Abstract
The electron temperature and density pedestals tend to vary in their relative radial positions, as observed in DIII-D (Beurskens et al 2011 Phys. Plasmas 18 056120) and ASDEX Upgrade (Dunne et al 2017 Plasma Phys. Control. Fusion 59 14017). This so-called relative shift has an impact on the pedestal magnetohydrodynamic (MHD) stability and hence on the pedestal height (Osborne et al 2015 Nucl. Fusion 55 063018). The present work studies the effect of the relative shift on pedestal stability of JET ITER-like wall (JET-ILW) baseline low triangularity (δ) unseeded plasmas, and similar JET-C discharges. As shown in this paper, the increase of the pedestal relative shift is correlated with the reduction of the normalized pressure gradient, therefore playing a strong role in pedestal stability. Furthermore, JET-ILW tends to have a larger relative shift compared to JET carbon wall (JET-C), suggesting a possible role of the plasma facing materials in affecting the density profile location. Experimental results are then compared with stability analysis performed in terms of the peeling-ballooning model and with pedestal predictive model EUROPED (Saarelma et al 2017 Plasma Phys. Control. Fusion). Stability analysis is consistent with the experimental findings, showing an improvement of the pedestal stability, when the relative shift is reduced. This has been ascribed mainly to the increase of the edge bootstrap current, and to minor effects related to the increase of the pedestal pressure gradient and narrowing of the pedestal pressure width. Pedestal predictive model EUROPED shows a qualitative agreement with experiment, especially for low values of the relative shift.
Highlights
The edge pedestal structure in H-mode plasmas strongly affects the fusion power [Maget NF2013, Urano NF2014]
The increased gas fuelling has led to a reduction of the pedestal pressure top values, mainly due to the lower pedestal temperature compared to JET carbon wall (JET-C), as described in [Beurskens NF2014, Maggi NF2015]
This work studies the role of the pedestal relative shift in the pedestal stability of JET ITER-like wall (JET-ILW) plasmas and compares the results with JET-C
Summary
The edge pedestal structure in H-mode plasmas strongly affects the fusion power [Maget NF2013, Urano NF2014]. The increased gas fuelling has led to a reduction of the pedestal pressure top values, mainly due to the lower pedestal temperature compared to JET-C, as described in [Beurskens NF2014, Maggi NF2015]. In DIII-D discharges with the injection of Li, an enhanced pedestal pressure and global energy confinement has been observed This has been linked with the increase of the pedestal temperature. The density in the HFSHD region can be reduced by applying nitrogen seeding, allowing the density profile to shift back inwards and improving pedestal stability [Dunne PPCF2017, Dunne IAEA2016]. The changes in pedestal structure, when the electron temperature and electron density have different relative pedestal positions, are correlated with a reduction in the nor malized pressure gradient, playing a significant role in pedestal stability.
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