Mapping neutral, ion and electron number densities within laser ablated plasma plumes

Spatially and temporally varying neutral, ion and electron number densities have been mapped out within laser ablated plasma plumes expanding into vacuum. Ablation of a magnesium target was performed using a KrF laser, 30 ns pulse duration and 248 nm wavelength. During the initial stage of plasma expansion (t <EQ 100 ns) interferometry has been used to obtain line averaged electron number densities, for laser power densities on target in the range 1.3 - 3.0 X 108 W/cm2. Later in the plasma expansion (t equals 1 microsecond(s) ) simultaneous absorption and laser induced fluorescence spectroscopy has been used to determine 3D neutral and ion number densities, for a power density equal to 6.7 X 107 W/cm2. Two distinct regions within the plume were identified. One is a fast component (approximately 106 cm-1) consisting of ions and neutrals with maximum number densities observed to be approximately 30 and 4 X 1012 cm-3 respectively, and the second consists of slow moving neutral material at a number density of up to 1015 cm-3. Additionally a Langmuir probe has been used to obtain ion and electron number densities at very late times in the plasma expansion (1 microsecond(s) <EQ t <EQ 15 microsecond(s) ). A copper target was ablated using a Nd:YAG laser, 7.5 ns duration and 532 nm (2 (omega) ) wavelength, with a power density on target equal to 6 X 108 W/cm2. Two regions within the plume with different velocities were observed. Within a fast component (approximately 3 X 106 cms-1) electron and ion number densities of the order 5 X 1012 cm-3 were observed and within the second slower component (approximately 106 cms-1) electron and ion number densities of the order 1 - 2 X 1013 cm-3 were determined.

A silent witness? Medieval urban landscapes and unfolding their mapping histories

This paper offers a critical reflection on the place of maps in writing medieval urban histories. Using findings from recent research on medieval Swansea, the case is made that mapping provides an interpretative space for exploring alternative narratives about past places. To do so the paper draws upon current critical debates on cartography, particularly the idea that mapping is fluid and iterative, to suggest that Swansea's medieval urban origins are open to a range of alternative interpretations. This approach to mapping differs from that often used by historians to map medieval urban landscapes, where historic maps are simply used as ‘sources’, and the landscapes they represent used as ‘witnesses’ to past events, for creating maps, both through digital and analogue media, instead opens up – or unfolds – a landscape's past. The paper uses past attempts to map medieval Swansea to highlight difficulties in interpreting its urban landscape features, and uses multiple mappings of the medieval townscape, resulting from recent research, to question how far any sources about the past really provide a coherent narrative. Instead, multiple mappings of the medieval urban landscape – reflecting different and competing perspectives – are more attuned with how places were perceived and understood during the Middle Ages.