Albergante, Mattia

Coda, Stefano

de Mejeire, C. A.

Kraemer-Flecken, Andreas

Soldatov, Sergey

Infoscience - École polytechnique fédérale de Lausanne

Observations on turbulence dynamics and beam-ion driven modes on theTEXTOR tokamakC.A. de Meijere1, S. Coda1, A. Krämer-Flecken2, S. Soldatov2, M. Albergante11 Ecole Polytechnique Fédérale de Lausanne (EPFL), Centre de Recherches en Physique desPlasmas, Association Euratom-Confédération Suisse, CH-1015 Lausanne, Switzerland2 Institut für Energieforschung - Plasmaphysik, Forschungszentrum Jülich GmbH, AssociationEURATOM-FZJ, Trilateral Euregio Cluster, 52425 Jülich, GermanyIntroductionTo achieve H-mode the TEXTOR tokamak [1] is operated at weaker than usual toroidal mag-netic field (Bt = 1.3 T). In both L- and H-mode this leads to quasi periodic bursts of two dis-tinct types of modes (fishbone and Alfvén), both driven unstable by suprathermal beam ions.Although this phenomenology is well known from other experiments [2], it had not yet beendocumented for the case of TEXTOR. A presentation to this end constitutes the first part of thepresent paper. We then discuss observations of interaction between the fishbone and the Alfvénmodes, and their statistical link to ELMs. Finally, we discuss a third type of mode, unrelatedto either the fishbone or the Alfvén modes, which is shown to modulate the amplitude of broadband turbulence in the pedestal during periods leading to ELM crashes.Observations of beam-ion driven modesdB/dt (a.u.)f (kHz)010203.2 3.2005 3.201 3.2015 3.202D α (a.u.)t(s)#110568 (H−mode)n=1 (fishbone)n=−2 (ELMprecursor)ELM crashIp=230kA; Bt=1.3T;ne=3.5×1019m−3; Te=1keVFigure 1: Contours (4 dB interval) ofspectral power B˙p; ∆twin = 0.5 ms.Figure 1 shows magnetic-probe measurements of thepoloidal field fluctuation (B˙p(t)) and the correspondingspectrogram for two consecutive bursts. Such bursts areonly seen for Bt < 1.6 T and start roughly 20 ms afterthe neutral beam (1 MW tangential injection of 56 keVdeuterium atoms) is switched on. The magnetic spectraduring bursts are dominated by a down-chirping n = 1mode, propagating in the ion-diamagnetic drift direc-tion. All this phenomenology matches that of the ‘fish-bone’ mode [3], which is driven by suprathermal beam-ions.The fishbone is accompanied by several Alfvénmodes with frequencies between 100 and 200 kHz inthe lab frame. These modes are driven by beam ions, which have parallel velocities up tovb = 2× 106 m/s, compared to a core Alfvén velocity vA = 5× 106 m/s. For these experi-38th EPS Conference on Plasma Physics (2011) P1.1371 1.5 2 2.5 3 3.5120140160180200t (s)f (kHz) n=5n=4  0.10.20.30.41 1.5 2 2.580100120140160180200t (s)f (kHz)#112731 (L−mode)γ2min=0.04+ n=5° n=4expected forTAE in plasmaframe∇: observed inplasma frame(a) (b)Figure 2: Contours of coherence between phase signals from two reflectometer antennas sepa-rated toroidally by 90◦; features correspond to Alfvénic modes.ments the sampling rate of the Mirnov coil signals was insufficient to resolve the Alfvén modes,which have instead been studied using a heterodyne, O-mode reflectometer [4]. This diagnosticfeatures two equatorial antenna arrays, toroidally separated by 90◦, and one array viewing thetop of the plasma (Fig. 4). The available range of microwave frequencies (28-36 GHz) results incut-off layers with r/a> 0.75 in L-mode and r/a> 0.9 in H-mode; the signals were sampled at1 MHz. During ELMs and L-mode, small-scale density fluctuations strongly scatter the probingmicrowave beam, so that the signature even of coherent plasma fluctuations is almost totallyobscured in the spectral power of the phase signals from the reflectometer. However, the Alfvén(and fishbone) modes are evident as peaks in the spectral coherence between phase signals fromthe various antenna pairs. Figure 2 gives an example for an L-mode discharge in which Bt wasramped down from 1.5-1.3 T on 1.0< t < 1.6 s, while ne increased from 2.7−4.0×1019 m−3on 1.0 < t < 3.5 s. As expected for Alfvén modes the mode frequencies scale with B/√ne towithin the error bars of the measurements.The cross-phase spectrum corresponding to Fig. 2 is consistent with the Alfvén modes havingtoroidal mode numbers n1 = 4 and n2 = 5, propagating in the ion-diamagnetic drift direction.Assuming that the frequency difference between the two observed modes is caused solely bydifferent Doppler shifts we use our tentative estimates of n to calculate the mode frequencyin the plasma frame [5], which is shown in Fig. 2b. The obtained frequency is significantlylower than what would be expected for a toroidicity induced Alfvén eigenmode (TAE) drivenat q = 1.5 [2]. Future attempts to identify these modes, which might be β -induced Alfvéneigenmodes (BAE) [6], should take the high value of normalized plasma-β in these experiments(up to βN ≈ 3) into account.A large majority of the fishbones coincide with ELM crashes, like the first one in Fig. 1. In38th EPS Conference on Plasma Physics (2011) P1.137102030f2 range fishbonef 2 (kHz)  0.050.10.150.20 50 100 150 200 250 300 350 400 450 500102030f2 range n=2 ELM precursorf1 (kHz)f 2 (kHz)  00.020.040.060.08f1 rangeAlfvenic modesf1 range modulated turbulence#110568 (H−mode)#110568 (H−mode)condition: fishbone but no ELMcondition: 0.5 ms window before any ELM(a)(b)Figure 3: Bispectral coherence of the reflectometer phase signal, averaged over two differentconditionally constructed ensembles. Dashed lines indicate the principal domain.this case the fishbone systematically starts to grow in amplitude tens of µs before the ELM isvisible in the Dα signal, so one can speculate that it may actually trigger the ELM crash [3, 7].However, a minority of the fishbones, like the second one in Fig. 1, clearly do not coincide withan ELM. Selecting windows of length 0.5 ms around the latter type of fishbones, we constructedan ensemble of 298 sub-records of a reflectometer phase signal for H-mode discharge 110568.Fig. 3a shows the bispectral coherence, averaged over this ensemble. The spectral features with100 < f1 < 200 khz and f2 = 14 kHz indicate the presence of non-linear interaction betweenthe fishbone and the Alfvén modes [8].Measurement of turbulence propagationBetween ELMs there are periods of strongly reduced turbulence amplitude, during which thestandard deviation of the reflectometer phase signals drops to below 1 radian. Selecting theseperiods, we construct an ensemble of phase signals from poloidally closely spaced reflectometerantennas (Fig. 4a) and evaluate the cross-spectra. The coherence is low (but significant) andfeatureless for f > 200 kHz, up to the Nyquist frequency of 500 kHz. Let ∆θi and ∆Ψi( f )denote the angular separation of the detection volumes and the cross-phase, respectively, ofantenna combination i. Assuming that the fluctuations obey a single-valued dispersion relationm( f ) in the lab-frame (i.e. neglecting turbulent broadening), we have m( f ) = ∆Ψi( f )/∆θi,where m is the poloidal mode number. Figure 4b suggests that broad band fluctuations with30< m< 80 propagate poloidally past the reflectometer, at a group velocity of 26 krad/s in theelectron-diamagnetic drift direction.Turbulence modulation by n= 2 ELM precursor modeIn addition to the fishbone, the spectrogram of B˙p in Fig. 1 shows an n= 2 feature at 23 kHzaround the time of the ELM crash, which is also visible in the reflectometer phase signals. The38th EPS Conference on Plasma Physics (2011) P1.137  θ200 300 400 500−150−100−500f (kHz)m=ΔΨ/Δθ  Δθ=0.075 radΔθ=0.075 radΔθ=0.05 radΔθ=0.025 radΔθ=0.025 radfit: 26 krad/s inelectron diamagneticdrift direction#110568betweenELMs(b)Figure 4: (a) Poloidal section showing one antenna array. (b) Measured dispersion relation ofbroad band density fluctuations.mode propagates in the electron-diamagnetic drift direction and poloidal harmonics have m= 4and m= 5, giving it a poloidal propagation velocity of roughly 30 krad/s. Since it is only visiblein periods of duration 0.5 ms leading up to ELM crashes, we label this mode ‘n = 2 ELMprecursor’.Selecting these 0.5 ms periods before every ELM, we constructed an ensemble of 890 sub-records of the reflectometer phase signal. Figure 3b shows the corresponding averaged bis-pectral coherence, which features a conspicuous ridge for f2 = 23 kHz (corresponding to then = 2 ELM precursor) and f1 > 200 kHz, corresponding to broad-band density fluctuations.The biphase (not shown) is constant along this ridge. These observations strongly suggest thatthe n = 2 ELM precursor modulates the amplitude of turbulent electron density fluctuations inthe H-mode pedestal, whose poloidal group velocity matches that of the n= 2 mode.This work was supported in part by the Swiss National Science Foundation.References[1] B. Unterberg et al, J. Nucl. Mater. 390-391 (2009) 351[2] W.W. Heidbrink, Phys. Plasmas 15 (2008) 055501-1[3] T. Kass et al, Nucl. Fusion 38 (1998) 807[4] A. Krämer-Flecken et al, Plasma Phys. Controlled Fusion 51 (2009) 015001[5] E.J. Strait et al, Plasma Phys. Controlled Fusion 36 (1995) 1211[6] W.W. Heidbrink et al, Phys. Rev. Lett. 71 (1993) 855[7] G. Matsunaga et al, Proceedings of the 23rd IAEA Fusion Energy Conference (2010)EXS/5-3[8] N.A. Crocker et al, Phys. Rev. Lett. 97 (2006) 04500238th EPS Conference on Plasma Physics (2011) P1.137

Observations on turbulence dynamics and beam-ion driven modes on the TEXTOR tokamak

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Observations on turbulence dynamics and beam-ion driven modes on the TEXTOR tokamak

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