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Electron cyclotron resonance heating (ECRH) is a technique being widely utilized nowadays in magnetic fusion experiments. It is also considered for application in ITER, for plasma heating and neoclassical tearing mode control. The analysis [1-3] of different parametric decay instabilities (PDIs) potentially dangerous for mm-waves at the ECR frequencies in the presence of the monotonous plasma profiles revealed the typical microwave power threshold in the range much higher than 1 MW. This is due to huge energy loss of the interacting waves from the decay region. The typical heating system output power range available up to now, even if the gyrotrons are joined into a group, is substantially lower than the predicted PDI power threshold value. Thus, it has been taken as known until very recently that the ordinary or extraordinary electromagnetic wave propagation and absorption in the absence of the UHR being well described by linear theory is predictable in detail. However, during the last decade many observations have demonstrated the anomalous phenomena during ECRH. Firstly, the fast ion generation and ion heating were observed at the TJ-II stellarator [4] and the TCV tokamak [5] during the 2 harmonic Xmode ECRH pulse under conditions when energy exchange between electrons and ions should be very low. 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 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 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 [10-13]. 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 cornerstone of them is taking full account of a nonmonotonous plasma density profile often observed in the ECRH experiments and originated due to different physical mechanisms, among which are features of plasma confinement in the magnetic island [14] or the electron pump-out effect manifesting itself as a consequence of the anomalous convective particle flux from the ECR layer at the intensive ECRH. The poloidal magnetic field inhomogeneity is a key element of the theory as well. Based on these two elements it was shown that twoor even three-dimensional (2D or 3D) trapping of either a daughter ion Bernstein wave (IBW) or a daughter electron Bernstein wave (EBW) due to a specific multidimensional plasma inhomogeneity can be responsible for excitation of several convective and absolute PDIs possessing thresholds drastically less than those predicted by the standard model. These theoretical results expanded significantly the standard theory which ignored peculiarities of a density profile and geometry of toroidal devices. Though the developed theoretical models demonstrated the principal possibility of the low-threshold PDI excitation in the 2 harmonic X mode ECRH experiments at toroidal devices, they couldn't describe the main features and details of the observed anomalous effects. Predicted fast parametric instabilities were shown to be convective. The most dangerous absolute PDIs were demonstrated to have the growth rates comparable with the inverse characteristic times of the MHD and drift processes. The relative crisis in development of the theory was, apparently, overcome in [15, 16] pointing out to the possibility that the backscattering signal generation as well as the anomalous absorption in ion channel could be a consequence of secondary nonlinear process, that accompanies a primary low – threshold absolute PDI of a different nature. A hint to this primary instability is provided experimentally in [7] demonstrating the most intensive backscattering at the plasma density in the magnetic island slightly exceeding the UHR value for half the pump frequency. In [15, 16] it was realized that at these conditions the low-threshold pump wave decay into the two upper hybrid (UH) waves is possible. In the present paper we review recent results of the two-plasmon PDI theory development [15-24]. We introduce a model taking into account, as distinct from the standard theory [1-3], the presence of a nonmonotonous density profile, which always exist on the discharge axis or may be present due to the magnetic island or the density pump-out effect. We interpret the generation of backscattering signal and the anomalous ion heating, as a result of secondary nonlinear processes that accompany a primary low– threshold two–UH–plasmon PDI driven by the pump X-mode. The threshold of the primary PDI is shown to be smaller than the one predicted in [1] due to the UH wave trapping in radial direction [15,16]. It is also shown that in the presence of the finite-size pump beam the most dangerous absolute instability could be excited. The threshold of this instability is determined in hot fusion plasma not by the UH wave damping, but by diffractive losses along the magnetic field and could be as small as several tens kW. The twoplasmon decay growth rate is very high, in the range  DOI: 10.1051/ , 01012 (2017) 71490 1 1012 EPJ Web of Conferences epjconf/201 49

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