The development of a superconducting traveling wave accelerating cavity with high gradient

The required accelerating gradient in the ILC project is over 30 MeV/m [1]. For current technology, the maximum accelerating gradient in superconducting (SC) structures is limited mainly by the value of the surface RF magnetic field. In order to increase the gradient, the RF magnetic field is distributed homogeneously over the cavity surface (low-loss structure), and coupling to the beam is improved by introducing aperture and cell shape (re-entrant structure). These features allow gradients in excess of 56 MeV/m to be obtained for a single-cell cavity. Further improvement of the coupling to the beam may be achieved by using a traveling wave (TW) SC structure with small phase advance per cell. Calculations show that an additional gradient increase by up to 46% is possible if a π/2 TW SC structure is used. However, a TW SC structure requires a SC feedback waveguide to return the few GW of circulating RF power from the structure output back to the structure input. We discuss variants of the superconducting traveling wave ring (STWR) with one and two feeding couplers. The text was submitted by the authors in English.     

Berry phase in superconducting charge qubits interacting with a cavity field

We propose a method for analyzing Berry phase for a multi-qubit system of superconducting charge qubits interacting with a microwave field. By suitably choosing the system parameters and precisely controlling the dynamics, novel connection found between the Berry phase and entanglement creations.     

The evolution of field distribution in a dynamic cavity

The results of calculations of the dynamics of a field in a cavity that consists of a stationary mirror and a periodically displacing mirror are presented. The initial distribution of the field corresponds to the equilibrium thermal (Planck) distribution. It is shown that, if conditions of a resonant excitation are implemented, pulses can be formed the degree of unipolarity of which increases from the initial zero degree to an almost maximum (unit) degree.     

ac Stark shift and dephasing of a superconducting qubit strongly coupled to a cavity field

We have performed spectroscopy of a superconducting charge qubit coupled nonresonantly to a single mode of an on-chip resonator. The strong coupling induces a large ac Stark shift in the energy levels of both the qubit and the resonator. The dispersive shift of the resonator frequency is used to nondestructively determine the qubit state. Photon shot noise in the measurement field induces qubit level fluctuations leading to dephasing which is characteristic for the measurement backaction. A crossover in line shape with measurement power is observed and theoretically explained. For weak measurement a long intrinsic dephasing time of T2>200 ns of the qubit is found.     

Multipartite entanglement in the interaction system between a single-mode microwave cavity field and superconducting charge qubits

This paper proposes a method of generating multipartite entanglement through using d.c. superconducting quantum interference devices (SQUID) inside a standing wave cavity. In this scheme, the d.c. SQUID works in the charge region. It is shown that, a large number of important multipartite entangled states can be generated by a controllable interaction between a cavity field and qubits. It is even possible to produce entangled states involving different cavity modes based on the measurement of charge qubits states. After such superpositions states are created, the interaction can be switched off by the classical magnetic field through the SQUID, and there is no information transfer between the cavity field and the charge qubits.     

Controlled hole burning in the photon-number distribution of field states in a cavity

A feasible experimental scheme to generate states of a field mode in a high-Q cavity possessing holes in their photon-number distributions at controlled positions is proposed. Such kind of states may be of interest to optical data storage and communications with a possible readout system consisting of an atomic deflection setup.     

Information entropy and entanglement of a superconducting qubit coupled to a cavity field with its spontaneous decay

The quantum entanglement between superconducting qubit and cavity field is described quantitatively in the presence of spontaneous decay. Depending on how how a system is quantum correlated with its environment, the entanglement dynamics between the qubit and cavity is evaluated and investigated during the dissipative process. The motivation based on recent experiments wherein the Cooper box can be used to probe the decay of the resonator superposition state due to environmental decoherence, we theoretically investigate the dynamics of entanglement measured by the negativity. Wehrl entropy and Wehrl phase distribution of a superconducting qubit coupled to a cavity field induced by a superconducting qubit-damping reservoir governed by a master equation.     

The design of a five-cell high-current superconducting cavity

Energy recovery linacs are promising for achieving high average current with superior beam quality. The key component for accelerating such high-current beams is the superconducting radio-frequency cavity. The design of a 1.3 GHz five-cell high-current superconducting cavity has been carried out under cooperation between Peking University and the Argonne National Laboratory. The radio-frequency properties, damping of the higher order modes, multipacting and mechanical features of this cavity have been discussed and the final design is presented.     

Electromagnetic characteristics of the CH cavity

In this paper, we present the RF simulation, the fabrication and the normal RF test of a six-cell copper model cross bar H mode (CH) cavity. The CH cavity was researched and developed at the Institute of Modern Physics for Injector II of the superconducting linac of the accelerator driven system of China, operating at a frequency 162.5 MHz, β=0.065. The deep drawing and electron beam welding were employed to fabricate this cavity, which would be used to develop the superconducting CH cavity in the future. The results of the normal RF test agree with the simulation of the electromagnetic properties, such as the electric field distribution on the cavity axis, frequency and Q factor.     

有限厚无限大平板超导体模型场分布的严格解

给出了有限厚无限大平面板超导体模型场分布的严格解，确证了文献中近似结果的有效性. 关键词： 严格解 Jacobi椭圆函数 超导     

The development of a TESLA recirculator with superconducting accelerating structures in Ukraine

The recirculator project, which is to be built at the National Science Center, Kharkov Institute of Physics and Technology (NSC KIPT), is presented. The basic solutions incorporated in the design are given. The TESLA superconducting section is chosen as the accelerating structure of an accelerating complex. The text was submitted by the authors in English.     

CSNS-II超导椭球腔形变电场平坦度仿真分析

超导椭球腔是一种通过冲压零件和电子束焊接而成型的薄壁结构,在加工过程中存在不可避免的形变,如单元倾斜、偏轴,单元长度偏差,从而影响椭球腔的轴向电场平坦度,进而降低椭球腔的运行电场梯度。因此,在椭球腔加工成型后要进行预调谐处理。椭球腔预调谐是通过对椭球腔整形,包括倾斜和偏轴矫正,单元长度矫正,以达到提高轴向电场平坦度的目的。采用COMSOL多物理场耦合软件对CSNS-II（China Spallation Neutron Source Phase II）超导椭球腔可能存在的形变进行了仿真计算,研究了各单元形变量对电场平坦度的影响,分析了腔体调谐位移量对电场平坦度的影响,为椭球腔预调谐提供数据参考,也为预调谐机的设计研制提供指导。另外,确定了椭球腔机械形变的要求,确保椭球腔机械加工质量满足CSNS-II升级的实际工程需求。     

射频超导腔加速性能的改进

 为提高超导加速腔的加速梯度和Q值，改进了薄膜型超导腔的加速性能。研究证明，对于铜铌溅射腔，在无氧铜衬底和铌膜之间加入NbN 层可以提高铌膜的超导转变温度，改善晶格结构；对纯铌超导腔提出了改进方法，在传统的纯铌超导腔表面制备多层的超导-绝缘-超导复合膜可以屏蔽Nb腔表面的界面场，提高超导腔的临界磁场，从而提高了铌腔的加速梯度。     

射频超导腔加速性能的改进

为提高超导加速腔的加速梯度和Q值,改进了薄膜型超导腔的加速性能。研究证明,对于铜铌溅射腔,在无氧铜衬底和铌膜之间加入NbN 层可以提高铌膜的超导转变温度,改善晶格结构;对纯铌超导腔提出了改进方法,在传统的纯铌超导腔表面制备多层的超导-绝缘-超导复合膜可以屏蔽Nb腔表面的界面场,提高超导腔的临界磁场,从而提高了铌腔的加速梯度。     

Analytical solution to the transient beam loading effects of a superconducting cavity

Transient beam loading is one of the key issues in any high beam current intensity superconducting accelerators, and needs to be carefully investigated. The core problem in the analysis is to obtain the time evolution of effective cavity voltage under transient beam loading. To simplify the problem, the second order ordinary differential equation describing the behavior of the effective cavity voltage is intuitively simplified to a first order one, with the aid of two critical approximations which lack proof of their validity. In this paper, the validity is examined mathematically in some specific cases, resulting in a criterion for the simplification. It is popular to solve the approximate equation for the effective cavity voltage numerically, while this paper shows that it can also be solved analytically under the step function approximation for the driven term. With the analytical solution to the effective cavity voltage, the transient reflected power from the cavity and the energy gain of the central particle in the bunch can also be calculated analytically. The validity of the step function approximation for the driven term is examined by direct evaluations.After that, the analytical results are compared with the numerical ones.     

Towards measuring variations of Casimir energy by a superconducting cavity

We consider a Casimir cavity, one plate of which is a thin superconducting film. We show that when the cavity is cooled below the critical temperature for the onset of superconductivity, the sharp variation (in the far infrared) of the reflection coefficient of the film engenders a variation in the value of the Casimir energy. Even though the relative variation in the Casimir energy is very small, its magnitude can be comparable to the condensation energy of the superconducting film, and this gives rise to a number of testable effects, including a significant increase in the value of the critical magnetic field, required to destroy the superconductivity of the film. The theoretical ground is therefore prepared for the first experiment ever aimed at measuring variations of the Casimir energy itself.     

Quantum nondemolition measurement of photon—number distribution for a weak cavity field with resonant atoms

We propose a quantum nondemolition measurement of the photon-number distribution for a weak cavity field with no more than two photons. The scheme is based on the resonant interaction of atoms with the cavity field, and thus the required interaction time is much shorter than that using dispersive interaction. This is important in view of decoherence. Our scheme can also be used to generate even and odd coherent states for a weak cavity field with resonant atoms.     

六极装置电场分布及对CH3Cl分子束选态会聚的模拟

通过数值获得的六极装置内部电势分布数值解,拟合得出电势分布公式中的系数,给出各项系数随着装置参数ρ0/r0的变化规律及拟合函数形式,结果表明ρ0/r0=0.5516时,六极装置的电场分布最接近理想六极电场. 对于其它ρ0/r0值时,推导出电势、电场更精确的计算公式. 并利用这些公式,在ρ0/r0不满足最优配置时给出了对CH3Cl分子转动态的会聚所需的实验参数. 这些方法为利用六极装置对分子束会聚和转动态选择的理论模拟及试验提供了更准确的理论依据.     

Color superconducting matter in a magnetic field

We investigate the effect of a magnetic field on cold dense quark matter using an effective model with four-Fermi interactions. We find that the gap parameters representing the predominant pairing between the different quark flavors show oscillatory behavior as a function of the magnetic field. We point out that due to electric and color neutrality constraints the magnetic fields as strong as presumably existing inside magnetars might induce significant deviations from the gap structure at a zero magnetic field.