Low Threshold Parametric Decay Backscattering Instability in Tokamak Electron Cyclotron Resonance Heating Experiments

Abstract

Abstract. The experimental conditions leading to substantial reduction of backscattering decay instability threshold in ECRH experiments in toroidal devices are analyzed. It is shown that drastic decrease of threshold is provided by non monotonic behavior of plasma density in the vicinity of magnetic island and poloidal magnetic field inhomogeneity making possible localization of ion Bernstein decay waves. The corresponding ion Bernstein wave gain and the parametric decay instability pump power threshold is calculated.

Modification of the collective Thomson scattering radiometer in the search for parametric decay on TEXTOR

“…11 The observations have been suggested to originate from the parametric decay instability and has motivated a re-evaluation of the parametric decay instability power threshold. [12][13][14][15] In this article we describe the modified CTS radiometer used to detect the newly discovered strong signals during ECRH operation. The calibration of the system is discussed, and examples of time resolved measurements of strong signal in the presence of tearing modes are shown for different plasma densities.…”

Anomalous phenomena in ECRH experiments at toroidal devices and low-threshold parametric decay instabilities

“…As it was shown in [9], [11][12][13], the PDI threshold decreases substantially when one of the daughter waves, IB wave in the particular case of these references, is trapped at least in x -direction. The poloidal dependence of the magnitude of the magnetic field can ensure the localization of the EB wave also in poloidal direction.…”

“…It was shown that the local maximum of the plasma density, which is usually observed in the O-point of magnetic island at Textor [10], can lead to localisation of the low frequency ion Bernstein (IB) decay wave and thus to suppression of IB wave convective losses in radial direction. A more complicated 2D analysis of the IB wave propagation accounting for the poloidal inhomogenuity of magnetic field in toroidal plasma have shown possibility of IB wave localization in the poloidal direction, as well [11], [12]. The threshold of the backscattering PDI was calculated in this case and shown to be more than four orders of magnitude lower than predictions of standard theory (in the range of 50 kW for the Textor experiment parameters).…”

Anomalous absorption in ECRH experiments due to parametric excitation of localized UH waves

“…Secondly, the observations of the backscattering signal down -shifted from the pump frequency by approximately 1 GHz and correlated to the MHD mode rotation were reported [6,7] in the 200 -600 kW level 2 nd harmonic X-mode ECRH experiment in the TEXTOR tokamak and interpreted in terms of anomalous backscattering of the pump EC wave. The anomalous scattering of the X-mode in the ASDEXUpgrade tokamak 2 nd harmonic ECRH experiment has been recently reported as well [8,9].The attempts of theoretical explanation of these anomalous phenomena have been undertaken during recent years [10][11][12][13]. Several theoretical models demonstrating the possibility of excitation of the lowthreshold PDIs which lead to generation of the electron and ion Bernstein waves had been proposed.…”

“…The attempts of theoretical explanation of these anomalous phenomena have been undertaken during recent years [10][11][12][13]. Several theoretical models demonstrating the possibility of excitation of the lowthreshold PDIs which lead to generation of the electron and ion Bernstein waves had been proposed.…”