Calibration Stability Tests

In order to test the overall stability of the interferogram calibration to ambient conditions, images of a diffuser illuminated by a HeNe laser were recorded every 5 minutes for 48 hours in the lab. For the first 24 hours the temperature stabilised cell was turned off to observe the stability without active temperature stabilisation;

for the second 24 hours the active temperature stabilisation was enabled. Measure-ments of the ambient temperature and interferometer optics temperature were also obtained using thermocouples. Time histories of the ambient temperature, crystal plate temperature, fringe phase and fringe contrast are shown in figure 5.15. Phase and temperature measurements are the mean from a central 100x100 pixel region of the image.

The ambient temperature shows similar levels of fluctuation with a range of 4 - 5^◦C over both the stabilisation off and on periods. The plate temperature shows fluctuations of the same magnitude with the temperature control off, however the response to faster temperature changes is smoothed out due to the (passive) thermal insulation of the cell. With the temperature stabilisation enabled, the plate tem-perature varies with a range of 0.4^◦C, consistent with the controller specifications.

With the temperature control off, the fringe phase varies with a range of 2.2

ra-5.3. Interferometer Calibration 120

Tambient(º C)

0 5 10 15 20

20 22 24

Tplates(º C)

0 5 10 15 20

20 25 30 35

Δϕ9(rad)

0 5 10 15 20

-2 -1 0

Time9(hours)

ζ

0 5 10 15 20

0.7 0.8 0.9 1

Temp.9Control9off Temp.9Control9on

Figure 5.15: Time histories of ambient temperature, polarisation optics tempera-ture, fringe contrast and fringe phase over 24 hour periods with active temperature stabilisation of the optics off (blue lines) and on (green lines).

5.4. Summary 121 dians, equivalent to a flow calibration shift of almost 80km/s. With the temperature control on this is reduced to 0.3 rad or 12km/s in flow, somewhat larger than the 5km/s range expected from the stand-alone temperature sensitivity measurements.

This could be due to effects other than changes in the crystals themselves, such as thermal expansion in the mechanical structure of the system causing the crystals to shift slightly relative to the detector. The amount of fringe movement on the de-tector for a phase range of 0.3 rad at 633nm is 0.74 pixels, of which approximately 0.17 rad is expected due to changes in the crystals.

The fringe contrast shows approximately the same amount of variation with the temperature control on as off. This indicates that the contrast calibration drift is dominated by changes in the camera offset level, which will be the same in the stabilisation on and off cases. The range of contrast variations in both cases is ap-proximately 16 percentage points. Since this effect is primarily due to camera offset drifts, it is not expected to be observed during plasma operations since offset level calibration is performed per-shot, whereas it was performed only once for these sta-bility measurements. It is also observed that the contrast is consistently lower with the temperature control on than off. This is due to increased delay inhomogeneity in the crystals when their temperature is raised from ambient to a higher stabilised temperature, and this behaviour is consistent with the measurements in section 5.3.4. It is therefore desirable to operate the temperature controller at the lowest temperature where good stabilisation can be achieved, in order to avoid lowering the fringe contrast and therefore SNR unnecessarily.

Overall the stability measurements show considerable variation in the calibration parameters, at a level at which calibration monitoring during plasma operations is required. In order to improve the stability, it would be desirable to more carefully account for thermal expansion effects in the mechanical design of any future instru-ment, and to investigate methods of better stabilising the thermal effects in the plates.

5.4 Summary

In this chapter, various aspects of the behaviour and performance of the MAST CIS diagnostic hardware have been investigated experimentally. The detector response and noise level have been measured, and in the next chapter will be shown to result in flow measurement noise of around 1km/s under typical measurement conditions on MAST. Difficulties with accurate contrast calibration due to dark level drift of the camera preclude ion temperature measurements with the MAST instrument.

5.4. Summary 122 The line selection band pass filters were found to meet the requirements set out in chapter 3, and are not expected to cause any significant flow measurement errors. The absolute light sensitivity and vignetting of the assembled diagnostic were measured and were within approximately 20% of the design calculations.

The interferometer group delay has been calibrated and found to agree well with the design specifications for two of the three delay plates, although the 9.8mm plate produces a delay around 5% larger than the specifications. This makes the 9.8mm plate less useful for C II measurements but does not affect the baseline flow measurement configuration. Errors on the group delay calibration are ≤ 1.1% and are not expected to be a significant source of measurement error. Measurements of the fringe contrast show the contrast to be highest at the edges of the field of view, which is qualitatively explained by non-uniformities in the interferometer components. Measurements of the calibration stability in the lab confirmed the need for active temperature stabilisation of the birefringent interferometer optics, and the need for monitoring of the calibration during plasma operations. The procedures for calibration of the instrument have been presented, including a hybrid scheme using online and offline measurements to calibrate the instrument phase.

Chapter 6

Coherence Imaging Measurements