Turbulence in the edge of tokamaks

Nuclear fusion is a clean, inexhaustible source of energy. It only requires specific isotopes of hydrogen and it does not emit carbon dioxide or leave nuclear waste. It is a complex technology because fusion reactions require extremely high temperatures and, under such conditions, the hydrogen gas is ionised. Fusion energy is then faced with the challenge of confining ionised gas (known as plasma) of high energy density. However, the prize at hand is so attractive (and more so now in an increasingly energy-conscious world) that the fusion community is involved in a worldwide effort to develop a viable fusion reactor. As a result, ITER is being built in Cadarache (France) and several other experiments (JET, in UK, among the most important) are currently working intensively on different aspects of the problem. This international collaboration can only be compared to the previous experience in particle accelerators.The concept behind ITER and JET is magnetic confinement (the plasma is held in equilibrium by magnetic fields). Magnetic confinement has proven the most effective fusion method so far and, among all possibilities, the tokamak is the most successful (ITER and JET are tokamaks). A tokamak is an axisymmetric toroidal configuration with a dominant toroidal magnetic field and a smaller poloidal magnetic field. Tokamaks were proposed in Russia in the 1950s, and since then they have been subject to extensive research. Even so, there is still a crucial area where our knowledge is incomplete, namely, turbulent transport of energy and particles to the walls of the vessel. Understanding the processes that lead to the observed energy and particle losses, and controlling them, are necessary steps prior to the construction of a successful fusion energy plant. Over the last ten years, there has been a notable effort directed at unravelling the physics of turbulent transport in tokamaks. As a result, we have learnt that the energy stored in the tokamak mainly depends on the temperature at the edge of the plasma.In tokamaks, in what is known as a high confinement mode or H-mode, the temperature at the edge of the plasma is determined by a transport barrier that sits just at the edge of the plasma. The physics in this transport barrier, known as the pedestal, determines the difference in density and temperature between the hot core and the cold, partially ionised edge. Unfortunately, the pedestal is still poorly understood. My research programme focuses on the physics of the pedestal, in particular, on how phenomena specifically relevant to the plasma edge affects turbulence. Most turbulence research has been aimed at the plasma core, where the plasma is fully ionised, the walls of the vessel are far away and the defects of the magnetic field geometry are small. In the pedestal, however, these features matter. To study them, I am developing analytical models that will be useful to gain physical insight and to construct meaningful benchmarks for more complex works (several groups around the world are intensively working on large numerical simulations). Most of my research is on the effect of defects in the magnetic geometry on the level of saturation of the turbulence. Magnetic geometry is an important ingredient in the torodial velocity evolution, and toroidal velocity shear decorrelates the turbulence, controlling the level at which the turbulence reaches its equilibrium. I attempt to understand this relationship using both linear and nonlinear analyses.