Strong Scattering of High Power Millimeter Waves in Tokamak Plasmas with Tearing Modes

Westerhof, E.; Nielsen, S. K.; Oosterbeek, J.W.; Salewski, M.; Baar, M.R. de; Bongers, W.A.; Bürger, A.; Hennen, B.A.; Korsholm, S. B.; Leipold, F.; Moseev, D.; Stejner, M.; Thoen, David J.

doi:10.1103/physrevlett.103.125001

Cited by 116 publications

(140 citation statements)

References 22 publications

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“…Such signals were first observed in TEXTOR [3,4], where a strong dependence of the measured power from the magnetic field and the plasma density was evidenced, as described in [4]. Similar evidences were subsequently found also in ASDEX and FTU.…”

Section: Introductionsupporting

confidence: 57%

First operations with the new Collective Thomson Scattering diagnostic on the Frascati Tokamak Upgrade device

et al. 2015

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Anomalous emissions were found over the last few years in spectra of Collective Thomson Scattering (CTS) diagnostics in tokamak devices such as TEXTOR, ASDEX and FTU, in addition to real CTS signals. The signal frequency, down-shifted with respect to the probing one, suggested a possible origin in Parametric Decay Instability (PDI) processes correlated with the presence of magnetic islands and occurring for pumping wave power levels well below the threshold predicted by conventional models. A threshold below or close to the Electron Cyclotron Resonance Heating (ECRH) power levels could limit, under certain circumstances, the use of the ECRH in fusion devices. An accurate characterization of the conditions for the occurrence of this phenomenon and of its consequences is thus of primary importance. Exploiting the front-steering configuration available with the real-time launcher, the implementation of a new CTS setup now allows studying these anomalous emission phenomena in FTU under conditions of density and wave 1 Corresponding author.

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Section: Introductionsupporting

confidence: 57%

First operations with the new Collective Thomson Scattering diagnostic on the Frascati Tokamak Upgrade device

et al. 2015

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“…The analyzed parametric decay possesses a very low threshold power of several tens kW and the growth rate being extremely large , which is similar to [3]. The saturation of this strong instability could occur at the level leading to substantial power transfere into UH waves affecting the power budget of the ECRH as it according to [3] was the case in ECRH experiments in TEXTOR tokamak [1,2]. The nonlinear coupling of the UH plasmons is a possible candidate for explanation of the anomalous scattering effect observed in the ECRH experiments in ASDEX Upgrade tokamak [9].…”

Section: Discussionmentioning

confidence: 85%

“…Then, we assume the weak nonlinear coupling of UH waves and utilize the perturbation theory approach [10]. In the first step of the perturbation procedure we put 1 , the system operates at 140 GHz with X2 scheme) [9]. The chosen edge density profile is also depicted in the figure 1 in thick solid curve.…”

Section: Parametric Excitation Of the 2d Trapped Uh Waves By The Pumpmentioning

confidence: 99%

“…The most eloquent among them is the anomalous backscattering phenomena studied in detail in the second harmonic X-mode heating experiments on TEXTOR [1,2]. The theoretical model [3] proposed recently explains this anomalous backscattering as a result of the two upper-hybrid (UH) plasmon parametric decay instability (PDI) possessing very low threshold due to trapping of nonlinearly excited plasmons in the vicinity of the density maximum that accompanies the magnetic island [4].…”

Section: Introductionmentioning

confidence: 99%

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Anomalous backscattering and absorption in second harmonic ECRH experiments due to low-threshold two UH plasmon decay instability in blobs

2017

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“…2 the spectrum of shot #39848 is shown in correlation with a Mirnov Coil signal and an MHD frequency spectrum. Sudden bursts of emissions detected by CTS at frequencies multiple of the ion cyclotron frequency above and below the gyrotron frequency (at 0.8 GHz in the spectrum) show a fast time evolution (at microsecond scale) which differs with respect to phenomena observed in [1,2]. Simultaneous variations of the stray radiation level in the vessel close to the launching port have been recorded.…”

mentioning

confidence: 89%

Fast events detection with the CTS diagnostic on FTU and plans for improvement

et al. 2017

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The capability of the CTS diagnostic to detect emissions at frequencies close to the injected EC power can be used to study the occurrence of parametric decay instabilities (PDI) excited by the ECH power. In this way, from the detection of back-scattered power in the case of 2nd harmonic X-mode injection [1,2], the sources of the signals were well explained by waves generated by PDI mechanisms [3]. On FTU, during the work with the CTS diagnostics focused on studying the effects of possible PDI occurrence on EC beams (under Enabling Research project ER-ENEA-06), other still unexplained fast signals were detected in the spectra, in experiments with injection of EC power in O-mode at the fundamental EC harmonic [4]. The possibility to analyze data with a high time resolution is crucial for the study of these fast events, and the use of a fast rate (12 GS/s max) digitizer and of the FFT of the sampled intermediate frequency signal (taken at the output of the 140 GHz heterodyne radiometer front-end) over different time-scales is mandatory [5]. In this paper the observations obtained with new experiments are compared to the previous ones. The design of the new receiving line equipped with a RS antenna and the status of the work are presented. Experiment preparationOn the diagnostic side an effort is ongoing to exclude any parasitic effect of gyrotron spectrum, of highfrequency front-end components, of local breakdown in the antenna and other disturbances. The gyrotron frequency spectrum, previously obtained from a lossy pickup line 50 m long used to measure the gyrotron main line frequency vs time (without using the notch filter), was now obtained with a direct pick-up from the stray radiation in the launcher region while shooting the gyrotron on the local dummy load. In this way a higher signal is collected and a sensitive spectrum of the gyrotron radiation is obtained, using the notch filter to suppress the main line. The resulting spectra from the gyrotron often include spurious lines that appear also in some CTS spectra from plasma, as shown in Fig. 1. The lines of this type have always the same behavior, starting from the beginning of the gyrotron pulse and lasting tens or hundreds of ms with a frequency decreasing with oscillations around an average speed of 2 GHz/s (Fig. 1 top right).Whether these are due to gyrotron spurious mode or to a signal treatment in the HF diagnostic section is under evaluation. The level of the spurious line, not observed in the 2015 shots, is estimated at least 50 dB below the main gyrotron line.The calibration of spectra received from the plasma is now performed comparing the spectra with the ones obtained during Ohmic pulses with the EC resonance inside the plasma (where the emission is only thermal ECE) and subtracting the noise spectra obtained during the same shot with the aid of a chopper/modulator working at around 10 kHz. The spectra obtained during the two chopping phases (on-off) are evaluated by proper data analysis, summed separately and then the noise part is subtracted....

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Strong Scattering of High Power Millimeter Waves in Tokamak Plasmas with Tearing Modes

Cited by 116 publications

References 22 publications

First operations with the new Collective Thomson Scattering diagnostic on the Frascati Tokamak Upgrade device

First operations with the new Collective Thomson Scattering diagnostic on the Frascati Tokamak Upgrade device

Anomalous backscattering and absorption in second harmonic ECRH experiments due to low-threshold two UH plasmon decay instability in blobs

Fast events detection with the CTS diagnostic on FTU and plans for improvement

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