Study of the parametric instabilities in the lower hybrid frequency range in the FT tokamak

Publisher Website
Google Scholar
Abstract
An overall view of the phenomenology observed with radiofrequency (RF) probes during lower hybrid wave injection is presented for several operational regimes of the FT tokamak. The main emphasis is on the high plasma density regime where the effects of the launched waves on the main plasma become negligible. A rather rich spectrum from about 0 MHz to beyond the injected frequency is observed in response to the RF pulse. Many secondary peaks, or sidebands, shifted from the pump by multiples of the ion cyclotron frequency evaluated at the outer torus edge appear, while the pump spectrum is strongly broadened. Evidence of the primary importance of the border plasma conditions is recognized. Using a numerical code, the authors satisfactorily reproduce part of the experimental data concerning the decay process giving rise to the sideband. The decay of the pump into a quasi-mode strongly damped on ions is considered, and the convection losses due to the finite extension of the pump are taken into account. The key to the good agreement between the numerical simulations and the experiment is to attribute a prominent role to the plasma edge and to its variation with the main plasma parameters, i.e. density and current.