52 5. Methodology
a machine size dependence remains
∂ΨN
∂R ∝ 1 a
1 +ǫ
ǫ , (5.4)
where ǫ = a/R0 is the inverse aspect ratio. The estimations which led to Eq. (5.4) neglect the real shape of the plasma and the local poloidal field. However, a correction can be introduced in form of the shaping factor fq which will be discussed in detail in Section 6.3.
For the scaling to AUG, DIII-D and JET the machine size dependence is found to be inverse but slightly less than linear.
∂ΨN
∂R ≃1.65a−0.70±0.05fq1.07±0.10. (5.5) The aspect ratio dependence in Eq. (5.4) could not be reproduced since there is no variation in ǫ for the available database. The quality of the approximation Eq. (5.5) is illustrated in Figure 5.5.2 and shows reasonable agreement for all three machines. In this representation the shaping factor was introduced instead of βp,ped, because it is the better quantity to describe the problem. With the regression parameters a and βp,ped the RMSE increases by 50%. When applying the regression to all three parameters a, fq and βp,ped Eq. (5.5) does not change but the exponent ofβp,pedb is found to be b <0.03.
Applying a coordinate transformation - e.g. from real space to normalized flux space (∂ΨN/∂R) - will influence all comparisons of widths and gradients. This coordinate trans-formation is dependent on machine size, aspect ratio, magnetic field, shaping and plasma pressure. All these parameters are also possible candidates in determining the pedestal width.
In particular, following Eq. (5.5), the two dimensionless descriptions of the width flux space
∆ΨN and in real space∆m/aare not equivalent and will result in different scalings withfqor βp,ped. For multi-machine comparisons it is important to use the correct coordinate system which is set by the underlying physics. For example normalized flux coordinates for MHD related physics and real space coordinates when atomic processes dominate the plasma edge, as described in Chapter 3.
5.2 Pedestal Building 53
electron temperature [keV]
normalized radius ρp AUG #25119
0.0 0.2 0.4 0.6
0.90 0.95 1.00 1.05 1.10
(a) electron temperature [keV]
normalized radius ρp AUG #25119
0.0 0.2 0.4 0.6
0.90 0.95 1.00 1.05 1.10 (b)
Figure 5.6: Comparison of a single measurement (a) with a composite profile of 10 measurements (b).
electron density [1019m-3]
normalized radius ρp AUG #22898: 3.15-3.30s
0.0 2.0 4.0 6.0 8.0 10.0
0.90 0.95 1.00 1.05 1.10 no ELM sync.
(a) 19-3 electron density [10m]
normalized radius ρp AUG #22898: 3.15-3.30s
0.0 2.0 4.0 6.0 8.0 10.0
0.90 0.95 1.00 1.05 1.10 with ELM sync.
(b)
Figure 5.7: Comparison of composite profiles without (a) and with (b) ELM synchronisation.
5.2.1 ELM synchronisation
The main goal of this thesis is to characterise the H-mode edge pedestal at the stability limit of ELMs. In Section 2.1 the impact of an ELM on the plasma edge was described as a collapse of the whole pedestal region in temperature and density. Taking an average profile, including parts of this collapse, would yield extremely scattered composite profiles as shown in Figure 5.7 (a) for ne profiles from the Li-beam diagnostic. In such a case no reasonable characterization of the pedestal is possible. Therefore, the data is synchronised in time relative to the onset of an ELM. The restriction to time points which lie immediately before an ELM gives a sharper pedestal (b). In this case the pedestal can be characterized accurately as will be described in Section 5.3. The synchronisation is only helpful with the assumption that the pedestal parameters reach the same limit values before each ELM.
This can be expected because of the similar characteristics the ELMs show when the global plasma parameters are not varied.
The decision for the actual values for the ELM synchronisation is motivated with knowl-edge of the ELM cycle as discussed in Section 2.1. The pre-ELM time window for AUG is illustrated in Figure 5.8. Before the ELM onset 1.5 ms are excluded as margin for possible errors in the ELM recognition and time delays between different diagnostics. From this time mark a 2.0 ms interval is chosen and only measurements are selected as pre-ELM whose exposure time lies completely within this interval. In the case of diagnostics with longer or
54 5. Methodology
0.0 0.5 1.0 1.5 2.0
3.225 3.230 3.235 3.240 3.245 3.250
Ipol,sol [104 A]
t [s]
AUG #22900
-1.5ms -3.5ms -6.5ms
+4.0ms
Figure 5.8: Schema of ELM synchronisation using the divertor current signalIpol,sol. The red shaded region is always excluded from the analysis. For diagnostics with high temporal resolution the dark green shaded region from 3.5 to 1.5 ms before an ELM is analysed. The light green shaded region with up to 6.5 ms before the ELM onset is only included for diagnostics with lower time resolution.
comparable exposure times, like the CXRS, this interval is broadened to 5.0 ms. In order to avoid problems at high ELM frequencies (>100 Hz) a distance of at least 4.0 ms is set to the previous ELM. At DIII-D and JET the pre-ELM time window is defined as the last 20% of the ELM cycle. At an ELM frequency of 100 Hz this corresponds fairly well to the definition used at AUG.
5.2.2 Mapping of Multiple Diagnostics
In order to use data from more than one diagnostic the measurements have to be mapped to a common coordinate system. In general diagnostics do not measure at the same position in machine coordinates R, z, φ which was illustrated in Figure 4.1. The common coordinate system used throughout this thesis is the normalized radius ρp, based on the poloidal flux.
With equilibrium reconstructions (see Section 5.1) and the assumption of toroidal symmetry all diagnostics are mapped onto ρp coordinates.
After the mapping procedure diagnostics regularly appear to be radially displaced up to 1 cm with respect to each other. Because some of the measured quantities are constant on flux surfaces like the pressure, all diagnostics should measure the same values for these in flux coordinates. Since this radial displacement is unphysical, an error has occurred on the way to ρp coordinates. Three possible sources exist for such an error. First, uncertainties in the determination of the observation volumes in machine coordinates could cause a dis-placement. Second, a deviation of the toroidal symmetry could account for shifts between diagnostics, however, the deviations are not expected to be as large as the observed shifts.
Still, when the toroidal symmetry is deliberately broken, for example, by the application of additional magnetic perturbations with magnetic field coils, a diagnostic shift of around 5 mm was observed at AUG [82]. The third possibility is an inaccuracy in the equilibrium reconstruction. At DIII-D shifts of the TS measurements with respect to the separatrix of up to 3 cm are regularly observed. The main reason for this are uncertainties in the plasma shape above the midplane. The DIII-D TS is affected strongly because it measures at the top of the plasma. For diagnostics at AUG, which are concentrated around the outer midplane, the plasma shape is a minor problem because the separatrix is well defined at the midplane.
Identifying the source of this was not part of this thesis, therefore, only procedures for cor-rection of the misalignment were applied. The main tool is the TS diagnostic which measures
5.2 Pedestal Building 55
diagnostic shift [cm]
fit nb.
shift ECE-TS shift LIN-TS
-1.0 0.0 1.0 2.0 3.0
0 50 100 150
(a) diagnostic shift [cm]
fit nb.
shift LIN-ECE
-1.0 0.0 1.0 2.0 3.0
0 50 100 150
(b)
Figure 5.9: Radial displacement of diagnostics over the database. Relative shift necessary to match ECE and Li-Beam with the TS (a). The resulting shift between ECE and Li-Beam determined with TS (b).
electron temperature [keV]
normalized radius ρp AUG #25123: 2.10-2.70s
ECE TS
0.0 0.2 0.4 0.6 0.8
0.90 0.95 1.00 1.05
(a) electron temperature [keV]
normalized radius ρp AUG #25123: 2.10-2.70s
ECE TS +9mm
0.0 0.2 0.4 0.6 0.8
0.90 0.95 1.00 1.05
(b)
Figure 5.10: Example of diagnostic alignment with ECE and TS at AUG. The measurements of the electron temperature directly after mapping ontoρp(a) and with a 9 mm shift of the TS diagnostic (b).
temperature and density at the same position and is not influenced by mapping, toroidal symmetry or position in machine coordinates. This allows to align all temperature and den-sity measurements. This is crucial to determine the pedestal pressure. In Figure 5.9 the shifts between ECE, Li-beam (LIN) and TS are plotted for the AUG database. The mean value of the shift between ECE and TS is 8±2 mm, between LIN and TS it is 11±3 mm and this gives a shift between LIN and ECE of 3±3 mm. It has to be noted that within the AUG data set no systematic dependency between the global plasma parameters and the diagnostic shift was observed. The 1σ-error does seem to have its source only in the uncertainty of aligning the diagnostics.
The procedure to align the diagnostics was done manually. Several automated techniques were tested, however, none could guarantee a satisfactory low error rate. An example for the alignment is given in Figure 5.10, directly after the mapping ontoρp coordinates (a) the ECE and TS diagnostic measure the steep gradient region at different radii. The aligned profile (b) is obtained by applying a 9 mm radial shift to the TS diagnostic inR, z coordinates and then repeat the mapping toρp. In the case of the electron temperature the alignment is done by matching the measurements in the upper pedestal region, because, the ECE generally does not provide measurements of the plasma temperature at the separatrix. For the density measured with the Li-beam and the TS also the profile shape at the separatrix should be included in the alignment procedure. The absolute density value of the TS has uncertainties in the calibration of around 20%, therefore, these uncertainties would be transfered to the diagnostic shift when only the steep gradient region is used for the alignment. This is
56 5. Methodology
toroidal rotation vtor [krad/s]
normalized radius ρp AUG #23223: 4.55-4.75s
Raus const
0 10 20 30 40
0.90 0.92 0.94 0.96 0.98 1.00
(a) toroidal rotation v [krad/s]tor
normalized radius ρp AUG #23223: 5.20-5.40s
Raus varying
0 10 20 30 40
0.90 0.92 0.94 0.96 0.98 1.00
(b) toroidal rotation v [krad/s]tor
normalized radius ρp 0
10 20 30 40
0.90 0.92 0.94 0.96 0.98 1.00 (c)
Figure 5.11: Effect of an Raus scan on the toroidal rotation measurement with the CXRS system. With constant outer radius (a), varying outer radius (b) and both cases as comparison (c).
ommitted when the value and position of the pedestal bottom can be utilized for an alignment of these diagnostics. A detailed discussion on the profile shape at the separatrix is found in Section 7.1.
5.2.3 R
ausScan
At AUG the so called Raus scan is an important tool to improve the radial coverage of edge diagnostics. Diagnostics generally view fixed positions in the plasma with a limited number of lines of sight. With a shift of the whole plasma column new virtual lines of sight are created. The improved radial coverage can also lead to better radial resolution, however, not better than the limits of the diagnostic itself as discussed in Section 4.7. One example where the radial resolution is improved is shown in Figure 5.11 with the toroidal rotation measured with the CXRS system described in Section 4.5. When the plasma column is not moved and Raus is constant (Figure 5.11 (a)) the different diagnostic channels are separated from each other. With only this measurement one could think of a monotonously decreasing rotation with smaller gradients at larger radii. However, with movement of the plasma column in (b) and the information of additional lines of sight this is proven wrong. The rotation shows a real minimum at aroundρp = 0.97and the gradient is increasing up to this radius rather than decreasing. In (c) both cases are plotted together for better comparison. In this example Raus was scanned over about 1.5 cm which corresponds roughly to the spacing between the diagnostic channels.
At AUG the Raus scans are included in many discharges routinely and typically consist of an Raus movement from 2.14 m to 2.16 m and back to 2.14 m in 600 ms. The pendant at DIII-D is called breathing and results in roughly 1 cm movement of the outer plasma radius. The majority of the time slices analysed for the database include scans of Raus. In [26] it was suggested that the plasma edge is significantly perturbed by moving the plasma column. For the database in this work each Raus scan was divided in intervals of 150 ms and then the intervals were compared to each other. No systematic deviation in the pedestal parameters derived from the different intervals could be identified. The major difference in the two approaches was the use of individual measurements in [26] and the composite profiles of the present work. The analysis, which showed no systematic impact of the Raus scan in