ECE Imaging for KSTAR ECE Imaging for KSTAR G. S. Yun, W. Lee, M. J. Choi, H. K. Park @ POSTECH B. Tobias, C. W. Domier, T. Liang, N. C. Luhmann, Jr. @ UC Davis T. Munsat @ Univ. Colorado, Boulder 19 th Toki Conference, Dec. 8, 2009 UC DAVIS P LASMA D IAGNOSTICS G ROUP
Abstract Fluctuating plasma quantities are important subjects of tokamak diagnostics in the context of plasma instabilities, transport, MHDs, and waves. Electron cyclotron emission (ECE) radiometry has been widely used to measure 1D radial temperature ( ) y y p profiles and fluctuations, providing useful but limited information on complex phenomena such as sawtooth crash, which involves magnetic reconnection and heat transport in 3D geometry. Recently, visualization of temperature fluctuations in 2D poloidal cross section via ECE imaging (ECEI) in the TEXTOR revealed the poloidal cross-section via ECE imaging (ECEI) in the TEXTOR revealed the physics of sawtooth crash in unprecedented detail, including the first observation of high-field-side crash [1]. This paper introduces the ECEI system being developed for the KSTAR. The KSTAR ECEI system is aimed at visualizing both high- and y g g low-field sides of q~1 flux surface simultaneously to capture the full picture of sawtooth crash dynamics. Spatial coverage and resolution required for the visualization are achieved by combining two large arrays of heterodyne detectors and imaging optics with flexible zoom and focus controls Benefits of simultaneous and imaging optics with flexible zoom and focus controls. Benefits of simultaneous measurement of high- and low-field sides for resolving the sawtooth dynamics in 3D geometry are discussed. *Work supported by NRF of Korea under contract no. 20090082507. pp y [1] H. K. Park, et al., PRL 96 , 195003 (2006).
Background • MHD fluctuations and micro-turbulence – Complex “3-D” phenomena – Often lead to radial energy transport and disruptions: major factors Often lead to radial energy transport and disruptions: major factors limiting the stability and confinement of fusion plasmas. – Difficult to control due to lack of understanding of the underlying physics. • 2-D/3-D Visualization of MHD fluctuations/turbulence is essential for physics study. – enables direct comparison with theoretical models and numerical enables direct comparison with theoretical models and numerical simulations. Visualization of Sawtooth Oscillation
Microwave Diagnostics on KSTAR • Recent advances in microwave technology enabled 2D Visualization of T e and n e fluctuations: fluctuations: – T e : Electron Cyclotron Emission Imaging ( ECEI ) – n e : Microwave Imaging Reflectometry ( MIR ) n : Microwave Imaging Reflectometry ( MIR ) • KSTAR offers a unique environment for studying the MHD and turbulence. – Fully superconducting magnets & Variety of heating mechanisms (Ohmic, ECH, ICRH, NBI) g ( ) � Long-pulse High-temperature plasma – Large-window port access for microwave imaging diagnostics. g g g
Sawtooth Oscillation (m/n=1/1 MHD instability) • Periodic growth (slow) and decay (sudden) of the core pressure of the toroidal plasma† • • Magnetic self organization of m/n=1/1 Magnetic self-organization of m/n=1/1 instability via reconnection – Stable m/n=0/1 mode in the initial stage – m/n=1/1 mode develops as the instability grows p y g • Tearing instability: slow evolution of the island/hot spot • Kink instability: sudden crash sawtooth oscillation depicted in helical coordinate system X-point Hot-spot Island † S. von Goeler et al, “Studies of Internal Disruptions and m =1 Oscillations in Tokamak Discharges with Soft X-Ray Techniques”, Phys. Rev. Lett. 33 (1974) 1201 - 1203.
Full Reconnection Model † • Helically symmetric reconnection zone (hypothesis) – Y-point reconnection process (2-D Sweet-Parker model) 1 τ ≈ τ ⋅ τ – Long reconnection time (monotonic full growth of the island) η η k k A A 2 2 – New model is inevitable because � Measured crash time << τ k � Crash without full growth of island � ICRF driven giant sawtooth and precursor-less sawtooth ICRF driven giant sawtooth and precursor less sawtooth Sykes et al. PRL, 36, 140, 1976 based on MHD fluid in cylindrical geometry † B.B. Kadomtsev, “Disruptive instability in tokamaks,” Sov. J. Plasma Phys. 1 (1976) 389.
Advances in Sawtooth Physics Understanding • Recent advances are driven by experimental observations via Kadomtsev: Full reconnection (q 0 � 1) microwave imaging di diagnostics. ti • Observation of sawtooth crash on the high field side triggered significant Ballooning model: 3-D localized crash at modification of the low-field side (q 0 < 1) physical model. h i l d l H. Park: 3-D localized crash everywhere
Reconnection (“X-point”) at Low Field Side •Sharp T e point ( #4) similar to Pressure Finger of ballooning mode mode. •Reconnection starts with “X-point” (#5) and the poloidal opening g grows to ~15 cm (#6) ( ) • Highly Collective Heat Flow in contrast to stochastic heat diffusion in Ballooning model. – Direction of heat flow is random. •Reconnection time scale < 100 μ s << τ K ~ 600 μ s.
Observation of High Field Side Crash (kink) • Localized Reconnection in the poloidal plane similar to the low field side case •A few attempts (pointed T e finger near the mid-plane) before the final puncture (#6 & #7) ( ) •Reconnection starts with a small hole and it grows up to ~10 cm (#10) • Highly Collective Heat Flow •Nested field line pushes the heat out and an island sets in (#11 and #12)
ECE Diagnostic – Local T e Measurem ent B(R) EM wave R • Electron Cyclotron Emission (ECE) : Spiraling electrons radiate EM waves at a series of discrete harmonic frequencies. • In tokamaks, a band of ECE frequency originates from a narrow resonant layer. • ECE radiation approaches the blackbody ECE di ti h th bl kb d limit for optically thick and Maxwellian plasma. • Temperature of the resonant layer,
ECE Resonant Layer • A thin layer of plasma radiates ECE at the resonant frequency and its harmonics. – For nominal KSTAR parameters, the thickness of the layer ~ 1 cm for F i l KSTAR t th thi k f th l 1 f 2 nd harmonic X-mode. = B B 2 2 T T 0 = ≈ = f 2 f 110 GHz R 180 cm ce 0 = a 50 cm = × − 13 2 - 3 n 5 10 ( 1 ( r / a ) ) cm e = × − 2 T 1 . 0 ( 1 ( r / a ) ) keV e
Profiles of Absorption Coefficient (or E Emissivity ) showing the i i it ) h i h existence of Thin Resonant Layers *color scales are arbitrary. 2 nd X-mode
ECE Imaging System – Microwave camera with zoom lens Conventional 1-D ECE Radiometer 2-D ECE Imaging System • 1-D ECE radiometer: well established tool for T e measurement • 2-D ECE Imaging system for T e fluctuation – requires imaging optics, wide-band mm-wave antenna, IF electronics, and sensitive detector array. , y – high spatial resolution, large field-of-view � 2-D correlation
KSTAR ECEI View Window (B 0 =2.0 T) High Field Side HFS LFS Low Field Side
KSTAR ECEI System II. Antenna Array I. Zoom/Focus Optics I. Zoom/Focus Optics + III. Heterodyne + III. Heterodyne Electronics
System Characteristics • Independent zoom and focus control • Wide vertical zoom range : 1—3 • Simultaneous coverage of low- and high-field sides • Temperature resolution ~ 3% of Te ~ 30 eV – thermal noise • Space resolution: – Vertical ~ 1—3 cm – Radial ~ 2—4 cm (LFS) and 1—3 cm (HFS) – 24 vertical x 8 radial channels in each array. Time resolution ~ 2 μ sec, limited by the video bandwidth B V . •
I. Triplet Zoom and Focal Optics • Independent zoom and focus control f 0 0 array array • Wide range of zoom (1~3) Wid f (1 3) (LF/HF) mirror er plasma cente ge plane f 2 f 1 f 3 f 0 2 1 imag 3 0 p array (HF/LF) y x s d - x beam splitter z=0 z z=L z z 3 z z z 2 z z 1 z 0 z
Parametric Representation: Independent Zoom and Focus green: focal position from the plasma center blue: vertical zoom factor
21 Example coverage: Wide Zoom
22 Example coverage: Narrow Zoom
Vertical Zoom in Practice (TEXTOR) TEXTOR ECEI data by T. Munsat, University of Colorado, Boulder Comparable sawtooth crashes imaged with different vertical zoom
Simultaneous Imaging of LFS and HFS • Correlation study between LFS crash and HFS crash. • Extension of cross-coherent analysis among LFS and HFS channels. HFS LFS
Simultaneous Imaging of LFS and HFS • Direct 2D visualization of core MHD perturbation structures • Smaller amplitude perturbations (<10 eV) such Alfven eigenmodes may be possible to image by integrating the ECEI signal over time. p g y g g g Tearing mode structure at DIII-D n=3 Toroidal Alfven Eigenmode M.A. Van Zeeland et al, Nucl. Fusion 48 (2008) 092002 M.A. Van Zeeland et al, PRL 97, 135001 (2006)
II. Antenna Array and LO Coupling by Ben Tobias and Calvin Domier, UC Davis • Mini lens array & Single sided • Mini-lens array & Single-sided ECE signal ECE signal coupling � High LO coupling efficiency Notch Filters • Separation of Even/Odd channels � High space Dichroic Filter resolution • Dichroic high-pass filter � Mini-Lens IF LO single-sided coupling • Notch filter � ECH rejection Array IF Mini-Lens Array
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