Speaker
Carlos Silva
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P2.1093.pdf
GAM evolution in L-mode approaching the L-H transition on JET
C. Silva1, J. C. Hillesheim2, L. Gil1, C. Hidalgo3, C.F. Maggi2, L. Meneses1, E.R. Solano3, and
JET Contributors*
EUROfusion Consortium, JET, Culham Science Centre, Abingdon, OX14 3DB, UK
1
Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade Lisboa, PT
2
CCFE, Culham Science Centre, Abingdon, OX14 3DB, UK
3
Laboratorio Nacional de Fusión, CIEMAT, 28040 Madrid, Spain
* See the author list of “X. Litaudon et al 2017 Nucl. Fusion 57 102001
The interaction between zonal flows (ZFs) and turbulence is a self-regulating mechanism.
Understanding this interaction is crucial to control plasma confinement. The shearing due to
ZFs is thought to dominate in regimes when the mean shear flow is modest as before and
during the L-H transition [e.g. 1]. This was corroborated by findings in different devices
demonstrating the importance of both the oscillating and mean flow shear and their
interaction in triggering the transitions [1-4]. While on AUG the sheared flow below the L-H
threshold is dominated by Geodesic Acoustic Modes (GAMs) [1], on devices such as DIII-D
[2], EAST [3] and HL-2A [4] GAMs do not appear to be important on the way to H-mode.
The reported results reveal that no clear picture exists on the relevance of GAMs in the
turbulence collapse required for the formation of steep pressure gradients at the transition.
This contribution focuses on the characterization of GAMs in JET L-mode plasmas when
approaching the L-H transition aiming at understanding its possible role in triggering the
transition. Doppler backscattering has been used to investigate GAMs by measuring
oscillations in the E × B flow velocity. Experiments were performed in NBI heated discharges
for different values of plasma current (2.2 < Ip < 3.2 MA) and line-averaged density (1.6 < n <
3.1×1019 m-3). The dataset also includes variations in toroidal magnetic field, magnetic
configuration and hydrogen isotopes.
Results demonstrate that parameters such as plasma current and density have a strong effect
on the GAM amplitude. By assessing the importance of critical parameters such as safety
factor and collisionality, experimental evidence is found for the different mechanisms
determining the GAM amplitude: turbulence drive, collisional and collisionless damping.
GAMs have been studied along the power ramp used to induce the L–H transition, taking
advantage of the unique JET dataset. As the heating power increases, the GAM amplitude
first increases but then is reduced as the L-H transition is approached. GAMs are either
suppressed or have a modest amplitude at the transition. The cause of this reduction is
however unclear as the GAM damping rate is expected be reduced along the heating power
ramp and density fluctuations levels and the E × B shear flow display modest changes.
Experimental investigation of isotope effects in hydrogen and deuterium plasmas was also
performed. Stronger GAMs were found in D than in H plasmas at low heating power (PNBI <
4 MW) associated with larger edge density fluctuations in H. However, above PNBI 4 MW,
the GAM amplitude is reduced in D plasmas while increases for H plasmas. Unfortunately,
the L-H transition could not be achieved in H plasma due to heating power limitations.
[1] G.D. Conway et al., PRL 106, 065001 (2011) [2] Z. Yan et al, NF 53 113038 (2013)
[3] G.S. Xu et al, PRL 107, 125001 (2011) [4] M. Xu et al, PRL. 108, 245001 (2012)
