Magnetic-field amplification in turbulent laser-plasmas

Thesis

Understanding magnetic-field generation in turbulent plasma is essential for explain- ing the presence of dynamically significant magnetic fields in astrophysical environments such as the intracluster medium. Seemingly plausible theoretical frameworks attributing the origin and sustainment of these fields to amplification by the so-called fluctuation dynamo are somewhat hampered by conceptual uncertainties concerning the validity of the models in which these frameworks are formulated. A recent experiment on the OMEGA laser facility attempted to overcome some of these uncertainties by demonstrating the feasibility of magnetic-field amplification by stochastic motions up to dynamical strengths in actual turbulent plasma. In order to realise the scientific goals of this experiment, accurate measurements of stochastic magnetic fields arising in turbulent laser-plasmas were required. This thesis reports on the development of an analysis technique which meets this requirement by recover- ing the magnetic-energy spectrum from proton imaging data, as well as the mean magnetic-energy density and characteristic structure sizes. The general applicability and reliability of the technique is considered in depth. On application to data derived from the OMEGA experiment, the magnetic-energy density is found to increase over five hundred times in the experiment from its initial value; in addition, estimates of the maximum magnetic field strength indicate that the field is likely to be dynamically significant. The experiment therefore constitutes the first demonstration in the laboratory of the fluctuation dynamo. The results of a second experiment on the OMEGA laser facility – in which a remodelled variant of the previously employed experimental platform is used to provide a time-resolved characterisation of a plasma dynamo’s evolution, measuring temperatures, densities, flow velocities and magnetic fields – are also described. It is shown that the initial growth of the dynamo-generated fields occurs exponentially at a rate which significantly exceeds the turnover rate of the driving-scale stochastic motions in the plasma. Both experiments validate the claim that the fluctuation dynamo is indeed capable of amplifying magnetic fields significantly.

Authors: Bott, A

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Astrophysical plasmas; Proton radiography; Plasma dynamo; Proton imaging
• Subjects: Laser plasmas