Parametric decay instabilities near the second-harmonic upper hybrid resonance in fusion plasmas

Abstract

Parametric decay instabilities (PDIs) lead to the generation of strong frequency-shifted radiation when powerful X-mode polarized microwaves, injected for electron cyclotron resonance heating (ECRH) of fusion plasmas, cross a region of nonmonotonic electron density near the second-harmonic upper hybrid resonance (UHR). For the standard second-harmonic X-mode ECRH scenarios used at the ASDEX Upgrade tokamak, the second-harmonic UHR occurs near the plasma edge, meaning that PDIs occur in connection with phenomena leading to non-monotonic edge electron density profiles, e.g., blobs, edge-localized modes (ELMs), and inter-ELM modes. We present the first study of strong signals near half the ECRH frequency in fusion-relevant plasmas and demonstrate their PDI-like features, such as an ECRH power threshold required for their occurrence, through analog modulations of the ECRH power. The signals near half the ECRH frequency are found to be far more prevalent than expected based on previous theories, and we hence present a qualitative explanation of their origin, in terms of cross-polarized microwaves generated through nonlinear interactions of the waves involved in the PDIs, which captures the basic features of the experimental observations. We additionally present examples of the PDI-related microwave signals just below the ECRH frequency which indicate the location of the microwave source. Furthermore, based on a nonlinear magnetohydrodynamics simulation of an ELM crash in ASDEX Upgrade, performed using the JOREK code, the duration of the PDI-like microwave spikes observed in connection with ELMs is shown to match the transit time of an ELM filament though an ECRH beam. Finally, the PDI power threshold expected theoretically, calculated using the electron density and temperature profiles from the JOREK simulation, is shown to match the experimentally observed values.

The JOREK non-linear extended MHD code and applications to large-scale instabilities and their control in magnetically confined fusion plasmas

“…Here, ELM losses, magnetic measurements [204], and measurements for coldfront penetration [205] agree well with experiments. Based on such simulations, the role of density fluctuations onto ECEimaging measurements was studied [206] and the excitation of parametric decay instabilities during electron cyclotron resonance heating was investigated [207]. Further simulations confirmed the trend of decreasing dominant toroidal mode numbers with an increasing edge safety factor like experimentally observed [208].…”

Nonlinear decay of high-power microwaves into trapped modes in inhomogeneous plasma

“…Similarly, in magnetically confined fusion plasmas, nonmonotonic density profiles are generated by magnetic islands and edge localized modes. In these cases, anomalous microwave signals observed in TEXTOR [9] and ASDEX Upgrade [10] tokamaks during second-harmonic extraordinary (X2) electron cyclotron resonance heating (ECRH) were explained with APDI into a pair of trapped UHWs at half the pump frequency [11]. However, the wave trapping mechanism could not be assessed, since the decay region was poorly diagnosed.…”

The JOREK non-linear extended MHD code and applications to large-scale instabilities and their control in magnetically confined fusion plasmas

“…Here, ELM losses, magnetic measurements [183], and measurements for cold-front penetration [184] agree well with experiments. Based on such simulations, the role of density fluctuations onto ECE-Imaging measurements was studied [185] and the excitation of parametric decay instabilities during electron cyclotron resonance heating was investigated [186]. Further simulations confirmed the trend of decreasing dominant toroidal mode numbers with an increasing edge safety factor like experimentally observed [187].…”