(TH/P8-11) Diamagnetic GAM Drive Mechanism

K. Hallatschek1)
1) Max-Planck-Institut für Plasmaphysik, Garching, Germany

Abstract.  Geodesic Acoustic Modes (GAM), poloidal flows oscillating at the characteristic acoustic frequency of a tokamak or stellarator, are an ubiquitous edge plasma phenomenon. In recent years, they have dramatically gained experimental interest and are candidates for applications ranging from plasma diagnostics to transport control. GAMs oscillate between states of strong rotation and up-down asymmetric plasma compression. Hence, at first glance the natural turbulent drive (or damping) mechanisms for them is either a direct boost of the rotation - via Reynolds stress - or the creation of asymmetric pressure distributions - via transport. However, an up-down asymmetric pressure may also be created by the divergence of the diamagnetic drifts, if there is a perturbation in the diamagnetic drift velocity, i.e., of the overall pressure gradient. That in turn may fluctuate due to any modulation of the flux surface averaged turbulent transport. On the other hand, a modulation of the turbulent transport due to the GAMs themselves is expected to happen in the tokamak edge, and has been observed early on in simulations and recently in many experiments. In turbulence simulations for edge parameters, the described effect tendencially is a strong driver of the GAMs of equal importance to the other two. As a striking consequence, the coupling of diamagnetic velocity and GAM can produce propagating fronts of high flow velocity and transport, which closely resemble avalanches - without necessity of a critical gradient. The diamagnetic flow drive is strong enough to advance the flow and transport layer in radial direction - although the linear dispersion relation would just result in a localized oscillation! An interesting consequence of the diamagnetic drive mechanism is that it offers the possibility of direct excitation of GAMs by resonantly modulated external heating (replacing turbulent transport with heating power). If the GAMs are detected by Doppler reflectometry, the achievable efficiency is certainly enough for diagnostic purposes such as to actively probe the GAM frequencies or to measure the turbulence response to the GAMs. Particularly exciting however is the prospect of a way to artificially set up a GAM pattern to control the transport.

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