Low threshold parametric decay instabilities in ECRH experiments at toroidal devices

Experimental conditions leading to a substantial reduction in the backscattering decay instability threshold in electron cyclotron resonance heating (ECRH) experiments in toroidal devices are analysed. It is shown that the drastic decrease in threshold due to suppression of convective losses is provided by the nonmonotonic behaviour of plasma density, which is often observed due to the so-called density-pump-out effect or the presence of magnetic islands, and by poloidal magnetic field inhomogeneity making localization of ion Bernstein decay waves possible. The corresponding ion Bernstein wave gain and the convective parametric decay instability pump power threshold are calculated. The possible experimental consequences of easy backscattering decay instability excitation are discussed.