Speaker
Evgeniy Z. Gusakov
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P2.1070.pdf
Anomalous absorption in ECRH experiments due to parametric excitation
of localized UH waves
E.Z. Gusakov, A.Yu. Popov, A.N. Saveliev
Ioffe Institute, St. Petersburg, Russia
Electron cyclotron resonance heating (ECRH) is widely used in toroidal plasmas and is
considered for application in ITER for heating and neoclassical tearing mode control. In
contradiction to the theory predictions [1] during the last decade many experiments have
demonstrated excitation of the anomalous phenomena at the ECRH. The clearest evidence of
the nonlinear effects was obtained at TEXTOR [2] where the strong backscattering signals
down–shifted in frequency and amplitude modulated by the magnetic island were observed. A
demonstration of the anomalous ion heating during the ECRH pulse was obtained at TCV [3].
In the present paper we develop further the theoretical model [4] taking into account, as
distinct from the standard theory [1], the presence of a non-monotonous density profile, which
always exists on the discharge axis or is often present due to the magnetic island or the
density pump-out effect. The model interprets the generation of backscattering signal and the
anomalous ion heating, as a result of secondary nonlinear processes that accompany a primary
low – threshold two–upper-hybrid (UH) – plasmon PDI of the pump X mode. The threshold
of the primary PDI is shown to be smaller than the one predicted in [1] due to the trapping of
one UH wave in the presence of the non-monotonous density profile. The primary PDI is
absolute due to the finite-size of the pump beam. Its growth enhancing the UH wave
fluctuations from the thermal noise level is saturated in our theory due to both the pump wave
depletion and the decay of the trapped daughter UH wave that leads to excitation of the
secondary UH wave, which is also localized and ion Bernstein (IB) wave. The coupling of
different daughter UH waves is responsible in the theory for generation of the backscattering
signal. This mechanism appears capable of reproducing the fine details of the frequency
spectrum of the anomalously reflected X wave and the absolute value of the observed
backscattering signal in TEXTOR experiment. It also predicts substantial (up to 20%)
anomalous absorption in the electron channel and explains the anomalous ion heating at TCV
by the generation of the secondary IB 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 parametric excitation of the trapped UH wave is discussed as well.
[1] M. Porkolab, B.I. Cohen 1988 Nucl. Fusion 28 239
[2] E. Westerhof, S. Nielsen, J. W. Oosterbeek et al. 2009 Phys. Rev. Lett. 103 125001
[3] A. N. Karpushov et al. 2013 33rd EPS Conference on Plasma Physics, 2006 30I P–1.152
[4] E. Z. Gusakov and A. Yu. Popov 2016 Physics of Plasmas 23, 082503
