We analyze both analytically and numerically, two cases of low-power-threshold two-upper hybrid-plasmon parametric decay instability saturation when the odd-or even-step cascade of secondary decays is possible. We have found that giant anomalous absorption (more than 80%) of pump power leads to its depletion in the regime when the instability is saturated due to the even-step cascade of decays of primary upper hybrid waves. This can make the power deposition profile different from that predicted by linear theory and provides an alternative explanation for the evident broadening observed under electron cyclotron resonance heating in toroidal devices.

We discuss the experimental conditions responsible for a drastic decrease in the power threshold of parametric decay instabilities under auxiliary electron cyclotron resonance heating (ECRH) in toroidal magnetic fusion devices when the upper hybrid (UH) resonance for the pump wave is absent. We show that for a finite-width pump in the presence of a nonmonotonic (hollow) density profile occurring due to plasma equilibrium in the magnetic islands or anomalous particle fluxes from the ECR layer, 3D localization of one or both daughter waves is possible. This localization leads to the full suppression of daughter wave energy losses from the decay layer and a substantial increase in the nonlinear pumping efficiency. This decreases the power threshold of nonlinear excitation, which can be easily overcome in current ECRH experiments utilizing 1 MW microwave beams. Different scenarios of extraordinary and ordinary wave decays are investigated. The secondary decays of primary daughter waves and pump wave depletion are considered as the most effective mechanisms leading to the transition of primary instability to the saturation regime. The proposed theoretical model was shown to be able to describe the anomalous phenomena discovered in ECRH experiments in different toroidal fusion devices all over the world.

Correlation enhanced scattering (CES) near the upper hybrid resonance has been applied for studying small-scale plasma density fluctuations excited by the rf fields in a helicon discharge. The turbulent fluctuations are diagnosed for conditions where the electron plasma frequency exceeds the electron cyclotron frequency considerably. The frequency and wave number spectra of the fluctuations are measured both in the plasma core as well as in outer region of the helicon discharge. The spectral measurements evidence the short-scale fluctuations to originate from a parametric decay instability. The low-frequency fluctuations obey the ion-sound dispersion relation while the lower sideband of the helicon wave frequency satisfies the Trivelpiece–Gould wave dispersion relation. In order to gain more insight in the experimental results and, in particular, to estimate the fluctuation level the backscattering process is analyzed both numerically and analytically for high-density plasma conditions. A fully electromagnetic model was developed that takes into account the radial density distribution of the plasma column as well as the antenna diagram of the rectangular emitting/receiving horn. Using this model the relative amplitude of ion-sound density fluctuations in the core of the helicon discharge is estimated as 11%. The role of nonlinear effects on formation of the scattering spectra due to the high fluctuation level in the plasma center is discussed. The findings also demonstrate that the CES diagnostic can be applied to diagnose fluctuations in spherical tokamak plasmas where the probing conditions resemble those of high-density helicon discharges.

We present novel experimental evidence of parametric decay instability of microwave beams in the plasma edge of Wendelstein 7-X stellarator. We propose that the instability is sustained by trapping of only one daughter wave in the non-monotonic density profile measured with high spatial resolution within a stationary magnetic island. The power levels and spectral shapes of the detected microwave signal are reproduced by numerical modelling and a theoretical power threshold is predicted around 300 kW, comparable with observations. We predict a fraction of power drained by daughter waves around 4% in the experiments, potentially increasing above 50% for minor modifications of the density bump. Such absorption levels could significantly reduce the efficiency of the microwave heating and current-drive system in tokamaks and stellarators.

In the present paper the localization of poloidal correlation reflectometry (PCR) is treated analytically for arbitrary density and turbulence profiles. The 3D WKB approach is used to calculate the fluctuation reflectometry signal and the PCR cross correlation function produced by long-scale turbulence. The study is performed in cylindrical geometry correctly accounting for the plasma curvature effects. The probing wave propagation is described in paraxial approximation accounting for the refraction and diffraction effects. The obtained explicit expressions for the PCR characteristics can be used for determination of the turbulence level and the velocity distribution from the experimental data for arbitrary density profiles.