The picture of the behaviour of the pedestal in medium D2 gas injection rate pulses, is
more complex than at low D2 gas injection rate. However, from the analysis of pulses
87338, 87341 and 87339 a pattern is emerging. Firstly lowest βN pulse 87338 just
reaches the PB boundary, this agrees with previous results [96]. The pedestal widens throughout the ELM cycle until the 80-99% pre-ELM interval, where the pulse suddenly narrows. This coincides with the pedestal gaining second stability access, forcing the pedestal into a narrower region governed by the n= ∞ ballooning limit, which grows in height and provides the second stability access. Secondly the middle βN pulse 87341,
clearly does not reach the PB limit, and does not have second stability access. This pulse is constrained by the n = ∞ ballooning limit. Interestingly this pulse also has
Chapter 6. JET-ILW ELM cycle study 6.8. Analysis: high gas fuelling
the highest ELM frequency of all the pulses. Finally, the highest βN pulse 87339 is
characteristically similar to 87338, and also reaches the PB stability boundary. This is contrary to the pre-ELM 70-99% previous results [96]. This pulse displays the same pedestal evolution as 87338, with the pedestal widening until it narrows just before the ELM. This corresponds to the gaining of second stability access in the 80-99% pre-ELM interval. Both 87338 and 87339 show that in this phase of the ELM cycle that the width of the pressure pedestal is governed by the region of the pedestal which has second stability access. Therefore, this suggests that increasing the region with second stability access will increase the pedestal width, and therefore the achievable height. This is an important result, with implications for optimising the pedestal.
6.8
Analysis of dataset: high deuterium gas injec-
tion rate and low triangularity
This section considers the high D2 gas injection pulses 87346, 87350 and 87342, which
are all at low triangularity δ= 0.2 and high D2 gas injection rate, ΓD = 18 × 1021es−1.
6.8.1
Pressure pedestal height and width evolution
Figure 6.21: As for figure 6.17, but for high gas injection, low triangularity pulses (a) 87346, βN =1.16, (b) 87350, βN =1.7, and (c) 87342, βN =1.95.
This section details the evolution of the 20-40%, 40-60%, 60-80% and 80-99% OPs for the high gas injection pulses 87346, 87350 and 87342, compared to the PB boundary. The results of this study for the three high D2 gas injection rate (high gas)
pulses is shown in figure 6.21. Figure 6.21 (a) shows the PB boundary and the OPs for pulse 87346. This is the lowest βN pulse of the high gas dataset and βN = 1.16,
Zef f = 1.24 and the ELM frequency in the stationary phase is fELM = 48Hz. The
evolution of this pulse does show EPED-like widening of the pedestal between the final two phases, 60-80% and 80-99%, of the ELM cycle. However, before this the pedestal narrows during the first three phases of the ELM cycle, with no significant change in height. The 80-99% OP is not at the PB boundary, and therefore not at the PB boundary when the ELM occurs. The OP is significantly further from the PB boundary than the low βN medium gas pulse, 87338: and therefore does not appear to
agree with the pre-ELM 70-99% analysis [96]. The pedestal width evolution between the final two phases is EPED-like. However, this result shows that additional physics is required to understand the ELM trigger here.
Figure 6.21 (b) shows the peeling-ballooning (PB) boundary and the OPs for pulse 87350. This is pulse has βN = 1.7, which is identical to the low gas injection pulse
84795, Zef f = 1.44 and an ELM frequency in the stationary phase of fELM = 98Hz.
This fELM is significantly higher than any pulse seen in this analysis so far. There is
an initial narrowing of the pedestal between the 20-40% and 40-60% phases. However, for the remainder of the evolution there no clear trend and the 80-99% OP is very far from the PB boundary, and therefore not at the PB boundary when the ELM occurs. This agrees with the pre-ELM 70-99% results [96]: pulses at high gas rate with a βN
greater than 1.5, achieve a pedestal height far below the height of the PB boundary. This shows that the pedestal does not show broadening before the ELM onset, and additional physics is also required to understand the ELM trigger here.
Finally, figure 6.21 (c) shows the peeling-ballooning (PB) boundary and the OPs for pulse 87342. This is the highest βN pulse of the high gas dataset, with βN = 1.95
which is much lower than the highest βN achieved at low gas injection. It also has
Zef f = 1.52 and an ELM frequency in the stationary phase of fELM = 122Hz, which is
the highest frequency of any of the low triangularity pulses analysed in this dataset. The range of 48 < fELM < 122Hz at high gas injection rate illustrates the stronger
scaling of fELM with Psep compared to at low gas injection rate, illustrated in figure
6.16 (b). Initially between the first two phases of the ELM cycle, 20-40% and 40-60% there is significant narrowing of the pedestal. Throughout the rest of the ELM cycle, between the 40-60%, 60-80% and 80-99% phases the pedestal grows in height and widens. This evolution where the pedestal grows in height and width monotonically is characteristic of the widening described in the EPED model. However, as figure 6.21 (c) shows, the 80-99% is significantly far from the PB boundary. This result agrees with the pre-ELM 70-99% results [96]. Therefore, the evolution of the pedestal is EPED-like, but additional physics is again required to understand the ELM trigger.
6.8.2
KBM proxy: n=∞ ballooning stability
Now the KBM constraint is considered in more detail using the ideal n= ∞ ballooning limit. The results for the three high D2 gas injection pulses, (a) 87346, (b) 87350 and
Chapter 6. JET-ILW ELM cycle study 6.8. Analysis: high gas fuelling
(c) 87342, are shown in figure 6.22.
Figure 6.22: As for figure 6.18, but for low triangularity high gas injection pulses (a) 87346, βN =1.16, (b) 87350, βN =1.7, and (c) 87342, βN =1.95.
Firstly figure 6.22 (a) shows the results for pulse 87346. Recall that this pulse does not reach the PB boundary. The figure shows it can be seen that at the beginning of the ELM cycle, in the 20-40% phase, the pedestal is very small and n= ∞ ballooning limit is well above the pressure gradient: therefore, the pressure gradient is not constrained by the KBM. The pressure gradient then grows in the 40-60% phase and is against the n = ∞ ballooning limit, which constrains the pedestal gradient. In the 60-80% phase of the ELM cycle the n = ∞ ballooning limit has grown rapidly, providing the beginning of second stability access for the pressure gradient and controlling the width of the pedestal. Finally in the 80-99% pre-ELM phase an unknown change in the plasma occurs, causing the n= ∞ ballooning limit to drop and second stability access is lost. The pedestal in the pre-ELM phase is clearly constrained by the the n = ∞ ballooning limit, does not have second stability access and is unable to reach the PB boundary. Therefore, the pedestal widens before the ELM occurs as described by the EPED model and the pedestal is KBM constrained, but the pedestal does not reach the PB boundary.
Next figure 6.22 (b) shows the results for pulse 87346. Recall that this pulse does not reach the PB boundary. This shows that the pressure gradient is limited by the n = ∞ ballooning limit. Studying the ELM-cycle in (b), it can be seen that at the beginning of the ELM cycle, in the 20-40% phase the pedestal is very small and the n= ∞ ballooning limit is above the pressure gradient, and as such the pressure gradient is not constrained. The pressure gradient then increases to become much closer to the n= ∞ ballooning limit in the 40-60%, 60-80% and 80-99% phases. In the final pre-ELM phase, there is a slight increase of the n= ∞ ballooning limit above the pedestal, but
there is not an indication of second stability access in this pulse, and a lower pedestal height is achieved. Therefore, despite the width evolution in this pulse, the pedestal is likely KBM constrained and fulfils the KBM criteria for EPED, but does not reaching the PB boundary.
Finally, consider pulse 87342, shown in figure 6.22 (c). Recall that this pulse does not reach the PB boundary. This figure shows that the pressure gradient is also broadly constrained by the n= ∞ ballooning limit. Initially, in the first 20-40% phase the pedestal is small and the pressure gradient is below the n = ∞ ballooning limit, suggesting it is not yet constrained. The pressure gradient then increases in the 40-60% phase so that the pedestal becomes close to the n= ∞ ballooning limit. This limits the achievable pedestal height in this pulse. There is no sign of second stability access in this pulse, and the pulse does not reach the PB boundary. This indicates a role of the KBM in this pulse, and the pedestal width shows EPED-like widening. This suggests that the KBM criteria are satisfied for this pulse, but PB theory does not explain the ELM trigger here.