivity of the La_0.85Ag_0.15MnO₃ manganite. The results have been discussed within the model of a phase-separated manganite in terms of the percolation theory. It has been shown that, from analyzing the temperature and magnetic field dependences of the electrical resistivity, it is possible to predict the behavior of the magnetization and specific heat, as well as the change of magnetic entropy near the phase transition temperature, and that the obtained agreement with experimental data for this analysis is quite reasonable.     

Magneticresonant three dimensional kinetic theory of the free electron laser with waveguide cavity

Vlasov kinetic theory of the free electron laser employing waveguide frifting tube and helical pump is presented. Distinct from the former theories this theory has (1) taken into account the 3-d structure of the scattering field; (2) applied the plasma kinetic theory, which is more accurate than the other methods, to analysing the effect of the longitudinal. field; (3) taken account of the influence of the annular structure of the beam. The magnetic resonant effect appearing when the cyclotron frequency of electrons approaches the effective frequency of the magnetic pump is analysed and the growth rate formula in the linear regime derived. Results show that on taking into account the 3-d structure of the field and beam, one cannot arbitrarily choose the radius of the hollow beam, or else within certain range the instability may disappear altogether. Lastly, the equation for determination of the optimum beam radius is given and the possibility of raising growth rate by operating in high order modes shown.     

Energy fluxes in helical magnetohydrodynamics and dynamo action

Renormalized viscosity, renormalized resistivity, and various energy fluxes are calculated for helical magnetohydrodynamics using perturbative field theory. The calculation is of firstorder in perturbation. Kinetic and magnetic helicities do not affect the renormalized parameters, but they induce an inverse cascade of magnetic energy. The sources for the large-scale magnetic field have been shown to be (1) energy flux from large-scale velocity field to large-scale magnetic field arising due to non-helical interactions and (2) inverse energy flux of magnetic energy caused by helical interactions. Based on our flux results, a primitive model for galactic dynamo has been constructed. Our calculations yield dynamo time-scale for a typical galaxy to be of the order of 10⁸ years. Our field-theoretic calculations also reveal that the flux of magnetic helicity is backward, consistent with the earlier observations based on absolute equilibrium theory.     

Oscillations of the magnetoresistance of a two-dimensional electron gas under microwave irradiation: Influence of the irradiation frequency

We present measurements of the magnetoresistance of a two-dimensional electron gas (2DEG) under continuous microwave as a function of the irradiation frequency. In a previous work by Simovič et al. [Phys. Rev. B 71 (2005) 233303], the magnetoresistance under microwave was shown to be modulated by oscillations of large amplitude that are periodic with magnetic field, their period and phase depending strongly on the electron density. Here we show that the phase and the amplitude of the microwave-induced oscillations also depend on the frequency of irradiation and the sign of the magnetic field.     

自由电子激光的单粒子理论及其应用

本文建立了圆极化摇摆场自由电子激光的单粒子理论,导出了电子未扰轨道及其稳定性判据和自由电子激光单程增益的表达式。单程增益由三项构成,其中第一项即自由电子激光不稳定性的增益与动力学理论得到的指数增益相符。第二项和和三项表明存在一对新的不稳定性——正不稳定性和负不稳定性。该理论没有对电子未扰轨道纵向速度作任何假设,不仅可以更合理地用于常规自由电子激光的研究,而且可以用于短周期摇摆器弱相对论自由电子激光研究。后者由于电子未扰轨道纵向速度比较低,已有的单粒子理论中所作的电子纵向速度约等于光速的假设不再成立。借助数值分析,我们发现:(1)稳定轨道Ⅱ的弱导引场区域也出现了动力学理论描述过的与自由电子激光互作用机理相悖的现象。(2)正不稳定性和负不稳定性在稳定轨道Ⅰ的导引磁场临界值附近可能严重影响自由电子激光的工作。(3)可以使用较弱的短周期摇摆场和较强的导引场产生高频率高增益相干受激辐射。     

Characteristics of a reflex discharge in hydrogen in a magnetic field

The properties of an extensive reflex discharge of large power with one heated and one cold cathode in a magnetic field of 10 to 1500 Gauss were measured. At a neutral hydrogen pressure of 10^–4 to 10^–3 torr a plasma column was obtained which was 100 cm long, 10 cm in diameter and had a density greater than 10^–11 cm^–3. The dependence of the radial profile of the potential in the plasma was measured by a heated probe, and the temperature and electron density by Langmuir probes calibrated by a microwave interferometer.The dependence of the origin of low-frequency rotational instability on the parameters of the discharge was studied and the measured critical magnetic field was compared with theory. Apart from this low-frequency instability, intensive oscillations were also found in the discharge current in a frequency band up to 30 MHz, and the dependence of their spectrum on the magnetic field was measured.     

Dynamic range and linearity improvement for zero-field single-beam atomic magnetometer

Zero-field single-beam atomic magnetometers with transverse parametric modulation for ultra-weak magnetic field detection have attracted widespread attention recently. In this study, we present a comprehensive response model and propose a modification method of conventional first harmonic response by introducing the second harmonic correction. The proposed modification method gives improvement in dynamic range and reduction of linearity error. Additionally, our modification method shows suppression of response instability caused by optical intensity and frequency fluctuations. An atomic magnetometer with single-beam configuration is built to compare the performance between our proposed method and the conventional method. The results indicate that our method's magnetic field response signal achieves a 5-fold expansion of dynamic range from 2 nT to 10 nT, with the linearity error decreased from 5% to 1%. Under the fluctuations of 5% for optical intensity and ±15 GHz detuning of frequency, the proposed modification method maintains intensity-related instability less than 1% and frequency-related instability less than 8% while the conventional method suffers 15% and 38%, respectively. Our method is promising for future high-sensitive and long-term stable optically pumped atomic sensors.     

Photoabsorption oscillations in semiconductors with sharply anisotropic effective mass of electrons in quantizing magnetic fields

A theory of photoabsorption on magnetic-impurity states of carriers with sharply anisotropic effective mass is developed. Both in crossed and parallel electric and magnetic fields the photoabsorption coefficient is investigated for intraband (in terms of Landau bands) and interband transitions of an electron. It is shown that the resonant component of the photoabsorption coefficient at electron detachment and the resonant component of cyclotron resonance harmonics have periodically dependence by the magnetic field as √H. Influence of parity of the magnetic-impurity states on frequency dependence of the resonant component of photoabsorption coefficient is studied.     

Observation of pulsed fast electron precipitations and the cyclotron generation mechanism of burst activity in a decaying ECR discharge plasma

We have detected and investigated quasi-periodic series of pulsed energetic electron precipitations in the decaying plasma of a pulsed ECR discharge in a mirror axisymmetric magnetic trap. The observed particle ejections from the trap are interpreted as the result of resonant interaction between energetic electrons and a slow extraordinary wave propagating in the rarefied plasma across the external magnetic field. We have been able to explain the generation mechanism of the sequences of pulsed precipitations at the nonlinear instability growth phase in terms of a cyclotron maser model in which the instability threshold is exceeded through a reduction in electromagnetic energy losses characteristic of the plasma decay.