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It is instructive to compare measurements of the magnetic axis that are independent of MSE to corroborate the inconsistencies between the IMSE and conventional MSE for different shot types. The magnetic axis is an excellent comparison point because ambiguity between different MSE views is removed as Er = 0 and measurements of other plasma

parameters such as electron temperature are expected to peak on the axis. The ‘EFIT1’ equilibrium reconstruction is constrained with external magnetic pickup coils and directly provides a prediction of the magnetic axis independent of MSE. The electron cyclotron emission (ECE) diagnostic measures the electron temperature with channels spaced closer than 3cm along the midplane and the peak temperature indicates the position of the magnetic axis. Other diagnostics such as ‘soft x-rays’ or charge exchange ion temperature measurements could also be used to measure the magnetic axis position but ECE offered the greatest resolution. Calculation of the magnetic axis position from MSE measurements is straightforward as the polarisation orientation profile is roughly linear and the magnetic axis corresponds to the zero crossing. ‘EFIT2’ is constrained with the conventional MSE and is therefore expected to indicate the same axis position. The location of the magnetic axis using these various diagnostics is shown in Fig. 4.35 for three shots covering the different combinations of Bt and Ip directions. Small differences ∼ 1◦ ∼= 2cm are to be

expected between the IMSE and conventional MSE measurements due to line-integration and beam attenuation effects.

As expected the MSE constrained EFIT2 magnetic axis location agrees with the 315◦ MSE measurements. Definitive measurements of the axis position from the ECE electron temperature are challenging as the peak is often too broad to provide the resolution needed to make a conclusive assessment. Furthermore with non-inductive current drive the electron temperature signal is often peaked on both sides of the axis. Therefore ECE data is only shown when there is a clear peak. As detailed earlier, for shots with normal toroidal field and plasma current there is close agreement between the IMSE and conventional MSE, this is again evident in the top plot of Fig. 4.35. Typically the EFIT1 and ECE measurements are in agreement for the ohmic part of the discharge and when supplementary heating is applied the ECE and MSE data suggests that EFIT1 under- estimates the magnetic axis position. In shot 166153 there is a discrepancy between the IMSE and conventional MSE after 1.8s but this likely results from a line-integration effect from the IMSE view. The ECE peak identification is also noisier during this period.

For the reverse toroidal field shots with IMSE measurements there is limited consis- tency in the ECE data to corroborate the accuracy of the IMSE and conventional MSE measurements. This is evident in Fig. 4.35 for shot 166093 where initially the ECE data is in agreement with the conventional MSE neart= 800ms before it more closely follows the IMSE axis measurements neart= 2000ms. At other periods during the shot there is no clearly resolvable peak in the ECE data, either because it is too broad or too noisy. The EFIT2 axis position in this case is inside the EFIT1 prediction which is opposite to the normal Bt example.

For shot 166263 with reverse plasma current, both the EFIT1 and ECE measurements suggest that the axis position measured by the conventional MSE is too low and the IMSE is too high. In particular the EFIT2 axis position is well below the EFIT1 position, which is the opposite of the case with normal Bt and Ip when there was agreement between all

internal measurements. Furthermore in Fig. 4.35 for the reverse Ip shot, it can be seen

Shot 166153 ‘Normal’

Shot 166093 ReverseBt

Shot 166263 Reverse Ip

Figure 4.35: Comparison of the magnetic axis position measured using EFIT (EFIT1 mag- netics only, EFIT2 MSE constrained), 315◦ MSE, IMSE and electron cyclotron emission. (Top) Shot 166153 with normal Ip and Bt. (Middle) Shot 166093 with reverse Bt (the

2.94s IMSE data point is noisy due to a failed beam blip shorter than 1ms). (Bottom) Shot 166263 with reverse Ip.

Figure 4.36: (Left) Position of magnetic axis given from EFIT1 for normal, reverse toroidal field and reverse plasma current shots. (Right) Magnetic axis position for EFIT2 (con- ventional MSE constrained) and IMSE relative to EFIT1 (external magnetics only). The median value is taken from points in time where all three measurements are available.

‘Normal’ ReverseBt ReverseIp

EFIT1 1.751m 1.755m 1.729m EFIT2 1.784m 1.749m 1.691m IMSE 1.785m 1.799m 1.787m

Table 4.5: Median magnetic axis position inferred with EFIT1 (magnetics only), EFIT2 (MSE constrained) and IMSE for different shot types.

there is an inconsistency between the external magnetic measurements and conventional MSE the equilibrium solver will struggle to converge to a solution.

Given the challenges in obtaining routine measurements of the magnetic axis from ECE it was decided that EFIT1 provides the most unbiased and consistent comparison point. In the left of Fig. 4.36 the average magnetic axis position given by EFIT1 for each shot is shown. The average is only taken during time intervals in a shot where EFIT1, EFIT2 and IMSE are all available and therefore the intervals are different for each shot. Then in the right hand side of Fig. 4.36 the position of the magnetic axis measured with EFIT2 and IMSE is plotted relative to the EFIT1 measurement. In this plot there is a clear discrepancy between the IMSE and conventional MSE for the reverse Ip measurements

and to a lesser degree for the reverseBtshots. This trend is quantified in Table 4.5 where

the median value from all shots of a given type is displayed, confirming the general trends seen in Fig. 4.35.

The shots with reverseIp have the greatest discrepancy between the MSE diagnostics

for the magnetic axis measurements and while there are a number of uncertainties for the IMSE measurement the discrepancy is significant. The possibility of a systematic error in the conventional MSE measurement for reverse Ip shots should be further investigated

given that: EFIT2 struggles to converge to a solution; the discrepancy with the ECE, IMSE and EFIT1 measurements of the magnetic axis position; and the conventional MSE pitch angles do not appear to be originating from 0◦pitch at the start of the shot. However there is no obvious source of error that would arise with only a change in the direction of the plasma current. Less commonly considered effects such as stress induced birefringence in the port window or Faraday rotation from a poloidal field coil may change with direction

of the current. For example ramped currents used to induce the plasma current may cause changes in magnetic flux through the port window resulting in stresses from any metal enclosing the window, however any fringing fields near the midplane will not have a significant component normal to the window. Movements between the beams, tokamak and collection optics are unlikely but could possibly give rise to a current dependent offset. IMSE has the advantage that the any movement can be seen in the images and some minor movements that were observed with the IMSE optics are discussed in the next section. There are smaller discrepancies between the different measurements for shots with reverse

Bt and there is less evidence casting doubt over the well-established conventional MSE

measurements for these shots.