Speaker
Description
See the full abstract here:
http://ocs.ciemat.es/EPS2019ABS/pdf/P1.1075.pdf
Parametric decay instabilities (PDIs) are phenomena in which a large-amplitude pump wave decays to two lower-frequency daughter waves once its amplitude exceeds a nonlinear threshold. PDIs are ubiquitous in situations where plasmas interact with strong waves, including ionospheric modification experiments and wave heating of laboratory plasmas, and may lead to a significantly different wave response than the one expected from linear theories [1]. Here, we focus on PDIs occurring during electron cyclotron resonance heating (ECRH) at the ASDEX Upgrade tokamak; particularly, PDIs near the 2nd-harmonic upper hybrid resonance (UHR). For these PDIs, trapping of the daughter waves is necessary to reduce the threshold to a level accessible with ECRH [1]. Such trapping may occur near a local maximum of the electron density, e.g. in connection with magnetic islands and edge localised modes (ELMs). We demonstrate the occurrence of these PDIs during ELMs at high electron densities through measurements Figure 1: Wave trapping near the 2nd-harmonic with the collective Thomson scattering sys- UHR during an ELM crash, simulated with tem. Further, in Fig. 1, we show the first con- JOREK [2]. A PDI with an absolute ECRH firmation of the existence of wave trapping beam power threshold of approximately 2 kW and excess of the PDI threshold during such and a convective ECRH beam power threshold ELMs for electron density and temperature of approximately 300 kW exists in this case [1]. profiles simulated using the JOREK code [2].
References
[1] A.Yu. Popov and E.Z. Gusakov, Plasma Phys. Control. Fusion 57, 025022 (2015)
[2] M. Hoelzl et al., Contrib. Plasma Phys. 58, 518 (2018)
