Numerical modelling of geodesic acoustic mode relaxation in a tokamak edge

Abstract

Geodesic acoustic modes (GAMs) are an important phenomenon in a tokamak edge plasma. They regulate turbulence in a low confinement (L-mode) regime and can play an important role in the low to high (L–H) mode transition. It is therefore of considerable importance to develop a detailed theoretical understanding of their dynamics and relaxation processes. The present work reports on the numerical modelling of collisionless GAM relaxation, including the effects of a strong radial electric field characteristic of a tokamak pedestal in a high confinement (H-mode) regime. The simulations demonstrate that the presence of a strong radial electric field enhances the GAM decay rate, and heuristic arguments elucidating this finding are provided. The numerical modelling is performed by making use of the continuum gyrokinetic code COGENT.

Energetic particle driven geodesic acoustic mode in a toroidally rotating tokamak plasma

“…The second fluctuation appears in the region q ∈ (3.5, 4.5). Similar results have been reported in the previous simulation and theory study [5][6][7]10]. The first fluctuation is induced by the 2nd FOW resonance effect and the second fluctuation is mainly caused by the 3rd FOW resonance effect.…”

“…The classical Landau damping rate of GAM is found to be ∝ e −q 2 Ω 2 and independent of δ i , where Ω is the GAM frequency normalized by R/v T i with major radius R and ions thermal velocity v T i . Later, some theoretical analysis[2-4], numerical evaluation [5], and simulation [6,7] all indicate that the high-order FOW effect plays a key role in the collisionless damping of GAM, specifically in the large q region. The resonant damping rate is sensitive to and significantly enhanced by k r ρ i , where k r is the radial wave number.…”

“…There is no fluctuations on the curves. As a comparison, we then let k = 0.1375 as done in the TEM-PEST simulation [6], COGENT simulation [7] and theoretical calculation [10,16]. It is found that in this condition, the damping rate increases with M first.…”

Continuum kinetic modelling of cross‐separatrix plasma transport in a tokamak edge including self‐consistent electric fields

“…Motivated in part by the success of continuum (Eulerian) delta‐F codes for core physics and in part by their potential for high accuracy, the Edge Simulation Laboratory collaboration has been developing a full‐F code called COGENT for the edge . The code is based on a high‐order mapped multi‐block finite‐volume discretization scheme that involves the use of multiple grid blocks to represent complex magnetic topologies, including a divertor geometry .…”

Finite-orbit-width effects on the geodesic acoustic mode in the toroidally rotating tokamak plasma

“…The classical Landau damping rate of GAM is found to be ∝ e −q 2 Ω 2 and independent of δ i , where Ω is the GAM frequency normalized by R/v T i with major radius R and ions thermal velocity v T i . Later, some theoretical analysis [2][3][4], numerical evaluation [5], and simulation [6,7] all indicate that the high-order FOW effect plays a key role in the collisionless damping of GAM, specifically in the large q region. The resonant damping rate is sensitive to and significantly enhanced by k r ρ i , where k r is the radial wave number.…”