Towards cooling of high-intensity ion beams

Buffer gas beam coolers may become excellent beam preparation devices for high-resolution mass separation. The small beam emittance provided makes efficient isobar resolution a realistic goal. In order to fulfill the needs of future facilities providing high-intensity beams of rare isotopes, it is desirable to increase the beam intensity limit of such devices from typically several tens of nanoamperes to microamperes. This requires the usage of high-voltage radiofrequencies in a low-pressure gas environment. A buffer gas beam cooler, dedicated to this purpose, is under development at the NSCL. The study of voltage breakdowns under such conditions and the design of an electrode system minimizing them is mandatory.      

金属表面下氦泡融合与释放研究

采用分子动力学方法研究了钛金属表面下不同深度处氦泡的行为,分析了氦泡融合与释放的竞争,对比了不同深度处氦泡的释放对金属的影响。结果表明：在接近金属表面处,氦泡很难通过融合无限长大,当达到临界尺寸后,氦泡将会释放而不再与邻近的氦泡发生融合;植入深度对氦泡的融合有一定的影响,深度越大,越有利于形成具有较高氦密度的大氦泡;较深处氦泡的释放会在金属表面形成较大的突起和表面针孔。实验中观察到的不同尺寸的表面孔,其部分原因来自于金属表面下不同深度处氦泡的释放。     

直流磁控溅射沉积含He钛膜的研究

研究了用He/Ar混合溅射气体的直流磁控溅射制备钛膜中，He的掺入现象.分析结果表明，大量的He原子（He/Ti原子比高达56%）被均匀地引入到Ti膜中，其He含量可由混合溅射气体的He分量精确控制.通过调节溅射参数，可实现样品中He的低损伤引入.研究还发现，溅射沉积的含氦Ti膜具有较高的He成泡剂量和高的固He能力，这可能是溅射沉积形成了纳米晶Ti膜所致.纳米晶Ti膜较粗晶材料具有很高浓度的He捕陷中心，使He泡密度增大而泡尺寸减小.随He引入量的增加，Ti膜的晶粒尺寸减小，He引起的晶体点阵参数和X射线衍射峰宽度增大，晶体的无序程度增加.Helium trapping in the Ti films deposited by DC magnetron sputtering with a He/Ar mixture was studied. He atoms with a surprisingly high concentration (He/Ti atomic ratio is as high as 56%)incorporate evenly in deposited film. The trapped amount of He can be controlled by the helium partial amount. The introduction of the helium with no extra damage(or very low damage) can be realized by choosing suitable deposition conditions. It was also found that because of the formation of nanophase Ti film a relative high He flux for bubble formation is needed and the amount of the retain He in sputtering Ti films is much higher than that in the coarse grain Ti films. The nanophase Ti film can accommodate larger concentration of trapped sites to He, which results in a high density and small size of the He bubbles. With the increasing He irradiation flux, the grain size of Ti film decreases and the lattice spacing and width of the X ray diffraction peak increase due to the He introduction, and the film tends to amorphous phase.     

Circular polarization of the (1s2p)2 → (1s2)0 line emitted by 205Tl79+ ions after collisional excitation by an unpolarized electron beam

The circular polarization of the (1s2p)_J=2 → (1s²)_J=0 line emitted by the helium-like ion ²⁰⁵Tl⁷⁹⁺ following impact excitation by a nonpolarized electron beam is theoretically studied. The probability for the hyperfine induced E1 transition (1s2p)₂F=3/2→1s² has been determined from the first order perturbation theory. Various collision strengths for excitation of thallium ions to the (1s2p)₂M_J=0, 1 and 2 magnetic sublevels have been used. It is found that the interference between M2 and E1 radiation occurring in the line can give rise to a significant degree of circular polarization. This revised version was published online in August 2006 with corrections to the Cover Date.     

激光二极管抽运氦气冷却钕玻璃叠片激光放大器热致波前畸变和应力双折射的数值模拟和实验研究

热效应仍是限制激光放大器向高功率、高光束质量进一步发展的瓶颈问题, 高效的热管理是抑制热效应的重要技术途径. 研究了基于激光二极管抽运氦气冷却的钕玻璃叠片激光放大器的热效应. 利用有限元数值模拟的方法, 分析了钕玻璃的温度、应力应变和应力双折射分布, 并计算了热致波前畸变和退偏损耗, 与实验结果符合得较好, 在热沉积为0.7 W/cm³的条件下, 6片钕玻璃总的热致波前畸变为6.77λ , 最大退偏量大于90%.     

冲击加载下金属铝中氦泡演化行为的相场模拟

氦泡等缺陷对金属材料动态强度的影响一直是动态强度研究关注的重点。将相场方法引入冲击加载下氦泡演化行为研究中,通过与晶体塑性理论耦合,建立了可描述冲击下氦泡早期演化行为的介观模拟技术。应用该方法,针对含氦泡的金属铝材料,从介观尺度对氦泡的演化行为及其对位错集体演化行为的影响进行了研究。结果表明:氦泡结构的非均匀性导致局域应力集中和塑性变形集中,局域塑性变形集中会导致沿冲击波传播方向发射稀疏波;从能量守恒角度上看,在材料变形过程中氦泡生长与塑性变形呈竞争关系,塑性耗散的快慢直接影响氦泡的生长速率,使其发生改变。研究结果可为解读含氦泡材料的宏观屈服强度和层裂行为提供理论支撑。     

高温高压金刚石单晶生长界面的研究

 利用扫描电镜及Auger电子谱技术研究了高温高压下金刚石单晶生长界面的特性，观察到了金刚石单晶表面及金属膜表面的沟槽结构及金刚石-金属、金属-石墨两个主界面间的过渡层结构及界面间C原子电子组态的变化。     

静压及范德瓦尔斯压力对超流氦传热的影响

1引言在过冷态超流氦（HellP）被成功地获得并应用在超导磁体冷却后，超流氦的工程应用引起了低温工程和低温物理研究者的重视，各种各样的HellP制冷机（恒温器）相继出现。同时出现了很多有关Hell浴中传热的论文山；然而从未有过有关压力对传热影响的系统研究。本文以Hell传热的基本规律及Hell的相图为基础，结合传热实验结果系统地研究了压力对Hell传热的影响。2P＜PA时Hell中的最大临界热负荷软2·1饱和态起流氧2·1·1无噪声沸腾Hell传热中试样达到其最大临界热负荷伽的前提条件是必须克服一过冷度凸Tin。x。如图1氦的相图所示，…     

Feature of the magnetotransport of a quasi-one-dimensional electrons on the superfluid helium

The magnetotransport in a nondegenerate quasi-one-dimensional (Q1D) electron system over superfluid helium has been investigated experimentally. The measurements are performed in the presence of a perpendicular magnetic field B up to 2.6 T in the temperature range T=0.48–2.05 K in the system of conducting channels of 100–400 nm width. It is shown that the value of longitudinal magnetoresistance ρ_xx increases with B. In the electron-gas scattering region (T>0.9 ), the behaviour of ρ_xx agrees with classical Drude law. In the quantum transport regime, the self-consistent Born approximation (SCBA) theory for a 2D electron system over liquid helium describes the experimental data qualitatively. The deviation due to the difference of the experimentally studied Q1D system of the electrons in a parabolic potential well differs from theoretically analysed one. The experimental data agree with the theoretical calculation for the Q1D electron system at the weak magnetic field and the low temperature.

The negative magnetoresistance of the conducting channels has been observed in both the gas- and the ripplon-scattering region. These effects have been explained by weak carrier localization on the gas atoms at high temperature and by display of the quantum magnetotransport features in a mesoscopic system at low temperature.     

磁场中液氦薄膜表面电子涟波子系统的性质

研究了磁场中液氦薄膜表面电子与涟波子强耦合和弱耦合的性质。采用线性组合算符方法导出磁场中液氦薄膜表面电子 涟波子系统的振动频率和基态能量。讨论磁场对表面电子 涟波子系统的振动频率和基态能量的影响。