Detection of T2 changes in an early mouse brain tumor

The aim of the study was to determine the effect of early tumor growth on T₂ relaxation times in an experimental glioma model. A 9.4-T magnetic resonance imaging (MRI) system was used for the investigations. An animal model (n=12) of glioma was established using an intracranial inoculation of U87MGdEGFRvIII cells. The imaging studies were performed from Day 10 through Day 13 following tumor inoculation. Tumor blood vessel density was determined using quantitative immunochemistry. Tumor volume was measured daily using MR images. T₂ values of the tumor were measured in five areas across the tumor and calculated using a single exponential fitting of the echo train. The measurements on Days 10 and 13 after tumor inoculation showed a 20% increase in T₂. The changes in T₂ correlated with the size of the tumor. Statistically significant differences in T₂ values were observed between the edge of the tumor and the brain tissue on Days 11, 12 and 13 (P=.014, .008, .001, respectively), but not on Day 10 (P=.364). The results show that T₂-weighted MRI may not detect glioma during an early phase of growth. T₂ increases in growing glioma and varies heterogenously across the tumor.     

Jamming and diode effects for vortices in nanostructured superconductors

We examine jamming and ratchet effects for vortex matter in superconductors with asymmetric funnel geometries. We show that the vortex–vortex interactions can induce a clogging or jamming effect where it becomes increasingly difficult for the vortices to move through the system. We also find that commensurability effects can arise when certain vortex configurations form highly symmetrical structures in the funnel plaquettes. Due to the asymmetry, the critical currents are different for driving in different directions, leading to a diode effect. We also discuss other possible geometries and approaches that could be used to explore jamming in vortex matter, such as an analog to a granular hopper and a single driven vortex probe moving through an array of other vortices.     

Driving an individual vortex in the presence of a periodic pinning array

Recently it has been demonstrated experimentally that it is possible to manipulate an individual vortex in a type-II superconductor using a magnetic force microscope tip. Using numerical simulations, we investigate the dynamics of a single driven vortex in the presence of a periodic pinning array and other vortices. Remarkably, we find that the effective drag on the driven vortex is reduced at the matching fields, which is opposite from the behavior of the critical current when all the vortices are driven. We discuss this effect in the context of the type of dynamics that occur at matching and nonmatching fields.     

Superconducting transport in an LED with Nb electrodes

Superconducting transport is measured between two superconducting electrodes at the n-type semiconductor side of a superconductor-based LED where a Josephson junction is formed. The characteristics of the Josephson junction are found to be modulated by applying voltage to the normal electrode at the p-type semiconductor side. The Josephson junction characteristics show an extraordinary sensitivity to the radiative recombination process, which we estimate as the recombination efficiency.     

Vortex structures in nano-scaled superconducting networks

Using the de Gennes–Alexander equation, we have investigated the stable vortex structures in finite superconducting networks (10 × 10 holes) with disordered wires under an external magnetic field. Vortex structures change gradually with increasing magnetic field. For the network with a disordered wire at the edge, vortices are not pinned disordered hole, but enter into the network at the holes with the disorder. But for the network with two disordered wires, the vortex enters at the hole between two disordered wires. This behavior can be considered as the result of the nonlocality of superconductivity.     

Multi-vortex versus interstitial vortices scenario in superconducting antidot arrays

In superconducting thin films, engineered lattice of antidots (holes) act as an array of columnar pinning sites for the vortices and thus lead to vortex matching phenomena at commensurate fields guided by the lattice spacing. The strength and nature of vortex pinning is determined by the geometrical characteristics of the antidot lattice (such as the lattice spacing a₀, antidot diameter d, lattice symmetry, and orientation) along with the characteristic length scales of the superconducting thin films, viz., the coherence length (ξ) and the penetration depth (λ). There are at least two competing scenarios: (i) multiple vortices sit on each of the antidots at a higher matching period and (ii) there is nucleation of vortices at the interstitial sites at higher matching periods. Furthermore, it is also possible for the nucleated interstitial vortices to reorder under suitable conditions. We present our experimental results on NbN antidot arrays in the light of the above scenarios.     

Terahertz transmission of NbN superconductor thin film

Transmission of terahertz waves through a thin layer of the NbN superconductor deposited on a sapphire substrate was studied as a function of temperature in zero field as well as in magnetic field perpendicular to the sample. For photon energies lower than optical gap, detailed temperature measurements in zero field provide BCS-like curves with a pronounced peak below the critical temperature. In accordance with the BCS model, the temperature peak disappears as the energy of incident radiation is increased above the gap. In non-zero field, the temperature behavior of transmission is modified because the gap is suppressed and vanishes at upper critical field. In addition, the presence of quantized vortices in the superconducting film substantially changes shape of the temperature curves.     

轴对称磁透镜设计

以中国科学院近代物理研究所正在研制的轴对称磁透镜为例,设计了两种结构的轴对称磁透镜:带屏蔽铁壳的螺线管透镜和极靴形状为锥形的Glaser透镜。带屏蔽铁壳的螺线管透镜产生的轴对称磁场比较均匀,而极靴形状为锥形的Glaser透镜产生的轴对称磁场比较集中。分别采用了两种物理模型计算磁场,同时应用有限元分析软件ANSYS进行数值模拟,最后从物理和工程角度对这两种透镜作了全面比较,得出带屏蔽铁壳的螺线管透镜结构更符合设计要求。     

宽平顶低横向场分量螺线管线圈设计

介绍了非均匀绕线技术和匀场环技术在螺线管线圈设计中的应用。分析表明这两种技术的采用可以减小线圈间隙的磁场波动、屏蔽线圈磁场的横向分量,从而设计出宽平顶低横向场分量的螺线管线圈。针对中国工程物理研究院12 MeV直线感应加速器螺旋管线圈的设计结果表明,非均匀绕线技术可以使线圈间隙处的轴向磁场波动减小约70%,匀场环的采用则能使线圈磁场的横向分量减小96.5%。     

50kA超导变压器初级绕组设计制作及低温通电实验

主要讨论了超导变压器初级绕组的设计、绕制以及低温通电实验,初级绕组主要采用2 942匝NbTi超导股线绕制,并经过真空压力浸渍而成。利用Ansys软件分析了其磁场分布。采用LHe浸泡式制冷,最大电流可达190A,中心场约为2T。