Speaker
Description
Е.Z. Gusakov, A.Yu. Popov, A.A. Nagovitsyn
Ioffe Institute, 194021 Saint-Petersburg, Russia
ECRH and ECCD are widely used in toroidal devices for magnetic plasma confinement. Recently, however, a number of anomalous parasitic effects have been detected during ECRH mode. Among them are the powerful plasma microwave radiation [1, 2], which can damage the measuring equipment [3], and the generation of suprathermal ions [4, 5]. These phenomena have been observed when a microwave beam passes through a plasma region with a non-monotonic density profile arising from magnetic islands [2, 6] or a pump-out effect [7]. They were interpreted as consequences of low-threshold parametric decay instability (PDI) of microwaves, resulting in the excitation of daughter upper-hybrid (UH) waves localized near the local density maximum [8].
In the present work, it is shown analytically, based on the results of [9, 10], and numerically that the pump X2-mode can be unstable to low-threshold parametric decay even at monotonic density and magnetic field profiles, giving rise to a pair of daughter nonlocalized UH waves. The one-dimensional instability model developed in [9, 10] is generalized to the realistic case of two-dimensional pump wave decay geometry.
The scenario is implemented in the presence of two decay points of the pump wave when the excited daughter UH waves have oppositely directed group velocities. In this case, a positive feedback is possible for both daughter waves between the decay points, which can lead to the exponential growth of the amplitudes of UH waves with time [11]. The results obtained allow to explain the phenomenon of a significant broadening of the ECRH power deposition profile, recently discovered when analyzing data accumulated over decades at the T-10 tokamak [12, 13], as a consequence of excitation of the low-threshold two-plasmon decay of a sub-MW X2-mode pump.
ACKNOWLEDGEMENTS
The analytical treatment is supported under the RSF 22-12-00010 grant, the numerical modelling is supported by the Ioffe Institute state contract 0040-2019-0023 whereas the code for the PDI modeling was developed due to the Ioffe Institute state contract 0034-2021-0003.
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