Influence of the movement of cylindrical samples with variable internal diameter and variable length along thex-axis of a double TE104 and a single TE102 rectangular cavity on the EPR signal intensity: A sample shape study

The response of the cavity to the movement of cylindrical samples with internal diameters from 0.7 to 4 mm and lengths from 5 to 50 mm along thex-axis of the Bruker double TE₁₀₄ and single TE₁₀₂ rectangular cavity has been analyzed. Independently of sample internal diameter, the experimentally observed dependences of the electron paramagnetic resonance (EPR) signal intensity versus sample position in the cavity showed the following: (i) a sharp maximum for sample lengths from 5 to 20 mm; (ii) a “plateau”, over which the signal intensity remained constant within experimental errors of 0.47–1.16%, for lengths from 30 to 40 mm; and (iii) a “sloping plateau” region, which could be approximated by the linear function (correlationr = 0.96–0.98) for the 50 mm sample. Theoretical predictions of the experimental dependences of the signal intensity versus sample position in the cavity were calculated with the “modified” and “revised” sine-squared function, and the correlation between observed and theoretically computed dependences is very good. Additionally, the experimental dependence of the signal intensity versus the sample internal diameter and length for cylindrical samples situated at the position in the cavity at which the signal intensity was a maximum was likewise numerically approximated by the surface fitting with the Lorentzian cumulative additive function (correlationr = 0.999). The experimental dependence of the signal intensity versus the sample internal diameter for the given sample length is nonlinear. The samples with internal diameters of 0.7 and 1 mm gave the total maximum of signal intensity for the 40 mm sample, however, the samples with internal diameters of 2, 3 and 4 mm gave the total maximal value of signal intensity, which was identical for both the 30 and 40 mm samples. The experimental dependence of the EPR signal intensity versus the sample volume clearly showed that the samples with identical volumes, however, with different shapes, can give significantly different signal intensities (with differences ca. 200–400%). Then, the comparison of cylindrical samples with identical volumes but different shapes may be a serious source of significant errors in quantitative EPR spectroscopy. Cylindrical samples to be compared should be of identical shape. Accurate and precise positioning of each sample in the microwave cavity is essential.     

Influence of the diameter and wall thickness of a quartz pipe inserted in the EPR cavity on the signal intensity

Some dependences between the EPR signal intensity of point samples and the diameter and wall thickness of the quartz tubes with which a standard rectangular TE₁₀₂ EPR cavity is loaded are reported. It is found that the EPR signal intensity linearly depends on the wall thickness of the container and can be increased approximately twice when thick sample tubes are used. The microwave power in the location of the sample increases similarly whereas the normalized bell-shape distribution of the signal intensity along thez-axis of the cavity remains unchanged. The effective microwave power necessary to saturate the sample decreases and the cavity filling factor increases.     

Experimental Study of a Slot Antenna Excited by a TE011 Mode Coaxial Cavity

A slot antenna is developed to excite the high harmonic waveguide mode for generating large-area plasmas. This antenna consists of a TE₀₁₁ mode coaxial cavity with the axial slots positioned on equal interval on the inner wall. The waves radiated from those slots can excite the high harmonic mode in the central area. With the azimuthal symmetric wave field of the TE₀₁₁ mode, the number of the slots can be chosen to match the field pattern of the high harmonic mode. In this report, the dispersion relation of the coaxial waveguide, the coupling scheme and the mode competition of the cavity are studied. A method has been successfully developed to suppress the TE₁₂₁ mode which is the most competing mode to the TE₀₁₁ mode.     

Application of obliquely interfering TE10 modes for electron energy gain

Two fundamental TE₁₀ modes are considered to interfere at a small angle θ and then propagate along the z-axis in an evacuated rectangular waveguide. The electron trajectory in the resultant field and the expressions for energy gain and the acceleration gradient are obtained, when the electron is injected along the z-axis. A 50 keV electron gains 718 keV energy in a 4.0 cm × 2.5 cm waveguide, when the microwave with intensity of 1 × 10¹⁰ W/cm² and frequency 5.577 GHz is used and the modes superpose at an angle of 10°; here the maximum acceleration gradient is obtained as 251 MeV/m. The energy gain and acceleration gradient are decreased with increasing width of the waveguide and microwave frequency. Higher gradient and larger energy gain are obtained for the higher microwave intensity, smaller angle of superposition and also when the electron is injected with larger initial energy.     

非均匀扰动结构TE0n模式变换器研究

本文提出一种半径和周期双重扰动的非均匀圆波导TE_0n模式变换器. 通过耦合波理论(CWT)和数值优化方法对该模式变换器进行研究, 计算结果与电磁仿真软件基本一致. 与传统的均匀结构半径微扰模式变换器比较, 非均匀扰动结构TE_0n模式变换器可以在更少的波纹周期内实现高于均匀结构的模式转换效率, 95%功率转换绝对带宽增加150%, 器件长度减小接近一半. 本文的研究工作为设计轴向尺寸短、工作带宽大、 转换效率高的高功率回旋管外接模式变换器提供了重要参考.     

Theoretical and Numerical Investigation of a Four-cavity TE021-Mode Gyroklystron

Using self-consistent field theory and PIC simulation, the interaction of electron flow with HF fields in a four-cavity Gyroklystron with TE₀₂₁-mode has been analyzed. Self-consistent field theory includes both linear theory and nonlinear theory. Optimized parameters and their corresponding efficiency, gain and bandwidth of the optimized Gyroklystron have been found. Numerical investigation using PIC simulation is also given. Parameters of the cavities which are operating in TE₀₂₁ mode are optimized to minimize TE₀₁₁ mode and to suppress parasitic self-oscillations. The results of theory are in good qualitative agreement with PIC simulation. This work is supported by the Chinese Natural Science Foundation under Contract 60532     

Experimental investigation of a compact relativistic magnetron with axial TE11 mode radiation

As one of the relativistic electron tubes having compact configuration and high efficient output, the relativistic magnetron with direct axial radiation is very attractive in pulsed power and high power microwave fields for industrial and military applications. In this paper, the experimental investigation of a relativistic magnetron with axial TE11 mode radiation is reported. Under a total length of - 0.3 m, volume of - 0.014 m3, working at an applied voltage of 508 kV and a magnetic field of - 0.31 T, the relativistic magnetron radiates a microwave of 540 MW with the TE11 mode at 2.35 GHz in the axial direction. The power conversion efficiency is 15.0%. After a lot of shots, the detected amplitudes of microwaves are nearly the same. The fluctuations of wave amplitudes are less than 0.3 dB.     

宽带非均匀半径渐变TE0n-TE0(n+1)模式转换器的设计研究

基于耦合波理论,对两类半径渐变圆波导TE_0n-TE_0(n+1)模式转换器进行理论分析、数值计算和仿真模拟.均匀半径渐变波导高功率模式转换器,采用中心频率为17.14GHz、六周期TE₀₂-TE₀₃模式和中心频率为34.30GHz、六周期TE₀₁-TE₀₂模式两种设计参数.非均匀半径渐变波导高功率模式转换器,采用中心频率为34.30GHz、六周期TE 关键词： 模式转换器 耦合波理论 非均匀半径渐变 转换带宽     

Half-field EPR line in the two-dimensional ferromagnet K2CuF4

Room temperature measurements of the half-field EPR line in the two- dimensional ferromagnet K₂CuF₄ are reported. Changes of the intensity and line-width vs the angle between the applied field and the c-axis indicate the necessity of an additional term in the perturbing Hamiltonian besides the dipolar interaction. Its appearance at half the main Zeeman field at each angle means that there is no observable dynamic effect on the shift of the resonance position in this salt.     

Uniform Field Re-entrant Cylindrical TE$$_{01\text {U}}$$ Cavity for Pulse Electron Paramagnetic Resonance Spectroscopy at Q-band

Uniform field (UF) resonators create a region-of-interest, where the sample volume receives a homogeneous microwave magnetic field (\(B_1\)) excitation. However, as the region-of-interest is increased, resonator efficiency is reduced. In this work, a new class of uniform field resonators is introduced: the uniform field re-entrant cylindrical TE\(_{\text {01U}}\) cavity. Here, a UF cylindrical TE\(_{\text {01U}}\) cavity is designed with re-entrant fins to increase the overall resonator efficiency to match the resonator efficiency maximum of a typical cylindrical TE\(_{011}\) cavity. The new UF re-entrant cylindrical TE\(_{\text {01U}}\) cavity is designed for Q-band (34 GHz) and is calculated to have the same electron paramagnetic resonance (EPR) signal intensity as a TE\(_{011}\) cavity, a 60% increase in average resonator efficiency \(\Lambda _\mathrm{ave}\) over the sample, and has a \(B_1\) profile that is 79.8% uniform over the entire sample volume (98% uniform over the region-of-interest). A new H-type T-junction waveguide coupler with inductive obstacles is introduced that increases the dynamic range of a movable short coupler while reducing the frequency shift by 43% during over-coupling. The resonator assembly is fabricated and tested both on the bench and with EPR experiments. This resonator provides a template to improve EPR spectroscopy for pulse experiments at high frequencies.     

Analysis of the radial and longitudinal effect in a double TE(104) and a single TE(102) rectangular cavity

The response of the cavity to the rotation of a point-like sample in the horizontal (y-z) plane passing through the center of the Bruker double TE(104) and single TE(102) rectangular cavities in concentric circles of radii rho = 0, 1, 2, 3, 4, and 5 mm from the cavity center (radial effect) has been analyzed. The experimentally observed dependencies of the EPR signal intensity, I(pp), showed the following: (i) for rho = 0 mm (a sample position in the cavity center), I(pp) is independent of the angle of rotation; (ii) for rho = 1, 2, and 3 mm, the I(pp) dependence progressively changes from circular to oval; (iii) when the radius is further increased to rho = 4 and 5 mm, the I(pp) dependence changes dramatically, giving a figure eight shape. These experimental observations are in very good agreement with the theoretical calculations, in which the response is modeled using modified Cassinian curves, K(rho, phi). Similar trends were observed for any position of the horizontal (y-z) plane at which the sample is situated along the vertical x axis of the cavity; however, the amplitude of the signal decreases with increase in the absolute value of the x coordinate, ||x ||. The variation in the signal amplitude along the cavity x axis (longitudinal effect) can be calculated theoretically using a modified sine-squared curve, G(x). In general, the response of the cavity to a point-like sample situated at any position, P(rho, phi, x), can be represented as a product of the mentioned Cassinian curve, K(rho, phi), and sine-squared curve, G(x), giving for the signal intensity I(pp)(rho, phi, x) approximately K(rho, phi)G(x). The response to a large cylindrical sample which is concentrically situated on the cavity x axis can then be obtained by integrating the above product, K(rho, phi)G(x), over the sample volume. The nonlinear radial effect may give rise to a serious source of systematic error in quantitative EPR spectroscopy and shows that accurate and precise positioning of the sample in the microwave cavity is essential.     

An EPR study of different Gd3+ centers in LiNbO3

Electron Paramagnetic Resonance (EPR) and Zero-(magnetic)-Field (paramagnetic) Resonance (ZFR) spectra have been observed for LiNbO₃:Gd. EPR spectra measured at 9 and 35 GHz consisted of at least seven independent patterns; all of predominantly axial symmetry and all ascribed to Gd³⁺. The two most intense patterns have been analyzed in detail, and while one was found to be axially symmetric the other, though predominantly axial, had a small rhombic crystal field term. The ZFR spectrum, measured between 1 and 18 GHz, consisted of at least two patterns of zero field transitions. Both EPR and ZFR spectra were found to be insensitive to variations in LiNbO₃ stoichiometry, c- of a-axis crystal growth direction, and various sample treatments.     

Design of a Quasi-Optical System Converting the TE06 Output Mode of a Gyrotron into a Gaussian-Like Beam

Gaussian optics can be used to design a quasi-optical system converting the TE₀₆ mode output (f=388 GHz) of a submillimeter wave gyrotron into a well-collimated, linearly-polarized free-space beam with a circular cross-section. A quasi-optical antenna produces a main beam with an elliptical cross-section, which is then converted by two mirrors into a well-collimated beam with a circular cross-section.     

Influence of the lens effect in a sample with large dielectric constant in a loop-gap resonator on the EPR signal intensity at 700 MHz

The influence of the lens effect on the electron paramagnetic resonance (EPR) signal intensity was investigated in a loop-gap resonator (LGR) with an inner diameter of 41 mm. TheQ- value and EPR signal intensity were measured when the phantoms containing 3-carbamoyl-2,2,5,5-tetramethyl-pyrrolidin-l-yloxy dissolved in sodium chloride aqueous solutions were put in the LGR. TheQ- value and signal intensity reduced with increasing concentrations of sodium chloride in the phantom, indicating that the imaginary part of the dielectric constant is larger in the phantom with the higher concentration of sodium chloride. However, relationships betweenQ-values of the resonator and EPR signal intensities were not proportional and signal intensities were relatively higher compared with theQ-values. These findings suggest that the signal reduction due to lowQ is slightly compensated by the lens effect in the sample with the large real part of the dielectric constant. In the distribution of the signal intensities of a pinpoint sample made of diphenylpicrylhydrazyl in the agar medium containing sodium chloride in the LGR, it was found that the signal intensity decreased according to the distance from the center and the difference in the signal intensity within 10 mm from the center was about 20%, indicating the inhomogeneity of the alternating magnetic field at the center and marginal region in the sample with the large dielectric constant caused by the lens effect.     

A Mode Converter with Multi-waveguide Output from the TE01 Circular Waveguide Mode to the TE10 Rectangular Mode for Millimeter Wave Gyro-device Applications

A mode converter with multi-waveguide output for millimeter wave gyro-device applications is proposed in this article, which is used to convert the TE₀₁ circular waveguide mode into the TE₁₀ rectangular waveguide mode. Computer simulations with Ansoft HFSS code show that the energy transmission coefficient of larger than 95.96% may be reached in the frequency range from 34.094 GHz to 35.8 GHz (a bandwidth of 1.706 GHz) at a VSWR of lower than 1.5 for the input operating mode.     

8mm TE13 Mode Gyrotron

The design of a 8mm TE ₁₃ mode gyrotron is given in this paper. Discussions about the selection of the dimensions of the RF structure and electron gun are presented. Calculated results of the operating parameters of the gyrotron are also given. At 37.5GHz, pulse power 56kW is obtained for the gyrotron manufactured according to our design. Mode pattern obtained by scorching method shows that the mode of the output millimeter wave is TE ₁₃.     

高功率速调管TE_(11)-HE_(11)模式变换器的设计及实验验证

基于模式匹配法编制了分析内壁刻槽TE11-HE11圆波导模式变换器数值计算程序;采用该程序为工作频率为30.5GHz的高功率速调管设计了一半径为16mm的TE11-HE11模式变换器。计算表明该变换器在2.6%的带宽内转换效率在98.8%以上,实验结果表明该变换器性能良好。     

Antiferroelectricity in TID2PO4

Polarization versus electric field double hysteresis loops along the b-axis of TID₂PO₄ were observed. From this fact and the permittivity versus temperature characteristic, the low temperature phase of TID₂PO₄ was confirmed to be antiferroelectric.     

Design of a TE42,7 coaxial cavity for a 1 MW, 280 GHz gyrotron

To provide the required mode selectivity for a 1 MW, 280 GHz gyrotron, a coaxial resonator operating in the high order TE_42,7 mode is considered. Mode discrimination is achieved both by exploring selective ohmic effects and examining the electrodynamic properties of the coaxial cylindrical waveguide. Analytic results indicate the effectiveness of a coaxial insert with a low electrical conductivity in rarefying the mode density of overmoded, megawatt hollow cavities.     

Intermodal interference of LP0j modes in optical fiber with liquid core

We present intermodal interference of modes LP_0j in silica optical fiber with liquid core. An appropriate detection method is proposed for observation of intermodal interference of LP_0j modes. Measured spectral dependencies reveal a clear local minimum, where position is a function of optical fiber parameters. We focused on investigation of dependence of that minimum position on length of optical fiber with liquid core. From measured spectral dependencies the evident blue shift of local minimum is observed as length of optical fiber increases. Character of spectral characteristics correspond with intermodal interference of modes LP_0j and is demonstrated by numerical calculations. Presented intermodal interference in combination with liquid-filled core of optical fiber can be attractive for sensing applications.