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The study of GAM properties in the T-10 tokamak

The study of GAM properties in the T-10 tokamak. A.V. Melnikov1 , L.G. Eliseev1, S.V. Perfilov1, S.E. Lysenko1, S.A.Grashin1, V.A. Mavrin1, R.V. Shurygin1, D.A. Shelukhin1, V.A. Vershkov1, V.P. Budaev1, G.N. Tilinin1, S.A. Grashin1, L.I. Krupnik2, A.D. Komarov2, A.S. Kozachek2,

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The study of GAM properties in the T-10 tokamak

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  1. The study of GAM properties in the T-10 tokamak A.V. Melnikov1, L.G. Eliseev1, S.V. Perfilov1, S.E. Lysenko1, S.A.Grashin1, V.A. Mavrin1, R.V. Shurygin1, D.A. Shelukhin1, V.A. Vershkov1, V.P. Budaev1, G.N. Tilinin1, S.A. Grashin1, L.I. Krupnik2, A.D. Komarov2, A.S. Kozachek2, A. Kraemer-Flecken3, S.V. Soldatov3, G. Ramos4, C.R.Gutierrez-Tapia5, H. Hegazy6, A. Singh7, J. Zajac8, G. Van Oost9 and M. Gryaznevich10 1 Nuclear Fusion Institute, RRC "Kurchatov Institute", 123182, Moscow, Russia, 2 Institute of Plasma Physics, NSC “Kharkov Institute of Physics and Technology”, Kharkov, Ukraine 3 Institut für Plasmaphysik, Forschungszentrum Jülich, EURATOM Association, Jülich, Germany 4 CICATA, Instituto Politécnico Nacional, Mexico 5 Instituto Nacional de Investigaciones Nucleares, Mexico 6 Egyptor project, Egypt 7 University of Saskatchewan, Canada 8 Institute of Plasma Physics, Prague, Czech Republic 9 Ghent University, Belgium IAEA Joint Experiment at RRC “ Kurchatov Institute”

  2. Outline • Experimental set-up on T-10 • Introduction • GAM features on T-10. • HIBP potential-density correlations • HIBP-Correlation Reflectometry long-distance correlations • Summary

  3. Introduction - what is GAM • History:C- stellarator, strongly dominated  oscillations L. Ornstein and K. Young. Bull Am Phys Soc. 13, 286 (1968) Windsor N, Johnson J L, Dawson J M (1968) Phys. Fluids11 2448 • Theory: • Electrostatic acoustic oscillations in toroidal plasmas • Radially localized zonal flow ( m = 0, n = 0) – high frequency branch of ZF • Coupled to m = 1, n = 0 pressure perturbation p = p0sin (), ( m = 0, n = 0) • Δn/n < < e∆ /Te • Frequency: f ~ Cs/2R , correction terms of order unity • Driven by turbulence, possibly via Reynolds stress • Turbulence self-regulating mechanism via oscillating poloidal flow V = ErxBtor • GAM affects the anomalous transport in magnetic fusion devices • References: • P H Diamond, S-I Itoh, K Itoh, T S Hahm P H 2005 Plasma Phys. Control. Fusion 47 R35 • PPCF Special issue on ZF/GAM 2006, NF Fujisawa et al, 2007 • ZF/GAMs : More than 500 refs (theory), < 30 (experiment)

  4. Recent GAM- experiments • Specific oscillations of the plasma electric potential with frequencies near 20 kHz, called the“20 kHz-mode” (15 – 40 kHz) have been discovered in the ТЕХТ tokamak by the Heavy Ion Beam Probe and Langmuir probe [ Tsui, et al, 1993]. Later careful data analysis allowed the authors to interpret them as zonal flows, GAMs [ P. Schoch, RSI, 2003]. • The DIII-D team observed a peak on the spectrum of poloidal rotation velocity oscillation in the range f=13-16 kHz. They proposed to link it with the GAM, while it is characterized by the square root dependence on Te [G. McKee, PPCF 2003]. • On the T-10 tokamak, the presence of the typical “near 20 kHz” quasicoherent peak was observed first on plasma densityneby Correlation Reflectometry [Vershkov, et al, EPS 1994]. , then by Langmuir probes (density/potential) and HIBP (potential), and found to be linked with GAM/ZF [Melnikov, et al, EPS 2003]. The GAM features on T-10 were found to be very similar to those on TEXT and DIII-D. It was clearly seen that the GAM-like oscillations influences the total turbulence level [Vershkov, et al, IAEA 2005]. • HIBP is found to be the most direct tool to study GAM/ZF[Diamond P, et al, PPCF 2005]. • It has been effective one for ZF/GAM study in the CHS stellarator with dual HIBP [Fujisawa A, et al, PRL 2004], in the JIPP T-IIU [Hamada Y, et al, NF 2005 ] and in the JFT-2M tokamaks [Ido T, et al, NF 2006 ] • Recently, ASDEX-U tokamak also observed the similar GAM features on Er with Doppler Reflectometry [Convay C D, et al , PPCF 2005], TEXTOR with CR, H-1 heliac and Castor tokamak with probes. • In all mentioned experiments the spectra exhibit a strongly pronounced sharp monochromatic peak dominating the noisy background at the appropriate GAM frequencies. • Here we discuss the features of the GAMs in plasma potential and density measured by HIBP and CR in the regimes with Ohmic and ECR heating on T-10.

  5. Experimental set-up General Layout The typical plasma parameters: Ip - from 150 to 330 kA, BT - from 1.95 to 2.5 T, q(a)~ 2.5-4, ne - from 1 to 51019 m-3, TeOH(0)~ 800-1100 eV, Ti(0)~400-500 eV.

  6. Experimental set-up – reflectometry & probes A section A section

  7. Experimental set-up - HIBP D section Tl+1 beam with energies Eb up to 240 keV. The measurements were made over the upper outer quadrant of the plasma cross-section. The sample volume position in plasma in a single discharge can be either fixed or scanned radially with a period of 10 ms, producing a series of profiles during a single shot. Sampe volumes look like elliptical disks, approximately 1cm 1.5 cm 0.5 cm thick. Radial size of the sample volume was about 1.5 cm. The uncertainty in the sample volume position was up to 2 cm. HIBP was able to operate in 0.57<ρ<1.0, (depending on B0) while CR was able to observe whole plasma. At the plasma edge (0.95<ρ<1.2) they both overlapped with Langmuir probes.

  8. GAMs have a scale of dozens volts in plasma potential (HIBP)

  9. GAMs have pronounced peak in potential power spectrum (HIBP) HIBP measurements The typical power spectra of potential oscillations for the Ohmic phase. Measurements in the fixed position of the sample volume, r= 0.57 show that during the OH phase the spectrum exhibit the sharp dominating peak between 15-30 kHz, full width at half maximum (FWHM) of about 5 kHz, Δf/f = 1/4, amplitude typically several times larger than that of the background.

  10. GAMs have pronounced peak in density power spectrum (CR) At the plasma core HIBP overlapped with CR. The typical power spectrum of density oscillations for Ohmic phase . Measurements show that the low frequency part of spectrum has the sharp dominating peak with FWHM of about 2 kHz, Δf/f < 1/10.

  11. GAMs have pronounced peaks in both floating potential and ion saturation current spectra (MLP) At the edge (0.95<r<1) HIBP with Eb =200 keV overlapped with Langmuir probes. The position of HIBP sample volume was fixed at 28 cm with an uncertainty of 2 cm and a radial size of ~ 1.5 cm. The sampling rate was chosen at 10 s. steady-state phase of OH discharge, t=700-800 ms, are presented for the following plasma conditions: BT = 2.42 T, Ip = 290 kA, q(a) =2.5,ne = 41019 m-3.

  12. GAMs evolution with ECRH HIBP single point. ρ= 0.57. #36819 ВT=2.05 Т, Ip=270kA, ne~1.51019 m-3, off-axis 129 GHz ECRH at ρ ~0.4, PERCH=0.4 MW. During the ECRH phase, (700 ms <t<850 ms) the frequency of oscillations increases, FWHM and Δf/f remain the same. After switch-off the initial spectra are recovered. The peak amplitude is notably higher in ECRH.

  13. #37253 HIBP potential peak frequency follows Te within a millisecond timescale. the characteristic time of frequency variation coincides with the characteristic time of local electron temperature variation, ~ 7 ms.

  14. Sawtooth modulation of the GAMs HIBP single point. The modulation of the GAMs by sawteeth was found in the OH and ECRH phases. The GAMs were observed at the outer part of the phase reversal radius, rs = 12 cm. Peak frequency follows the oscillating temperature with the same period of several msec. The mode amplitude is also modulated by the reversed phase of Te sawteeth oscillations.

  15. Dependence of the GAM frequency on Te Data set: various shots with OH, on-axis and off-axis ECRH. f - rather weakly growing function of the electron temperature, f ~ Te1/2. Such dependence is an indication of the nature of the sound waves. HIBP single point. Te - 2nd harmonic of ECE

  16. Intermittent character of GAM amplitude/frequency HIBP single point. Eb ~240 kV at r ~ 0.73 While the GAM spectral peak is generally continuous and quite stable in time, it was found some irregular modulations, presenting the bursty character. GAM intensity is a sequence of irregular intermittent bursts of ~2-4 ms in length with a “frequency” of ~ 0.2 -1 kHz. The intensity modulation is in the range of 50%. GAM peak also varies slightly in frequency within the limit of 2 kHz. spectrum evolution over 100 ms using a sliding FFT of 256 points with 50% overlapping of the Hanning window. Note: There were no any variations observed in the ECE channels ( Te ) and the plasma column position data. The possible explanation - modulation of GAM by the low frequency zonal flows. Obtained numerically by two fluid model for Alfvén - drift wave turbulence [Ramish M, Stroth U, Niedner S and Scott B 2005 New. J. Phys. 5 1222].

  17. Potential-density correlations (HIBP) Single point. Long (hundreds ms) time averaging. HIBP shows that GAM oscillations are more pronounced on the potential than on the density. In contrast, the amplitude of 7 kHz MHD m=2 oscillations is larger on the density and much smaller on the potential. The clear correlations between potential and density are seen at the GAM frequency. The phase shift is constant for GAM frequency interval. Const =/2 In contrast, for MHD m=2 peak, the phase shift is zero. #44166

  18. Intermittent character of GAM amplitude/frequency leads to bursty correlations

  19. Density-potential cross-phase if Coh > 0.3, color If Coh <0.3 green

  20. Cross-phase histogram -> -0.5

  21. Long distance potential - density correlations (HIBP-CR) Single point. Long (hundreds ms) time averaging. Fourier correlation analysis reliably shows a clear correlations between HIBP potential and CR density at the GAM oscillations frequency. The phase shift is a topic for further more accurate analysis. This observation suggests a global character of the GAMs observation pointsr = 250.5 cm

  22. Long distance potential - density correlations (HIBP-CR) “short” (a few ms) time averaging observation points r = 250.5 cm # 43285

  23. Radial correlation length for GAM via potential - density correlations HIBP single point at r = 25 0.5 cm. CR varies position with density decay.OH plasma Correlation coefficient GAM Variation of the CR observation position in a few cm shows the constancy of correlation coefficient at the GAM frequency .

  24. Summary GAM correlation study was performed by HIBP and CR for the first time on T-10 tokamak in the framework of IAEA JE. It shows: ·       The most pronounced manifestation of GAM is quasi-coherent oscillation in plasma electric potential, while GAM also seen in density. ·      GAM power spectra of Dj and ne looks like a narrow dominant coherent peak between 9 – 33 kHz with a high contrast to the noisy background. The GAM frequency depends on the electron temperature as Te1/2. The absolute values of the frequencies are close to the theoretical ones. ·  GAM has an intermittent character in amplitude and phase, • the long-distance correlation was found for GAM, suggesting GAM is global mode, • radial correlation length for GAM has a range of a few cm • strong potential density correlation was found for GAM at the sample volume by HIBP • potential density phase shift is constant for GAM frequency

  25. Open issues • GAMs around lower m, m = 2, 1.5, 1. • Density limit for potential (not density) manifestation of GAMs • Nature of satellites • Ion mass scaling for GAMs frequency, comparison with TEXTOR data • Note that the discussed T-10 measurements were done with quite low q value. This seems to be in agreement with the tendency of decrease of the GAM frequency with the rise of Ip (decrease of q), which was observed in ASDEX . The q-scaling should be the point for the further study. • The ZF/GAM oscillation is predicted to have n=m=0 mode structure with short kr. It was shown in experiments m=0 and kr in a range of few cm. Nevertheless, to complete the GAM features we should demonstrate that the described oscillations have n=0. This needs the toroidally and poloidally resolved measurements, which are planned for the future correlation experiments with full existing diagnostic set: HIBP and CR.

  26. Boltzman relationship HIBP single point. ECRH HIBP and Langmuir probes show that GAM oscillations are more pronounced on the potential than on the density. In contrast, the amplitude of 7 kHz MHD m=2 oscillations is larger on the density and much smaller on the potential. Dj = A(25 kHz) ~ 60 V, Te ~ 400 eV, eDj /Te = 1.510-1; Δn/n < 10-2 ; 10Δn/n < eDj /Te, KGAM ~ 10. In contrast, for Broadband Turbulence : KBB ~ 1 [Vershkov V A et al 2001]

  27. GAMs are visible in HIBP raw signals i = (iLU + iRU - iLD - iRD) / Itot Itot = iLU + iRU + iLD + iRD

  28. Long distance potential correlations. Core-edge plasma (HIBP-MPL) HIBP at fixed point vs Limiter probe floating potential. OH We did not see linear correlation. at the GAM frequency. Radial location of the observation points?

  29. Wavelet correlation Intermittent character of GAMs suggests the wavelet analysis 5 ms N44631, 905 ms 5 ms Burst correlation t, ms

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