Study of plasma edge turbulence in tokamaks

The plasma edge in tokamaks is known to be very turbulent. The nonlinear stability of a test mode is investigated in the presence of a helical potential perturbation, i.e., a ‘‘pump mode,’’ which simulates the plasma turbulence. The particle trajectories in this perturbed equilibrium are derived using a Hamiltonian formalism. The electrons appear to have trapped trajectories in the potential well of the pump mode, while the ions experience a large convective motion. These two effects have a large influence on the test mode stability. First, nonlinearly trapped electrons supply an energy source for the test mode. Second, the ion convective motion introduces a radial scale of the test mode larger than the ion Larmor radius, in agreement with experimental data. These two phenomena allow a bifurcation in the turbulence level and therefore provide an explanation for the L–H transition.