Development of Superconducting Coaxial Cables for Cryogenic Detectors

Fast readout with low noise is essential to apply superconducting detectors to such experiments as time-of-flight mass spectrometry or X-ray spectroscopy, where high counting efficiency is required. We have developed a thin seamless superconducting coaxial cable employing NbTi alloy in both of the outer and inner electrical conductors. The outer diameter is 1.60 mm, and both conductors are separated by dielectric material of PTFE. The coaxial cable revealed to become superconductive below about 10 K. The thermal conductance was measured between 0.4 and 6 K and consistent with literature. Performance at high frequency was also measured at 4.2 K. The attenuation was very small and less than 1 dB up to about 7 GHz. The effect of mechanical treatment to thermal and electrical properties of NbTi alloy seems to be small in the present forming process of coaxial cable.      

Miniature coaxial resonator and related bandpass filter with ultra-wide stopband

Resonator of a new type is described, which is formed by two coaxial conductors applied onto the internal and external surfaces of a ceramic tube arranged in a metal case. An equation for the resonant frequencies is derived. In comparison to a standard coaxial quarter-wave resonator, the proposed device has a shorter length and manifold increased ratio of frequencies of the second resonance to the first resonance. The new resonator can be used for the creation of bandpass filters with ultra-wide stopband. The frequency response of a working prototype four-resonator filter is presented, in which the stopband at a −90 dB level extends up to a frequency that is 47 times greater than the central frequency of the passband.     

A New High-Frequency Current Standard

A short-circuited-ring electrodynamic ammeter is described. The short-circuited ring is supported by a fine quartz fiber and is suspended midway between the inner and outer conductors of a coaxial transmission line. The current on the line is found by measuring the torque exerted against the ring by the current on the coaxial line. A new technique is given for determining the relationship between the torque on the ring and the current on the transmission line, and is compared with the technique used in the past. The new method is an application of the resonator action theorem used by Cullen for calibrating a torque-operated microwave wattmeter. The major weakness in the previous evaluations of the torque-current relation is discussed. The ammeter is useful for measuring current from 1 to 100 amperes over the frequency range 1 MHz to 1 GHz, with an uncertainty of the order of 0.5 percent.     

SE同轴测试装置中阶梯电容的研究与优化

在屏蔽材料的屏蔽效能(shielding effectiveness,SE)测试装置中,通过对锥形连接器内导体进行微小移位,优化固定同轴装置的支撑介质的内、外径比值,并在支撑介质中挖去一定厚度的环形沟道构成小电感等技术对阶梯电容进行补偿,减小由截面不连续引起的回波损耗,设计了频率范围为10 MHz至3 GHz的SE同轴测试装置.利用软件仿真验证了表面电阻为5Ω的屏蔽材料在该装置中测试有稳定的SE值,证明该装置不仅改善了基于ASTM D4935测试装置频率范围窄的不足,同时具备良好的阻抗匹配特性.     

Development of superconducting niobium accelerating structures for heavy ions

Results of tests of two new designs for superconducting niobium resonant cavities are presented. Both types resonate at 145.5 MHz and accelerate most efficiently for particle velocities β= v/c = 0.16. One resonator is of the split-ring type, but of a simpler design than a previously reported β=.16 unit. Although the surface fields are higher, the performance is somewhat better than for the earlier design: an accelerating field Ea= 4.3 MV/m has been obtained at 4.2K with 4w of rf input, where Eais defined as the energy gain per unit charge for a synchronous particle averaged over the interior resonator length. The other resonator is an 8-inch OD tapered coaxial quarter-wave line terminated with a drift tube of 1.50 inch aperture. At 4.2K, this resonator exhibits a low-level Q of 2×10⁹, and has achieved Ea= 4.7 MV/m with 2.8w of rf input.     

Design and Construction of a Helical Resonator for Delivering Radio Frequency to an Ion Trap

Supplying high voltage radio frequency (RF) is a critical part of ion trapping system due to impedance mismatching between RF source and the ion trap. A helical resonator has been constructed in order to deliver narrow bandwidth and high voltage RF to the ion trap for stable confinement of ions. The performances of the helical resonator have been studied for different capacitive load of the ion trap. Both the resonant frequency and quality factor of the resonator show strong dependence on external capacitive loads.     

Misalignment of Coaxial Conductors

Compressed gas insulated coaxial transmission lines are being developed which withstand 1200-kV rms phase-to-phase voltage and carry 3000 A continuous current. As a method of instrumenting the voltage on a single-phase of coaxial line, an isolated capacitive section approximately one meter in length is inserted between the conductors. The resulting displacement current due to line voltage is detected with low voltage electronics. In the present work, misalignment of the coaxial conductors is investigated theoretically and experimentally to determine the effect of small relative displacements on the amount of capacitance of the isolated section. Capacitance change due only to conductor misalignment is found not to restrict severely the metering accuracy of the isolated section, provided reasonable tolerances for the position of the isolated section are maintained. Forces due to imbalance of surface charge when the conductors are slightly misaligned are calculated and shown to be small, suggesting that a "voltage coefficient" of capacitance also should not severely restrict the metering accuracy of the isolated section.     

High Q-factor distributed bragg reflector resonators with reflectors of arbitrary thickness

The Bragg reflection technique improves the Q-factor of a resonator by reducing conductor and dielectric losses. This is achieved by designing a low-loss inner resonant region (usually free space) surrounded by an outer anti-resonant region made of distributed Bragg reflector layers. In this paper we develop a simple non-Maxwellian model and apply it to design three distinct cylindrical Bragg resonators based on the same set of single-crystal sapphire plates and rings by changing only the dimension of the cavity that supports the structure. To accomplish this, the simple model allows an arbitrary thickness for either the horizontal or the cylindrical dielectric reflectors by relaxing the condition that they must be lambda/4 thick. The model also allows for higher-order field variations in both the resonant and the anti-resonant regions. The resonators were constructed and experimental results were compared with the simple model and the rigorous method of lines analysis. For the fundamental mode, an unloaded Q-factor of 234,000 at 9.7 GHz was obtained. This is larger than that for a whispering gallery mode resonator. The resonator also exhibited a greatly reduced spurious mode density when compared to an overmoded whispering gallery mode resonator.     

基于SOI-MEMS技术的静电驱动-电容检测硅谐振器

提出一种基于SOI-MEMS技术的静电驱动-电容敏感检测的横向硅谐振器,对其进行设计、MEMS工艺加工制作实现、微弱电容检测及开环测试．该新型静电激励谐振器结构主要包含一个从中间受力点向两侧引出两个电极板的双端固支梁,这种设计使得此谐振器的静电驱动电压远小于具有相同电极板面积和极板间距的同类静电驱动谐振器,且检测电容更大,降低了检测难度．以器件层电阻率很低（0．001～0．002Ω·bcm）的SOI晶圆为基础材料,其SOI—MEMS加工工艺流程简单,仅需要2块掩膜版,有4个主要单步工艺．实验测试结果表明：在真空度为0．1—1．0Pa环境下,直流偏置电压低至30V,交流驱动电压峰-峰值为20mV时,该谐振器在其谐振频率点52261．99Hz处的Q值依然高于11800．     

Equivalent circuit for the microstrip ring resonator suitable for broadband materials characterisation

A simple equivalent circuit of the edge coupled microstrip ring resonator is developed based on both circuit and electromagnetic theory. The new model extends the work done by previous authors by including the effects of radiation loss, the coupling gap and the feed network as well as extending the frequency range by including higher-order modes. The model accurately predicts the resonant frequencies of the ring including the effects of dispersion and thick conductors. The equivalent circuit allows measurement of the resonance frequencies and the Q factors to be made independently of the coupling gap dimensions, which traditionally have been difficult to accurately model. A method of determining radiation loss is also presented, which has often been incorrectly neglected in the past. Experimental results support the accuracy of the equations and measurements on alumina rings that demonstrate a frequency accuracy of better than 1% over 12 resonant modes in the frequency range 3-33 GHz.     

Scattering Parameters Representing Imperfections in Precision Coaxial Air Lines

Scattering parameter expressions are developed for the principal mode of a coaxial air line. The model allows for skin-effect loss and dimensional variations in the inner and outer conductors. Small deviations from conductor circular cross sections are conformally mapped by the Bergman kernel technique. Numerical results are illustrated for a 7 mm air line. An error analysis reveals that the accuracy of the scattering parameters is limited primarily by the conductor radii measurement precision.     

Low Temperature Properties of a Superconducting Niobium Coaxial Cable

Semirigid coaxial cables with seamless metal shields are promising for readout from sensitive devices operating below liquid helium temperature. Low thermal conduction of such cables are also essential to reduce heat penetration into cryogenic temperature. We have developed thin semirigid coaxial cables employing niobium-titanium and niobium in both center and outer conductors, taking advantage of low thermal conductivity and extreme small electrical resistivity of superconductors. We assembled an adiabatic demagnetization refrigerator and measured thermal and electrical characteristics of those superconducting coaxial cables below T _c. Thin niobium coaxial cable with an outer diameter of 0.86 mm showed two-orders lower thermal conduction than expected, which is considered as the effect of impurity of niobium and forming process. Small attenuation was observed up to high frequency above 10 GHz at 3 K.     

A High Accuracy Self-Calibrating Bridge with Coupled Inductive Ratio Arms Used for Standard Inductors Comparison

A bridge with inductively coupled ratio arms based on a seven decade inductive voltage divider is described. It is used for mutual comparisons between the standard inductors which form the IEN reference bank of inductance. Provided some auxiliary balances are made, this bridge is able to measure its own residual inductances, both the inductances of the standard-to-bridge connections (about 2 ?H), made constant by the rigidity of the system geometry within a few nanohenries, and those of the variable resistance elements, also known within a few nanohenries. The self-calibrations are sufficiently accurate because the bridge has a high sensitivity even in comparisons of 1-?H inductors and in the use of ratios up to 1:100. Three variable resistance components are used to balance the bridge: a commercial box of 1-? steps, with calibrated inductance increments; a two-material helicoidal-motion wire resistor (0 to 1 ?), suitably designed with inductance variations less than ±1 nH; and a constant-geometry three-decade Kelvin-Varley divider, also designed for this specific purpose, with its total and partial inductance measured, and its total inductance kept constant within 1 nH. Where possible, connections between bridge components are realized by rigid tetra-axial conductors (4 coaxial tubular conductors). To avoid inductive coupling between the standards and the third tubular conductor, it is made of coated wires aligned along the generatrices of a cylinder. Provided the phase errors of the inductive voltage divider are known only within 1 ?rad, the bridge has an estimated ratio accuracy of ±[2.     

High performance distributed Bragg reflector microwave resonator

A new resonator device structure is described that achieves Q-factors well above those currently realizable for conventional room temperature microwave structures. The new structure consists of a microwave cavity, for which the enclosure walls consist of distributed Bragg reflectors (DBR) made of low-loss sapphire. For this configuration, most of the resonant field resides in empty space, with small field strengths in the thin layers of sapphire which comprise the DBR structure. The physical structure takes the form of interpenetrating concentric rings and plates of low-loss sapphire contained in a cylindrical metal enclosure. The theoretical analysis of the DBR resonant structure allows the positions and dimensions of the component rings and plates to be precisely determined for a specified resonant frequency. The resonator Q can be accurately calculated, and plots of the resonant fields clearly show the physical mechanism leading to the observed efficiency of this resonator structure. Experimental results are given for resonators designed at 9.0 and 13.2 GHz. The measured unloaded Q's at room temperature are over 650000 and 450000, respectively.     

A capacitance-graded cryogenic high voltage bushing for vertical or horizontal mounting

The cryogenic high voltage bushing for the terminations of the 138 kV, 4100 A superconducting cable prototype at the Brookhaven National Laboratory (BNL) is described. The bushing is energized with the line-to-ground voltage between the coaxial centre and outer surrounding conductors, in the axial direction there is a temperature difference from ambient to about 6 K.The capacitively graded bushing can be mounted in any position. It is vacuum tight and withstands pressures of more than 20 bar. The heat leak of the insulation body in axial direction is only 8.5 W.The electrical specifications of the bushing are: 825 kV, 1.2 × 50 μs impulse voltage; 240 kV rms ac 2h test voltage: and > 160 kVs partial discharge inception voltage.This bushing concept may easily be modified to meet most conceivable thermal, mechanical and electrical requirements of cryogenic high voltage engineering.     

Planar ferrite-piezoelectric composite microwave resonator with electric and magnetic frequency tuning

A planar composite microwave resonator based on a bilayer structure comprising a single-crystal yttrium iron garnet (YIG) ferrite film on a lead magnesium niobate-lead titanate (PMN-PT) piezoelectric plate has been studied. The possibility of electric and magnetic tuning of the resonance frequency in this structure is demonstrated. The magnetic tuning in the frequency range of 2–18 GHz is effected by varying the magnetic bias field and the electric tuning within a 20 MHz bandwidth is achieved by applying a dc bias voltage to the piezoelectric layer. An increase in the central frequency of a YIG/PMN-PT composite resonator is accompanied by a growth in the resonator Q from 110 to 990, while the electric tuning bandwidth remains unchanged     

A sensitive beam-bunch phase detector

A sensitive heavy-ion beam-bunch phase detector has been developed by first examining the relationship between the sensitivity of an rf resonant cavity as a particle bunch detector and the shunt impedance of the same cavity as an accelerating structure. Then the various high shunt impedance rf cavities previously developed for accelerating heavy ions were evaluated for use as bunch detectors. A spiral-loaded geometry was chosen, built, and tested with beam. The sensitivity obtained, 14 μV per electrical nA of beam, is a factor 3 higher than previously reported.     

一种谐振式传感器频率特性测试平台

谐振式传感器的谐振频率和Q值可以通过其频率特性计算得到,因此需要有一种通用的测试平台来测量各种不同谐振式传感器的频率特性.本文描述了一种谐振式传感器频率特性测试平台,并提出了一种采用间歇激励方法测试谐振式传感器谐振频率的新方法,采用线性调频信号激励谐振式传感器,传感器自由振动状态下的振动频率即是传感器的谐振频率;测得传感器自由振动状态下的振动频率,即可得到谐振式传感器的谐振频率.     

Measurements of High Voltages by Means of Electron Speed Filtering

A cavity resonator is used as an electron speed filtering system. Discrete resonance values of the accelerating voltage can be calculated analytically. The device can, therefore, be used to build up a high-voltage scale with low uncertainty. Experimental investigations have shown good agreement with theory. Five current peaks generated by electrons which passed the resonator could be detected in the voltage range between 40 and 100 kV. They indicated a balanced condition of the electron speed filter, at which the accelerating voltages are calculable. These discrete voltages are nonequidistant, thus the measurement is unambiguous. Further developments are necessary and envisaged in order to considerably reduce the uncertainty. The goal is a measuring device using a superconducting niobium resonator which can be used to realize an absolute voltage scale up to 100-kV dc with a relative uncertainty of better than ±10-5.     

A 25.5 MHz double-coaxial λ/4-resonator as a rebuncher in heavy ion linac system

A 25.5 MHz double-coaxial λ/4-resonator has been constructed to be used as a rebuncher between a 25.5 MHz RFQ linac and a 51 MHz interdigital-H linac for the acceleration of short-lived nuclei. By employing a double coaxial structure, the resonator length is only 130 cm; the length of the natural λ/4-resonator is 294 cm. The resonator, 69 cm in inner diameter, has six acceleration gaps. The bare shunt impedance is 40.6 MΩ/m. The ions with a charge-to-mass ratio 1/10 can be rebunched by feeding an rf power of 1.4 kW.