Session G16: Sherwood I

The Future of Boundary Plasma and Material Science

The boundary of magnetic confinement devices, from the pedestal through to the surrounding surfaces, encompasses an enormous range of plasma and material physics, and their integrated coupling. It is becoming clear that due to fundamental limits of plasma stability and material response the boundary will largely define the viability of an MFE reactor. However we face an enormous knowledge deficit in stepping from present devices and ITER towards a demonstration power plant. We outline the future of boundary research required to address this deficit. The boundary should be considered a multi-scale system of coupled plasma and material science regulated through the non-linear interface of the sheath. Measurement, theory and modeling across these scales are assessed. Dimensionless parameters, often used to organized core plasma transport on similarity arguments, can be extended to the boundary plasma, plasma-surface interactions and material response. This methodology suggests an intriguing way forward to prescribe and understand the boundary issues of an eventual reactor in intermediate devices. A particularly critical issue is that the physical chemistry of the material, which is mostly determined by the material temperature, has been too neglected; pointing to the requirement for boundary plasma experiments at appropriate material temperatures. Finally the boundary plasma requirements for quiescent heat exhaust and control of transient events, such as ELMs, will be examined.   

Turbulent saturation and transport in global 3D two-fluid simulations of the tokamak edge

Based on nonlinear, global, three-dimensional two-fluid simulations, we explore the physics of turbulent saturation and transport in the tokamak scape-off layer and other magnetic geometries. We find that the global simulations can produce larger relative fluctuation amplitudes and more Bohm-like (versus Gyro-Bohm-like) transport scalings than those of non-global (flux-tube) simulations. In both the global and local simulations, small-scale primary instabilities such as driftwaves or resistive ballooning modes produce radial streamers, which grow until they are broken up by secondary modes such as the Kelvin-Helmholz instability or related modes. In a local simulation with fixed, radially constant equilibrium gradients and periodic boundaries, for example, the radial elongation of these primary-mode streamers is, in principle, unlimited. In the global simulations, however, the radial extent of the primaries is truncated by the nonlocal radial variations of the equilibrium profile gradients, typically to the geometric mean of the equilibrium profile scale-length and poloidal scale-length of the primary modes. This radial truncation can have a strongly stabilizing effect on the KH mode, leading to larger primary-mode fluctuation levels and non-Gyro-Bohm transport scaling.   

The nonlinear dispersion relation of geodesic acoustic modes

The energy input and frequency shift of geodesic acoustic modes (GAMs) due to turbulence in tokamak edge plasmas are investigated in numerical two-fluid turbulence studies. Surprisingly, the turbulent GAM dispersion relation is qualitatively equivalent to the linear dispersion but can have drastically enhanced group velocities. Such propagation velocities, which are comparable to the diamagnetic drift, allow for the formation of global GAM eigenmodes of several centimeters width. These would certainly be recognizable in experiments and might explain the GAM frequency plateaus in ASDEX Upgrade. In case of equilibrium magnetic fields with broken up-down symmetry the energy input due to the turbulent transport may favor the excitation of GAMs with one particular sign of the radial phase velocity relative to the magnetic drifts and may lead to periodic bursts of GAM and turbulence activity. This behavior resembles strikingly the experimentally observed pulsation during the I-phase in ASDEX Upgrade or the quiet periods in NSTX.