Speaker
Tim Happel
(Max-Planck-Institut für Plasmaphysik)
Description
The I-mode is an improved confinement regime characterized by the presence of a temperature pedestal, while the density profile remains comparable to that in L-mode [1]. This means that in I-mode a transport barrier is formed in the heat transport channel, while the particle transport channel is not affected. This has several benefits for the operation of a tokamak: both impurity accumulation and ELMs are absent in I-mode, resulting in a comparably benevolent plasma. After substantial I-mode research by the fusion community in the last years, the mechanism which creates a transport barrier in only one of the transport channels is still not understood.
In ASDEX Upgrade (AUG) I-modes, the edge fluctuation amplitude is reduced substantially, which leads to the dominance of the so-called weakly coherent mode (WCM) in the density turbulence spectrum [2,3]. It will be shown that both poloidal correlation reflectometry [4] and correlation electron cyclotron emission [5] diagnostics detect the WCM in the L-mode phase long before I-mode starts, showing that the WCM is not exclusive to the I-mode. A newly installed thermal helium beam diagnostic on AUG [6] confirms these results and allows a precise radial determination of maximum impact of the WCM. It will be reported that the reduction of fluctuation amplitudes at frequencies different from the WCM is accompanied by the emergence of solitary and intermittent structures with large amplitudes, even larger than in L-mode [7]. Hence the PDF of density fluctuations in I-mode exhibits a strong tail towards large amplitudes, which is observed for all wavenumbers investigated ($k_\perp = 5$-$12~{\rm cm}^{-1}$, with $k_\perp$ the perpendicular wavenumber of the density flucutations) [8]. These density turbulence bursts are linked to the WCM and their amplitudes scale with the I-mode confinement quality. The increased dominance of the WCM during improving I-mode, connected to the increased intensity of turbulence bursts could be an indication that the WCM is responsible for the generation of the turbulence bursts.
[1] D. G. Whyte et al., Nucl. Fusion **50**, 105005 (2010).
[2] A. E. Hubbard et al., Phys. Plasmas **18**, 056115 (2011).
[3] P. Manz et al., Nucl. Fusion **55**, 083004 (2015).
[4] D. Prisiazhniuk et al., Plasma Phys. Control. Fusion **59**, 025013 (2017).
[5] S. J. Freethy et al., Rev. Sci. Instrum. **87**, 11E102 (2016).
[6] M. Griener et al., Plasma Phys. Control. Fusion **60**, 025008 (2018).
[7] T. Happel et al., Nucl. Fusion **56**, 064004 (2016).
[8] T. Happel et al., Plasma Phys. Control. Fusion **59**, 014004 (2017).
Primary author
Tim Happel
(Max-Planck-Institut für Plasmaphysik)
Co-authors
Dmitrii Prisiazhniuk
(Max-Planck-Institut für Plasmaphysik)
Dominik Brida
(Max-Planck-Institut für Plasmaphysik)
Luis Guimarais
(Instituto de Plasmas e Fusao Nuclear, Instituto Superior Tecnico, Universidade de Lisboa, Portuga)
Michael Griener
(Max-Planck-Institut für Plasmaphysik / Physik-Department E28, Technische Universität München, James-Franck-Str. 1, 85748 Garching, Germany)
Prof.
Pascale Hennequin
(Laboratoire de Physique des Plasmas, Ecole Polytechnique, 91128 Palaiseau, France)
Dr
Peter Manz
(Max-Planck-Institut für Plasmaphysik)
Simon Freethy
(Plasma Science and Fusion Center, Massachusetts Institute of Technology, CambridgeMax-Planck-Institut für Plasmaphysik /)
Prof.
Ulrich Stroth
(IPP Garching)
the ASDEX Upgrade Team
(Max-Planck-Institut für Plasmaphysik)
the MST1 Team
(not available)
