The design and experimental characterisation of a stigmatic XUV instrument for laser plasma spectroscopy

Abstract

The optical system of an extreme ultra-violet (XUV) spectrograph was modified to produce high speed, spatially resolved spectra of small sources m the spectral range of about 5 to 25 nm. This was achieved by the use of a toroidal focusing mirror placed before the spectrograph. The instrument was used to record photoabsorption spectra of laser-produced plasmas in a technique employing a continuum emitted by a second laser plasma, known as the dual laser-produced plasma (DLP) method. The optical theory of the spectrograph and the toroidal mirror are examined in chapter 1 to analyse the imaging properties of the system and determine how best to use a toroidal mirror to compensate for the main aberrations of the concave grating. The spectrograph and experimental arrangement, as previously used to achieve DLP photoabsorption, is described in detail in chapter 2. This description includes a characterisation of the optical system by raytracing. The modifications to the expenment required to effect the improvements are described m chapter 3. This includes a descnption of the design of the optical system, housing and mounting structures and vacuum system. The alignment process and experimental method are detailed in the form of an user’s guide and the chapter is concludconcluded by an in-depth analysis of the imaging properties of the spectrograph system by ray-tracing. The final chapter contains a presentation of the experimental results obtained after careful testing and optimisation of the instrument. The combination of time and space resolution has facilitated the isolation of singly ionised aluminium, the photoabsorption spectrum of which is presented here for the first time The experimental results are compared to the ray-tracing analyses to test the validity of the modelling process and the spatial characteristics of the photoabsorption expenment are thus deduced The salient features of the optical system before and after modification are compared to determine the performance gain achieved The thesis is concluded by a cntical appraisal of the instrument as a tool for studying the photoabsorption spectroscopy of laser plasmas