Calibration Stability

Phase Offset

As was discussed in section 5.3.2, the flow calibration offset φ_offs is subject to drifts due to ambient temperature changes and mechanical disturbance of the instrument, and is monitored using radial sight-lines as a zero flow reference. During plasma operations, offset shifts were monitored over two different timescales. The first were intra-shot, i.e. frame-to-frame changes, due to mechanical vibration of the instrument during the discharge. When the system was first installed on MAST it was supported on a rail cantilevered from the tokamak vacuum vessel. During plasma shots, movement and vibrations of the vacuum vessel were transmitted to the instrument and caused movement of the interferometer components relative to the

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Detected0Signal0(ADU) Estimated0measurement0noise0(km/s)

X0pixel0 X0pixel0

Figure 6.1: Signal level and flow noise images for LSND plasma shot 28841 during H-Mode, using the C III filter and 2 ms exposure time. (a) Raw image from the camera and (b) estimated noise on the flow measurement, estimated from the results in section 4.3.

detector, making the entire fringe pattern move across the detector. This appears in the data as large oscillations in the measured flow (with peak-to-peak amplitudes of up to 40km/s) with a frequency of around 100Hz. The dominant cause was identified as vibration of the temperature control cell and its mount, which was secured at one end from the main base plate. This motion is illustrated in figure 6.2. The problem was mitigated by mounting the instrument from the floor next to

Camera l3 l2 l1

Pol.

Optics

Figure 6.2: Side view of a CAD model of the instrument, illustrating the mechanical vibrations causing frame-to-frame calibration oscillations. Motion is indicated by red arrows.

the tokamak rather than the vacuum vessel, and by adding an additional support for the temperature cell at the end not supported by the primary mount. Floor mounts could only be implemented for the HL07 divertor and HM02 midplane views, due to space restrictions and clashes with other structures & diagnostics. Fig 6.3 shows the flow measured on the φoffs reference chords, relative to its mean over the data shown, during two similar plasma shots on the HL07 divertor view: one before and one after the modifications to alleviate the vibration problem. The peak-to-peak

6.1. Instrument Performance 126 amplitude of the oscillations with no mitigation corresponds to a fringe motion of up to 1.4 pixels on the detector, or an angular motion of the crystals of up to 0.01^◦, and is reduced to 0.15 pixels or 0.001^◦ after the modifications. The sensitivity of the system to such small changes in the angular alignment of the crystals is due to the use of small beam angles through the collimated region, driven by minimising filter blue-shift effects. Larger oscillations were observed on data using the HM07 midplane views, with peak-to-peak amplitudes as high as 40km/s.

In data from the views where floor mounts could not be implemented (and to remove small remaining oscillations seen in some of the data even after the modifi-cations), subtraction of the oscillating φ_offs usually provided satisfactory correction for the vibrations across the whole field of view, since the motion of the polarisation optics was essentially constrained to 1 axis and relatively small amplitude, making the fringe pattern move systematically up and down on the detector.

Timed(s)

Referencedsight-linedflowd(km/s)

0.25 0.3 0.35 0.4 0.45

-10 -5 0 5 10

Vesseldmountedd(F29153) Floordmounted(F29411)

Figure 6.3: Effect of mechanical vibrations on flow measurements before subtraction of φoffs, during two comparable LSND plasma discharges viewed from the HL07 divertor port. The mean flow has been subtracted to show only the vibration effect.

Variations of the calibration offset on longer timescales, i.e. between different shots over the course of days and weeks, were monitored using the mean offset within each shot. Variation over a one week period, in km/s, is shown in figure 6.4. On any given day, the offset drifted by between around 2km/s - 15km/s. The range of all values shown in figure 6.4 is around 16km/s.

Both the inter- and intra- shot variations in calibration offset demonstrate the need for the calibration monitoring of the MAST instrument, and potential improve-ments in future diagnostic design. Namely, it would be desirable to improve both the thermal stabilisation (using an improved temperature controller, and/or passive

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Figure 6.4: Calibration offset variations over a 1 week period. Inset shows the offsets over a single day.

thermal stabilisation using multiple plate materials) and mechanical design in order to try to stabilise the calibration. In addition, an in-situ calibration system not relying on plasma light is highly desirable to maintain accurate calibration of the diagnostic independent of the plasma view.

Phase Shape

As described in section 5.3.2, calibration of the instrument phase shape φ_shape for a given run of measurements was performed offline in the lab before the diagnostic was installed on the tokamak. This calibration method relies on the phase shape not being significantly affected by thermal drifts or mechanical disturbance during the installation of the instrument or subsequent plasma operations. To test this, phase shape calibrations were performed in the lab before and after a 7 day period of operation on the wide angle divertor view, and compared after subtracting the mean offset over the image. Discrepancies between the two calibrations were found to be up to 3 km/s, and showed the form of a gradient across the image perpendicular to the fringes. As such the largest deviations were at the image corners, and in the central part of the image where most of the spatial flow information is obtained the discrepancies were up to around 1km/s. This shows that the phase shape does in fact suffer changes during installation and operation of the diagnostic, and in-situ, preferably per-shot calibration over the whole image frame would be desirable for accurate calibration of future instruments. Since the spatial structure of the changes is a gradient across the entire image, this does not significantly effect observations of more complex spatial structure in the flows or flow structure at fine spatial scales.

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