Parametric decay instability of an obliquely propagating ordinary wave in the electron cyclotron frequency range

Abstract

The possibility of the low-power-threshold parametric decay of an obliquely propagating ordinary wave to an upper hybrid wave and a low-hybrid wave is analysed under conditions of nonmonotonic plasma density profile in a magnetic trap. The instability threshold and growth rate are derived explicitly. The analytical results are illustrated under the conditions typical of the ordinary mode fundamental electron cyclotron resonance heating harmonic experiments at the FTU tokamak.

Side emissions during EC injection for PDI studies in FTU tokamak

“…Type-2 emissions are more frequent (see Fig.3) at higher densities and when a higher dB/dt of the rotating island is measured by the reference Mirnov coil, i.e. when the magnetic island is larger, and thus the density gradient can be more easily found inverted, condition to facilitate the PDI occurrence as predicted by theory [6,7]. In any case emissions appear as spiky, with a few microseconds time-scale of temporal evolution (Fig.4).…”

“…The aim was to add experimental evidences to confirm the hypotheses of low-threshold excitation of waves generated by PDI mechanisms based on the observation of back-scattered power in the case of 2 nd harmonic Xmode injection in TEXTOR [4 and reference therein], and on other observations in ASDEX-U [5]. Theoretical analysis [6,7] predicts the possibility of low-threshold parametric decay also for O-mode pump-wave injection, which can be explored at frequencies close to the first Harmonic resonance in FTU. Experiments were made at magnetic fields of 4.7 T and 3.6 T, injecting the 140 GHz probe and observing the emission from the second antenna of the EC launcher in symmetric and asymmetric configurations (see section 1.1), in presence of MHD islands mainly stimulated by Ne injection.…”

Anomalous absorption in ECRH experiments due to the parametric low-threshold excitation of localized UH waves

“…The pump ordinary wave can be represented by means of the WKB approximation as follows κ are the coupling coefficients of the waves involved in the primary and secondary decays presented explicitly in [20] and [24], correspondingly. Assuming the nonlinear coupling is weak we treat the set of equations (18) using the perturbation procedure [23].…”

“…The possibility of anomalous absorption of the Omode pump in the ECRH experiment due to parametric excitation of the trapped UH wave [19,20] is discussed as well.…”

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

“…Such localization leads to the complete suppression of energy losses by the daughter wave from the decay layer and, consequently, to a drastic decrease (several orders of magnitude) in the decay instability power-threshold. 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].…”