ECE Imaging for KSTAR ECE Imaging for KSTAR G. S. Yun, W. Lee, M. - - PowerPoint PPT Presentation

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

19th Toki Conference, Dec. 8, 2009

UC DAVIS

PLASMA DIAGNOSTICS GROUP

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 Te and ne fluctuations: fluctuations:

– Te: Electron Cyclotron Emission Imaging (ECEI) n : Microwave Imaging Reflectometry (MIR) – ne: 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

X-point

sawtooth oscillation depicted in helical coordinate system

†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.

Island Hot-spot

Full Reconnection Model†

• Helically symmetric reconnection zone (hypothesis)

– Y-point reconnection process (2-D Sweet-Parker model) – Long reconnection time (monotonic full growth of the island)

η τ τ τ ⋅ ≈ A k 2 1

– New model is inevitable because

Measured crash time << τk Crash without full growth of island ICRF driven giant sawtooth and precursor-less sawtooth

η A k 2

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

Kadomtsev: Full reconnection (q0 1)

driven by experimental

• bservations via

microwave imaging di ti diagnostics.

• Observation of sawtooth

Ballooning model: 3-D localized crash at low-field side (q0 < 1)

crash on the high field side triggered significant modification of the h i l d l physical model.

• 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 grows to ~15 cm (#6) g ( )

• Highly Collective Heat Flow in

contrast to stochastic heat diffusion in Ballooning model. – Direction of heat flow is random.

• Reconnection time scale < 100μs

<< τK ~ 600 μs.

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 Te 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
• riginates from a narrow resonant layer.

ECE di ti h th bl kb d

• ECE radiation approaches the blackbody

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. F i l KSTAR t th thi k f th l 1 f – For nominal KSTAR parameters, the thickness of the layer ~ 1 cm for 2nd harmonic X-mode.

T 2 B cm 50 cm 180 T 2 a R B = = =

GHz 110 2 ≈ =

ce

f f

keV ) ) / ( 1 ( . 1 cm ) ) / ( 1 ( 10 5

2 3

• 2

13

a r T a r n

e e

− × = − × =

Profiles of Absorption Coefficient (or E i i it ) h i h Emissivity) showing the existence of Thin Resonant Layers

*color scales are arbitrary.

2nd 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 Te measurement
• 2-D ECE Imaging system for Te 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 (B0=2.0 T)

HFS

Low Field Side High Field Side

LFS

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 μsec, limited by the video bandwidth BV.

• I. Triplet Zoom and Focal Optics

f0

array

• Independent zoom and focus

control Wid f (1 3)

er mirror array (LF/HF)

• Wide range of zoom (1~3)

plasma cente

f3 f2 f1 f0

ge plane array (HF/LF) p

3 2 1

x d - x y

imag

s

beam splitter

z=0 z z z z z=L z z0 z1 z2 z3 z

Parametric Representation:

Independent Zoom and Focus

green: focal position from the plasma center blue: vertical zoom factor

Example coverage: Wide Zoom

21

Example coverage: Narrow Zoom

22

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

M.A. Van Zeeland et al, Nucl. Fusion 48 (2008) 092002

n=3 Toroidal Alfven Eigenmode

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

ECE signal

• Mini-lens array & Single-sided

coupling High LO coupling efficiency

ECE signal Notch Filters

• Separation of Even/Odd

channels High space resolution

Dichroic Filter

• Dichroic high-pass filter

single-sided coupling

LO IF Mini-Lens

• Notch filter ECH rejection

IF Array Mini-Lens Array

Antenna Array Box

Improved Antenna Patterns

Antenna Array Designs by Xiangyu Kong, UC Davis

Old New

• Single-sided coupling allows dual-dipole antenna elements to be optimized to a

single wavelength

• Dramatically reduced side-lobes.

Dramatically reduced side lobes.

Quasi-Optical Planar Components

Notch filter design by T. Liang, UC Davis g y g,

• Inexpensive and wide-band dielectric

film beam-splitters with well-predicted p p splitting ratios

• Planar, stackable notch filters provide

better than 60 dB ECRH rejection Notch Filter Stack

Dichroic Plate High-Pass Filters

• Single-Sideband operation is

d ibl b di h i made possible by dichroic plate filters

• Rejection to -20 dB at 3 GHz

from cutoff.

• 3dB point is 500 MHz

accurate.

RF and IF Electronics

1 4 3 2 3