Instabilities in non-thermal plasmas

了解等离子体的不稳定性是很重要的，因为它们在受控核聚变方案的行为、产生高功率辐射的设备的性能以及发生在地球磁层、恒星和更奇特的天体物理物体中的现象中起着至关重要的作用。在许多情况下，这些不稳定性是由高能电子束产生的，本工作的主要目的是在实验室实验中研究这种电子束驱动的不稳定性，并通过详细的理论和计算分析来支持这一研究。这项研究建立在我们早期工作的基础上，这些工作集中在与强辐射源中寄生振荡开始相关的不稳定性，极光无线电发射和各种其他天体物理现象。实验结果与极光辐射的观测结果和理论相符。现在，研究的范围正在显著扩展到一个雄心勃勃的计划，旨在研究各种不稳定性，这些不稳定性是由不同实验几何中不同的高能电子分布所驱动的。这需要对原来的实验设备进行实质性的修改，以在产生快速电子时引入额外的灵活性，修改引导它们的磁场结构，并引入背景等离子体。我们将产生控制良好的等离子体，其中可以详细研究不稳定振荡的增长和最终饱和。结果将与计算机模拟和理论模型进行比较，以检查其准确性。我们将研究与一系列应用相关的这些不稳定性的不同方面，例如托卡马克。在这些限制等离子体达到聚变条件的环形装置中，射频驱动电流的方案产生了具有高速度和高能量的重要电子群。需要了解这些种群的稳定性以及它们在应对不稳定性时如何进化。在涉及激光压缩目标的核聚变方案中，进入目标中心区域的快速电子束是非常重要的，对它们行为的理解主要基于计算机模拟。在我们的大规模实验中，模拟结果与实际情况的关系将更加明显。高功率射频辐射的产生在很大程度上取决于高功率电子束的不稳定性及其能量转化为电磁波，从雷达到医疗都有广泛的应用。我们的工作应该对其中的一些过程，特别是对意外不稳定产生寄生辐射的过程有更深入的了解。最后，高能粒子束在空间和天体物理等离子体中非常常见，从地球的磁层到脉冲星和伽马射线暴。从这项研究中得到的对不稳定性的基本物理的理解将给理论和数值技术在其他真实等离子体环境中的应用带来信心，包括聚变实验。该项目汇集了一个在实验束等离子体系统方面具有广泛专业知识的团队，以及在应用于磁约束等离子体、激光产生等离子体和空间等离子体的理论和计算建模方面的广泛经验。该实验的目的是允许对广泛的条件进行调查，并隔离对磁和惯性聚变方案以及高功率辐射产生直接重要的特定不稳定性。

It is important to understand instabilities in plasmas since these play a crucial role in the behaviour of schemes for controlled nuclear fusion, the performance of devices for generating high power radiation and in phenomena taking place in the earth's magnetosphere, stars and more exotic astrophysical objects. In many cases these instabilities are generated by beams of high energy electrons, and the main objective of the present work is to study such beam driven instabilities in a laboratory experiment and support this study through detailed theoretical and computational analysis. This research builds on the foundation of our earlier work which concentrated on an instability of relevance to the onset of parasitic oscillations in powerful radiation sources, to auroral radio emissions and a variety of other astrophysical phenomena. Experimental results showed good agreement with the observations of auroral radiation and with theory. Now the range of the research is being significantly extended to an ambitious programme aimed at the study of a variety of instabilities driven by different distributions of energetic electrons in varying experimental geometries. This entails substantial modifications of the original experimental facility to introduce extra flexibility in the generation of the fast electrons, modifications to the magnetic field structure which guides them and the introduction of a background plasma.We will generate well-controlled plasmas in which the growth and eventual saturation of the unstable oscillations can be studied in detail. The results will be compared with computer simulations and with theoretical modelling with a view to checking their accuracy. We will investigate different aspects of these instabilities relevant to a range of applications including for example tokamaks. In these toroidal devices which confine plasmas are hoped to reach fusion conditions, schemes for radiofrequency driving of current produce an important population of electrons with a high velocity and energy. The stability of these populations and how they evolve in response to instabilities needs to be understood. In schemes for fusion involving laser compressed targets, beams of fast electrons moving into the central region of the target are very important and the understanding of their behaviour is largely based on computer simulation. The relation between simulation results and reality will be more easily seen in our large scale experiment. Generation of high power RF radiation, which has a wide range of applications ranging from RaDAR to medical treatment, largely depends on the instability of high power electron beams and the conversion of their energy into electromagnetic waves. Our work should give an enhanced understanding on some of these processes, in particular on the production of parasitic radiation from unintended instabilities. Finally, beams of high energy particles are very common in space and astrophysical plasmas, ranging from the earth's magnetosphere to pulsars and gamma ray bursters. The understanding of the basic physics of instabilities derived from this research will give confidence in the application of theoretical and numerical techniques to other real plasma environments, including Fusion experiments.The project brings together a team with extensive expertise in experimental beam plasma systems, together with wide experience in theory and computational modeling applied to magnetically confined plasma, laser produced plasma and space plasma. The experiment, designed to allow the investigation of a wide range of conditions and to isolate particular instabilities of direct importance to magnetic and inertial fusion schemes and high power radiation generation.