The goal of this paper is to assess the possibility of using a new three-ion ICRF heating scheme on the spherical tokamak Globus-M2. In the first place, the operating frequencies and positions of cyclotron harmonics were found. Matching with frequency band of RF system of Globus-M2 was made. The absorption of electromagnetic waves was modeled using a one-dimensional full-wave code. Since the appearance of high-energy particles is expected in the three-ion ICRF scheme of heating special attention is paid to a comparing the estimates for generated particles energies with their confinement in tokamak Globus-M2.

The kinetic approximation was used to obtain an expression for the bilinear component of the nonlinear charge density, which is used to describe the parametric decay of a microwave with ordinary polarization whose frequency corresponds to the second harmonic of the electron cyclotron resonance. As a result of the decay, an upper hybrid wave and a lower hybrid wave are excited.

The paper presents observations of plasma intermediate frequency range wave emission ((200--1000) MHz) performed in ohmic discharge of the FT-2 tokamak. The dependencies of the RF signal amplitude and spectrum on the plasma parameters and on the location of the probes are presented. Two possible explanations for this effect is proposed. One of them is related to the distortion of the electron distribution function due to the strong ripple of the magnetic field in the FT-2 tokamak. Second one is related to the existence of a beam of run-away electrons, the buildup of a fan instability responsible for the generation of synchrotron radiation in the range (10-80) GHz, and a complex chain of nonlinear decay instabilities leading to such a strong decrease in the frequency of the observed oscillations. Keywords: tokamak, lower hybrid current drive, high frequency oscillations, parametric decay instabilities.

The nonlinear theory of radial correlation reflectometry (RCR) predicts that the cross correlation function of a signal follows a Gaussian distribution with a cutoff separation, where the correlation length is determined by both the turbulence amplitude and its radial correlation length. In contrast, phase spectrum analysis provides information solely on the turbulence amplitude. This work describes the possibility of applying both signal analysis methods and demonstrates that turbulence amplitude and radial correlation length can be simultaneously measured using RCR diagnostics in the nonlinear regime.

The saturation of the low-threshold parametric decay instability of an ordinary wave during electron cyclotron resonance heating in the edge transport barrier of a tokamak due to the stochastic damping of a daughter two-dimensionally localized oblique Langmuir wave has been considered. It has been found that the saturation of instability in modern devices occurs at a relatively low level and does not affect the energy balance during plasma heating. It has been shown that the efficiency of nonlinear pump under expected conditions of microwave power injection in the International Thermonuclear Experimental Reactor will exceed the maximum efficiency of stochastic damping, which will result in the breaking of amplitude-dependent saturation and can significantly modify the power deposition profile.

The phenomenon of two-dimensional localization of the upper hybrid (UH) wave in a magnetic island was discovered. It was studied how this phenomenon affects the threshold and saturation level of the absolute parametric decay instability of the extraordinary wave, as a result of which a couple of two-dimensionally localized UH waves are excited.

In this paper, we analyze theoretically the low-threshold parametric decay instability (PDI) that can be excited at the tokamak à configuration variable (TCV tokamak, Lausanne, Switzerland) during X2 electron cyclotron resonance heating experiments producing a two-dimensionally localized upper hybrid (UH) wave and a daughter extraordinary wave running from the decay layer outward to the plasma edge. The primary instability is then saturated due to the cascade of secondary decays into two-dimensionally localized UH and ion Bernstein waves. The level of plasma microwave emission in the frequency range substantially below half the pump wave frequency and of anomalous power losses of the pump are predicted.

The saturation level of the two Upper-Hybrid-plasmon low-threshold parametric decay instability (PDI) depends on the pump wave beam width. The edge plasma turbulence may lead to drastic distortion of the pump beam, thus generating PDI characteristic changes. Numerical evaluations of the PDI process are provided in this work for different turbulence scenarios. The average PDI characteristics are evaluated and compared to the simulation results for the pump beam unperturbed by the edge plasma turbulence. It is shown that up to the intermediate turbulence level, the PDI characteristics are only gently modified. But at high edge turbulence level, which can happen in tokamaks, the threshold increases and the growth rate reduces. It is important to note that the anomalous absorption keeps a little reduction value, whatever the edge turbulence regimes.

Poloidal correlation reflectometry (PCR) diagnostic is studied within linear analytical theory. The emphasis is given to the comparison of the two different antenna configurations possessing translational and mirror symmetry. Analytical expressions for measured quantities are obtained and the qualitative equivalence of the two configurations is proven in linear regime. Results of the theoretical analysis are confirmed with linear numerical modelling.

Исследованы последствия эффекта локализации ионных бернштейновских (ИБ) волн, которые возбуждаются при вторичных распадах, сопровождающих низкопороговую параметрическую распадную неустойчивость необыкновенной волны накачки, приводящую к генерации необыкновенной волны и локализованной в плазме верхнегибридной (ВГ) волны. Показано, что обнаруженный эффект приводит к уменьшению как уровня насыщения первичной неустойчивости, так и уровня аномального поглощения волны накачки в экспериментах по электронному циклотронному нагреву плазмы на второй гармонике резонанса.

The effect of considerable broadening of the power deposition profile in on-axis X2 electron cyclotron resonance heating experiments observed in the L-2M stellarator is interpreted in the paper as one of the consequences of the low-threshold parametric decay instability of the pump wave, resulting in excitation of ion and electron Bernstein daughter waves. The instability is considered under the specific L-2M experimental conditions and shown to be absolute. The threshold of this instability is exceeded in the experiment. This finally allows explaining, at least qualitatively, the effect of nonlocal microwave power deposition detected in L-2M.

It is shown that when a pump extraordinary wave is obliquely incident onto the plasma in electron cyclotron resonance heating (ECRH) experiments, the threshold of parametric decay instability (PDI), leading to the generation of two localized upper-hybrid waves, increases. It is proposed to use a small tilt of the heating beam to suppress this PDI, the most dangerous and often observed in ECRH experiments.

The induced scattering parametric decay instability of O-mode polarized microwaves leading to excitation of lower hybrid or electron plasma waves localized within a tokamak edge transport barrier is investigated. Numerical estimates of the instability threshold and its growth rate for this scenario are given for the conditions of O2-mode ECRH experiments at ASDEX-Upgrade, where experimental investigation of these phenomena is possible.

The consequences are studied of the localization effect of ion Bernstein (IB) waves excited in secondary decay processes accompanying the development of low-threshold parametric decay instability of the extraordinary pump wave, which results in the excitation of the extraordinary wave and the upper-hybrid (UH) wave localized in the plasma. It is shown that in experiments on electron cyclotron resonance plasma heating at the second resonance harmonic, the effect discovered results in a decrease in both the saturation level of primary instability and anomalous absorption level of the pump wave.

The paper presents observations of plasma intermediate frequency range wave emission ((200-1000) MHz) performed in ohmic discharge of the FT-2 tokamak. The dependencies of the RF signal amplitude and spectrum on the plasma parameters and on the location of the probes are presented. Two possible explanations for this effect is proposed. One of them is related to the distortion of the electron distribution function due to the strong ripple of the magnetic field in the FT-2 tokamak. Second one is related to the existence of a beam of run-away electrons, the buildup of a fan instability responsible for the generation of synchrotron radiation in the range (10-80) GHz, and a complex chain of nonlinear decay instabilities leading to such a strong decrease in the frequency of the observed oscillations.

It is shown that, when a pump extraordinary wave is obliquely incident onto the plasma in the electron cyclotron resonance heating (ECRH) experiments, an increase takes place in the threshold of parametric decay instability (PDI) leading to generation of two localized upper-hybrid waves. It is proposed to use a small tilt of the heating beam to suppress this PDI which is most dangerous and often observed in the ECRH experiments.

A quasilinear equation which allows describing evolution of the electron distribution function and generation of non-inductive currents by helicons is obtained. It is shown that in the analyzed case the Fokker-Planck equation can be approximated by a one-dimensional equation in the longitudinal electron velocity space with a diffusion coefficient proportional to the helicon power absorbed by electrons due to Landau damping. Keywords: tokamak, current drive, helicon, quasilinear diffusion

A detailed interpretation of the effect of anomalous radiation is given, which was first observed at the TEXTOR tokamak in experiments on electron cyclotron resonance heating by the extraordinary wave at the second resonance harmonic in a wide range of plasma densities.