Contamination from stray reflections

Stray reflections in the chamber and also reflection internal to the optical system can cause phase mixing from light from different regions of the plasma which will result in contamination of the temperature measurements.

Reflections in MAGPIE are tricky to quantify as this involves knowing the reflective prop- erties of the chamber. To investigate the extent of these reflections, coherence imaging measurements of the plasma were made with and without blackened view-dump sheeting installed in the chamber. A radial cross-section for the contrast in the high magnetic field region at 3 mT with and without the view-dump sheeting is shown in figure 5.13 (c). The same study at 10 mT is also shown in figure 5.13 (d). The corresponding brightness profiles are shown in (a) and (b).

§5.2 Contamination effects 107 photon coun t 0 1000 5000 4000 3000 2000 6000 0 200 400 600 800 1000 j (pixel number) 0 2000 10000 8000 6000 4000 photon coun t 0 200 400 600 800 1000 j (pixel number) a) b) d) c) 0.2 0.6 0.4 0.0 Ion temper ature (e V) 0 200 400 600 800 1000 j (pixel number) Ion temper ature (e V) 0 200 400 600 800 1000 0.8 0.4 1.2 0.0 1.0 0.6 0.2 j (pixel number)

Figure 5.13: Figures (a) and (b) are plots of the measured radial brightness in MAGPIE at 3 mT and 10 mT and plots (c) and (d) show the corresponding ion temperature measurements, respectively. The plots are taken with (blue) and without (red) anti-reflective shielding installed.

core temperatures increase in the presence of reflections. This suggests that bright light from the cold plasma center reflects into the edge region and decreases the apparent tem- peratures at the edge while the low-light high-temperature edge light will reflect into the center region and increase the core apparent temperature. The 10 mT contrast study shows the same decrease in the apparent edge temperatures however no change the ap- parent central temperatures. From the brightness profile in figure (d) we see that the 10 mT plasma has a beam-like brightness profile and so the bright cold center will to have a much larger effect on temperature contamination on the edge features than vice-versa. These results confirm that reflections within the chamber can affect the apparent tem- perature profile (high edge-temperatures reduce and low core-temperatures increase) and demonstrate the need to use blackened view-dump sheeting.

Reflections within the optical system also present a concern in this study due to the large optical systems used (∼100 mm). Collimated light passing through the crystal cell (even at small angles) can be cut-off and internally scattered by the blackened optical cell walls. The diagram in figure 5.14 shows an example where the rays from two points in the plasma are traced through the imaging system. In this image the blue rays are generated at the edge of the plasma source and red rays are generated at the center of the plasma. The green arrow identify where rays from each of these points hit the internal walls of the crystal cell and become reflected (shown by the dashed rays). These reflected rays are directed into another part of the image on the CCD and have a different path length compared with the collimated rays focused at the same position. The path length difference can result in a phase difference and, once averaged with the unperturbed rays, will result in a decrease

108 Coherence imaging of the MAGPIE argon plasma: single condition study

in the contrast at that point. This effect can be minimised by decreasing the aperture of the front lens in order to stop edge vignetting effects in the instrument.

Figure 5.14: Diagram shows an illustrative example of how internal reflections within a coherence imaging system can be reflected by the walls of the system (indicated by the green arrows) and result in decreased instrumental contrast. Generally this effect can be minimised by reducing the aperture of the front lens.

Figure 5.15: shows (left) the radial brightness profile taken in the high magnetic field region at the reference condition. For each of these measurements the central segment of the image has blocked by a card (with increasing width). The second image (right) shows the corresponding

projected ion temperatures from radial positions of 2 cm (×) 2.5 cm (+) and 3 cm (?) shown

plotted against the percentage of the image blocked by the card.

An attempt to quantify the effect on the edge temperatures due to internal reflections was made by placing a card in the view of the imaging system so that the central light of the plasma was blocked. This eliminated the source of bright light which could reflect within the imaging system. By varying the card width the dependence on the edge temperature was observed. The brightness profiles measured in the high magnetic field region, with the central segment blocked by different card widths is shown in the left figure of 5.15. The corresponding projected ion temperature at radial positions of 2 cm (×), 2.5 cm (+) and 3 cm (?) are shown plotted against the percentage of the image blocked by the card, is shown in the right figure.

It was found that the edge temperatures decreased by blocking the central light (∼ 0.2 eV over the measurement). This suggests that phase-mixing due to internal reflections in the imaging system will result in an instrumental increase in the measured temperature.

§5.2 Contamination effects 109

Even though this is an instrumental effect, it is a product of the phase information in the plasma light and hence cannot be removed with calibration.

It is important to note that while this measurement blocks the central bright plasma light from reflecting into the edge of the image, it does not completely eliminate all internal reflections within the optical cell. For example light from the plasma edge (the blue rays in image 5.15) can still be internally reflected within the optical cell. This study therefore examines some of the effects of internal reflections but is not able to show the full extent of this contamination. In this study, the effects of reflections have only been examined for measurements of the temperature. It is expected that the phase will be ‘washed-out’ in presence of reflections due to phase-mixing resulting in an underestimate of the flows.