Tomographically Inverted Data

Data from several shots have been tomographically inverted to obtain 2D profiles of C III parallel flow and emissivity in the divertor, covering the region from just above the lower X-point to the divertor targets, using the methods described in section 4.6. Inverted profiles based on the C III data presented in section 6.2.3 are shown in figure 6.13, for the L-Mode, ELM-free H-Mode and ELMy H-Mode discharge phases (the ELMy H-Mode data is from the period between ELMs). Here the sign convention for the flow is that positive parallel flows have a toroidal component in the same direction as the plasma current. Each profile was generated from a

6.2. Plasma Observations 142

Zswm)

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Rswm)

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Zswm)Zswm)

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L-ModeELM-freesH-ModeELMysH-Mode v||swkm/s)

wNormalised)sEmissivitysProfile Parallelsflow

Figure 6.13: Tomographically inverted emissivity (left) and C III parallel flow (right) profiles for periods of different operation modes in shot #29541: L-Mode (top), ELM-Free H-Mode (centre) and ELMy H-mode (bottom).

6.2. Plasma Observations 143 single 1ms exposure. The general features of these images are the same as shown in the line-integrated data: parallel flow towards both divertor targets and increased flow speeds going from L-Mode → ELM-free H-Mode → ELMy H-Mode. Note the stripe pattern of positive and negative flows at the high field side in the L-Mode reconstruction (at Z ≥ −1.3m): this is an artefact which was propagated through the inversion from the line integrated images, and is a known behaviour of the FFT demodulation in the presence of reflections from the poloidal field coils.

There is also a rectangular artefact due to reflections from a supporting bracket of these coils (at R ∼ 0.65m, Z ∼ −1.3m), particularly noticeable on the ELMy H-Mode inversion. Reconstruction artefacts such as these, and those due to response matrix errors caused by line-of-sight calibration errors and some sight-lines being poorly constrained by the data, appear to be the dominant source of error in the inverted profiles (the effect of noise in the line-integrated data on the inversions was investigated numerically with a Monte-Carlo technique and found to be typically

 1km/s). These types of reconstruction artefacts can typically be readily identified since they correspond to known artefacts in the input line integrated data, and/or appear as static features in the images which do not respond to changing plasma behaviour.

In conjunction with suitable 2D SOL modelling, these profiles and similar inver-sions for C II and He II could in principle be used to investigate the physics of the flow differences seen between the different species. In practise, however, the C II and He II images are less amenable to inversion, and profiles for these impurities could not be reliably produced with sufficient quality using the inversion method described in the previous chapter. One of the main difficulties with these inversions is the geometry of the outboard divertor PFCs on MAST, consisting of a set of toroidally discrete tiles separated by relatively large gaps. For C II and He II the emission is peaked close to the target plates, which leads to a toroidally periodic, rather than toroidally symmetric emission pattern in the images. This can be seen in figure 6.10, where the C II and He II measurements show a much stronger toroidally periodic pattern than C III. Since toroidal symmetry is assumed in the formulation of the tomography problem, this causes artefacts in the inversions. In future it may be possible to implement an inversion geometry with toroidal periodicity matched to that of the target plates, in order to reduce this effect.

6.2.6 Summary

The MAST CIS flow diagnostic was operated successfully throughout the experimental campaign in May September 2013. Using integration times between 1

-6.2. Plasma Observations 144 16ms, data was obtained for all three impurity species and flow image noise levels were as low as < 1km/s. The phase calibration offset was successfully monitored using radial sight-lines, on both inter- and intra-shot timescales. This revealed ini-tial problems with vibrations causing frame-to-frame changes in the flow calibration offset, however this problem was alleviated by re-designing the mounting scheme for the instrument. In future, a more robust mechanical design taking into account the vibration environment could improve this aspect of the calibration stability. On longer timescales, variation of the calibration offset from shot-to-shot were up to 16km/s over the course of a week. Since these slower drifts are likely to be domi-nated by thermal effects, improved thermal stabilisation is required to reduce this effect. Using phase shape calibrations performed before and after a week long period of operation, the phase shape calibration was found to be stable to within 3km/s over the whole frame. Although this has not caused significant problems with the present measurements, an in-situ calibration procedure is desirable to monitor the phase shape calibration more accurately. Comparisons between CIS and dispersive Doppler spectroscopy measurements using the ECELESTE diagnostic show reason-able agreement in the flow differences between different discharges, however the absolute calibration of the instrument could not be properly benchmarked due to absolute calibration difficulties with both instruments on these observations.

In plasma measurements on MAST, the CIS diagnostic demonstrated strong capabilities in revealing spatially complex flow patterns which would be very difficult to diagnose and interpret using traditional dispersive systems. In limited plasmas, impurity flows consistent with flow towards the limiter in the SOL and co-current rotation of the confined plasma were observed. Strong responses of flows to high field side gas puff fuelling were also observed for the first time, most notably in the form of field-aligned, counter-rotating patterns around the centre column under certain conditions. In the divertor, data was obtained for multiple impurity species in different plasma conditions, suggesting a sudden increase in flow towards the divertor targets in H-Mode which is greatest at the strike point and less prominent in the far SOL and PFR. The CIS diagnostic was also used to perform the first measurements of divertor flow perturbations due to the application of RMPs for ELM control. Tomographically inverted data was successfully obtained for C III in a variety of shots, however toroidally periodic emission profiles from C II and He II prevented the same high quality of inversion for these species. Interpretation of the physics behind these more complex plasma observations is still ongoing, and in general will require comparisons with sophisticated modelling using codes such as EDGE2D, SOLPS and EMC3.

Chapter 7