Supported by Experimental studies of ion-scale fluctuations via microwave imaging reflectometry in KSTAR 5 th EAST-Asia School and Workshop (EASW) on Laboratory, Space, Astrophysical Plasma (POSTECH, Pohang, August 17-22, 2015) Woochang Lee, H. K. Park (UNIST), J. Leem, J. A. Lee, M. J. Choi, G. S. Yun (POSTECH), S. H. Ko, K. D. Lee, W. H. Ko, (NFRI), R. V. Budny, W. Wang (PPPL), Y. S. Park (Columbia U.), K. W. Kim (KNU), C. W. Domir, N. C. Luhmann, Jr. (UCD)

UNIST Contents Microwave imaging reflectometry (MIR) system on KSTAR –2-D measurement of electron density fluctuation in the poloidal cross section –Ion gyro-scale turbulence study Fluctuation measurements in ohmic, L-mode, and H-mode plasmas –Spectral analysis –Correlation analysis Estimation of the ExB flow velocity and radial electric field in NB heated L-mode plasmas –Poloidal velocity of fluctuation pattern –ExB flow velocity from the poloidal pattern velocity EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 2

UNIST Fluctuation measurement for turbulence study Turbulent fluctuations induced by micro-instabilities (ITG, TEM, ETG, MTM, …) are responsible for anomalous heat and particle transport in fusion plasmas. –Turbulence is unavoidable in high pressure plasmas for fusion reaction Accurate measurements of fluctuations (of temperature and density) and profiles (of temperature, density, rotation, current, and electric field) are important for micro turbulence and transport studies. –Fluctuations: ECE, reflectometry, BES, collective scattering system, … –Profiles: ECE, reflectometry, Thomson scattering, charge exchange recombination spectroscopy, MSE, soft X-ray, … 2-D/3-D visualization of turbulent structure is essential to understand micro turbulence. –3D nature of fluctuation (k r, k θ, and k φ ) EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 3

UNIST Microwave reflectometry for ne fluctuation EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 4 Incoming wave is reflected at a critical density layer. –This layer is called the cut-off layer. Reflected waves contain information of the shape of the cut-off layers. Fluctuating phase of the reflected wave is linearly proportional to the fluctuating electron density where k 0 is the probe beam wave number, is plasma permittivity. R. Nazikian et al., POP 8, 1840 (2001)

UNIST Why imaging reflectometry? The interpretation of the fluctuating phase is straightforward in 1-D fluctuation. Multi-dimensional fluctuations can lead to interference in the phase front. –Interference is stronger in shorter wavelength and larger amplitude fluctuations So, conventional reflectometry has limitation. –It works only on long wavelength and small amplitude fluctuations Imaging with an optics can restore phase front. –Imaging optics collects scattered beams and form cut-off layer images at the detector plane. EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 5 1-D fluctuation T. Munsat et al., PPCF 45, 469 (2003) k θ = 1.25 cm -1, Δφ = 2 π k θ = 1.25 cm -1, Δφ = 4 π

UNIST Microwave imaging reflectometry (MIR) EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 6 With the imaging optics, detector collects diffracted beams from a small area of the cut-off layer and reconstruct the phase Concept of MIR system

UNIST KSTAR MIR system ( ) EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 7 Channel: poloidal 16 and radial 2 (4) Tunable probing frequency: –86 – 96 GHz (until 2013) –78 – 96 GHz (from 2014) Poloidal wave number –k  = up to 3 cm -1 (available for Ion gyro-scale turbulence structure measurement) Poloidal spatial resolution: –spot size: ~ 1.6 cm (1/e 2 width) –channel spacing: ~ 0.6 cm Radial coverage: 0 < r/a < 0.8 (0.9) Temporal resolution: μs W. Lee et al., JINST 8, C10018 (2013) ~9 cm ~ 5 cm W. Lee et al., NF 54, (2014) Imaging optics Source Detector array

UNIST Spectra of fluctuations in ohmic and L-mode EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 8 r/a ~ 0.6

UNIST Coherent and quasi-coherent modes in ohmic phase A quasi-coherent (QC) mode (f ~ 22 kHz) is observed in low-density phase. –The QC-mode disappears in high-density phase. Coherent modes (f ~ 2 kHz and 22 kHz) are also observed. –It is not clear yet what these coherent modes are. EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 9 QC-modeCoherent mode (22 kHz) Coherent mode (2 kHz)

UNIST Spectra of fluctuations in L- and H-mode EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 10 r/a ~ 0.6 (L) r/a ~ 0.9 (H)

UNIST 25 kHz coherent mode in L-mode A 25 kHz coherent mode is observed at r/a ~ 0.6 in L-mode. –It is stronger in low-density phase. –It disappears in high-density L-mode and H-mode. –It is only observed in the negative frequency in all poloidal channels. –It is electrostatic EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 11

UNIST Summary of spectral analysis The spectrum width seems to be proportional to the plasma flow velocity. Low-frequency (~2 kHz) coherent mode is observed only in ohmic plasma. QC-mode (f ~ 22 kHz) is observed in low-density ohmic plasma. Coherent mode is observed at in ohmic (f ~ 22 kHz) and low-density L- mode (f ~ 25 kHz) plasmas. –The coherent mode in NB heated L-mode appears in the negative frequency for all poloidal channels. –This mode is observed at r/a ~ 0.6 but not observed at r/a ~ 0.4. –This mode is not observed in NB+ECH heated L-mode plasmas. EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 12

UNIST Correlation analysis Plasma fluctuations are correlated in a close space. –However, radiation or electrical noises are uncorrelated. Correlation techniques can provide useful information. EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 13 W. Lee et al., NF 54, (2014)

UNIST Images of density fluctuation patterns EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 14 Ohmic plasma (#8224), r/a ~ 0.5 NB heated L-mode plasma (#9010), r/a ~ 0.7

UNIST Poloidal motion in the laboratory frame EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 15 v θ * of fluctuation were obtained by the time delayed cross correlation. –+1.9 km/s (upwards or electron diamagnetic direction) in ohmic plasma –-5.5 km/s (downwards or ion diamagnetic direction) in NB heated L-mode plasma Ohmic plasma (#8224) NB heated L-mode plasma (#9010) r/a ~ 0.5 r/a ~ 0.7 W. Lee et al., JINST 8, C10018 (2013)

UNIST Poloidal velocity of fluctuation pattern, v θ * EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 16 Poloidal motion of fluctuation patterns measured by MIR (or ECEI or BES) is not a poloidal motion of turbulence or poloidal plasma flow. Instead, it is an apparent motion. The turbulence in the plasma, which flows toroidally and poloidlly, appears to move in the poloidal direction in the laboratory frame: (1) Modified version of Fig 2 in reference of Y.-c. Ghim et al., PPCF 54, , (2012). Poloidal velocity by “- U φ tanα”

UNIST ExB flow shear on turbulence Reduced turbulence and transport in transport barriers are associated with the ExB flow shear. –The ExB flow shear is a well known turbulence suppression mechanism. Accurate measurements of the ExB flow velocity (or radial electric field) and corresponding radial gradient are important in understanding of the turbulence behavior. The most widely used method is to measure toroidal and poloidal rotation velocities and density of impurity ion (C6+) or main ion (D+) –toroidal and poloidal charge exchange recombination spectroscopy (CES) –evaluate the radial electric field from the radial force balance relation A new method for direct measurement of the ExB flow velocity is based on the poloidal velocity of fluctuation patterns. EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 17

UNIST The perpendicular turbulence velocity also can be decomposed into –the perpendicular flow velocity (U ┴ ) and (note ) –the phase velocity of turbulence in the perpendicular flow frame (ω / k ┴ ) (2-2) Two phase velocities are related to each other as (3) –Since (this is generally true), the direction of ω / k ┴ is always the electron diamagnetic direction (or upward). Perpendicular velocity of turbulence, v ┴ * The perpendicular velocity of turbulence in the lab. frame (v ┴ *) can be decomposed into (K. L. Wong et al., Phys. Lett. A 236, 339 (1997)) –the equilibrium ExB flow velocity (U ExB ) and –the intrinsic phase velocity of turbulence in the ExB flow frame (ω 0 / k ┴ ). (2-1) EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 18

UNIST ExB flow velocity from poloidal pattern velocity Two velocities in Eqs. (1) and (2-1) are related to each other as (4-1) since pitch angles are often small (α < 7 deg). This equation can be rewritten as (4-2) If the intrinsic phase velocity is much smaller than the ExB flow velocity or poloidal pattern velocity (i.e., ω 0 / k ┴ << U ExB or v θ *), then (4-3) If not, we have to obtain the intrinsic phase velocity by simulation. EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 19 NBI only NBI+ECH Electron mode Ion mode NBI+ECH NBI only ω 0 / k θ ~ 0.4 km/s Linear GYRO simulation for 9009 by S. H. Ko (NFRI)

UNIST ExB flow velocities in NBI L-mode plasmas Five L-mode discharges heated by 1.43 MW neutral beam were analyzed. –Bt = 3.0 T (3.3 T for one shot) and Ip = 600 kA (700 kA for another shot) EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 20 U ExB ~ v θ * = 5 – 10 km/s (in the ion direction) –ω 0 / k ┴ < +/- 0.4 km/s W. Lee et al., will be submitted

UNIST Modelling of an L-mode plasma using GTS EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 21 MIR measurement positions (r/a = 0.6 – 0.7) δphi / phi Shot 9010 has been modelled with a non-linear gyrokinetic simulation code GTS (in collaboration between UNIST and PPPL). –GTS modelled shot 9010 for ~ Alfven times (the non-linear saturation phase starts from ~19000 Alfven times) with sufficient spatial grid. –The collisionless TEM (CTEM) turbulence occurs at R = 2.1 – 2.2 m –In the saturated phase, k θ ~ 3 cm -1, which is consistent with that from the GYRO calculation. –From GYRO calculation, turbulence are most unstable at k θ ρ s ~ 0.4 GTS simulation for an L-mode plasma (#9010) by R. Budny and W. Wang (PPPL).

UNIST Comparison of v θ * The measured v θ * was compared with that from GTS simulation (in the saturated phase). –There are ~ 1 km/s difference between MIR and GTS. –Turbulence effect on ExB velocity? EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 22 W. Lee et al., will be submitted

UNIST Summary of correlation analysis Poloidal correlation lengths of measured fluctuations are L θ ~ 1 cm. –Measured fluctuations are ion gyro-scale since L θ / ρ i ~ 5. Poloidal velocity of fluctuation patterns were obtained by the time-of-flight method. –The time-of-flight was determined using a time delayed cross correlation analysis. A method for direct measurement of the ExB flow velocity was investigated based on the poloidal velocity of fluctuation pattern. –The measured poloidal velocities of fluctuation patterns are good representation of the ExB flow velocities if the intrinsic phase velocity of the dominant turbulence is significantly smaller than the poloidal pattern velocity. EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 23

UNIST Backup EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 24

UNIST Low-frequency coherent mode in another ohmic plasma EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 25 Coherent mode (1 kHz) B = 2.7 T, = 0.6 x 10 19, r/a ~ 0.3

UNIST Coherent mode in NB heated L-mode plasmas EASW 2015 (POSTECH, Pohang, August 17-22, 2015) 26 r/a ~ 0.6 r/a ~ 0.4 r/a ~ 0.6 r/a ~ 0.4 r/a ~ 0.7 r/a ~ 0.6 r/a ~ 0.5 r/a ~ 0.6 r/a ~ 0.4