Anomalous phenomena in ECRH experiments at toroidal devices and low-threshold parametric decay instabilities

Abstract

Abstract. In the paper the possibility of total 3D trapping of electron Bernstein (EB) waves in the tokamak equatorial plane in the vicinity of the local density maximum produced by electron pump-out-effect is demonstrated. Thresholds and growth rates of the associated absolute (temporally growing) parametric decay instability (PDI) leading to anomalous absorption is predicted in the range of less than 100 kW. Its possible role in explanation of ion acceleration observed in ECRH experiments as well as in redistribution of the deposited power is discussed.

Low-threshold absolute two-plasmon decay instability in the second harmonic electron cyclotron resonance heating experiments in toroidal devices

“…As has already been shown in [19][20][21][22][23][24][25][26] the pump second harmonic X-wave PDI power threshold decreases substantially when one of the daughter waves is trapped at least in the direction in which it has maximal energy loss from the decay region. The daughter UH plasmons are demonstrated in [27] to be easily trapped in the radial direction in the vicinity of both the UHR and the local maximum of the non-monotonous density profile x max .…”

“…The poloidal magnetic field inhomogeneity appears to be a key element of the theory as well. The two-dimensional [20,21,24] or even three-dimensional [22,23,25,26] trapping of the ion or electron Bernstein waves (depending on both the experimental conditions and the machine parameters) are shown to be responsible for the low threshold excitation of different PDIs, the thresholds of which are drastically smaller than those predicted by the standard theory [6][7][8].…”

“…Secondly, the observations that the backscattering signal down-shifted in the frequency range by approximately 1 GHz and correlated to the MHD mode rotation were reported [17,18] in the 200-600 kW level second harmonic X-mode ECRH experiment in the Textor tokamak and interpreted in terms of anomalous backscattering of the EC pump waves. Recently, the theoretical models to explain these anomalous effects and to advance the standard theory [6][7][8] have been developed [19][20][21][22][23][24][25][26][27]. They all utilize the fact that the plasma density profile at the ECRH is often non-monotonic due to different physical mechanisms.…”

On a saturation mechanism of two-upper-hybrid-plasmon parametric decay instability in the second harmonic ECRH experiment

“…In response to these experimental challenges, the theoretical models [20][21][22][23][24][25][26][27], which advance the standard theory [1][2][3] dealing with the monotonous background profiles and are incapable of describing the anomalous phenomena, have been developed. Distinct from the standard theory, the cornerstone of these models is taking full account of a nonmonotonous plasma density profile often observed in the ECRH experiments and originating due to different physical reasons.…”

Experimental characterization of anomalous strong scattering of mm-waves in TEXTOR plasmas with rotating islands

“…The observations have been suggested to originate from parametric decay, even though the gyrotron frequency was far from the upper hybrid frequency. The TEXTOR results have motivated a re-evaluation of the parametric decay power threshold [18][19][20][21][22].…”