Investigating the radial structure of axisymmetric fluctuations in the TCV tokamak with local and global gyrokinetic GENE simulations

Abstract

Axisymmetric (n=0) density fluctuations measured in the TCV tokamak are observed to possess a frequency f0 which is either varying (radially dispersive oscillations) or a constant over a large fraction of the plasma minor radius (radially global oscillations) as reported in a companion paper [Z. Huang et al, this issue]. Given that f0 scales with the sound speed and given the poloidal structure of density fluctuations, these oscillations were interpreted as Geodesic Acoustic Modes, even though f0 is in fact smaller than the local linear GAM frequency fGAM. In this work we employ the Eulerian gyrokinetic code GENE to simulate TCV relevant conditions and investigate the nature and properties of these oscillations, in particular their relation to the safety factor profile. Local and global simulations are carried out and a good qualitative agreement is observed between experiments and simulations. By varying also the plasma temperature and density profiles, we conclude that a variation of the edge safety factor alone is not sufficient to induce a transition from global to radially inhomogeneous oscillations, as was initially suggested by experimental results. This transition appears instead to be the combined result of variations in the different plasma profiles, collisionality and finite machine size effects. Simulations also show that radially global GAM-like oscillations can be observed in all fluxes and fluctuation fields, suggesting that they are the result of a complex nonlinear process involving also finite toroidal mode numbers and not just linear global GAM eigenmodes.

Experimental observations of modes with geodesic acoustic character from the core to the edge in the TCV tokamak

“…Spatial correlation analysis on 9 radially separated TPCI chords shows that the radial wave number of the GAM is k ρ = 2 cm −1 , outward propagating, which is consistent with previous observations of single-frequency radially coherent GAMs on TCV [11]. Local and global nonlinear gyrokinetic simulations have been performed with GENE [26] with reference q, n e , T e and T i profiles from TCV experimental plasmas of both GAM continuum and radially coherent mode cases, as reported in the companion paper [1]. Nonlinear flux-tube simulations show that the E × B velocity oscillation frequency is always dependent on the local temperature with both reference profiles, however downshifted from the local analytical GAM frequency.…”

“…A characterization of the oscillations at the edge GAM frequency in the SOL is presented in section 5, followed by conclusions in section 6. A companion paper [1] presents an extensive theoretical and numerical analysis of these modes in TCV.…”

Complex-Hamiltonian paraxial description of damped geodesic acoustic modes

“…Experimentally, GAMs are known to appear also in form of eigenmodes, which oscillate with constant frequency across a finite radial extent [64,65] also in non-uniform equilibria. This feature is known from theory [5,66,67] and has been reproduced also numerically [68,69]. Here, we suppose the GAM behaves as a continuum [32], i.e.…”

Nonlocal effects in negative triangularity TCV plasmas

“…See for instance Refs. [15,16] which consider other TCV conditions (standard positive triangularity shapes) in which global effects appear to be less relevant, or Ref. [17] showing a unexpected global effects for machines of bigger size.…”