Electrostatic instabilities, turbulence and fast ion interactions in simple magnetised plasma
Seminar Room 1, Newton Institute
Electrostatic turbulence, related structures and their effect on particle, heat and momentum transport are investigated in TORPEX simple magnetized plasmas using high resolution diagnostics, control parameters, linear fluid models and nonlinear numerical simulations. The nature of the dominant instabilities is controlled by the ratio between vertical and toroidal magnetic field intensities. For Bv/BT>3%, only ideal interchange instabilities are observed. A critical pressure gradient to drive the interchange instability is experimentally identified. Interchange modes give rise to blobs, radially propagating filaments of enhanced plasma pressure. The measured values of blob velocities and sizes span a wide range and are described by a single analytical expression, from the small blob size regime in which the blob velocity is limited by cross-field ion polarization currents, to the large blob size regime in which the limitation to the blob velocity comes from parallel currents to the sheath. As a first attempt at controlling the blob dynamical properties, limiter configurations with varying angles between field lines and the conducting surface of the limiter are explored. Mach probe measurements clearly demonstrate a link between toroidal flows and blobs. To complement probe data, a fast framing camera and a movable gas puffing system are installed. Density and light fluctuations show similar signatures of interchange activity, but the fast camera images can be obtained with higher spatial resolution, proving data on small turbulence scales. The effect of interchange turbulence on fast ion phase space dynamics is studied using movable fast ion source and detector. A theory validation project is conducted in conjunction with TORPEX experiments, based on quantitative comparisons of observables that are defined in the same way in the data and in 2D and 3D local and global simulations.