Comparison between experiment and computer modeling for simple MILOconfigurations

The MILO is a crossed-field HPM, high power microwave source which uses its self-generated magnetic field to cut off electron flow to the anode. A detailed comparison of experimental results and a computer simulation has been made for a number of simple axisymmetric MILO structures designed to operate at 1 GHz. The structures were built from demountable components which enabled the number of cavities and their dimensions to be rapidly altered. Measurements were made of the fluctuating magnetic fields at the end of each cavity. The amplitude and depth of RF modulation of the magnetic fields, although repeatable, changed drastically from one configuration to the next; these parameters were compared with predictions from VIPER, a 2-D electromagnetic PIC code. Good quantitative agreement was obtained between experiments and the simulation in most situations, although, late in the current pulse, after about 100 ns, the level of RF began to decay; a phenomenon which became more pronounced as the applied voltage was increased. The decay was attributed to plasma formation on the cavity vanes and subsequent electron emission; this explanation was verified by computer modeling electron emission and by using vanes make from polished stainless steel in place of aluminum vanes     

三维射频加速腔程序包MAFIA的开发和应用

介绍了三维射频加速器计算机辅助设计(CAD)程序包MAFIA的特点、功能及其应用。用CAD的方法实现了射频加速器特别是非轴对称带耦合腔的加速器的三维腔形设计。同时介绍了节省内存、节约机时的模型方法,使得在中小型机器上运行大程序(八万条)成为可能。文章主要对L波段的加速腔进行了数值模拟,并和实验进行了比较,结果符合较好;通过三维数值模拟及图形模拟的方法比较了各种形状谐振腔的性能;计算分析了输入耦合腔及高阶模耦合器对加速腔电磁场分布等参量的影响;提出了加耦合器后谐振频率改变的补偿方法。     

Single-stage depressed collectors for gyrotrons

Two 140 GHz gyrotrons with a single-step depressed collector have been operated. The different position of the isolating collector gap in the stray magnetic field causes the electron motion in the retarding region to be in one case adiabatic and in the other case nonadiabatic. The kind of motion within the retarding field influences strongly the behavior of the gyrotron with a depressed collector. In the case of nonadiabatic motion a significant amount of transverse momentum is given to the electrons reflected at the collector potential. This causes the reflected electrons to be trapped between the magnetic mirror and the collector. The electrons escape from the trap by diffusion across the magnetic field to the body of the tube thus contributing to the body current. Despite the high body current there is no observable influence of the collector voltage on the RF output power. In the case of adiabatic motion the reflected electrons do not gain a sufficient amount of transverse momentum to be trapped by the magnetic mirror. They pass the cavity toward the gun and they are trapped between the negative gun potential and the collector. The interaction with the RF field by electrons traveling through the cavity enhances the diffusion in the velocity space thus enabling the trapped electrons to overcome the potential barrier and escape toward the collector. Therefore the body current stays at low values since in this case the reflected electrons do not contribute to it. However, at higher collector voltages a reduction of RF power occurred and some noise in the electron beam was observed     

The symmetry,inferable from Bogoliubov transformation,between processes induced by a mirror in two-dimensional and a charge in four-dimensional space-time

We consider the symmetry between creation of pairs of massless bosons or fermions by an accelerated mirror in (1+1)-dimensional space and emission of single photons or scalar quanta by an electric or scalar charge in (3+1)-dimensional space. The relation of Bogoliubov coefficients describing the processes generated by a mirror to Fourier components of the current or charge density implies that the spin of any disturbances bilinear in the scalar or spinor field coincides with the spin of quanta emitted by the electric or scalar charge. The mass and invariant momentum transfer of these disturbances are essential for the relation of Bogoliubov coefficients to invariant singular solutions and the Green functions of wave equations for both (1+1)-and (3+1)-dimensional spaces, and especially for the integral relations between these solutions. One of these relations leads to the coincidence of the self-action changes and vacuum-vacuum amplitudes for an accelerated mirror in two-dimensional space-time and a charge in four-dimensional space-time. Both invariants of the Lorentz group, spin and mass, play an essential role in the established symmetry. The symmetry embraces not only the processes of real quanta radiation, but also the processes of the mirror and charge interactions with fields carrying spacelike momenta. These fields accompany their sources and determine the Bogoliubov matrix coefficients α _ω′ω ^{B, F} . It is shown that the Lorentz-invariant traces ±trα^B,F describe the vector and scalar interactions of the accelerated mirror with a uniformly moving detector. This interpretation rests essentially on the relation between propagators of the waves with spacelike momenta in two-and four-dimensional spaces. The traces ±trα^{B, F} coincide with the products of the mass shift Δm_{1, 0} of the accelerated electric or scalar charge and the proper time of the shift formation. The symmetry fixes the value of the bare fine structure constant α₀=1/4π.     

用于HIRFL-CSRm纵向压缩高频腔的磁合金环测试

在HIRFL-CSRm上进行高能量密度物理和等离子体物理的研究需要一个能提供高电压的高频腔来对束团进行纵向压缩, 而磁合金加载腔获得较高的加速场梯度可以满足这一要求, 且腔体不需要调谐, 从而简化了高频控制系统。为了选择磁导率和阻抗较大、Q值(品质因数)小于1的磁合金材料来加载压缩高频腔, 对型号为V和A的两种磁合金材料进行了测试。测试结果表明: 型号为V的材料具有较大的磁导率、 阻抗和小于1的Q值, 将被用来加载腔体以获得足够高的加速电场梯度。A high voltage RF cavity is required to compress the beam bunch in the longitudinal for high energy density physics and plasma physics research at HIRFL CSRm. A magnetic alloy (MA) loaded cavity which has high acceleration gradient and without tuning loop (can simplify the RF control system), can meet the requirement. In order to select a proper MA material with higher permeability and shunt impe dance, Q< 1 value to load the RF compression cavity, two different MA materials V and A have been tes ted. The results indicate that the material V will be better for the construction of the RF cavity than material A because of its higher permeability and shunt impedance, Q< 1 value.     

A symmetry relating certain processes in 2-and 4-dimensional space-times and the value α0 = 1/4π of the bare fine structure constant

The symmetry manifests itself in exact relations between the Bogoliubov coefficients for processes induced by an accelerated point mirror in 1 + 1 dimensional space and the current (charge) densities for the processes caused by an accelerated point charge in 3 + 1 dimensional space. The spectra of pairs of Bose (Fermi) massless quanta emitted by the mirror coincide with the spectra of photons (scalar quanta) emitted by the electric (scalar) charge up to the factor e ²/ħc. The integral relation between the propagator of a pair of oppositely directed massless particles in 1 + 1 dimensional space and the propagator of a single particle in 3 + 1 dimensional space leads to the equality of the vacuum-vacuum amplitudes for the charge and the mirror if the mean number of created particles is small and the charge e = √ħc. Due to the symmetry, the mass shifts of electric and scalar charges (the sources of Bose fields with spin 1 and 0 in 3 + 1 dimensional space) for the trajectories with a subluminal relative velocity β₁₂ of the ends and the maximum proper acceleration w ₀ are expressed in terms of the heat capacity (or energy) spectral densities of Bose and Fermi gases of massless particles with the temperature w ₀/2π in 1 + 1 dimensional space. Thus, the acceleration excites 1-dimensional oscillation in the proper field of a charge, and the energy of oscillation is partly deexcited in the form of real quanta and partly remains in the field. As a result, the mass shift of an accelerated electric charge is nonzero and negative, while that of a scalar charge is zero. The symmetry is extended to the mirror and charge interactions with the fields carrying spacelike momenta and defining the Bogoliubov coefficients α^B,F. The traces trα^B,F, which describe the vector and scalar interactions of the accelerated mirror with a uniformly moving detector, were found in analytic form for two mirror trajectories with subluminal velocities of the ends. The symmetry predicts one and the same value e ₀ = √ħc for the electric and scalar charges in 3 + 1 dimensional space. Arguments are adduced in favor of the conclusion that this value and the corresponding value α₀ = 1/4π of the fine structure constant are the bare, nonrenormalized values. The text was submitted by the author in English.     

Fusion reactor start-up by RF cavity mode heating of a gas

The application of high power microwaves to a gas-filled resonant cavity may be an ideal way to start up Tokamak, mirror or other thermonuclear reactors. RF coupling problems are avoided by input of microwave power prior to wave cut-off.     

Electric field measurements and computational modeling at ultrahigh-field MRI

While magnetic resonance images essentially contain a map of the both circularly polarized components of the RF transverse magnetic fields (B(1) field), the thermal heat and electromagnetic power deposition is generated by the associated electric fields. Measurement of electric field distributions/intensities across a sample yields an indirect indication of possible cause of heating within the sample and potentially enables the detection of "hot spots," which can be present within inhomogeneous radiofrequency (RF) fields, such as the case with magnetic resonance imaging at high field strength. As a result, establishing a valid technique for direct measurements of the electric field and its correlation, obtained using computational electromagnetics, is essential in assessing (1) the safety of the RF coil designs and (2) the validity of the calculations. In this work, a probe was built and used to measure the transverse electric field (E(1) field) distributions within an empty 8 T (tuned to 340 MHz) RF head coil and within a saline water phantom loaded in the same coil. The measured E(1) field distributions were favorably compared to the distributions obtained utilizing a finite difference time domain in-house package.     

A Resonant-Cavity ICRF Coupler for Large Tokamaks

A new resonant cavity ICRF coupler is proposed for large tokamaks. The design features a novel resonant cavity, an RF magneticfield orientation that effectively radiates fast Alfvén waves, matching to 40-? transmission lines, and an electric-field orientation so that the strongest RF electric fields are orthogonal to the main toroidal magnetic field, thereby benefiting from magnetic insulation. As a result, the power handling capability is excellent. For the case of the "Big-Dee" Doublet III tokamak, a single 35 × 50-cm coupler can launch 20 mW of fast Alfvén waves. Extrapolation to fusion reactor parameters is straightforward.     

Fabrication of a fiber-based microcavity with spherical concave fiber tips

A straightforward method for manufacturing concave fiber ends with optical quality surfaces is presented. This method allows for the indention of bare cleaved fiber ends of single-mode Bi₂O₃ fibers by way of conventional heating and subsequent contact with ultra-smooth sinter metal spheres. The resulting high quality surfaces show promise for microcavity experiments. Results for a simple plano-concave cavity composed of a single indented fiber and a plane mirror are presented.     

Possible high-frequency cavity and waveguide applications of high temperature superconductors

Conventional superconducting materials used at liquid helium temperatures have been employed to greatly reduce ohmic power loss in microwave cavities. If suitable high temperature superconductor surfaces can be developed, it could be possible to alleviate constraints due to relatively low energy gaps and limits due to critical fields, and operate at more convenient temperatures with larger thermal margins. These features could be used to improve performance of present superconducting microwave cavity devices. They may also facilitate new applications and device designs at millimeter-wave and far infrared frequencies. Possible cavity and waveguide applications are described. Sensitivity of RF surface resistance to boundaries and nonaligned grains in the material and to small amounts of nonsuperconducting material may be an obstacle to development. Moreover, critical RF fields can be limiting for high power applications. An additional problem is the adverse effect of D.C. magnetic fields.Supported by Department of Energy No. ET-51013-227. Reproduction, translation, publication, use and disposal in whole or in part, by or for the United States government is permitted.     

A 35-GHz low-voltage third-harmonic gyrotron with a permanent magnet system

A systematic theoretical and experimental study on a 35-GHz 45-kV third-harmonic gyrotron with a permanent magnet system is presented in this paper. A complex cavity with gradual transition and a diode magnetron injection gun (MIG) are employed in the gyrotron. A self-consistent field nonlinear theoretical investigation and numerical simulation for electron beam interaction with RF fields are given. The diode MIG is simulated numerically utilizing our code in detail. The permanent magnet system provided the maximum axial magnetic field of about 4.5 kG in the cavity region of the gyrotron. The Ka band third-harmonic complex cavity gyrotron with a permanent magnet system has been designed, constructed, and tested. A pulse output power of 147.3 kW was obtained at a beam voltage of 45 kV with beam current of 32.2 A, corresponding to an efficiency of 10.2%.     

Toroidal current generation by lower hybrid waves in a WT-1 device

A toroidal current driven by a lower hybrid wave (LHW) is obtained by applying RF power to a toroidal plasma produced by electron cyclotron resonance. The direction as well as the magnitude of the current depend on the supplementary vertical and horizontal magnetic fields, which is explained by confinement of current-carrying electrons in a toroidal device.     

A simple method for automatic cavity alignment of a solid-state laser

Misalignment of a laser cavity would result in the displacement of beam location on the cavity mirrors. We present a simple method for automatic cavity alignment of a solid-state laser through detecting the location of the laser beam on the rear mirror, with which neither external alignment beams nor modifications to the cavity are required. The rear mirror of the laser cavity is replaced by a tip/tilt mirror to compensate for the misalignment. Experiments show that this method could effectively correct the misalignment of the laser cavity and restore the output power to almost the original value when the resonator is well aligned.     

Towards a complete coil array

A complete RF coil system, as has been previously defined, is capable of generating any steady-state RF field, at the MR frequency, that is compatible with Maxwell's equations. A coil system is complete if it is capable of generating all basis vector fields in the multipole expansion of the electromagnetic fields. A complete coil system has the potential to reach the ultimate intrinsic signal-to-noise as an MRI receiver coil. It also offers maximum flexibility in tailoring the spatial RF field distribution as an excitation coil. Here, computer simulations have been performed on array coils employing composite coil elements, assuming the current loops are small and can be approximated by magnetic dipoles. We demonstrate that a coil array can be configured to approximate a truncated complete array coil and to generate the basis magnetic vector fields up to certain orders in the multipole expansion of the electromagnetic fields.     

激光陀螺本征模偏振态与磁敏感特性的理论研究

环境磁场的干扰是影响激光陀螺精度的重要因素之一. 为了减小二频机械抖动激光陀螺的磁敏感性, 推导了环形腔的琼斯矩阵, 其中考虑了非共面角、增益介质、腔损耗、腔镜反射各向异性和应力双折射, 采用矩阵本征问题求解的摄动理论分析了环形腔的偏振态和磁敏感特性. 研究表明, 为了减小激光陀螺的磁敏感性, 应减小非共面角和腔损耗, 增大腔镜的反射各向异性. 腔镜应力双折射究竟会增大还是减小磁敏感性与应力作用主轴、受应力腔镜的位置有关, 此外它还会引起顺时针和逆时针模式之间的偏振非互易性. 磁敏感性与腔失谐近似成线性关系, 导致激光陀螺工作于增益峰值时磁敏感性并非最小. 这些结果对减小二频机械抖动激光陀螺的磁敏感性具有较好的指导意义. 关键词： 激光陀螺 磁敏感性 椭圆度 琼斯矩阵     

A high-finesse fiber optic Fabry–Perot interferometer based magnetic-field sensor

A high-finesse extrinsic Fabry–Perot interferometric sensor for the measurement of weak dc magnetic fields is demonstrated. The Fabry–Perot cavity is formed by aligning the fiber end-face and the TbDyFe rod end-face, and each end-face is coated by a mirror with a micro-lens. The length of the TbDyFe rod is changed by the variation of an applied dc magnetic field, leading a change of the Fabry–Perot cavity length. By interrogating the white-light interferometric spectrum, the wavelength of the resonant peak is tracked and the length of the Fabry–Perot cavity is obtained. The sensor exhibits a high sensitivity of 1510 nm/mT with a magnetic resolution of 25 nT.     

Study of RF Excited CO 2 Waveguide Laser Array

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Cumulative interaction in the deflection cavities of a high-powermagnicon amplifier

A major factor limiting the output power of a high-power magnicon is RF breakdown in its penultimate cavity. Utilization of the principal of angle summing allows us to achieve cumulative interaction between a beam and RF fields of the magnicon deflection system, and thus to decrease the maximum electric field in the penultimate cavity. In this paper, the problems of implementing a cumulative interaction in a magnicon deflecting system are discussed. A deflecting system with coupled cavities working according to the principal of angle summing, which has been tested with success experimentally, has a limitation caused by instabilities that can limit pulse duration at a power level higher than a few tens of milliwatts. The method described employs a set of uncoupled penultimate cavities to obtain cumulative interaction at high power without sacrifice of pulsewidth     

A coaxial HOM coupler for a superconducting RF cavity and its low-power measurement results

A resonant buildup of beam-induced fields in a superconducting radio frequency (RF) cavity may make a beam unstable or a superconducting RF cavity quench. Higher-order mode (HOM) couplers are used for damping higher-order modes to avoid such a resonant buildup. A coaxial HOM coupler based on the TTF (TESLA Test Facility) HOM coupler has been designed for the superconducting RF cavities at the Proton Engineering Frontier Project (PEFP) in order to overcome notch frequency shift and feed-through tip melting issues. In order to confirm the HOM coupler design and finalize its structural dimensions, two prototype HOM couplers have been fabricated and tested. Low-power testing and measurement of the HOM couplers has shown that the HOM coupler has good filter properties and can fully meet the damping requirements of the PEFP low-beta superconducting RF linac.