Possibility of strong anomalous absorption of microwaves in electron cyclotron resonance heating experiments in fusion devices

Gusakov, E. Z.; Popov, A. Yu.

doi:10.1088/1741-4326/ab3c7f

Cited by 22 publications

(35 citation statements)

References 27 publications

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“…The corresponding anomalous absorption rate ranges from 20%-25% in the case of two trapped primary UH decay waves to 5%-10% in a more general case of only one trapped primary UH daughter wave. However in the case of even number of steps of the saturation decay cascade, as it was demonstrated recently [40][41][42] in the case of two localized primary UH waves the power fraction gained by all the daughter waves can achieve extremely high level of 60%-80% of the pump power. This effect can be responsible for a substantial variation of the power deposition profile, i.e.…”

Section: Introductionmentioning

confidence: 88%

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: 88%

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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“…As a result of the many PDIs, the heating power intended for an entirely different region of the plasma will deteriorate. Analytical estimates for plasma A c c e p t e d M a n u s c r i p t parameters taken from the TEXTOR tokamak predict that 60% 2nd harmonic ECRH power [22] was converted into undesired waves in unintended regions due to PDIs near the 2nd harmonic UH layer. Experiments in the linear device Granit [24] have shown a 45% absorption rate due to PDIs.…”

Section: Introductionmentioning

confidence: 99%

Trapped upper hybrid waves as eigenmodes of non-monotonic background density profiles

Senstius

Nielsen

Vann

2021

Plasma Phys. Control. Fusion

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Non-monotonic plasma density structures such as blobs and magnetic islands give rise to trapped upper hybrid (UH) waves. Trapped UH waves which satisfy Bohr–Sommerfeld quantization can be thought of as eigenmodes of a cavity. Using fully kinetic particle-in-cell simulations, we verify the existence of these UH eigenmodes and demonstrate their significance as only eigenfrequencies become unstable to three-wave interactions. The eigenmodes can be excited through parametric decay instabilities (PDIs) of an X-mode pump wave at approximately twice the UH frequency, as could be the case for a gyrotron beam traversing a blob in a magnetically confined fusion plasma. We derive a closed expression for the wavenumber of UH waves, which is accurate both close to the UH layer and to the electron cyclotron resonance. This allows for fast analysis of eigenmodes in a non-monotonic structure. An expression for the amplification of PDI daughter waves in an inhomogeneous plasma is extended to a decay region where the first several derivatives vanish. From the amplification in a convective PDI, we estimate the growth rate of the absolute PDI involving the trapped waves. We show that the excitation of eigenmodes through PDIs in our simulations are indeed absolute rather than convective due to the trapping of the daughter waves. Additionally, we show that only eigenmodes get excited through the PDIs, and that we are able to predict the growth rates of the daughter waves and how they scale with the pump wave intensity. This is evidence supporting a fundamental assumption of analytical theory describing low threshold strong scattering observed in magnetically confined fusion experiments during second harmonic electron cyclotron resonance heating (ECRH). Such low threshold instabilities can degrade ECRH performance but also offer novel uses for ion heating or as diagnostics.

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“…The strong, frequency-shifted radiation generated by the above PDIs poses a threat to microwave diagnostics such as electron cyclotron emission (ECE) in connection with magnetic islands [6]. In these cases the PDIs may even lead to the conversion of a very large fraction (up to 80 %) of the ECRH power to UH plasmon power according to current theories [14,15], changing the power deposition and current drive characteristics; PDI-mediated absorption of 45 % of the ECRH power has been demonstrated in lowtemperature plasma filament experiments [17]. On the other hand, as shown in the present paper, the PDIs allow detection of structures possessing non-monotonic electron density profiles along an ECRH beam near the edge of fusion plasmas, e.g., ELM filaments, inter-ELM modes [23], and blobs [24], since the wave amplitudes generated in these cases are generally insufficient to damage microwave diagnostics.…”

Section: Introductionmentioning

confidence: 99%

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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Possibility of strong anomalous absorption of microwaves in electron cyclotron resonance heating experiments in fusion devices

Cited by 22 publications

References 27 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

Trapped upper hybrid waves as eigenmodes of non-monotonic background density profiles

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

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