Overview of the Geodesic Acoustic Mode (GAM) studies on the T-10 tokamak

Transcription

1 Overview of the Geodesic Acoustic Mode (GAM) studies on the T-10 tokamak A.V. Melnikov 1,2, L.G. Eliseev 1, L.I. Krupnik 3, J. Chercoles 4, A.A. Chmyga 3, G. N. Dezhko 3, M.A. Drabinskij 1, P.O. Khabanov 1, N.K. Kharchev 1, A.D. Komarov 3, A.S. Kozachek 3, S.M. Khrebtov 3, V.P. Lakhin 1, S.E. Lysenko 1, A. Molinero 4, JL. De Pablos 4, D.A. Shelukhin 1, M.V. Ufimtsev 5, V. A. Vershkov 1, V.N. Zenin 1, and A.I. Zhezhera 3 1 National Research Centre Kurchatov Institute, Moscow, Russia, 2 National Research Nuclear University MEPhI, Moscow, Russia, 3 Institute of Plasma Physics, NSC Kharkov Institute for Physics and Technology, Kharkov, Ukraine, 4 CIEMAT, Madrid, Spain 5 Moscow State University, Moscow, Russia, 23-rd IAEA TM on the RUSFD, Santiago, Chile. March 2017

2 Layout Introduction * Моtivation * GAM concept Experimental set-up, tools to study GAM * Heavy Ion Beam Probing * Correlation reflectometry * Langmuir Probe Geodesic Acoustic Modes features in T-10 * phenomenology * T e dependence of frequency * correlation properties * radial and poloidal structure * evolution with density * interaction with broadband turbulence Summary 2

3 Motivation Understanding of the role of electric field in confinement is almost equivalent to the understanding of plasma confinement itself K. Itoh and S.-I. Itoh, PPCF 1996 The direct measurement of an electric potential and its fluctuations in a core plasma is of a primary importance for the understanding of the mechanisms of L-H transitions in toroidal plasmas and of the role of the electric field in plasma confinement. Both static and oscillatory (GAM, ZF) components of E r or potential play a role in plasma confinement. Heavy Ion Beam Probe is a unique diagnostic to study plasma potential in the core and at the periphery as well. It has been recently upgraded in the T-10 tokamak to study directly with a good spatial (up to 1cm) and temporal (up to 1 s) resolution the plasma electric potential and density. Correlation Reflectometry and Langmuir Probe contribute to the GAM studies on T-10 with density and edge plasma potential data 3

4 Concept of GAM Oscillating E r creates oscillating ExB poloidal rotation (Zonal Flow, GAM) Zonal Flow mechanism of turbulence self-regulation -> affects cross-field transport. GAM high frequency branch of Zonal Flow Oscillating Er, linked with ZF/GAM, results from nonlinear interaction between various spectral components of plasma turbulence P. Diamond et al, PPCF (2005) Zonal Flow is considered to be a possible mechanism affecting L-H transition F. Wagner, PPCF (2007) Theory: m=n=0, Δn/n < < e /T e, f ~ C s /2pR N. Winsor, PRL (1968)!! Experiment - since 2003: TEXT(P. Schoch et al, RSI), DIII-D (G. McKee et al, PRL), T-10 (A. Melnikov et al, EPS-2003)

5 Experimental set-up. General Layout. CR. LP. LP provides simultaneous measurements of fluctuations of electron density and potential at the plasma edge (0.95< ρ<1.2) with f BW < 500 khz. CR overlappes with LP The heterodyne O-mode CR provides poloidal, radial and toroidal correlation measurements in n cr range from 0.8 to m -3 over the whole plasma radius with f BW < 500 khz. [A.V.Melnikov et al, PPCF, 2006]

6 HIBP principles (T-10) Primary trajectory Tl+ Secondary fan Tl++ Detected particles * Sample volume the local area of the plasma potential measurements pl = E2+-E+ ~ pl- local I2+ = I+neσ12λA ~ne semilocal if r<<l Vz2+ ~ Ψ/R ~Bpol non-local

7 Heavy Ion Beam Probe on the T-10 tokamak [A.V.Melnikov, Nature Phys, 2016] 7

8 Machine and HIBP parameters Parameter T-10 <R> [m] 1.5 <a> lim [m] 0.3 B T [T] n e [10 19 m -3 ] P ECRH [MW] 3 E beam [kev] 300 HIBP ion species Tl + Observation area 0.2< 2.1T radial resolution [cm] ~ 1 Frequency <250 bandwidth [khz] 8

9 Observations of GAM oscillations - HIBP HIBP measurements The typical power spectra of potential oscillations for the Ohmic phase. Measurements in the fixed position of the sample volume, = 0.57 show that during the OH phase the spectrum exhibit the sharp dominating peak between 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 Observations of GAM oscillations - 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 Observations of GAM oscillations LP OH B T = 2.42 T, I p = 290 ka, q(a) =2.5, n e = m -3 At the edge (0.95< <1) HIBP with E b =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= ms

12 Observations of GAM oscillations CR versus Magnetic Probe Theoretically, GAM is considered as purely electrostatic mode, but in real plasma some electromagnetic effects may be involved. GAM magnetic component was observed in some discharges in T-10. Both MHD m=2, n=1 and GAM are clearly seen by Magnetic Probe and CR, with a high cross-coherence for GAM frequency. The similar observation were done on the TEXT tokamak with MP inserted closely to the plasma surface [Tsui, 1993]. In both machines this mode was not seen by the standard Mirnov coils, located outside the vacuum chamber. Magnetic component was also found for GAMs in JIPPTII-U [J. Hamada et al, NF 2005] and COMPASS [J. Seidl et al, EPS 2016]. [A.V.Melnikov et al, EPS 2003]

13 GAM is mainly potential oscillations Properties of GAM Dependence on T e Satellite = V, e / T e >> Δn/n ~ 10-2 f egam = (Te / mi ) 1/2 /2pR GAM frequency evolution during ECRH-on Experiment agrees with theory, except satellite 13

14 Properties of GAM - Satellite Satellites were later observed in TEXTOR by A. Kramer-Flecken, et al Integration over the variable frequency Interval f min (t) f max (t) [L.G. Eliseev et al. EPS 2016] 14

15 f GAM (khz) GAM frequency dependence Тuman-3М n e (10 19 m -3 ) Т-10 OH, ECRH I pl (ka) B t (T) f GAM ~ C s /2pR T e, 21 cm (kev) Overall dependence over whole operational domain was explored [A. Fujisawa et al, NF, 2007] Experiment agrees with theory: f egam = (Te / mi ) 1/2 /2pR

16 Coherency Coherency Cross-phase [rad] Cross-phase [rad] Ampl. [a.u.] Ampl. [a.u.] GAM correlation properties HIBP r = cm HIBP + CR r = cm, tor shift 1/4 4 2 MHD m=2 GAM n e 4 2 n e (CR) (HIBP) GAM 0 p 0 0 p 0 p 0.8 reliability level Frequency [khz] n e - correlation at the GAM frequency in the fixed position p reliability level Frequency [khz] Оservation of the n e - long-range correlations GAM - global mode. Experiment agrees with theory - q(n, ) = π/2 [A.V.Melnikov et al, J Phys. Conf Ser., 2015] 16

17 phase shift (rad) Potential-density phase shift q(n, ) Phase shift q(n, ) radial distribution Phase shift histogram in π p B(T) I(kA) q a T e (0)(keV) GAM Satellite # p/4 p/2 p/ r (cm) radial error bars scan Experiment agrees with theory - q(n, ) = π/2 17

18 GAM intermittency Wavelet spectrum of potential Both frequency and amplitude are modulated by sawteeth ECRH GAM has a complex structure, different from the harmonic oscillations with fixed frequency GAM looks like a stochastic sequence of the wave packages with a lifetime ~ ms [A.V.Melnikov et al. IAEA-FEC 2008] 18

19 HIBP single slit energy analyser Primary trajectory Tl+ Secondary fan Tl++ Detected particles * Sample volume the local area of the plasma potential measurements pl = E2+-E+ I2+ = I1+neσ12λA Vz2+ ~ Ψ/R Increasing the number of slits multichannel measurements

20 5 slits HIBP detector grid 20

21 phase shifts (rad) m GAM potential perturbation m= 0 dx = 5 cm 0,6 experimental shift 3 0,4 0,2 2 1 m = a q 0,0-0,2-0,4-0,6 slits 1 and [Two point correlation tecnique, L.G. Eliseev et al, PFR, 2012] r of central slit (cm) [V.N. Zenin et al, PAST, Plasma physics, 2014] Experiment agrees with theory: m=0 21

22 GAM radial structure Satellite GAM GAM presents the feature of the global eigenmode. Experiment disagrees with theory 22

23 GAM radial structure during the density raise n e (10-19 m -3 ) # time, ms B(T) I(kA) q a T e (0)(keV) B(T) I(kA) q a T e (0)(keV) GAM ( V) n e (10 19 m -3 ) r (cm) GAM ( V) n e (10 19 m -3 ) r (cm) GAM presents the feature of the global eigenmode for the whole range of the T-10 operational limits. 23

24 n e (10-19 m -3 ) f (khz) GAM radial structure during the density raise B=2.1 T radially the most extended GAM observations 3 # B(T) I(kA) q a T e (0)(keV) time, ms 10 5 n e (10 19 m -3 ) r (cm) During the density raise the GAM frequency decreases slowly 24

25 Ampl, V GAM amplitude evolution with density. 220 n=0.8 *4/3 = r,cm 25

26 Ampl, V 220 n=1.0 *4/3 = r,cm 26

27 Ampl, V 220 n=1.3 *4/3 = r,cm 27

28 Ampl, V 220 n=1.5 *4/3 = r,cm 28

29 Ampl, V GAM amplitude decreases with density. 220 n=1.8 *4/3 = r,cm Experiment agrees with theory collisional GAM damping 29

30 A GAM (V) A GAM (V) The model of the GAM evolution with density Drift-wave kinetic equation for GAM potential perturbation Longitudional collisional damping due to parallel ion viscosity Excitation by Reynolds Stress due to collisional trapped electrons F GAM ~ 1/n (1-n/n cr ) 1/2 GAM amplitude decreases with density k*n ~ 1/n [V.P. Lakhin to be published] GAM amplitude raises with plasma current 100 A GAM ~ I a /n I=220 ka B 0 =2.3, 2.4T a>0 I=300 ka B 0 =23.5, 2.4T n 20 I=150 ka B 0 = 1.6T [L.G. Eliseev et al. EPS 2016] I=150 ka B 0 = 2.4 T n e 30

31 GAM evolution due to the impurity puffing Amplitude of GAM abruptly decreases and then slowly restores. Also frequency of GAM decreases and then recovers. f egam = (Te/ mi ) 1/2 /2pR f GAM #65489 f GAM experimental f GAM theoretical time (ms) He ECRH [V.N. Zenin et al. EPS 2015] 31

32 GAM evolution due to the MHD activity Satellite GAM MHD m=2 large iseland Excited MHD m=2 destabilize the discharge strongly, that leads to decrease of the energy content, raise of plasma potential and GAM suppression [L.G. Eliseev et al. EPS 2015] 32

33 GAM evolution due to the MHD activity - 2 MHD m=2 large iseland appearance resonance phenomenon Gradual excitation of MHD m=2 mode leads to gradual GAM suppression m=2 Satellite GAM I pl = ka A m=2 ~ 0.1 mt 33

34 Radial correlation length cm range CR ZF/GAMs are theoretically predicted to have a finite radial wavenumber k r. To study radial correlations, two independent reflectometry systems with close frequencies were used. The radial separation of reflection layers is Δ= 0.3 cm at refl =0.88. Radial phase shift for GAM is q GAM = 40.10º (time delay τ = 2.26 μs) that gives a radial wavelength r ~3 cm. Extended database gives the values in a scale of r > 5 cm.

35 Radial correlation length cm range CR-HIBP CR observation radius. potential-density coherency HIBP radial position is r SV = cm. The radial separation of r SV and r refl is Δ= 1-2 cm, r > 2 cm. Extended database gives the values in a scale of r ~ 5 cm. Actual density should be enhanced by factor 4/3.

36 GAM-broadband turbulence interaction CR Spectra of the phase of reflected wave and the fluctuation of the turbulence level, integrated over the broadband frequency range khz. 10% of the total turbulence level oscillates with the GAM frequency (a) with the high coherency about 0.9, (c). It is important to point out the low frequency (close to zero) part of the spectrum, where one may see the significant coherency and constant cross-phase. Note that the signal exceeds the noise level significantly in this domain. The features may be indication of the low frequency coherent structure, like low frequency ZF. 36

37 f 2 (khz) GAM-broadband turbulence interaction HIBP Auto-bicoherence of plasma potential shows that GAM is linked with broadband potential turbulence up to 250 khz = f Nyquist via Three-Wave Coupling: f 1 = f 2 ± f GAM Auto-bicoherence (,, ) f 1 (khz) -f GAM +f GAM 2 It suggests a quadratic nonlinearity in the GAM-BBT interaction like Reynolds Stress in the Navier-Stox equation: F ( f ) F ( f ) F ( f f ) * b ( f1, f2) < b 2 < 1 F ( f ) F ( f ) F ( f f ) Three Wave Coupling - the wave propagation in the matter with quadratic nonlinearity (e.g. nonlinear optics) 37

38 GAM-broadband turbulence interaction-2 b 2 (f GAM, f 2 ), psd I tot (f) HIBP signals PSD at r/a=0.8 PSD density Quasicoherent mode PSD potential Cross-bicoherence (, n, n) -f GAM +f GAM 2 * F ( f ) F ( f ) F ( f f ) 2 1 n 2 n 1 2, nn, ( 1, 2) 2 2 b f f F ( f ) F ( f ) F ( f f ) 1 n 2 n 1 2 Non-uniformity of the bicoherency #62753 b ( f, f ) 2 x, y, z GAM 2 Statistically valuable bicoherence at f GAM was found for (, n, n), (n, n, n), (n, n, B pol ) for r/a > f 2 (khz) No b 2 for QC modes 38

39 GAM-broadband turbulence interaction-3 Auxiliary ECRH leads to the increase of b 2 in a factor of [A.V. Melnikov et al. IAEA-FEC 2016, EX/P8-34] 39

40 Conclusions GAMs were found on Т-10 and their features were studied GAMs were studied in the ECRH regimes for the first time, the satellites were found. GAM amplitude may exceed 100 Volts in OH and ECRH regimes, m = 0 for plasma potential perturbations. The long range toroidal density-potential correlation were found for the first time in the core tokamak plasma The intermittent character of GAM and GAM modulation by sawteeth was found for the first time The GAM amplitude decays with density raise due to the collisional damping GAM presents the feature of the global eigenmode over the whole observed radial interval 0.3<r/a<1, which claims the theoretical explanation. GAM is linked with broadband potential/density/b pol turbulence up to 250 khz via three-wave coupling