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M. Endler and M. Hirsch Max-Planck-Institut für Plasmaphysik, EURATOM Association, D Greifswald, Germany 1. General considerations 2. Confinement region 3. Scrape-off layer (SOL) Observation of Turbulence in Wendelstein 7-AS

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experiment theory theory with additional effects ion heat transportelectron heat transport From: M. Kick et al., IAEA 1996 (Montreal), vol. II, General Considerations Radial heat transport in W7-AS – comparison between neoclassical theory and observation: Reason for interest in plasma turbulence: Turbulent transport

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Turbulence and Transport T1T1 T0T0 T 1 > T 0 pot of boiling waterfusion plasma n, T coreedge n 1, T 1 n 0, T 0

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Twofold Motivation for Observing Plasma Turbulence Directly measuring the turbulent transport ( synchronised observation of ≥ 2 quantities required) Comparison with turbulence models, simulations, theory (aim: understanding parameters controlling turbulence; influencing turbulence)

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6 cm –1 1 cm Which structure sizes can be observed? Doppler reflectometry microwave scattering Measurement in k space: spatial band pass CO 2 laser scattering k s = 1 for 300 eV dissipation k [cm ] – [cm] Measurement with limited resolution: spatial lowpass Mirnov probes, SX ECE BES reflectometry Langmuir probes (multi-tip) driving instabilities a=17cm kinetic energy

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Raw data and statistical analysis raw data Example: Langmuir probe data from the W7-AS SOL (I sat ) probability distribution function (auto)correlation function

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Topics: T e fluctuations Doppler reflectometry Transient events Turbulence and transport Transition edge/SOL Topics: T e fluctuations Doppler reflectometry Transient events Turbulence and transport Transition edge/SOL 2. Turbulence in the W7-AS confinement region

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T e fluctuations in the plasma core by ECE decorrelate thermal fluctuations without decorrelating T e fluctuations Challenge: < 1 % T e /T e fluctuations are masked by thermal fluctuations of the radiation field ~ – by observing the same volume from two positions under sufficiently large angle (first demonstration on W7-AS) by observing at slightly different frequencies = shifted volume (first demonstration on TEXT) ^

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Decorrelation of thermal fluctuations Demonstration of the principle using an artificial source for “temperature fluctuations” but true thermal fluctuations lines of sight of observation below/above decorrelation angle From: S. Sattler and H.-J. Hartfuß, PPCF 35 (1993) 1285, figs. 9&10

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Different features in T e fluctuations 1.broadband fluctuations (bandwidth ~ 100 kHz) 2.low-frequency fluctuations (< 5 kHz) 3.quasicoherent modes From: S. Sattler et al., PRL 72 (1994) 653, fig. 2 normalized cross-correlation

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Broadband fluctuations disappear for T e = 0 From: H.-J. Hartfuß et al., PPCF 38 (1996) A227, figs. 9&10 In a region with T e = 0, only the low-frequency feature remains

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From: M. Häse et al., RSI 70 (1999) 1014, fig. 5 Correlation between n and T e ~ ~

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Topics: T e fluctuations Doppler reflectometry Transient events Turbulence and transport Transition edge/SOL 2. Turbulence in the W7-AS confinement region

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Doppler reflectometry – using turbulence as a tracer for poloidal rotation corrugated and fluctuating reflecting layer antenna microwave signal “ordinary” reflectometry: use of 0th diffraction order of reflected signal From: M. Hirsch et al., PPCF 43 (2001) 1614, fig. 1 Doppler reflectometry: use of (–1)st diffraction order of reflected signal

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Comparison of poloidal velocity from Doppler reflectometry and from spectroscopic data poloidal velocity of fluctuations ≈ poloidal velocity of impurities ≈ v E B From: M. Hirsch et al., PPCF 43 (2001) 1614, fig. 7

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Time resolution of Doppler reflectometry From: M. Hirsch et al., PPCF 48 (2006) S155, fig. 6 4 µs resolution reveals strong and fast changes in poloidal velocity and scattered power at HL backtransition

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Topics: T e fluctuations Doppler reflectometry Transient events Turbulence and transport Transition edge/SOL 2. Turbulence in the W7-AS confinement region

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ELM-like transient transport events profile flattening in ECE T e signals in region of strong pressure gradient causing cold pulses propagating inward on diffusive time scale simultaneously bursts in broadband Mirnov activity and small-scale density fluctuations From: S. Zoletnik et al., 32nd EPS (Tarragona, 2005) P-5.023, fig. 1 Correlation analysis: From: M. Hirsch et al., 25th EPS (Prague, 1998) 2322, fig. 1a

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Transient magnetic activity – poloidal mode structure Magnetic activity: poloidal mode number related to edge rotational transform bursts of ~ 100 µs See: M. Anton et al., J. Plasma Fusion Res. SERIES 1 (1998) 259 Arrangement of Mirnov coils in poloidal cross section: From: S. Zoletnnik et al., PPCF 44 (2002) 1581, fig. 24

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Correlation between magnetic and density fluctuations Complement poloidal resolution of Mirnov coils with radial resolution of Li beam See: S. Zoletnik et al., PoP 6 (1999) 4239, fig. 5 = radial ^ correlation of Mirnov signal with various BES channels along the Li beam

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Tentative model for transient transport events After: S. Zoletnik et al., 32nd EPS (Tarragona, 2005) P poloidally localised event (associated with broadband turbulence) causes radial transport of hot, dense plasma flattening of pressure (temperature, density) gradient initial poloidal gradient causes MHD oscillations with m = 1/ until gradients on flux surface are balanced (after a few ion transit times ~ 100 µs)

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Topics: T e fluctuations Doppler reflectometry Transient events Turbulence and transport Transition edge/SOL 2. Turbulence in the W7-AS confinement region

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Is fluctuation amplitude related to transport? Turbulent transport cannot be measured directly in the confinement region Turbulence and transport in the confinement region We may still be lacking important diagnostic information (phase between quantities? small scales, e. g., ETG turbulence?) -Sometimes, yes: n e, T e amplitude is correlated with heat diffusivity for density variation (at fixed heating power) ~ ~ -Sometimes, not in the expected way: n e, T e amplitude is anti-correlated with heat diffusivity for heating power variation (similar: for variation) ~ ~

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Topics: T e fluctuations Doppler reflectometry Transient events Turbulence and transport Transition edge/SOL 2. Turbulence in the W7-AS confinement region

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Density fluctuations inside and outside the last closed magnetic surface (LCMS) no significant radial correlation across the LCMS different character of density fluctuations in edge and SOL See: S. Zoletnik et al., PoP 6 (1999) 4239, fig. 4 (from fast Li beam diagnostic)

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Transport in the scrape-off layer Definition of last closed magnetic surface (LCMS): by a limiter by a magnetic separatrix scrape-off layer (SOL) B radial transport B transport B to the target plates confinement region

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3. Turbulence in the W7-AS scrape-off layer Topics: Spatial structure of turbulence Phase between fluctuating quantities Transport Topics: Spatial structure of turbulence Phase between fluctuating quantities Transport

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Langmuir probe heads: 1 cm Diagnostics – Langmuir probes Positions of Langmuir probes in W7-AS:

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Diagnostics – H fluctuation diagnostic emissivity: n e n 0 f (T e ) only weak temperature dependence plasma lens vacuum vessel window glass fiber 16 gas valve (H 2 /D 2 ) photomultipliers filters

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Raw data from the H fluctuation diagnostic (density fluctuations) Individual “fluctuation events” are propagating in poloidal direction lifetime:several 10 µs poloidal correlation length:1–5 cm poloidal velocity:O(100)–O(1000) m/s time poloidal position 500 µs 9 cm

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Frequency spectrum (floating potential data) Auto power density spectrum arb. units 10 3 10 4 10 5 10 6 10 f [kHz] f [kHz] same, double logarithmic From: J. Bleuel et al., NJoP 4 (2002) 38, fig. 5

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Poloidal-temporal correlation function d [cm] 32101233210123 20 [µs] 0.2 grey scale (floating potential data)

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Correlation/coherency || B fl data from SOL, 6.3 m probe tip separation || B, torus outboard side cross correlation cross coherency From: J. Bleuel et al., NJoP 4 (2002) 38, figs. 20&22

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Correlation || B in W7-AS – comparison of poloidal-temporal correlation functions Correlation function between single probe tip and the tips of the poloidal array displaced by 6 m || B (at radial position of maximum correlation) Correlation function between the tips of the poloidal array From: J. Bleuel et al., NJoP 4 (2002) 38, fig. 21

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Radial-poloidal correlation function – obtained from the angular array (floating potential data) 5 different time lags: radial separation d r [cm] From: J. Bleuel et al., NJoP 4 (2002) 38, fig. 11 poloidal separation d [cm]

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Comparison with spatial structure from model calculations 3D simulation of ITG/drift wave turbulence, T e fluctuations at fixed time: From: B. Scott, Phys. Plasmas 7 (2000) 1845–1856

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Topics: Spatial structure of turbulence Phase between fluctuating quantities Transport 3. Turbulence in the W7-AS scrape-off layer

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Particles: Energy (for each species): Transport due to “electrostatic” turbulence

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Correlation and phase between different fluctuating quantities Correlation between floating potential and ion saturation current: Typically, a phase of /2... /3 between these quantities is observed, maximising transport, if fl fluctuations are considered equivalent to pl fluctuations From: J. Bleuel et al., NJoP 4 (2002) 38, fig. 8

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p0p0 Phases between n and in interchange instability r B pp Target plate j ≠ 0 due to curvature j || EE vExBvExB fl I sat fl

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Phase between n, T e and pl fluctuations n - T e n - pl T e - pl From: M. Schubert, PhD thesis, Greifswald (2005), figs. 5.24&25 (accessible through I sat - fl

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Modelling of SOL turbulence The observed phases are consistent with a drift-interchange type of turbulence The impact of the target plate boundary conditions has not yet been fully explored The changes of the phases in radial direction are not yet understood in detail

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Topics: Spatial structure of turbulence Phase between fluctuating quantities Transport 3. Turbulence in the W7-AS scrape-off layer

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Fluctuation-induced radial energy transport From: M. Schubert, PhD thesis, Greifswald (2005), fig (accessible through Observed: (6.6 ± 1.5) kW/m 2 Expected from global energy balance: 24 kW/m 2 (assuming homogeneous transport across LCMS, taking into account local flux expansion)

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Summary improving knowledge about relations between different quantities capability to observe directly the turblence-induced transport qualitative agreement with transport to be expected from global confinement Confinement region: Progress to be expected from improvement of diagnostic capabilities SOL: detailed knowledge of spatial structure of turbulence

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Tentative outlook Confinement region: high temporal & spatial resolution required problem of intensity – could progress in lasers help? combine several methods to obtain information on different quantities, or complementary information on one quantity SOL: improve advanced methods (fast sweeping of electrostatic probes?) and perform parameter studies continue detailed comparison with theory and modelling “turbulence engineering” by suitable shaping of targets or by active methods?

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