Doppler refectometry in the TJ-II stellarator: design of an optimized Doppler reflectometer and its application to turbulence and radial electric field studies

Abstract

For several decades magnetic confinement of high-temperature plasmas has been investigated with the objective of building a burning fusion reactor. One of the main obstacles in reaching this goal is the energy and particle losses caused by radial transport processes in the plasma. Therefore, the identification and reduction of this radial transport is a demanding challenge faced by theoretical and experimental physicists. The transport processes in toroidal plasmas can be grouped into two categories, i.e. neoclassical and turbulent transport. Neoclassical theory is an extension of classical theory to include the toroidal geometry of magnetic confinement fusion experiments, which results in new particle drifts and magnetic field mirror effects, which trap particles and lead to an increased collision frequency. Neoclassical transport is an ubiquitous process, since it depends on the existence of background gradients in the plasma and Coulomb collisions between particles. The second type of transport, turbulent or anomalous transport, is fundamentally different from neoclassical transport due to the fact that the described particle losses are caused by microinstabilities. These microinstabilities occur irregularly in the plasma, hence turbulent transport is an intermittent process rather than a continuous one. The particle and energy losses observed in toroidal fusion plasmas are believed to be mainly caused by turbulent transport, making it one of the dominant fields of investigation of the fusion community in the last few decades. Plasma turbulence can basically be described as the incoherent motion of the plasma which arises from small-scale fluctuations in parameters such as plasma density, tem- perature, potential, and the magnetic field. Gradients in the plasma parameters are the driving forces of the turbulence, which leads to the conclusion that the better the plasma confinement (stronger gradients), the higher the turbulence level. However, this is not completely true: the discovery of the H-mode confinement regime in 1982 showed that the plasma can spontaneously self-organize and enter a mode of improved confinement (L-H transition), characterized by a steepening of plasma gradients accompanied by a significant reduction of the level of fluctuations and turbulent transport. This discovery led to an immense effort, from both the theoretical and the exper- imental sides, in trying to understand the L-H transition and the reduced turbulence level in the H-mode confinement regime. After more than a quarter century of active research, the prevailing paradigm to explain the turbulence level reduction consists in turbulence suppression via sheared flows. However, although these flows are observed in H-mode plasmas, their generation mechanisms are still unknown. Several candidates involving the edge pressure gradient or turbulence driven mean and oscillating flows exist, but elucidation is still pending.