Optimization of a Cryogenic System at Temperature 2 K for the PKU-FEL Facility

The conceptual design of a cryogenic system at temperature 2 K for the Peking University Free Electron Laser （PKU-FEL） facility is carried out. In order to minimize the scale of the cryogenic system and the running cost, the superconducting accelerator and the superconducting injector will mainly run at a long-pulsed mode. Optimization of the 2-K cryogenic system is carried out based on the heat load estimation and running parameters. Total cooling power of 52.5 W for the long-pulsed mode is necessary for the PKU cryogenic system. The PKU cryogenic system will be the first 2-K system for accelerators in China and will provide experience for similar facilities.     

Creation of a diagnostic complex for the characterization of cryogenic laser-fusion targets using the tomography method with probing irradiation in the visible spectrum

We present our results on developing a diagnostic complex for high-precision characterization of the parameters of laser-fusion microobjects, such as microspheres and cryogenic targets (the microobject size is 1–2 mm). The complex operates based on the principle of tomography. The complex consists of (a) the scanning system providing a set of shadow projections of a microobject in the visible range of radiation and (b) specially developed software for 3D reconstruction of the microobject from the set of projections. The spatial resolution of the optical system is 1 μm for a probing-radiation wavelength of 490 nm. The distinctive features of the diagnostic complex are (1) operation with both free-standing and mounted targets and (2) the possibility of scanning the targets from room to cryogenic temperatures. The operation of the complex was demonstrated in the reconstruction of polystyrene microspheres by a large set (80–90) of shadow projections at room and cryogenic temperatures.     

Physical design of superconducting magnet for ADS injection Ⅰ

A superconducting solenoid prototype magnet for Accelerator Driven Subcritical System (ADS) Injection Ⅰ has been designed and fabricated, which has also been tested in a liquid Helium state inside a vertical Dewar in the Haerbin institute of Technology in November 2012. The design current was 210 A, when the test current reached 400 A no quench occurred so the solenoid magnet was forced to quench by the embedded heaters. The integral field strength, leakage field at the nearby upstream and downstream superconducting spoke cavities all meet the design requirements. At the same time, it also checked the reliability of the vertical test Dewar and the quenched detection system. The superconducting prototype magnet has accumulated valuable experiences for the coming batch magnets production and cryogenic test.     

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.     

Control application of a superconducting magnet power supply based on EPICS

The superconducting quadrupole magnets （SCQs） are powered by 16 power supplies in the inter- action region of the BEPC Ⅱ. The control application of these power supplies must be interlocked with the quench protection system to protect the superconducting magnet and relevant devices. This paper describes the development procedures of this control application using EPICS and the operating result with the quench protection system on-site.     

Post-acceleration study for neutrino super-beam at CSNS

A post-acceleration system based on the accelerators at CSNS(China Spallation Neutron Source) is proposed to build a super-beam facility for neutrino physics.Two post-acceleration schemes,one using superconducting dipole magnets in the main ring and the other using room temperature magnets,have been studied,both to achieve the final proton energy of 128 GeV and the beam power of 4 MW by taking 10% of the CSNS beam from the neutron source.The main design features and the comparison for the two schemes are presented.The CSNS super-beam facility will be very competitive in long-baseline neutrino physics studies,compared with other super-beam facilities proposed in the world.     

A new muon-pion collection and transport system design using superconducting solenoids based on CSNS

A new muon and pion capture system is proposed for the China Spallation Neutron Source(CSNS),currently under construction. Using about 4% of the pulsed proton beam(1.6 Ge V, 4 k W and 1 Hz) of CSNS to bombard a cylindrical graphite target inside a superconducting solenoid, both surface muons and pions can be acquired. The acceptance of this novel capture system- a graphite target wrapped up by a superconducting solenoid- is larger than the normal muon beam lines using quadrupoles at one side of the separated muon target. The muon and pion production at different capture magnetic fields was calculated using Geant4. The bending angle of the capture solenoid with respect to the proton beam was also optimized in simulation to achieve more muons and pions.Based on the layout of the muon experimental area reserved at the CSNS project, a preliminary muon beam line was designed with multi-purpose muon spin rotation areas(surface, decay and low-energy muons). Finally, high-flux surface muons(10~8/s) and decay muons(10~9/s) simulated by G4 beamline will be available at the end of the decay solenoid based on the first phase of CSNS. This collection and transport system will be a very effective beam line at a proton current of 2.5 μA.     

The stability margin on EAST tokamak

The experimental advanced superconducting tokamak (EAST) is the first full superconducting tokamak with a D-shaped cross-sectional plasma presently in operation. Its poloidal coils are relatively far from the plasma due to the necessary thermal isolation from the superconducting magnets, which leads to relatively weaker coupling between plasma and poloidal field. This may cause more di?culties in controlling the vertical instability by using the poloidal coils. The measured growth rates of vertical stability are compared with theoretical calculations, based on a rigid plasma model. Poloidal beta and internal inductance are varied to investigate their effects on the stability margin by changing the values of parameters αn and γn(Howl et al 1992 Phys. Fluids B 4 1724), with plasma shape fixed to be a configuration with k = 1.9 and δ = 0.5. A number of ways of studying the stability margin are investigated. Among them, changing the values of parameters κ and li is shown to be the most effective way to increase the stability margin. Finally, a guideline of stability margin Ms(κ,li,A) to a new discharge scenario showing whether plasmas can be stabilized is also presented in this paper.     

RIKEN超导ECR离子源磁体系统的设计

Superconducting magnet system for a 28GHz ECR ion source has been designed.The maximum axial magnetic fields are 4T at the rf injection side and 2T at the beam extraction side,respectively.The hexapole magnetic field is about 2T on the inner surface of the plasma chamber.The superconducting coils consist of six solenoids and six racetrack windings for a hexapole field.Two kinds of coil arrangements were investigated:one is an arrangement in which the hexpole coil is located in the bore of the solenoids,and another is the reverse of it.The coils use NbTi-Copper conductor and are bath-cooled in liquid helium.The six solenoids are excited with individual power supplies to search for the optimal axial field distribution.The current leads use high Tc material and the cryogenic system is operated in LHe re-condensation mode using small refrigerators.The thermal insulated supports of the cold mass have also been designed based on the calculated results of the magnetic force.The heat loads to 70K and LHe stages were estimated from the design of the supports,the current leads and so on.     

Effect of cryogenic temperature characteristics on 0.18-μm silicon-on-insulator devices

The experimental results of the cryogenic temperature characteristics on 0.18-μm silicon-on-insulator(SOI) metaloxide-silicon(MOS) field-effect-transistors(FETs) were presented in detail. The current and capacitance characteristics for different operating conditions ranging from 300 K to 10 K were discussed. SOI MOSFETs at cryogenic temperature exhibit improved performance, as expected. Nevertheless, operation at cryogenic temperature also demonstrates abnormal behaviors, such as the impurity freeze-out and series resistance effects. In this paper, the critical parameters of the devices were extracted with a specific method from 300 K to 10 K. Accordingly, some temperature-dependent-parameter models were created to improve fitting precision at cryogenic temperature.     

The design simulation of the superconducting section in the ADS injector Ⅱ

The high-current superconducting proton linac is being studied for the accelerator-driven system (ADS) project undertaken by the Chinese Academy of Sciences. The injector Ⅱ will be operated at 162.5 MHz, and the proton out from the RFQ with an energy of 2.5 MeV will be accelerated to 10 MeV by two cryo-modules, which are composed of eight superconducting half wave resonance cavities and nine solenoids. In this paper, the design and beam simulation of the superconducting section of the injector Ⅱ, the acceptance calculation and a stability analysis are presented.     

Mechanical Analysis and Measurements of a Multicomponent NbTi/Cu Superconducting Magnets Structure for the Fully Superconducting Electron Cyclotron Resonance Ion Source

A fully superconducting electron cyclotron resonance(ECR) ion source(SECRAL Ⅱ) is currently being built in the Institute of Modern Physics,Chinese Academy of Sciences.Its key components are three superconducting solenoids(Nb-Ti/Cu) and six superconducting sextupoles(Nb-Ti/Cu).Different from the conventional superconducting ECR magnetic structure,the SECRAL Ⅱ includes three superconducting solenoid coils that are located inside the superconducting sextupoles.The SECRAL Ⅱ can significantly reduce the interaction forces between the sextupole and the solenoids,and the magnets can also be more compact in size.For this multi-component SECRAL Ⅱ generating its self Held of ~8T and being often exposed to the high self Geld,the mechanical analysis has become the main issue to keep their stress at 200 MPa on coils.The analytical and experimental results in mechanics are presented in the SECRAL Ⅱ structure.To improve the accuracy and efficiency of analysis,according to the composite rule of micromechanics,the equivalent uniform windings are used to simuiate the epoxy-impregnated Nb-Ti/Cu coils.In addition,using low temperature strain gauges and a wireless fast strain acquisition system,a fundamentai experiment on the strains developments of a sextupole is reported.Finally,based on our analysis,the stresses and deformations for its assembly of each SECRAL Ⅱ coil will be further optimized.     

A tuning system for BEPCⅡ

The tuning system plays a very important role when a superconducting cavity is in operation. It cooperates with other control loops to adjust the cavity frequency with high precision, reduce the reflection power, guarantee the stability of beam, and ensure the safety of the superconducting cavity. This paper focuses mainly on the tuning system working principle, the working state and problems that Beijing Electron Positron Collider (BEPC ) has encountered during operation.     

Practical 2.45-GHz microwave-driven Cs-free Hˉ ion source developed at Peking University

A practical 2.45-GHz microwave-driven Cs-free Hˉ source was improved based on the experimental Hˉ source at Peking University(PKU). Several structural improvements were implemented to meet the practical requirements of Xi'an Proton Application Facility(XiPaf). Firstly, the plasma chamber size was optimized to enhance the plasma intensity and stability. Secondly, the filter magnetic field and electron deflecting magnetic field were enhanced to reduce co-extracted electrons. Thirdly, a new two-electrode extraction system with farther electrode gap and enhanced water cooling ability to diminish spark and sputter during beam extraction was applied. At last, the direct Hˉ current measuring method was adopted by the arrangement of a new pair of bending magnets before Faraday cup(FC) to remove residual electrons. With these improvements, electron cyclotron resonance(ECR) magnetic field optimization experiments and operation parameter variation experiments were carried out on the Hˉ ion source and a maximum 8.5-mA pure Hˉ beam was extracted at 50 kV with the time structure of 100 Hz/0.3 ms. The root-mean-square(RMS) emittance of the beam is 0.25 Π·mm·mrad. This improved Hˉ source and extraction system were maintenance-free for more than 200 hours in operation.     

Updating the CSNS injector linac to 250 MeV with superconducting double-spoke cavities

In order to update the beam power from 100 k W to 500 k W in the China Spallation neutron source(CSNS) Phase, one of the important measures is to replace the 80 m long beam transport line between the present80 Me V linac injector and the rapid cycling synchrotron(RCS) to another kind of acceleration structure. In this paper, we proposed a scheme based on 324 MHz double-spoke superconducting cavities. Unlike the superconducting elliptical cavity and normal conducting coupled cavity linac(CCL) structure, the double-spoke cavity belongs to the TE mode structure and has a smaller transverse dimension compared with that of the TH mode one. It can work at base frequency as the drift tube Linac(DTL) section, so that the cost and complexity of the RF system will be much decreased, and the behaviors of the beam dynamics are also improved significantly because of the low charge density and larger longitudinal acceptance. Furthermore, because of the relatively longer interactive length between the charged particle and the electromagnetic field per cell, it needs relatively less cell numbers and it has larger velocity acceptance compared with the double frequency TH structures. The superconducting section consists of 14 periods, each of which includes 3 superconducting cavities encapsulated in one cryomodule and a doublet in room temperate. The general considerations on cavity and beam dynamics design are discussed and the main results are presented.     

Design and performance of the LLRF system for CSNS/RCS

The rapid cycling synchrotron(RCS) is part of the China Spallation Neutron Source(CSNS). The RCS provides 1.6 Ge V protons with a repetition rate of 25 Hz. The RF system in RCS is mainly composed of a ferrite loaded RF cavity, a high power tetrode amplifier, a bias supply of 3300 A and a digital low level RF(LLRF) system based on FPGA. The major challenge of the LLRF system is to solve problems caused by rapid frequency sweeping and the heavy beam loading effect. A total of eight control loops are applied to ensure the normal operation. An effective feedforward scheme is widely used to improve the dynamic performance of the system. The design of the LLRF system and high power integration test results with the prototype RF system are presented.     

The cryogenic control system of BEPCⅡ

A superconducting cryogenic system has been designed and deployed in the Beijing Electron-Positron Collider Upgrade Project (BEPCⅡ). The system consists of a Siemens PLC (S7-PLC, Programmable Logic Controller) for the compressor control, an Allen Bradley (AB) PLC for the cryogenic equipments, and the Experimental Physics and Industrial Control System (EPICS) that integrates the PLCs. The system fully automates the superconducting cryogenic control with process control, PID (Proportional-Integral-Differential) control loops, real-time data access and data storage, alarm handler and human machine interface. It is capable of automatic recovery as well. This paper describes the BEPCⅡ cryogenic control system, data communication between S7-PLC and EPICS Input/Output Controllers (IOCs), and the integration of the flow control, the low level interlock, the AB-PLC, and EPICS.     

Field measurement for superconducting magnets of ADS injector Ⅰ

A superconducting magnet prototype for Accelerator Driven Sub-critical System Injection-Ⅰ had been designed and fabricated, and tested in a new made vertical Dewar in November 2012. Batch magnet production was processed after some major revision from the magnet prototype, they include: removing off the perm-alloy shield, extending the iron yoke, using thin superconducting wire, etc. The first one of the batch magnets was tested in the vertical Dewar at the Harbin Institute of Technologyin in September 2013. A field measurement was carried out at the same time by the measurement platform that was seated on the top of the vertical Dewar,the measurement results met the design requirements. This paper will present the field measurement system design, measurement results and discussion on the residual field from the persistent current effect.     

Influence of Defect Particles on Configuration of a Two-Dimensionally Confined Dusty Plasma

The behaviour of defect particles in a two-dimensional （2D） confined dusty plasma system is investigated by molecular dynamics （MD） simulations. The mean square displacement （MSD） and the pair correlation function g（r） are used to characterize the structural and dynamical properties of the system. The influences of the number and the charge （mass） of the defect particles on the system configurations are simulated. All the defect particles with charges （masses） larger than the normal particles have the trend to move towards the system centre. The moving speed of the defect particles towards the centre increases with the increasing number and charge （mass） of defect particles and with the system temperature.     

Sm–Co high-temperature permanent magnet materials

Permanent magnets capable of reliably operating at high temperatures up to ~450?C are required in advanced power systems for future aircrafts, vehicles, and ships. Those operating temperatures are far beyond the capability of Nd–Fe–B magnets. Possessing high Curie temperature, Sm–Co based magnets are still very important because of their hightemperature capability, excellent thermal stability, and better corrosion resistance. The extensive research performed around the year 2000 resulted in a new class of Sm_2(Co, Fe, Cu, Zr)_(17)-type magnets capable of operating at high temperatures up to 550?C. This paper gives a systematic review of the development of Sm–Co permanent magnets, from the crystal structures and phase diagrams to the intrinsic magnetic properties. An emphasis is placed on Sm_2(Co, Fe, Cu, Zr)_(17)-type magnets for operation at temperatures from 300?C to 550?C. The thermal stability issues, including instantaneous temperature coefficients of magnetic properties, are discussed in detail. The significance of nanograin structure, nanocrystalline, and nanocomposite Sm–Co magnet materials, and prospects of future rare-earth permanent magnets are also given.