PREPARATION OF a-AXIS ORIENTED YBa2Cu3Ox THIN FILMS

The a-axis oriented YBa₂Cu₃O_x(YBCO) thin films could be grown on (100) SrTiO₃(STO) substrates with STO buffer layers by dc and rf magnetron sputtering either by low-ering the deposition temperature, or by using a self-template technique. For the latter, the resistivity of the thin film at 290K along the substrate [001] direction is about four times larger than that in the [010] direction. The zero resistance temperatures T_c0 are 89 K in both directions. So high quality a-axis oriented YBCO thin films can be prepared by the self-template technique. Also the T_c0 increase monotonously with the reduction of the thickness of the YBCO seed layer.     

射频激发热丝化学气相沉积制备硅薄膜过程中光发射谱的研究

采用射频(rf)激发,在热丝化学气相沉积(HWCVD)制备微晶硅薄膜的过程中产生发光基元,测量了rf激发HWCVD (rf-HWCVD)的光发射谱,比较了相同工艺条件下rf-HWCVD和等离子体增强CVD(PECVD)的光发射谱,分析了rf功率、热丝温度和沉积气压对rf-HWCVD光发射谱的影响.结果表明,在射频功率<0.1W/cm²时,rf-HWCVD发射光谱反映了HWCVD高的气体分解效率和高浓度原子氢的特点,能够解释气压变化与微晶硅薄膜微结构的关系,是研究HWCVD气相过程的有 关键词： HWCVD OES 微晶硅     

First Beam Measurements of the S-Band photocathode Radio-Frequency Gun at Tsinghua University

During the last decades, photocathode rf gun has been proven to be successful in generating the high brightness electron beam （-lnC,-lπ mmmrad,-l ps） which is required by the ILC, XFEL, Thomson scattering x-ra y source, etc. A photocathode rf gun system is built to develop electron source for the Thomson scattering x-ray source at Accelerator Laboratory of Tsinghua University. The system consists of a BNL/ATF-type 1.6 cell S-band rf cavity, a solenoid for emittance compensation, a laser system and some simple equipments for beam diagnosis. The first beam measurements of the photocathode rf gun, including the dark current, transverse beam profile, charge and quantum efficiency, are reported.     

Fast Growth of Polycrystalline Film in SiCl4／H2 Plasma

We report the discovery, of fast growth of polycrystalline silicon films under low temperature of 200-300℃ from SiCl4/H2 mixture gases by plasma enhanced chemical vapour deposition technique. The deposition rate strongly depends not only on the rf power and the flow ratio of H2/SiCl4, but also on the substrate temperature, while the crystalline fraction is mainly affected by both the rf power and the flow ratio of H2/SiCl4. The high film-growth rate is due to the enhancement of the gas-phase reaction in SICI4/‘H2 plasma. By gleans of adjusting the matchingrelation between the flow ratio of H2/SiCl4 and rf power, and optimizing the substrate temperasure, we obtain the potycrystalline silicon films deposited at a higher deposition rate over 3.5A/s, with a crystalline fraction of 75% and an average crystallite size of 400-500nm in diameter.     

Study of the beam loading effect in the CSNS/RCS

The China Spallation Neutron Source/Rapid Cycling Synchrotron (CSNS/RCS) accelerates a high-intensity proton beam from 80 MeV to 1.6 GeV. Since the beam current and beam power is high, the beam loading is a severe problem for the stability of the circulating beam in the RCS. To study the beam loading effect in the CSNS/RCS theoretically, the RLC circuit model of the rf cavity, the method of Fast Fourier Transform and the method of Laplace transform have been employed to obtain the impedance of the rf system, the beam spectrum and the beam-induced voltage, respectively. Based on these physical models, the beam dynamics equations have been revised and a beam loading model has been constructed in the simulation code ORIENT. By using the code, the beam loading effect on the rf system of the CSNS/RCS has been investigated. Some simulation results have been obtained and conclusions have been drawn.     

rf-偏置超导隧道结的周期次谐波和混沌现象研究

本文利用全电流超导隧道结电子模拟器研究了dc与rf电流激励超导隧道结 .在固定rf激励频率和阻尾的条件下 ,随着rf激励幅度增加 ,初步观察到了系统的周期、次谐波和混沌解的无穷尽序列现象 .     

On the theory of photocathode rf guns

In this paper we give a set of analytical formulae to describe the characteristics of photocathode rf guns at any rf frequencies, such as energy, energy spread, bunch length, out going current, and emittance etc. as functions of the laser injection phase, which are useful in the design and practical operation of rf guns.     

Influence of Radio-Frequecy Power on Structural and Electrical Properties of Sputtered Hafnium Dioxide Thin Films

Hafnium dioxide （HfO2） thin films are prepared by rf magnetron sputtering. The influences of rf power on the structure, chemical states and electrical properties of the thin films were investigated through x-ray diffraction, x-ray photoelectron spectroscopy, capacitance-voltage and leakage current density-voltage measurement and UV-VIS spectrophotometry. The results show that the HfO2 thin films have a mixed structure of amorphous and polycrystalline phases. With increasing rf power, the crystallinity is enhanced and the crystallite size of the thin films is increased. The oxidation of Hf atoms is improved with increasing rf power for the HfO2 thin films. The flat band shift, oxide charge density and leakage current density of the thin films all decrease as the rf power increases from 50 to 110 W, and then increase as the rf power is increased to 140 W. The band gap energy is smaller for the thin film deposited at 110 W.     

ICRF加热的等离子体反应率计算

本文介绍了ICRF加热的等离子体聚变反应率的计算,并对有关的<σV>-(rf)表达式作了修正。计算结果表明,ICRF等离子体的高能尾部能使反应率增加,有助于把等离子体加热到点火,节省所需要的加热功率。但<σV>_(rf)只在低温、低密度区域增加显著,<σV>_(rf)/<σV>_m随等离子体温度增加而减小,当T=10keV时<σV>_(rf)/<σV>_m大约等于3,当T=20keV时<σV>_(rf)/<σV>_m大约等于1。     

Development of 1.3GHz high-T-c rf SQUID

A new high-T_c (HT_c) rf SQUID working at around 1.3GHz has been developed to avoid electromagnetic interference such as growing mobile communication jamming. This new system works in a frequency range from 1.23 to 1.42GHz (centred at 1.3GHz), which is not occupied by commercial communication. The sensor used in the 1.3GHz rf SQUID is made of a HT_c coplanar superconducting resonator and a large-area HT_c superconducting film concentrator. We have achieved in the 1.3GHz HT_c rf SQUID system a minimal flux noise of 2.5×10^{-5}Φ_0/\sqrt{Hz} and a magnetic field sensitivity of 38fT/\sqrt{Hz} in white noise range, respectively. The effective area of the concentrator fabricated on a 15×15mm^2 substrate is 1.35mm^2. It is shown that the 1.3GHz rf SQUID system has a high field sensitivity. Design and implementation of 1.3GHz HT_c rf SQUID offers a promising direction of rf SQUID development for higher working frequency ranges.     

Development of 1.3GHz high-T-c rf SQUID

A new high-T_c (HT_c) rf SQUID working at around 1.3GHz has been developed to avoid electromagnetic interference such as growing mobile communication jamming. This new system works in a frequency range from 1.23 to 1.42GHz (centred at 1.3GHz), which is not occupied by commercial communication. The sensor used in the 1.3GHz rf SQUID is made of a HT_c coplanar superconducting resonator and a large-area HT_c superconducting film concentrator. We have achieved in the 1.3GHz HT_c rf SQUID system a minimal flux noise of 2.5×10^{-5}Φ_0/\sqrt{Hz} and a magnetic field sensitivity of 38fT/\sqrt{Hz} in white noise range, respectively. The effective area of the concentrator fabricated on a 15×15mm^2 substrate is 1.35mm^2. It is shown that the 1.3GHz rf SQUID system has a high field sensitivity. Design and implementation of 1.3GHz HT_c rf SQUID offers a promising direction of rf SQUID development for higher working frequency ranges.     

Performance of YBaCuO thick film rf SQUIDs at 77 K

RF SQUID behaviour has been observed at 77 K in YBaCuO thick films prepared by screen printing technique. A hole shunted with a microbridge type of geometry is patterned manually for observing rf SQUID behaviour. Flux noise spectrum is also studied and it is found to depend on the quality of the film. The spectral density of the flux noise in the white noise region is 1.7×10⁻³ Φ₀/√Hz at 77 K.     

Performance of 2-hole YBCO rf SQUID in flux-locked-loop mode at 77 K

A 2-hole rf SQUID has been fabricated using naturally present grain boundary Josephson weaklinks in a microbridge of bulk Y-Ba-Cu-O superconductor. Periodic oscillations in voltage-flux characteristics have been observed up to 80.5 K. The spectral density of flux noise of the SQUID is 8 × 10⁻⁴ Φ₀/√Hz at 100 Hz and 77 K for open-loop mode. The SQUID has been successfully operated in flux-locked-loop mode at 77 K, demonstrating the feasiblity of the device for practical applications. In the flux-locked-loop mode the stability as well as flux noise of the SQUID has been found to improve considerably as compared to that in the open-loop mode. Various SQUID parameters have also been estimated and reported here.     

Observation of rf SQUID characteristics in YBCO bulk at 77 K

RF SQUID behaviour due to grain boundary weak links in a bulk YBCO is observed at 77 K using modified commercial rf electronics. Porous samples with lowI _c are found to show this characteristic whereas dense samples with higherI _c do not show SQUID behaviour.V-B modulation characteristic is found to be better when the rf pumping frequency is kept slightly higher than the resonance frequency of the tank circuit. Designing of coil for tank circuit with appropriateQ has been found to be very crucial for seeing the SQUID behaviour. Estimation of parameters such as coupling constant, mutual inductance, inductance and radius of the SQUID loop, have been made and their significance is discussed. Flux noise spectrum of the bulk rf SQUID in flux locked mode is also reported.     

High-Tc planar SQUID gradiometer for eddy current non-destructive evaluation

This paper reports the fabrication and test of a high-Tc SQUID planar gradiometer which is patterned from YBCO thin film deposited on a SrTiO3 bicrystal substrate. The measurement of noise spectrum at 77K shows that the white noise at 200 Hz is about 1×10^-4 φ0/√Hz. The minimal magnetic gradient is measured and the results suggest that the minimal magnetic gradient is 94 pT/m. The planar gradiometer is used in non-destructive evaluation （NDE） experiments to detect the artifacts in conducting aluminium plates by performing eddy current testing in an unshielded environment. The effect of the exciting coil dimension on the NDE results is investigated. By mapping out the induced field distribution, flaws about 10mm below the plate surface can be clearly identified.     

Simulation of dielectric resonator for high-Tc radio frequency superconducting quantum interference device

Nowadays, the high-critical-temperature radio frequency superconducting quantum interference device (high-$T_{\rm c }$ rf SQUID) is usually coupled to a dielectric resonator that is a standard $10\times 10\times 1$~mm³ SrTiO$₃ (STO) substrate with a YBa₂Cu$₃O$_{7 - \delta }$ (YBCO) thin-film flux focuser deposited on it. Recently, we have simulated a dielectric resonator for the high-$T_{\rm c }$ rf SQUID by using the ANSOFT High Frequency Structure Simulator (ANSOFT HFSS). We simulate the resonant frequency and the quality factor of our dielectric resonator when it is unloaded or matches a 50-$\Omega$ impedance. The simulation results are quite close to the practical measurements. Our study shows that ANSOFT HFSS is quite suitable for simulating the dielectric resonator used for the high-$T_c rf SQUID. Therefore, we think the ANSOFT HFSS can be very helpful for investigating the characteristics of dielectric resonators for high-$T_c rf SQUIDs.     

用于射频超导量子干涉器的新型单CSRR磁通聚焦器和谐振器

文提出了一种新型磁通聚焦器和谐振器. 磁通聚焦器引入了单互补开环谐振器结构 (单CSRR); 谐振器则采用环绕着绕磁通聚焦器的发夹型3阶阶梯阻抗谐振器 (SIR). 采用ANSYS HFSSv.11高频结构仿真软件对提出的磁通聚焦器和谐振器进行了仿真, 并对单CSRR进行了理论分析. 仿真和理论分析表明: 单CSRR的引入, 提高了磁通聚焦器的聚磁效果, 改善了磁通聚焦器与RF SQUID垫圈的耦合(耦合系数k_sc为引入单CSRR前的2倍), 并且有效面积增大到1.227 mm².     

用于高温射频超导量子干涉器的介质谐振器的性质研究

介质谐振器是目前高温射频超导量子干涉器较常采用的一种高品质因数微波谐振器.它是由10 mm×10 mm×1 mm的SrTiO₃(STO)标准衬底及覆盖在其上的YBa₂Cu₃O_7-δ(YBCO)薄膜磁通聚焦器共同构成的.为探明磁通聚焦器构形对介质谐振器谐振频率的影响,本文采用Ansoft公司出品的HFSS高频结构仿真软件对磁通聚焦器构形不同的若干介质谐振器的谐振特性进行了仿真.结果表明:增大磁通聚焦器开     

Stable diode lasers for hydrogen precision spectroscopy

We report on an external cavity diode laser at 972 nmstabilized to a mid-plane mounted Fabry-Perot (FP) resonator with afinesse of 400000. The 0.5 Hz optical beat note line width betweentwo similar lasers (Allan deviation 2 × 10^-15) is limitedby thermal noise properties of two independent FP resonators. Thelong term drift of the FP resonator and mirror substrates made fromUltra-Low-Expansion glass (ULE) is small and can be well predictedon time intervals up to many hours if the resonator is stabilized atthe zero thermal expansion temperature T_c. Using a Peltierelement in a vacuum chamber for temperature stabilization allowsstabilization of the FP cavity to T_c which is usually below theroom temperature. Beat note measurements with a femtosecond opticalfrequency comb referenced to a H-maser during 15 hours have shown awell defined linear drift of the FP resonance frequency of about 60 mHz/s with residual frequency excursions of less than ±20 Hz.