Observation of the strong anomalous absorption of the X-mode pump in a plasma filament due to the two-plasmon decay

Abstract

The effect of strong anomalous absorption of the X-mode pump wave associated with the two-upper-hybrid-plasmon parametric decay is observed in a specially performed model experiment in a laboratory plasma. The strong microwave absorption takes place at the maximal plasma density higher than the upper-hybrid resonance value for the frequency equal to half of the pump frequency value. Its efficiency is determined at the level of 80% in the initial stage of the parametric decay instability saturation and 45% in the following steady state. The theoretical model of the two-upper-hybrid-plasmon decay in strongly inhomogeneous plasma is developed. The localization, threshold and growth rate of the instability are determined in agreement with experimental observations.

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

“…Neither of these effects are included in the basic theory of PDIs near the second-harmonic UHR in a plasma slab, since it assumes electrostatic trapped daughter waves propagating perpendicular to the background magnetic field. An inclusion of electromagnetic effects and oblique propagation of the daughter waves may thus be necessary to estimate the theoretically expected signal levels near half the ECRH frequency, which could be compared with the experimental observations of the present paper; we note that oblique daughter wave propagation has already been included in the theory of PDIs near the second-harmonic UHR in (circular) plasma filaments [11,16,17]. Further, the applicability of the PDI theory to the signals observed in connection with L-mode turbulence should be confirmed in a similar manner to that of the ELM-related spikes, using the output of an edge turbulence code, e.g., HESEL [24] or GRILLIX [65], which could also be used to confirm the possibility of direct tunneling of the half-frequency X-mode daughter waves through the cutoff region.…”

“…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.…”

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

“…Later, observations have been made at a number devices including during ECRH and EBW heating at the Wendelstein 7-AS 18,19 stellarator, the FT-1 20 tokamak, the TCA 21 tokamak, the MAST 22 tokamak, the TJ-K 23 stellarator and the LHD 24 stellarator; and during collective Thomson scattering (CTS) [25][26][27] at the ASDEX Upgrade tokamak. PDI has also been studied experimentally in other magnetically confined laboratory plasmas such as in tandem mirror machines 28 and linear devices 29,30 .…”

“…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.…”

Low-Threshold Induced-Side-Scattering Instability at the Tokamak Edge Transport Barrier Predicted in O-Mode Electron Cyclotron Resonance Heating Experiments

“…The new model was used to analyze the most dangerous scenarios of low power-threshold parametric decays of both the second harmonic extraordinary microwave [19,20] and the fundamental harmonic ordinary microwave [21,22] in the vicinity of the local maximum of a non-monotonic density profile, leading to the excitation of localized upper hybrid waves. The possibility of substantial anomalous absorption has also been demonstrated [23].…”