Theoretical and experimental investigation of ELM-free states in high performance tokamak regimes

We propose a novel interpretation of the Edge Harmonic Oscillation (EHO), which is seen as part of quiescent high-confinement (QH) mode, and compare its predictions to JET data. This new model for the EHOs will improve our ability to predict the QH state. Unlike the standard high confinement (Hmode) regime, QH plasmas avoid violent and periodic plasma eruptions known as Edge Localised Modes (ELMs) [1]. ELMs deposit unacceptable peak heat loads causing a severe deterioration of the plasma facing components. This poses a serious operational threat to reactor relevant plasma scenarios. Thus, the development of naturally ELM-free regimes has become of crucial importance [2]. One such regime is the so called QH mode. In QH plasmas, ELMs are replaced by dominantly low- steady mild MHD EHOs [3]. EHOs have been observed in DIII-D [3], ASDEX-U [4], JET [5], JT60 [6] and NSTX [7] at low edge collisionality over a fairly broad range in [3]. EHOs, which are typically characterised by multiple rotating toroidal harmonics localised in the pedestal region [8], enhance particle transport allowing density control and potentially ash removal without the impulsive heat load caused by ELMs [9]. A successful access to and control of this favourable regime, still requires a deeper understanding of its mechanisms in terms of basic stability concept. We thus propose, within the ideal MHD linear stability framework, a novel interpretation for the EHO onset mechanism. Provided by the possibility of edge infernal-type instabilities in QH plasmas [10], we show that the interplay of poloidal flows (MHD and diamagnetic) allows the suppression of short wavelength modes so that low- oscillations, namely EHOs, can emerge [11]. Our model retrieves several features measured experimentally such as mode rotation frequencies, radial struture [8], and amplitude of the critical shearing rate [12]. The theoretical understanding of the EHO mode onset is then applied to the interpretation of JET-C discharges exhibiting Outer Mode activity [5] which are subsequently compared with the JET-ILW database. [1] A. W. Leonard, Phys. Plasmas 21, 090501 (2014); [2] E. Viezzer et al., Nucl. Fusion 58, 115002 (2018); [3] K. H. Burrell et al., Phys. Plasmas 12, 056121 (2005); [4] W. Suttrop et al., Plasma Phys. Control. Fusion 45, 1399 (2003); [5] E. R. Solano et al., Phys. Rev. Lett. 104, 185003 (2010); [6] N. Oyama et al., Nucl. Fusion 45, 871 (2005); [7] K. F. Gan et al., Nucl. Fusion 57, 126053 (2017); [8] X. Chen et al., Nucl. Fusion 56, 076011 (2016); [9] K. H. Burrell et al., Phys. Rev. Lett. 102, 155003 (2009); [10] D. Brunetti et al., Nucl. Fusion 58, 014002 (2018); [11] D. Brunetti et al., Phys. Rev. Lett. 122, 155003 (2019); [12] T. M. Wilks et al., Nucl. Fusion 58, 112002 (2018)