Low-threshold parametric decay of the ordinary wave in ECRH experiments at toroidal devices

Gusakov, E. Z.; Popov, A. Yu.; Saveliev, A. N.; Sysoeva, E. V.

doi:10.1088/1361-6587/aa6c33

Cited by 21 publications

(25 citation statements)

References 36 publications

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“…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).…”

Section: Emissions Of Typementioning

confidence: 79%

“…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.…”

Section: Introductionmentioning

confidence: 99%

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Side emissions during EC injection for PDI studies in FTU tokamak

et al. 2019

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The evidence of Parametric Decay Instabilities (PDI) excited by the ECH power injected in O-Mode has been explored in FTU Tokamak, using the Collective Thomson Scattering (CTS) diagnostic. The experiments show evidences to support the hypothesis of low-threshold excitation of waves generated by PDI mechanisms, formerly proposed in the case of 2nd harmonic X-mode injection in TEXTOR and ASDEX-U. Theoretical analysis predicts low-threshold parametric decay also for O-mode pump-wave injection, which can be injected in FTU at frequencies close to the first Harmonic EC resonance. Experiments were made at different magnetic fields, injecting the 140 GHz probe and observing the emission from the second antenna of the EC launcher in poloidally symmetric and asymmetric configurations, in presence of MHD islands. The signal is detected by the CTS radiometers, with a fast digitizer allowing the spectral reconstruction at very fine time and frequency scales. Different types of emissions are studied in detail, comparing them with the magnetic island rotation frequency in different plasma conditions. In order to locate the plasma volume originating the emissions, a new antenna and receiving line has been installed.

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Section: Emissions Of Typementioning

confidence: 79%

Section: Introductionmentioning

confidence: 99%

Side emissions during EC injection for PDI studies in FTU tokamak

et al. 2019

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“…The threshold in this case is higher (several hundred kW depending on the plasma parameters) than for the Xmode pump whereas the growth rate is large enough (in the range of 10 7 s -1 ) to expect the non-linear saturation of the instability [26].…”

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confidence: 81%

Anomalous absorption in ECRH experiments due to parametric excitation of localized UH waves

Gusakov¹,

Popov²

2017

EPJ Web Conf.

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Electron cyclotron resonance heating (ECRH) is a technique being widely utilized nowadays in magnetic fusion experiments. It is also considered for application in ITER, for plasma heating and neoclassical tearing mode control. The analysis [1][2][3] of different parametric decay instabilities (PDIs) potentially dangerous for mm-waves at the ECR frequencies in the presence of the monotonous plasma profiles revealed the typical microwave power threshold in the range much higher than 1 MW. This is due to huge energy loss of the interacting waves from the decay region. The typical heating system output power range available up to now, even if the gyrotrons are joined into a group, is substantially lower than the predicted PDI power threshold value. Thus, it has been taken as known until very recently that the ordinary or extraordinary electromagnetic wave propagation and absorption in the absence of the UHR being well described by linear theory is predictable in detail.However, during the last decade many observations have demonstrated the anomalous phenomena during ECRH. Firstly, the fast ion generation and ion heating were observed at the TJ-II stellarator [4] and the TCV tokamak [5] during the 2 nd harmonic Xmode ECRH pulse under conditions when energy exchange between electrons and ions should be very low. Secondly, the observations of the backscattering signal down -shifted from the pump frequency by approximately 1 GHz and correlated to the MHD mode rotation were reported [6,7] in the 200 -600 kW level 2 nd harmonic X-mode ECRH experiment in the TEXTOR tokamak and interpreted in terms of anomalous backscattering of the pump EC wave. The anomalous scattering of the X-mode in the ASDEXUpgrade tokamak 2 nd harmonic ECRH experiment has been recently reported as well [8,9].The attempts of theoretical explanation of these anomalous phenomena have been undertaken during recent years [10][11][12][13]. Several theoretical models demonstrating the possibility of excitation of the lowthreshold PDIs which lead to generation of the electron and ion Bernstein waves had been proposed. The cornerstone of them is taking full account of a nonmonotonous plasma density profile often observed in the ECRH experiments and originated due to different physical mechanisms, among which are features of plasma confinement in the magnetic island [14] or the electron pump-out effect manifesting itself as a consequence of the anomalous convective particle flux from the ECR layer at the intensive ECRH. The poloidal magnetic field inhomogeneity is a key element of the theory as well. Based on these two elements it was shown that two-or even three-dimensional (2D or 3D) trapping of either a daughter ion Bernstein wave (IBW) or a daughter electron Bernstein wave (EBW) due to a specific multidimensional plasma inhomogeneity can be responsible for excitation of several convective and absolute PDIs possessing thresholds drastically less than those predicted by the standard model.These theoretical results expanded significantly the standard theory w...

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“…In particular it is being exploited to study possible anomalous phenomena originating from the interaction of the probe beam with the plasma, as already noticed in di erent experiments [3,4]. It has been observed that the anomalous phenomena can be driven by the inversion of the density profile induced by the presence of a well-developed magnetic island [5,6]. Since we are interested in e ects of the magnetic islands on the EC beams, in order to verify the conditions for such low-threshold PDIs and their link with an inversion of the density gradient in islands, we stimulate with various techniques the growth of a MHD rotating island, and observe the e ects of its passage through the crossing region of the probe and receiving beams.…”

Section: Introductionmentioning

confidence: 98%