时  间：   2017年10月30日（星期一）下午14:30

题  目：修改引力中的Noether对称性和Hojman对称性

报告人：     薛冬泽             指导老师：韦浩

摘要：对称性在理论物理中有着重要的地位，不仅可以利用对称性进行精确解的求解，也可以令我们在更基本的层面上理解理论，并指导我们对模型的选取。近年来，在修改引力领域中，有着大量的对称性应用实例。其中Noether对称性和Hojman对称性是被广泛应用的两种对称性。在Noether对称性中，由对称性条件可以判断对称性的存在与否，并进行精确解的求解。而point-like拉氏量可以作为Noether对称性的前提。Hojman对称性则不需要给定拉氏量，而是通过理论的运动方程进行求解。

题  目：  激光诱导击穿光谱信号增强技术及机理的研究

报告人：    李安                         指导老师：刘瑞斌

摘要：The inevitable problems in laser induced breakdown spectroscopy are matrix effect and statistical fluctuation of the spectral signal which can be partly avoided by utilizing proper confined unit. The dependences of spectral signal enhancement on relative permittivity were studied by varying materials to confine the plasma, which include Polytetrafluoroethylene(PTFE), Nylon/Dacron, Silicagel, and Nitrile-butadiene rubber(NBR) with the relative permittivity 2.2, ~3.3, 3.6, 8~13, 15~22. We found higher relative permittivity rings induce stronger enhancement ability which restricts the energy dissipation of plasma better and due to the reflected electromagnetic wave from the wall of different materials, the electromagnetic field of plasma can be well confined make the distribution of plasma become more orderly. The spectral intensity of the characteristic lines Si I 243.5 nm and Si I 263.1 nm increases approximately 2 times with increasing relative permittivity values from 2.2 to ~20. The size dependent enhancement of PTFE was further checked and the maximum gain was realized by using confinement ring with the size of diameter 5 mm and height 3 mm(D5mmH3mm), and the rings with D2mmH1mm and D3mmH2mm also show higher enhancement factor. In view of peak shift, peak lost and accidental peaks in the obtained spectra were properly treated in data progressing, the spectral fluctuation decreases drastically for various material with different relative permittivity as confined unit, which means the core of plasma is stabilized attribute to the confinement effect. Furthermore, the quantitative analysis in coal show a wonderful result, the prediction fitting coefficient R² reaches 0.98 for ash and 0.99 for both volatile and carbon.

题 目：Experiment study on high density plasma sources based on hollow cathode

报告人：  司新路                 指导老师：何锋

摘要：Material in plasma treatment, microwave-plasma interactions and other applications require large-scale high-density plasma source. High-density plasma have been pushed from the hole of hollow cathode because of its hollow cathode effect (HCE). The bright negative、 region from hole of hollow cathode extend along to anode. Due to an external axial magnetic field and configuration of the cathode, the out plasma has been formed a column with high density. In this paper, the plasma was immersed in an external magnetic field from a set of Helmholz coils in argon. The magnetic field was oriented parallel to the electric field between the anode ground plane and a cylindrical hollow cathode. And different depths of the cathode can be changed to form the beam column in different experimental parameters to aim to finding out the optimal rates of depth to diameter. Experimental results show that the maximum density (order of magnitude is 11) of the beam column formed from cathode to anode is 0.5 Torr with the depth h="3mm." Meanwhile, the plasma column is denser and more stable by the pulse power supply than DC power supply.