【作者】 景群；

【导师】 赵建明；

【作者基本信息】 山西大学 ， 原子与分子物理， 2009， 硕士

【摘要】 等离子体通常被认为是物质的第四种状态,是宇宙中最普通的物质态,分布范围也非常广泛,从温度约为1016K的白矮星磁化层到300K的地球电离层。我们通常所说的传统等离子体,大都是通过粒子之间的碰撞电离形成的,因为一般的碰撞势要求为几个电子伏特,一个电子伏特相当于11600开尔文,所以传统等离子体一般都是高温等离子体。近几年来,随着激光冷却和俘获技术的实现,为低温等离子体的产生奠定了坚实的基础,而我们所要研究的超冷等离子体就属于低温等离子体。与高温等离子体不同的是,低温等离子体中粒子之间的库仑相互作用能大于它们的平均动能,属于强耦合等离子体。所以能用于传统等离子体物理中的一些基本理论就无法来解释在低温等离子体物理中存在的一些奇特现象,如等离子体中的相变过程,多体空间相关联等。由于超冷等离子体的温度比较低,所以我们通常使用荧光探测、吸收成像探测和带电粒子探测技术三种探测方法对超冷等离子进行实验探测。超冷等离子体的产生大大拓展了传统等离子体物理研究的范围,使等离子体物理进入了一个新的研究领域,所以超冷等离子体的产生和应用将成为更有挑战性和开拓性的工作。另外,等离子体物理的发展还为材料、能源、信息、环境,空间物理和地球物理等科学的进一步发展提供了新的技术和工艺。本文首先扼要地介绍了等离子体的定义以及超冷等离子体的特征参量,其次详细地探讨了超冷等离子体产生的两种方法,在激光冷却和俘获铯原子的基础上分别利用双光子电离和里德堡原子自发演化的方法直接和间接地得到了超冷等离子体,最后还研究了超冷等离子体的扩散过程和里德堡原子的再复合过程。本文的主要内容如下：第一章为引言,介绍了等离子体的定义、特征参量以及超冷等离子体在国内外的研究进展。第二章介绍了超冷等离子体的产生方法、探测装置以及超冷等离子体实验的总光路图。第三章利用双光子直接电离的方法产生了超冷等离子体,并用一个简单的模型解释了超冷等离子体产生的物理机制,最后对实验结果进行了分析。第四章由双光子激发获得超冷里德堡原子,在一定条件下研究了由里德堡原子通过自发演化的方法间接获得了超冷等离子体,并对演化产生机制加以分析。第五章在超冷等离子体扩散30μs后,增加一个脉冲电场,又观测到了里德堡原子的再复合过程,并对超冷等离子体的扩散和再复合机制做了简要分析。其中有创新性的工作是：1、分别通过双光子电离冷原子的方法和超冷里德堡原子自发演化的方法成功地获得了超冷等离子体,并对两种产生超冷等离子体的物理机制做了进一步的分析；2、研究了超冷等离子体的扩散和再复合过程,并在超冷等离子体的扩散过程中又观察到了里德堡原子的再复合。更多还原

【Abstract】 Plasma is the fourth species state of matter. Plasma is the most common state of matter in the universe, spans an incredible range of parameters, temperatures ranging from 1016K in the magnetosphere of a pulsar to 300 K in the earth’s ionosphere. In a conventional neutral plasma, ions and electrons are created from atoms and molecules by ionizing collisions between particles. Because a typical ionization potential is on the order of an electronvolt, most neutral plasmas have temperatures of thousands of kelvin or more. So the plasma of low temperature was very difficult to obtain. However, the advanced technology of cooling and trapping of neutral atoms provided people a basis to research the ultracold plasma. At lower temperatures, the plasma becomes strongly coupled, the properties of which are expected to differ significantly. So it would be quite interesting that strongly coupled plasma, in which electrical interaction energy between charged particles exceeds the average kinetic energy, does not obey some fundamental assumptions of classical plasma and it will have some special properties, for example, many-body spatial correlations and phase transitions and so on. The dynamics behaviors in ultracold plasma are usually able to be easier to observe and study by using the optical probes and the charged particle detection. The ultracold plasmas extend the studied field of conventional plasma physics, in other words,they represent a new frontier in the study of neutral plasmas, which traditionally deals with much hotter systems, but they also blur the boundaries of plasma, atomic, condensed matter, and low temperature physics. In a word, the ultracold plasmas physics is a new field that is full of challenges. The definition and characteristic parameters of plasma is introduced firstly. Two methods to create ultracold neutral plasmas in cooled atoms are discussed in detail. The Cs atoms are cooled and trapped by a ultrahigh-vacuum chamber experimentally. Ultracold plasmas are created directly by photoionizing laser-cooled atoms near the ionization threshold and indirectly by spontaneous evolution of a dense Rydberg gas. A simple model has been introduced to explain the physical processes in creating ultracold plasma. The Rydberg atoms are formed by recombination in expanding ultracold plasmas. The main contents are as follows:The first chapter is introduction. Some some basic concepts on plasmas and the properties and parameters of plasmas will be introduced. The developing process and status of ultracold plasmas are reported in summary.In the second chapter, two methods to create ultracold neutral plasmas in cooled atoms are described. The experimental systems of ultracold plasmas will be introduced, including the detection of ultracold neutral plasmas.In the third chapter, the signals of an ultracold plasma are observed by photoionizing laser-cooled atoms in a cesium MOT. A simple model has been introduced to explain the creation of plasma, and the mechanism is further investigated by changing the energy of a pulsed dye laser and the number of initial cooled atoms, respectively.In the fourth chapter, the spontaneous evolution from ultracold Rydberg atoms to plasma will be studied by using the method of field ionization pulse. The processes of the evolution and the ionization mechanisms will be discussed partly.In the fifth chapter, We have observed the recombination of Rydberg atoms in the expansion process of the ultracold plasmas. The relevant recombination mechanisms will be analysed briefly.The innovative work among the above in the article is:1、The ultracold plasmas are obtained by photoionizing laser-cooled atoms near the ionization threshold and by the spontaneous evolution from ultracold Rydberg atoms in cesium MOT respectively. The physical mechanisms of creating ultracold plasmas are further described.2、The recombination of Rydberg atoms in the expanding ultracold plasmas is studied.更多还原