Heating of solid matter via laser generated electrons

Abstract

This thesis reports on numerical studies of the heating of solid targets via laser generated fast electron beams, which allows for better understanding and optimisation of the generation of hot dense matter in a laboratory setting. The work is split into 3 main investigations which are summarised below. In the first investigation, simple analytical scalings of target temperatures with laser-target parameters, I L , λ L , n i , produced by the Spitzer resistivity and a prescriptive low temperature for Ohmic heating were compared using a 3D Hybrid-PIC code. Results showed that the scalings from the Spitzer resistivity underestimate the overall dependencies and the numerical results are found to be better fit by the prescriptive low temperature resistivity model. Thus, not only is the Spitzer resistivity insufficient to characterise the full evolution of a target but it is also shown that the low temperature regime of the target plays a pivotal role in the evolution of the target. In the second investigation, assumptions made in the previous investigation, constant ionisation and specific heat capacity, were explored via a 0D heating model. Results showed that the assumption had little impact on the scalings from the 0D model. It was noted, however, that fixing the ionisation and specific heat capacity resulted in higher overall temperatures. This was expanded upon by varying these two independently while also considering a more complete resistivity model which accounted for electron statistics and collisions. It was found that fixing the specific heat capacity produces higher temperatures compared to fixing the ionisation state. These results show that the scaling relations derived in the previous Chapter will be incorrect due to the ideal gas capacity having no temperature dependence. Furthermore, these results indicate how important the selection of the specific heat capacity is for a correct characterisation of the target. In the third investigation, the generation of uniform heating for potential use in heating experiments was considered from various designs of the inverse conical taper using a 3D Hybrid-PIC code. Results showed that a 2-material design with a “square” geometry of an Al cone, which has a truncated inverse pyramid-like shape in this geometry, and Cu wire were found to produce regions of uniform heating. This occurred due to inhomogeneous transport in the fast electron beam, which is not present in a 1-material design, e.g. Al, where a monotonically decreasing current density is found. While the exact reasons were not fully elcuidated, various investigations were carried out to consider the optimum periods of uniform heating. First, the Z value of the wire was then varied to see its role in the uniform heating. It was found that both Z values smaller and larger than Cu resulted in less uniform heating. Work then considered varying the I L & λ L with these producing contrasting results. Due to the strong dependence of the current density on λ L , the variation results in dramatically different transport patterns. Increasing λ L , decreasing current density, results in uniform heating occurring in a different position. Decreasing λ L , increasing current density, results in larger in terior magnetic fields, producing a completely different transport pattern. On the other hand, varying I L produces similar transport patterns. Increasing I L , results in a smoother inhomogeneous transport pattern and thus an absence of uniform heating while decreasing I L enhances the inhomogeneous transport and results in periods of uniform heating. Further work considered whether the uniform heating was limited to certain values of the fast electron conversion efficiency, β. Results showed that the uniform heating was independent of β but the position of the uniform heating was varied due to the variation in current density. Finally, a cylindrical geometry of the same area was considered to investigate the production of this uniform heating. Results showed that uniform heating was also produced in this geometry but that the transport patterns differed greatly. This arised due to the larger radius of the cylindrical geometry, leading to a weaker field generation meaning that the fast electron beam went under a more stable transport compared to the “square” approach.

Date of Award	Dec 2018
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Marco Borghesi (Supervisor), Satyabrata Kar (Supervisor) & Alexander Robinson (Supervisor)