The possibility of anomalous gyrotron frequency sub-harmonics emission in the ECRH experiments at toroidal devices

Gusakov, E. Z.; Popov, A. Yu.; Tretinnikov, P. V.

doi:10.1088/1741-4326/ab3a70

Cited by 24 publications

(31 citation statements)

References 31 publications

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“…The developed theoretical model and its predictions (the power-threshold of primary instability, the spectrum of secondary waves) was confirmed by sophisticated PIC simulations [43]. The observation of a phenomenon-anomalous plasma emission in the ECRH experiments at ASDEX-UG at half the pump wave frequency [44,45]-predicted by the theoretical model [46] and interpreted as a consequence of two-plasmon decay of the pump extraordinary wave can also serve as a confirmation of the model as well as the strong X-mode anomalous absorption effect discovered in a model experiments carried out in a plasma filament [47].…”

Section: Introductionmentioning

confidence: 60%

On the substantial anomalous absorption regime at the extraordinary wave decay into trapped and propagating upper hybrid waves

Gusakov

Popov

2020

Nucl. Fusion

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We analyze the strong nonlinear regime of low power-threshold parametric decay instability (PDI) of a pump extraordinary wave. The PDI leads to the excitation of two upper hybrid (UH) waves only one of which is trapped in the vicinity of the local maximum of a non-monotonic density profile. The pump depletion and the multi-step secondary decays of the localized daughter UH wave are treated as the mechanisms leading to the instability saturation. It is shown that in the case of the even-step secondary cascade decay of trapped primary UH wave the pump wave depletion is responsible for the transition of a nonlinear phenomenon into a stationary regime, and the strong anomalous absorption is possible.

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Section: Introductionmentioning

confidence: 60%

On the substantial anomalous absorption regime at the extraordinary wave decay into trapped and propagating upper hybrid waves

Gusakov

Popov

2020

Nucl. Fusion

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“…Additionally, microwave signals are present above, as well as below, half the ECRH frequency. The above features are at odds with the mechanism of microwave emissions near half the ECRH frequency proposed in [13],…”

Section: Discussionmentioning

confidence: 87%

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

et al. 2020

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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.

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“…The resulting wave would be an escaping Xmode wave at approximately 3/2 f 0 = 210 GHz. The possibility of observing the 3 f 0 /2 signal due to trapping of TPD daughter waves in a magnetically confined plasma has also been proposed analytically 56 . On one hand, the escaping 3 f 0 /2 waves provide a diagnostic opportunity as indications of trapped waves may be observed without having to look near the pump frequency where stray radiation from the gyrotrons can damage sensors.…”

Section: Escaping Wavesmentioning

confidence: 99%

“…Secondary daughter waves could then build up and combine into escaping waves in tertiary PIs, bringing better agreement with the experimental observations to the theoretical models. The models suggest a number of complications to gyrotron operation in non-monotonic plasma density profiles such as a substantial unintentional power deposition into the islands 29,52 and excitation of potentially damaging escaping waves at different half-integer harmonics of the gyrotron frequency 45,46,56 . The non-monotonic density profile can be caused by a number of mechanisms which means that such cascades of PDIs might be common during gyrotron operation across many machines without being noticed simply because no diagnostic is looking for them.…”

Section: Introductionmentioning

confidence: 99%

Numerical investigations of parametric decay into trapped waves in magnetized plasmas with a non-monotonic density background

2020

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Parametric decay instabilities (PDIs) exciting daughter waves trapped inside a magnetized plasma with a non-monotonic density profile are investigated numerically. The investigation is motivated in particular by observations of low threshold PDI signatures during 2nd harmonic electron cyclotron resonance heating (ECRH) experiments in magnetically confined fusion experiments. We use the particle-in-cell (PIC) code EPOCH to study conversion of a fast X-mode pump wave into a combination of half frequency X-mode and electron Bernstein waves (EBWs), and identify two regimes where PDIs can excite trapped electrostatic waves. Above the 2nd harmonic upper hybrid (UH) density, a PDI known also as a two plasmon decay (TPD) instability excites a pair of UH waves which we locate in frequency and wavenumber space. At lower densities, a PDI known as stimulated Raman scattering may produce one trapped and one returning X-mode daughter wave with a much slower growth rate than the TPD instability. In both cases, we show that the frequency separation of the daughter waves depends on the density in a predictable manner. With little loss from the decay region, the trapped daughter waves become unstable with respect to secondary parametric instabilities (PIs), leading to distinctly different phases of the UH spectrum. Unlike the primary instability, the secondary PIs are shown to depend on ion dynamics. Furthermore, we observe escaping waves near the 3/2 pump frequency resulting from tertiary PIs in agreement with recently proposed backscattering during magnetically confined fusion experiments.

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The possibility of anomalous gyrotron frequency sub-harmonics emission in the ECRH experiments at toroidal devices

Cited by 24 publications

References 31 publications

On the substantial anomalous absorption regime at the extraordinary wave decay into trapped and propagating upper hybrid waves

On the substantial anomalous absorption regime at the extraordinary wave decay into trapped and propagating upper hybrid waves

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

Numerical investigations of parametric decay into trapped waves in magnetized plasmas with a non-monotonic density background

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