Evgeniy Z. Gusakov

The effect of low-threshold parametric decay instability within a coherent structure (blob) at the plasma edge is considered. The analysis focuses on the decay of the extraordinary pump wave into a pair of electron Bernstein (EB) waves with similar frequencies. The emission at frequencies corresponding to these daughter waves was observed recently in X2-mode electron cyclotron resonance heating (ECRH) experiments at ASDEX Upgrade and TCV tokamaks. The EB waves are found for the first time to be three-dimensionally trapped within a plasma edge blob, leading to a substantial decrease in the power threshold for their parametric excitation. To illustrate the findings, the model is applied to the X2-mode ECRH scenarios at the ASDEX Upgrade tokamak.

The low-threshold parametric decay of an obliquely propagating ordinary wave into electron and ion Bernstein waves in a plasma with a monotonic density profile is considered. This particular scenario is critical as it provides a mechanism for direct nonlinear energy transfer from the pump beam to both the electron and ion components of the plasma, even under conditions when the ECR absorption is expected by the linear theory to be localized elsewhere. Given its practical significance for future electron cyclotron resonance heating operations on ITER, the results of the analysis are illustrated under ITER-relevant conditions. The parametric decay instability threshold power is shown to be substantially smaller than that of a single gyrotron beam.

An explanation of the dependence of the efficiency of electron cyclotron resonance heating (ECRH) on chord-average plasma density discovered during the analysis of experiments on-axis ECRH in the TJ-II stellarator is proposed. It is shown that the effect of decreasing heating efficiency with decreasing density can result from the excitation of a two-plasmon parametric decay instability (PDI) of the extraordinary microwave. The decay of the microwave (pump wave) occurs in two spatially close points as a result of the development of an absolute PDI leading to the excitation of electron Bernstein (EB) waves at frequencies close to half the frequency of the pump wave. The saturation of the instability is caused by secondary decays of the primary EB waves followed by the excitation of daughter wave pairs consisting of an EB and a slow lower hybrid (SLH) wave. The power deposition region of the daughter waves can differ substantially from that of the pump wave, which can explain the discovered discrepancy between the efficiency of ECR power absorption and the predictions of the standard linear theory. The daughter SLH waves, after linear transformation into ion Bernstein (IB) waves, can interact effectively with the ion component, which allows for a qualitative explanation of the appearance of superthermal ions discovered at the TJ-II installation during the ECRH.

To study the characteristics of electron scale turbulence, an enhanced scattering (ES) diagnostic has been developed on the J-TEXT tokamak. This ES diagnostic consists of 8 detection channels covering 44-58 GHz with a 2 GHz step. It launches and receives X-mode microwaves from the high-field-side (HFS). The probing wave is scattered by electron-scale turbulence at the upper hybrid resonance (UHR) layer on the low-field-side (LFS), where the electric fields of both probing and scattering waves are enhanced. The ES diagnostic has been successfully applied to J-TEXT experiments. Through cross-validation with the Doppler backscattering diagnostic (DBS), it has been verified that the ES diagnostic can measure high wavenumber fluctuations, providing a new diagnostic tool for the study of electron-scale turbulence on J-TEXT.

This paper presents an overview of recent research advances on the Globus-M2 spherical tokamak over the past two years. Upgrades to the diagnostic complex are reported. A novel inter-shot boronization technique demonstrated superior wall conditioning, achieving effective charge <1.1 and doubling the neutron yield through enhanced fast ion confinement. The hot-ion mode was established as a natural regime, with ion temperatures up to 4.7 keV, revealing that D-neutral beam injection (NBI) provides superior ion thermal insulation compared to H-NBI due to higher toroidal momentum input. Ion cyclotron resonance heating experiments demonstrated effective bulk ion heating and beam ion acceleration. For the first time in a spherical tokamak, the high-field side high-density phenomenon was identified, with SOLPS-ITER modeling confirming the dominant role of plasma drifts. MHD stability studies characterized Alfvén eigenmodes causing localized wall loads up to ∼30 MW/m2, identified locked modes as a key performance limitation, and demonstrated access to small-edge localized modes (ELM) and ELM-free regimes at reduced triangularity. These results provide the foundation for the design of the next-generation Globus-3 spherical tokamak.

The proposed theoretical model interprets the discrepancy between the launched and actually absorbed microwave power discovered in X2 ECRH experiments at the TJ-II stellarator as a consequence of parametric decay instabilities. The instability leads to the excitation of a traveling extraordinary mode wave and an upper hybrid (UH) wave localized near a local maximum of the nonmonotonic density profile. The subsequent instability saturation occurs through a cascade of further decays of the primary UH wave into trapped UH and traveling ion Bernstein waves, as well as due to depletion of the pump wave. The model also provides a qualitative explanation for the fast ion generation observed at TJ-II and predicts intense plasma microwave emission at frequencies in the range of 20 GHz.

The possibility of excitation of absolute parametric decay instabilities in X2- and O1-mode electron cyclotron resonance heating experiments with monotonic density profile and their possible consequences for the EC absorption efficiency under conditions of the T-10 tokamak are considered. Theoretical predictions are used to explain the puzzling results of the energy balance analysis in T-10.

The effect of strong anomalous absorption of the X-mode pump wave associated with two upper-hybrid (UH) plasmon parametric decay is observed in a specially performed model experiment in a laboratory plasma. Strong microwave absorption takes place at a maximal plasma density in the plasma filament higher than the UH resonance value for the frequency equal to the half value of the pump frequency. The anomalous absorption efficiency is determined at the level of 70% in the beginning stage of the parametric decay instability and 40% in the following steady state. A theoretical model of the two UH plasmon decay in strongly inhomogeneous plasma filaments is developed. The localization, threshold, and growth rate of the instability are determined and found to be in agreement with experimental observations.

It is shown that the absolute parametric decay instability of a microwave beam of a finite width, resulting in the excitation of two electron Bernstein waves, can occur at monotonic parameters of the background plasma in X2-mode ECRH experiments at a modest pump power. The excitation of daughter waves, whose power deposition localization differs from that of the pump wave, can potentially explain the broadening of the power deposition profile and lack of the pump power absorbed in the electron cyclotron resonance layer that has been found in X2-mode ECRH experiments.