超导在加速器中的应用概况

文中简要回顾了世界上已建造的大型超导加速器,重点介绍未来的超导加速器方案,对超导新材料在加速器中的应用研究以及我国超导在加速器中的应用研究情况作了简要介绍.     

Development of a superconducting LINAC booster for the pelletron at mumbai

A superconducting linear accelerator (LINAC) to boost the energy of the heavy ion beams from the 14 UD pelletron accelerator at Mumbai is under development. The booster is based on quarter wave resonators (QWR) coated with lead which is superconducting at liquid helium temperature. The operating frequency is 150 MHz. Four resonators each are mounted in a cryostat module built indigenously. A total of seven such modules arranged in two arms with an isochronous and achromatic beam bend in the middle comprises the full LINAC. The transverse focusing of the beam through the LINAC is carried out using periodic quadrupole doublet magnets operating at room temperature. The present status of the project is described.     

Superconducting LINAC booster for the mumbai pelletron

We are in the process of constructing a superconducting linear accelerator (LINAC), to boost the energy of heavy ion beams from the 14UD Pelletron accelerator, at Tata Institute of Fundamental Research, Mumbai. The accelerating structures in the LINAC are quarter wave resonators (QWR) coated with lead which is superconducting at liquid helium temperature. With feasibility studies having been completed during the course of the 4th and 5th five-year plan periods, culminating with the demonstration of beam acceleration using one accelerating module, the construction of the LINAC is now under way.     

Researches and Preliminary Experiments on Copper-Niobium Sputtered Low-β Superconducting Resonator

Superconducting resonator is the important component of a superconducting accelerator. The best choice of the acceleration cavity for heavy ion superconducting accelerator is quarter wave resonator (QWR). By sputtering a layer of niobium films of several microns on the OFHC copper substrate, good superconducting properties and acceleration properties of the cavity can be obtained. However, it is very difficult to get uniform niobium films by sputtering because the inner surface of the QWR is complicated. To deal with this problem, we developed a multiparameter adjusting method to control the sputtering process of different parts of the QWR. A layer of uniform niobium film with nice superconducting performance is obtained. The low temperature experiments with liquid helium show that the Cu Nb QWR has good superconducting characters. The Q value of the cavity is about 5×10⁸.     

改进铜铌溅射型QWR超导腔性能的探讨

通过直流偏压二级溅射方法，在无氧铜腔体表面溅射一层铌膜，研制了铜铌溅射型射频超导1/4波长谐振腔(quarter wave resonator,QWR)，该腔主要用于重离子的加速，是北京放射性核束装置中后加速部分的预研项目.目前国际上很多实验室都在研究进一步提高铜铌溅射型QWR超导腔的性能，通过多种方法的实验研究，发现在无氧铜衬底与铌膜之间加入一层氮 化铌（NbN）薄膜，可以使得表面铌膜的超导温度转变点由原来的8.8K提高到了接近9.6K ，该方法有可能成为提高QWR腔加速性能的重要途径，目前进一步研究正 关键词： 溅射 QWR超导腔 氮化铌NbN 超导温度     

Deterministic entanglement of photons in two superconducting microwave resonators

Quantum entanglement, one of the defining features of quantum mechanics, has been demonstrated in a variety of nonlinear spinlike systems. Quantum entanglement in linear systems has proven significantly more challenging, as the intrinsic energy level degeneracy associated with linearity makes quantum control more difficult. Here we demonstrate the quantum entanglement of photon states in two independent linear microwave resonators, creating N-photon NOON states (entangled states |N0> + |0N>) as a benchmark demonstration. We use a superconducting quantum circuit that includes Josephson qubits to control and measure the two resonators, and we completely characterize the entangled states with bipartite Wigner tomography. These results demonstrate a significant advance in the quantum control of linear resonators in superconducting circuits.     

Design of a superconducting low beta niobium resonator

The proposed high current injector for the superconducting Linac at the Inter-University Accelerator Centre will have several accelerating structures, including a superconducting module which will contain low beta niobium resonators. A prototype resonator for the low beta module has been designed. The resonator has been carefully modelled to optimize the electromagnetic parameters. In order to validate them, a room-temperature copper model has been built and tested. In this paper we present details of the electromagnetic design of the low beta resonator, briefly discuss the mechanical and engineering design, and present results from the measurements on the room-temperature copper model.     

Superconducting linac and associated accelerator development at NSC

There has been significant progress in the programme to develop a superconducting linear accelerator as a booster for the Pelletron accelerator at the Nuclear Science Centre. This paper presents the current status of the development in all the major components of the accelerator.     

Experimental And Analytical Investigation Of Steady-State Thermal Profile Of Forced-Flow Ln2-Cooled Thermal Radiation Shield For Superconducting Linear Accelerator Cryomodule

Acceleration of heavy ions through the superconducting linear accelerator is achieved by superconducting resonators housed in the cryomodules operating at 4.2 K. In the cryomodule, radiation heat flow from the ambient to the 600 kg of cold mass at 4.2 K is thermally screened by the intermediate thermal shield made of copper. The six surfaces of 17 m² total surface area of the box-shaped thermal shield are cooled in series by the forced flow of liquid nitrogen at 3.2 bar absolute pressure. The boiling temperature of liquid nitrogen at 3.2 bar is 90 K. The steady-state temperatures of the different surfaces of the thermal shield are in the range of 96–115 K at 21.25 L/h flow rate. The steady-state thermal profile of the different surfaces of the thermal shield has also been estimated for different flow rates using a simple analytical technique. The analytical thermal profile of the thermal shield has been correlated with the experimental measurement.     

Experimental evidence of one-dimensional plasma modes in superconducting thin wires

We have studied niobium superconducting thin wires deposited onto a SrTiO3 substrate. By measuring the reflection coefficient of the wires, resonances are observed in the superconducting state in the 130 MHz to 4 GHz range. They are interpreted as standing wave resonances of one-dimensional plasma modes propagating along the superconducting wire. The experimental dispersion law, omega versus q, presents a linear dependence over the entire wave vector range. The modes are softened as the temperature increases close the superconducting transition temperature. Very good agreement is obtained between our data and the predicted dispersion relation of one-dimensional plasma modes.     

Great progress in developing 500 MHz single cell superconducting cavity in China

Superconducting cavities have been adopted in many kinds of accelerator facilities such as synchrotron radiation light source, hard X-ray free electron laser linac, colliders and energy recovery linacs (ERL). The 500 MHz superconducting cavities will be a candidate to be installed in the high current accelerators and high current ERLs for their large beam aperture, low higher order modes impedance and high current threshold value. This paper presents great progress in the whole sequence of developing 500 MHz superconducting cavity in China. It describes the first in-house successful development of 500 MHz single cell superconducting cavity including the deep-drawing of niobium half cells, electron beam wielding of cavity, surface preparations and vertical testing. The highest accelerating gradient of the fabricated cavity #SCD-02 higher than 10 MV/m was obtained while the quality factor was better than 4×10⁸ at 4.2 K, which has reached the world level of the same kind of cavities.     

Superconducting synchrotron project for hadron therapy

The project of a superconducting medical synchrotron for carbon therapy in the ion energy range from 140 to 400 MeV/n is discussed in this paper. This project is aimed at developing and building a medical synchrotron on the basis of superconducting technologies at JINR under the construction of the Nuclotron accelerator complex. A linear accelerator with alternating phase focusing is proposed for injecting carbon ion into the synchrotron, while it is planned to use a superconducting gantry weighing about 150 t for delivering radiation treatment to patients from all directions.     

First observation of intrabeam stripping of negative hydrogen in a superconducting linear accelerator

We report on an experiment in which a negative hydrogen ion beam in the Spallation Neutron Source (SNS) linear accelerator was replaced with a beam of protons with similar size and dynamics. Fractional beam loss in the superconducting part of the SNS accelerator was measured to be at least 2×10(-5) for the H(-) beam, and it was an order of magnitude lower for the protons. Also beam loss has a stronger dependence on intensity with H(-) than with proton beams. These measurements verify a recent theoretical explanation of unexpected beam losses in the SNS superconducting linear accelerator based on an intrabeam stripping mechanism for negative hydrogen ions. This previously unidentified mechanism for beam loss is important for the design of new high current linear ion accelerators and the performance improvement of existing machines.     

Beam focusing in a linear ion accelerator consisting of a periodic sequence of independently phased superconducting cavities

Focusing of a low-energy beam of heavy ions in a linear accelerator based on a periodic sequence of independently phased superconducting cavities is considered. The possibility of heavy ion beam focusing by a system of superconducting solenoids is analyzed. The advantages and disadvantages of this focusing technique are considered. It is shown that solenoids with a magnetic field exceeding B = 15 T are required for focusing ion beams with a small charge-to-mass ratio Z/A ≈ 1/66 in a superconducting accelerator.     

Surface studies of niobium chemically polished under conditions for superconducting radio frequency (SRF) cavity production

The performance of niobium superconducting radiofrequency (SRF) accelerator cavities is strongly impacted by the topmost several nanometers of the active (interior) surface, especially as influenced by the final surface conditioning treatments. We examined the effect of the most commonly employed treatment, buffered chemical polishing (BCP), on polycrystalline niobium sheet over a range of realistic solution flow rates using electron back scatter diffraction (EBSD), stylus profilometry, atomic force microscopy, laboratory XPS and synchrotron (variable photon energy) XPS, seeking to collect statistically significant datasets. We found that the predominant general surface orientation is (1 0 0), but others are also present and at the atomic-level details of surface plane orientation are more complex. The post-etch surface exhibits micron-scale roughness, whose extent does not change with treatment conditions. The outermost surface consists of a few-nm thick layer of niobium pentoxide, whose thickness increases with solution flow rate to a maximum of 1.3-1.4 times that resulting from static solution. The standard deviation of the roughness measurements is ±30% and that of the surface composition is ±5%.     

Acceleration complex for extreme ultraviolet nanolithography based on a free-electron laser with kilowatt-scale average radiation power

The project of an acceleration complex is described that is based on a 0.7-GeV superconducting linear accelerator for the free-electron laser used for extreme ultraviolet lithography at a 13.5-nm wavelength with a 0.5-kW average power of laser radiation, as well as for examination of materials using X-ray and vacuum-ultraviolet radiations.     

射频超导谐振腔中的电磁场研究

由高纯铌制成的射频超导谐振腔在高场时会出现Q值下降现象，这是高场下超导腔表面局部磁场增强所致. 为研究高场下超导腔表面的电磁场特性，用单cell超导腔设计了腔表面峰值磁场的研究实验. 通过对单cell超导腔表面峰值磁场的测量，可以确定超导腔焊缝及iris附近表面是否存在缺陷，以便进一步进行腔的处理来改进腔的性能.     

Explanation for the surface dependence of the tunneling gap in Nb3Sn

An explanation is proposed for the experimental results of Hoffstein and Cohen on tunneling into superconducting Nb₃Sn. In these experiments a strong dependence of the gap on the orientation of the surface was found. This dependence is explained here in terms of the linear chain model; a [100] surface cutting only one family of niobium atom linear chains, while a [111] surface cutting all three families of linear chains.     

Investigation on the fabrication of the 3rd harmonic superconducting cavity for the SSRF storage ring

A third harmonic superconducting niobium cavity has been proposed for installation in the Shanghai Synchrotron Radiation Facility （SSRF） storage ring to improve the Touschek lifetime. In order to investigate the feasibility of the superconducting cavity fabrication indigenously and the possibility to master the fabrica tion techniques, cavities were fabricated from copper and niobium sheets by deep drawing and electron-beam welding, and a series of measurements, such as resonant frequency, shape dimensions and wall thickness, were carried out during this process. After analysis of various problems existing in the fabrication process, technique improvements were proposed, and finally the precise shape as designed and resonant frequency within 1.2 MHz were achieved for the new completed cavities. In addition, full annealing was finally proved to be a good cure for niobium sheets＇ tearing up during deep drawing. By fabricating niobium cavities successfully, some problems to the next step were cleared. This paper introduces the process of cavity fabrication and its technique improvements towards forming, and the initial vertical test result of niobium cavity is also presented.     

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。