Dunne, M.

Frassinetti, L.

Labit, B.

Merle, A.

Nyström, H.

Saarelma, S.

Vianello, N.

English

MPG.PuRe

Pedestal structure and stability at low collisionality in TCV  L. Frassinetti1, B. Labit2, M. Dunne3, A. Merle2, H. Nyström1, S. Saarelma4, N. Vianello5, the TCV Team2. 1Division of Fusion Plasma Physics, KTH Royal Institute of Technology, Stockholm SE 2Ecole Polytechnique Fédérale de Lausanne (EPFL), Swiss Plasma Center (SPC), Lausanne, Switzerland 3Max-Planck-Institut für Plasma Physik, Boltzmannstr.2, 85748 Garching, Germany 4UKAEA Culham, Culham Science Centre, Abingdon, Oxfordshire, OX14 3DB, United Kingdom 5Consorzio RFX, Corso Stati Uniti 4, 35127 Padova, Italy  ITER will operate at low pedestal collisionality (ν*eeped) and high normalized separatrix density (nesep/neped). The ITER pedestal collisionality is supposed to be sufficiently low (ν*eeped<0.1-0.2) that the pedestal will be limited by peeling instabilities, rather than ballooning instabilities. Most of the present days machines, in particular in Europe, tend to operate at high pedestal collisionality, with ELMs typically triggered by the balloning modes. While pedestal physics has been well studied at the ballooning boundary, so far information on the pedestal behaviour at the peeling boundary has been described only in DIII-D [1]. This work investigates the pedestal behaviour at low ν*eeped in TCV, with emphasis to the pedestal performance and stability. Four datasets are used, as shown in figure 1. One dataset with high ν*eeped obtained at 170kA/-1.4T/low-δ and PNBH=1MW. Another dataset with medium/high ν*eeped obtained at 170kA/-1.4T/low-δ, 1.2MW NBH and 0.9MW X3 ECRH (blue squares). Finally, two datasets at low-δ and high-δ (red circles and yellow triangles) with low ν*eeped obtained at 155kA/-1.4T, 1.0MW NBH and 1.1MW X2 ECRH. As shown in figure 1, the low ν* datasets reach a collisionality range comparable to that expected in ITER. Within each dataset, all engineering parameters are constant apart the gas rate (ΓD) during the H-mode phase which has been changed from shot to shot to produce a variation in nesep (ΓD has been varied from zero up to a maximum value necessary to remain in Type I ELMy H-mode or to have a good ECRH absorption). Unfortunately, at low ν* it was not possible to reach ITER-relevant nesep/neped. The empty circles in figure 1 highlight two couples of shots at low ν* (red and orange circles) and high ν* (cyan and blue circles) with low and maximum gas rate.  The electron pressure pedestal height (peped) is shown in figure 2. Figure 2(a) shows the correlation of peped with ν*eeped and figure 2(b) with electron density pedestal height (neped). At low collisionality (ν*eeped<1), peped increases with increasing ν*eeped and with increasing neped. At high collisionality (ν*eeped>1) the opposite trend is observed. The behaviour at high collisionality has been already reported in many machines [2,3,4] and it is due to the outwards shift of the density pedestal position (nepos), driven by the increased gas rate, which shifts the pressure outwards and destabilizes the balloning modes. Instead, the behaviour at low ν* was never observed before in a European machine, only DIII-D had so far observed a peped increase with increasing ν*eeped in type I ELMy H-modes. In the rest of the work we will investigate the causes of this behavior by analyzing the pedestal structure and stability.  The increase of peped in the low ν* datasets is not due to the increase of the pedestal pressure gradient (∇pe). As shown in figure 3(a), ∇pe decreases with increasing neped both at low and  Figure 1. ν*eeped and nesep/neped for the four datasets used in this work. 48th EPS Conference on Plasma Physics O4.108high ν*. Instead, the positive correlation between peped and neped at ν*eeped<1 is due to the widening of the pedestal. As clearly shown in figure 3(b), at low ν* the pedestal pressure width (wpe) increases with increasing neped. At high collisionality, the width behavior is significantly different.   To understand the different pedestal behavior at low and high ν*, the pedestal stability has been investigated. Figure 4 shows the peeling-ballooning (PB) stability diagram for the low ν*eeped pulse 71718 and for the high ν*eeped pulse 73079 done using KINX [5]. The low  ν*eeped pedestal is near the nose of the PB stability, rather close to the peeling boundary, and limited by low-n modes (in the range n=5-10). Instead, the high ν*eeped pedestal isat the ballooning boundary, limited by high-n modes (ncrit=15-30). This suggests that the different pedestal behaviour at low and high collisionality is, at least in part, related to the different type of instabilities that trigger the ELMs.  To further investigate the link between experimental results and MHD stability, predictive pedestal modelling using the Europed code [6] has been done for the two low ν*eeped pulses and the two high ν*eeped pulses highlighted in figure 2. For each pulse, the predictions have been done for several values of the density. In each simulation, the density profiles has been rescaled and the ratio nesep/neped has been kept constant. The results are shown in figure 5. Figure 5(a) shows the predicted peped vs neped. Some qualitative similarities between predicted and experimental results can be observed. First, at low ν* the predicted peped tends to increase with increasing neped. The trend is however significantly weaker than experimentally observed (see figure 2). Second, at neped≈4×1019 (m-3) the predicted pressure has a sharp decrease with increasing neped. This transition is roughly consistent with the experimental results for which the different pedestal behaviour between low and high ν* pedestals occurs at neped≈3.5×1019   Figure 2. (a) peped versus ν*eeped and (b) peped versus neped.  Figure 3. (a) maximum ∇pe in the pedestal and (b) pressure pedestal width versus  neped. 48th EPS Conference on Plasma Physics O4.108shown. ncrit is in the range 5-10 (consistent with peeling instabilities) at neped<4×1019 (m-3) and in the range 50-70 (consistent with ballooning instabilities) at neped>4×1019 (m-3). Note that the predictive modelling has a qualitative agreement with the experimental results also in terms of pressure gradient. The predicted normalized pressure gradient, αcrit in figure 5(d), has a negative correlation with neped, as also experimentally observed in figure 3(a).  We can highlight at least two clear differences between the predictions and the experimental results: (A) the behaviour of the pedestal width and (B) the behaviour of the two high ν* pulses highlighted by the cyan and blue circles in figure 2(b). (A) The behaviour of predicted and experimental width are significantly different at low ν*, figures 5(c) and 3(b). Experimentally, the width increases with increasing neped, while the predictions show no clear increase. Instead, at high ν* the width predictions are roughly in agreement with the experimental results (wpe≈0.08ψN) The predicted pedestal width has been determined using the KBM constraint which assumes turbulent transport driven by kinetic-ballooning microinstabilities. This suggests that the pedestal turbulent transport might be driven by different instabilities at low and high ν*. A preliminary experimental insight into the turbulent transport can be given via the ηe parameter, which is defined as (Te/Te)/ (ne/ne). As sown in figure 6(a),ne/ne is clearly different between the low ν* datasets and the high ν* datasets.Te/Te is also different between low and high ν* datasets, but in this case the transition from low to high ν* is smooth. Finally, ηe is clearly higher in the low ν* datasets, suggesting that the turbulent transport might be different from that in the high ν* datasets. This result is likely related to the different behaviour of the pedestal width, as higher transport can lead to wider wpe [3]. (B) In the high ν* dataset, the increase of the  Figure 4. Peeling-ballooning stability diagram for the low ν*eeped pulse 71718 and for the  high ν*eeped pulse 73075  (m-3) as shown in figure 2(b). The transition in the predicted results is clearly due to the different type of instability, as shown in figure 5(b) where the most unstable mode vs neped is   Figure 5. Europed predictive modelling for two low ν* and two high ν* pedestals. (a) peped, (b) most unstable mode, (c) pressure width and (d) normalized pressure gradient versus neped.  48th EPS Conference on Plasma Physics O4.108gas rate leads to a sharp reduction of peped (via Teped reduction) but to no significant variation in neped (cyan and blue circle in figure 2).   This behavior cannot be seen in the Europed scans of figure 5(a), where above neped≈4×1019 (m-3) the predicted peped is not significantly affected by neped. This is due to the fact that both neped and nesep are input parameters in Europed and the scans in figure 5 are done assuming constant nesep/neped. Interestingly, the behavior of nesep/neped differs significantly between the low ν* and the high ν* datasets. This is shown in figure 7 where the pre-ELM density profiles for the pulses highlighted by circles in figure 2 are shown. At low ν*, the increase of the gas rate leads to the increase in both neped and nesep with nesep/neped remaining approximately constant (consistent with the modelling of figure 5). At high ν*, the increase of the gas rate leads to the increase in nesep and in nesep/neped, effectively shifting the density profiles outwards. As discussed in several earlier works [2,3,4], this destabilizes the balloning modes, decreases the pedestal stability and leads to lower peped. Obviously, this effect cannot be predicted by figure 5 as constant nesep/neped has been assumed. In conclusion, this work shows that at low collisionality (ν*eeped<1) the pedestal pressure increases with increasing gas rate, a behaviour opposite to what observed at high ν*. This is linked to at least two reasons. First, nesep/neped is approximately constant at low ν* (so the destabilizing effect of the increasing nesep/neped is not present). Second, the pedestal is limited by low-n instabilities. Predictive modelling has some similarities with the experimental result (a sharp peped reduction at neped≈4×1019 (m-3)) but also clear differences (no wpe widening is predicted at low ν*). This might be due to a different turbulent transport which suggests that a more advanced transport constraint needs to be included in Europed. Finally, the different behaviour of the density at low and high ν* further strengthen the fact that fully reliable pedestal predictions can be achieved only if coupled with density predictions.  This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No 101052200 — EUROfusion). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them. REFERENCES [1] Snyder P. et al., Nucl. Fusion 55, 083026 (2015) [2] Dunne PPCF 2017 [3] Sheikh PPCF 2019 [4] Frassinetti NF2019 [5] Degtyarev L et al 1997 CPC 103 10–27 [6] Saarelma S et al., PoP 26, 072501 (2019)   Figure 6. ne/ne (a), Te/Te (b) and ηe (c) averaged over ψN=0.98-1.00 versus neped  Figure 7. Electron pedestal pressure versus density for a low ν* pulse (red square) and corresponding Europed modelling (blue) and most unstable modes. 48th EPS Conference on Plasma Physics O4.108

Pedestal structure and stability at low collisionality in TCV

http://hdl.handle.net/21.11116/0000-000C-178C-D

Pedestal structure and stability at low collisionality in TCV

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