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

Alexei Yu Popov,Evgeniy Z Gusakov,E Poli,H Laqua,J Oosterbeek

doi:10.1051/epjconf/201920301002

Abstract

We describe the theoretical model which interprets the anomalous phenomena, i.e. the generation of backscattering signal observed in the ECRH experiments at TEXTOR, TCV, TJ-II, ASDEX-UG, LHD and FTU, as a consequence of the excitation of the parametric decay instability (PDI) leading to anomalous damping of the pump wave. The PDI power-threshold is shown to be extremely low due to the localization of both or one daughter upper hybrid (UH) waves in presence of a nonmonotonic (hollow) density profile, which is often observed in the ECRH experiments due to the magnetic island or the density pump-out effect. In the case of the extraordinary wave pump the model predicts substantial (up to 25%) anomalous absorption in the electron channel and explains the anomalous ion acceleration by the generation of secondary low frequency waves which directly transfer the pump power to the ion component. The possibility of anomalous absorption of the O-mode pump in the ECRH experiment due to the parametric excitation of trapped UH wave is also discussed and the anomalous absorption rate at the 10% level is predicted.

Highlights

Electron cyclotron resonance heating (ECRH) and current drive is widely used in toroidal plasmas and is considered for application in ITER for heating and neoclassical tearing mode control
In the present paper we describe the theoretical model taking into account, as distinct from the conventional theory [1], the presence of a nonmonotonic density profile, which always exists on the discharge axis or may be present due to the magnetic island or the density pump-out effect
We demonstrate that even one radially trapped upper hybrid (UH) daughter wave can be nonlinearly localized on the magnetic surface due to the finite-width pump, that leads to excitation of the most dangerous absolute parametric decay instability (PDI)

Summary

Introduction

Electron cyclotron resonance heating (ECRH) and current drive is widely used in toroidal plasmas and is considered for application in ITER for heating and neoclassical tearing mode control. This mechanism appears capable of reproducing the fine details of the frequency spectrum of anomalously backscattered X wave and the absolute value of the observed backscattering signal in TEXTOR experiment It predicts substantial (up to 25%) anomalous absorption in the electron channel and explains the anomalous ion heating at TCV by the generation of secondary IB waves, which directly transfer the pump power to the ion component. The nonlinear system relaxes to the stationary regime in which the saturation level of primary plasmons number | ans |2 can be evaluated from a condition indicating saturation of the secondary decay described by the last equation in (9). The number of the primary plasmons | ams | in the saturation regime could be estimated using the second equation of (9) as

This saturation level can be estimated as

Cn κ

Findings

Conclusions