Prospects for Heating Tokamaks by Parametric Decay of Intense Extraordinary Mode Radiation

Intense electron beam technology has developed coherent very high power (350-MW) microwave sources at frequencies which are a modest fraction of the electron-cyclotron frequency in tokamaks. Propagation into a plasma occurs via the extraordinary mode which is subject to parametric decay instabilities in the density range ¿02< ¿pe2<¿0(¿0+¿e), where ¿0 is the impressed frequency, ¿pe the electron plasma frequency,¿e the electron-cyclotron frequency. For an incident wave focused onto a hot spot by a dish antenna of radius ¿, the effective threshold power P0 required to induce effective parametric heating is roughly P0 ¿8 MW x/¿ ¿e/¿0 (Te/ 1 keV)3/2 where x denotes the distance to the hot spot. With present single-pulse technology, experiments which demonstrate nonlinear absorption are possible. Significant heating of tokamaks requires the development of a high repetition frequency intense microwave source.