Possible transition from modest to strong anomalous absorption in X2-mode electron cyclotron resonance heating experiments in toroidal devices

Abstract

We analyze both analytically and numerically, two cases of low-power-threshold two-upper hybrid-plasmon parametric decay instability saturation when the odd-or even-step cascade of secondary decays is possible. We have found that giant anomalous absorption (more than 80%) of pump power leads to its depletion in the regime when the instability is saturated due to the even-step cascade of decays of primary upper hybrid waves. This can make the power deposition profile different from that predicted by linear theory and provides an alternative explanation for the evident broadening observed under electron cyclotron resonance heating in toroidal devices.

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

“…We examine the pump depletion and the odd-step cascade of secondary decays of localized UH wave as the mechanisms responsible for the instability saturation and clarify their role in the transition to the quasi-stationary regime of the nonlinear interaction. As in the particular case of two primary daughter UH waves trapped in the direction of the plasma inhomogeneity [40][41][42], the even-step secondary decay cascade of only one trapped daughter UH wave seems not to be an effective mechanism of the instability saturation in these more general experimental conditions [27,28]. In the paper we focus on this scenario and demonstrate the pump depletion in this case to be responsible for the transition of a nonlinear phenomenon into a stationary regime.…”

“…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.…”

“…It should be underlined that the possibility of the strong anomalous absorption of microwaves has been demonstrated only in the particular case of the nonlinear excitation of two localized upper hybrid (UH) waves [40][41][42]. The latter is solely possible if the density at the local maximum of its non-monotonic profile slightly exceeds the upper hybrid value for half the pump wave frequency.…”

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

“…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.…”

Parametric Decay Instabilities during Electron Cyclotron Resonance Heating of Fusion Plasmas, Problems and Possibilities

“…This was explained in terms of ECRH PDIs involving trapped daughter waves [39], for which the convective losses normally limiting the occurrence of PDIs in inhomogeneous plasmas [31][32][33] are essentially removed. Specifically, the TEXTOR results at higher densities were explained in terms of decay of X-mode ECRH waves into two trapped UH waves near the O-point of the magnetic island [40][41][42][43][44][45][46][47]. PDIs of this kind have also been found at the ASDEX Upgrade tokamak [1,6,34], in low-temperature plasma filament experiments [2], and in particle-in-cell simulations [48][49][50].…”