On the possibility of reducing the instability threshold of a parametric decay of an extraordinary wave into two upper h
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TICAL, NONLINEAR, AND SOFT MATTER PHYSICS
On the Possibility of Reducing the Instability Threshold of a Parametric Decay of an Extraordinary Wave into Two Upper Hybrid Waves in an Inhomogeneous Plasma A. Yu. Popov and E. Z. Gusakov Ioffe Physical Technical Institute, Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg, 194021 Russia email: [email protected], [email protected] Received June 2, 2014
Abstract—A parametric decay instability (PDI) of an extraordinary wave leading to excitation of two upper hybrid (UH) plasmons at frequencies close to half the pump wave frequency is analyzed. It is shown that the twoplasmon PDI power threshold can be significantly reduced under conditions of electron cyclotron reso nance heating (ECRH) experiments in toroidal magnetic devices, where the plasma density profile is often nonmonotonic, which leads to the localization of UH waves. DOI: 10.1134/S1063776114110090
1. INTRODUCTION At present, electron cyclotron resonance heating (ECRH) is widely used in experiments on stellarators and tokamaks as an efficient method of local addi tional heating of the electron component of plasma. ECRH is also planned to be used in the ITER tokamak reactor both for heating and for controlling and sup pressing the neoclassical tearing mode. Note that the efficiency of this method is based on the use of reliable and efficient generators—gyrotrons, which allow gen erating the beams of electron cyclotron (EC) waves with a power level from 100 kW to 1 MW. According to the predictions of the standard theory [1, 2], the heating power, even if the gyrotrons are combined into groups, turns out to be much lower than the thresholds of typical parametric decay insta bilities (PDIs) that may accompany the propagation of EC waves. This is due to large convective loss of the energy of daughter waves generated in the parametric decay region. Note that the excitation of pump wave PDIs in ECRH experiments is nevertheless possible, as is shown in experiments where a linear transforma tion of extraordinary (X) waves in the vicinity of the upper hybrid resonance (UHR) is used for heating electrons in various setups [3–6]. In this case, a daughter electron Bernstein wave is excited by an X wave in the vicinity of this resonance, where its group velocity decreases and amplitude increases; this, in turn, leads to a decrease in the excitation threshold of a parametric instability [7] and makes it possible that this threshold can be exceeded in experiment. Until recently, it was assumed that traditional method of ECRH of plasma is quite routine and the propagation and absorption of an X wave at the second cyclotron harmonic and of an ordinary (O) wave at the
fundamental harmonic are predictable in detail and described within the linear theory of wave propaga tion. At the same time, a lot of experimental data have recently been obtained that provide evidence of the existence of anomalous phenomena that accompany the propagation of an EC wave. These include, first, the observati
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