Transverse wakefield suppression using cell detuning assisted by manifold damping for high gradient linear colliders

Transverse wakefields excited by multiple bunches in ultra-relativistic charged particle beams in a linear collider must be adequately damped in order to preserve the luminosity of the colliding beams and to prevent a beam break up instability developing. Means of achieving this wakefield suppression are discussed in detail for high gradient normal conducting linacs. In particular the features of a damped and detuned series of structures (DDS) are described. Analytical expressions for the wakefield on short time scales are presented together with the circuit analysis required to rapidly analyse the general long-range behaviour in these linacs. A conspectus of experimental results along with theoretical predictions for this series of accelerating structures developed for the NLC/GLC is presented together with recent results on wakefield suppression for CLIC.     

Spot F positron source electro-deposited on a graphite rod

2.2 GBq of ¹⁸F (half-life 110 min) was effectively deposited on a graphite electrode of diameter 3 or 5 mm. The fraction of the electro-deposited ¹⁸F was as high as 97% of the total ¹⁸F produced in the solution. The fraction of the positrons emitted out of the electrode was 44% of the total β⁺. Thus, as much as 36% of the β⁺ from the produced ¹⁸F is available for the source of slow positron beams.     

Nondestructive inspection of explosive materials using linearly polarized two-colored photon beam

A nondestructive inspection method for screening explosive materials that are hidden in passenger vehicles, trucks, and cargo containers with radiation shielding was presented. The method was examined experimentally using linearly polarized two-colored photon beam. A sample object was irradiated with the photon beam, followed by an emission of gamma-rays in nuclear resonance fluorescence. The gamma-rays from oxygen and nitrogen emitted through nuclear resonance fluorescence were measured using high-purity germanium detectors. We were able to evaluate the element concentration ratio.     

Dissociator for a pulsed source of polarized protons

The results of our investigation of a pulsed dissociator to be used in a polarized proton source are presented. Supersonic flow of hydrogen (2.8 ± 0.6) × 10²⁰ atoms·sr^?1·s^?1 was obtained at a Mach number M = 3.1 ± 0.2.     

Radiative regime of linear colliders, high repetetion rate free electron lasers and associated accelerating structures

A review of various scalings of high energy and high luminosity electron-positron colliders in the highly radiative regime is presented together with the electromagnetic and mechanical limitations of various associated acceleration structures to support operation with high gradients and high repetition rates. The potential for operation of normal conducting linacs in a mode of high repetition rate of 1 kHz or higher, relevant for today's linac-driven free electron lasers, is also discussed.     

Sensitivity of peak positions to flight-path parameters in a deep-inelastic scattering neutron TOF spectrometer

The effects of small changes in flight-path parameters (primary and secondary flight paths, detector angles), and of displacement of the sample along the beam axis away from its ideal position, are examined for an inelastic time-of-flight (TOF) neutron spectrometer, emphasising the deep-inelastic regime. The aim was to develop a rational basis for deciding what measured shifts in the positions of spectral peaks could be regarded as reliable in the light of the uncertainties in the calibrated flight-path parameters. Uncertainty in the length of the primary or secondary flight path has the least effect on the positions of the peaks of H, D and He, which are dominated by the accuracy of the calibration of the detector angles. This aspect of the calibration of a TOF spectrometer therefore demands close attention to achieve reliable outcomes where the position of the peaks is of significant scientific interest and is discussed in detail. The corresponding sensitivities of the position of peak of the Compton profile, J(y), to flight-path parameters and sample position are also examined, focusing on the comparability across experiments of results for H, D and He. We show that positioning the sample to within a few mm of the ideal position is required to ensure good comparability between experiments if data from detectors at high forward angles are to be reliably interpreted.     

Distillation of hydrogen isotopes for polarized HD targets

We have developed a new cryogenic distillation system to purify Hydrogen-Deuteride (HD) gas for polarized HD targets in LEPS experiments at SPring-8. A small amount of ortho-H₂ (∼0.01%) in the HD gas plays an important role in efficiently polarizing the HD target. Since there are 1-5% impurities of H₂ and D₂ in commercially available HD gases, it is necessary to purify the HD gas up to ∼99.99%. The distillation system is equipped with a cryogenic distillation unit filled with many small stainless steel cells called “Heli-pack”. The distillation unit consists of a condenser part, a rectification part, and a reboiler part. The unit is kept at the temperature of 17-21 K. The Heli-pack has a large surface area that makes a good contact between gases and liquids. An amount of 5.2 mol of commercial HD gas is fed into the distillation unit. Three trials were carried out to purify the HD gas by changing temperatures (17.5 K and 20.5 K) and gas extraction speeds (1.3 ml/min and 5.2 ml/min). The extracted gas was analyzed using a gas analyzer system combining a quadrupole mass spectrometer with a gas chromatograph. One mol of HD gas with a purity better than 99.99% has been successfully obtained for the first time. The effective NTP (Number of Theoretical Plates), which is an indication of the distillation performances, is obtained to be 37.2±0.6. This value is in good agreement with a designed value of 37.9. The HD target is expected to be efficiently polarized under a well-controlled condition by adding an optimal amount of ortho-H₂ to the purified HD gas.     

Development of a high-efficiency pulsed slow positron beam for measurements with orthopositronium in vacuum

We have developed a high-efficiency pulsed slow positron beam for experiments with orthopositronium in vacuum. The new pulsing scheme is based on a double-gap coaxial buncher powered by an RF pulse of appropriate shape. The modulation of the positron velocity in the two gaps is used to adjust their time-of-flight to a target. This pulsing scheme allows to minimize non-linear aberrations in the bunching process and to efficiently compress positron pulses with an initial pulse duration ranging from 300 to 50 ns into bunches of 2.3 to 0.4 ns width, respectively, with a repetition period of . The compression ratio achieved is 100, which is a factor 5 better than has been previously obtained with slow positron beams based on a single buncher. Requirements on the degree, to which the moderated positrons should be mono-energetic and on the precision of the waveform generation are presented. Possible applications of the new pulsed positron beam for measurements of thin films are discussed.     

光学计量用高速线阵CCD数据采集系统设计

介绍一种采用高速线阵 CCD 数据采集系统的设计过程。通过一片 MicroChip 公司的在线可编程微控制器对采集系统进行控制,并用标准 RS232 串行接口与外部进行数据通信和控制。在时序逻辑设计中,较好地解决了数据的存储、读取和点像元数据连续监测。实际应用表明该系统能满足光学干涉计量中线阵 CCD 数据采集要求。由于可以对线阵 CCD 进行单点、多点和整块数据的读取处理,因而可适应于不同周期、不同像素数目的线阵 CCD 传感器,以满足多种不同需求。     

PONTO—An instrument for imaging with polarized neutrons

An imaging instrument dedicated to radiography and tomography with polarized cold neutrons was installed and successfully tested at the BER II reactor at Helmholtz Zentrum Berlin. The spatial resolution for un-polarized neutrons was less than 85 μm for a point-to-detector distance of 5 mm, in the case of polarized neutrons it was less than 300 μm. With these resolutions, details of magnetic fields in different lead samples below the critical temperature being in the Meissner phase and magnetic flux pinning could be imaged.     

正电子湮没技术在高分子薄膜研究中的应用

介绍了正电子湮没技术在高分子薄膜研究中的最新进展.常规的正电子湮没技术和慢正电子湮没技术,可以准确地探测高分子薄膜微观缺陷(自由体积)尺寸、分数、浓度、深度分布,该测试技术将在研究各种高分子薄膜的微结构-性能关系、表面效应、界面效应等方面发挥积极的作用.     

Dynamic nuclear polarization in crystals of Nd:LaAlO3, a polarized La target for a test of time-reversal invariance

Dynamic nuclear polarization (DNP) has been performed in a magnetic field of 2.35 T at a temperature below 0.3 K in crystals of Nd³⁺ :LaAlO₃. A Q-meter NMR-system was used to measure the polarization of the ²⁷Al and ¹³⁹La nuclei and allowed to characterize the quality of the crystals. The ¹³⁹La polarizations of +47.5% and −56.5% achieved represent an important step in the development of a polarized ¹³⁹La target for a test of time reversal invariance in low-energy neutron reactions.     

改进的直线拟合线阵CCD图像边缘检测方法

为了检测经过光学系统成像所得图像的边缘,在直线拟合方法的基础上提出了一种新的边缘检测方法。该方法综合考虑边缘过渡区间所有点的信息,利用直线拟合方法求出一组可能的边缘值,再选择它们的中值作为最终的边缘值。基于该方法的采集系统由线阵 CCD 采集边缘的灰度图像,使用单片机来处理数据,并计算出边缘的位置。实验结果表明,这种改进方法的重复性误差为 0.89 个像素,而且能够在 0.039s 内完成一次边缘检测计算。     

The linear sampling method for a mixed scattering problem

This paper is concerned with the scattering problem of time-harmonic acoustic plane waves by a mixed scatterer, which is a combination of an open crack and an impenetrable obstacle. Firstly, the well-posedness of the solution to the direct scattering problem is established using the variational method. Then, a uniqueness result for the inverse scattering problem is proved, that is, both of the crack and the impenetrable obstacle can be uniquely determined simultaneously by the knowledge of the far-field pattern. Furthermore, a mathematical basis is given to reconstruct the shape of the crack and the impenetrable obstacle using the linear sampling method, and some numerical examples are given to establish the viability of our approach.     

Graphene nanoring as a tunable source of polarized electrons

We propose a novel spin filter based on a graphene nanoring fabricated above a ferromagnetic strip. The exchange interaction between the magnetic moments of the ions in the ferromagnet and the electron spin splits the electronic states, and gives rise to spin polarization of the conductance and the total electric current. We demonstrate that both the current and its polarization can be controlled by a side-gate voltage. This opens the possibility to use the proposed device as a tunable source of polarized electrons.     

An algorithm for computing a companion block diagonal form for a system of linear differential equations

In this paper we describe a new algorithm which reduces in a finite number of steps a linear system of differential equations to a companion block diagonal form. This form is particularly convenient if one wishes to compute invariants at singularities.     

用于多色光的波片的设计

在偏振光路中有时需要交替使用不同波长的光束,为使光路保持最佳状态,常需要根据不同的工作波长选用相应波长的波片。为此提出了多波长波片的概念,给出了多波长波片的设计方法,具体设计了有实用价值的双波长四边之一波片,并成功地应用于一个实际的光学系统中,从而验证了多皮长波片设计思想的正确性。虽然多波长波片不一定能完全等效于多个严格意义上的单波长波片,但在一定的误差范围内所设计的多波长波片是能满足实际需要的,从而避免了频繁更换不同波长波片的麻烦。     

正电子湮没技术在高聚物中的应用

本文介绍了正电子湮没技术的原理、技术方法 ,在自由体积测试方面的应用 ,以及国内研究进展。