PhD and MPhil Thesis Classes - Université Libre de Bruxelles

3Drift Waves3.1 IntroductionTo achieve the needed conditions for fusion in a tokamak it is necessary to confine theplasma for a sufficient time, as described in sec.1.6. However, confinement is limited bytransport processes of energy and particles across magnetic field lines. In the absence ofinstabilities, the confinement of a toroidally symmetric tokamak plasma is determinedby Coulomb collisions. However, the transport which actually occurs does not agreewith the calculated values from collisional models.The plasma in tokamak is inhomogeneous, therefore, the density and temperaturegradients give rise to electron and ion diamagnetic drifts vD s 1 across the magnetic fieldB. This drift will give rise to collective oscillations which are called drift waves. In thepresence of fluctuations of the plasma dynamical quantities (density, velocity, pressure,. . . ), these waves can become linearly unstable with their amplitude growing in time.These unstable modes play a crucial role in the mechanism of anomalous transport.The theoretical picture of anomalous transport is that the free energy released by aninstability drives a steady level of fluctuations in the associated perturbed quantities.In this turbulent state fluctuations result in a radial transport of particles and energy.1 The subscript s is used to label the species of plasma particle.25