Microwave Imaging and Visualization Diagnostics Developments for the

Study of MHD and Microturbulence

N.C. Luhmann, Jr. University of California at Davis

Work performed in collaboration with I.G.J. Classen, C.W. Domier, A.J.H. Donné, R. Jaspers, X. Kong, T. Liang, A. Mase, T. Munsat,

H.K. Park, Z. Shen, B.J. Tobias, M.J. van de Pol

The First International Workshop on “Frontiers In Space and Fusion Energy Sciences (FISFES)", National Cheng Kung University (NCKU), Tainan, Taiwan, November 6-8, 2008

T E C

Energy Transport in High Temperature Toroidal Plasmas

Dynamics of high temperature plasmas in toroidal devices is complex– Transport physics– Empirical scaling– Magnetic islands,

reconnection, transport barriers, turbulence, zonal flows, etc.

Need first principles based physics to successfully develop fusion energy– Advances in theoretical and

computational understanding

– Eliminate empirical scaling– Advanced diagnostics to

connect the theoretical models to reliable scale-up

ITER

Need to Measure Turbulent Fluctuations with

Good Spatial and Temporal Resolution • Important turbulence parameters for measurement

- correlation length c

- correlation time c

- density, potential, temperature fluctuation levels - velocity fluctuations (self regulation)

• Simple Random Walk Estimate: Diffusivity D c2 / c

Outstanding questions in fusion science • Is there a correlation between eddy size, fluctuation level and confinement? • What controls the turbulent scale length in fusion plasmas?

Small eddiesLow transport

Large eddiesHigh transport

c

Classical Fluids-Von Karmen Vortices

•Electron Cyclotron Emission Imaging

•Microwave Imaging Reflectometry

•Collective Scattering

•Interferometry and Polarimetry

•Supporting MMW and THz Technology Areas

UC Davis Millimeter Wave Plasma Diagnostics Program

Electron Cyclotron Emission (ECE) • Electron gyromotion results in Electron Cyclotron

Emission (ECE) at a series of discrete harmonic frequencies: ωn =nωce

• In an optically thick plasma, the ECE radiation intensity is the black body intensity (Rayleigh-Jeans Region):

• In tokamak plasmas, there is a one to onemapping between frequency and radial positiondue to 1/R dependence of magnetic field B.

ωce B 1/R

• ECE has become a standard techniqueto measure Te profiles and fluctuations inmagnetic fusion plasmas

B

R

ECEωce

2( ) ( )B eI I T

• In conventional 1-D ECE radiometry, a single antenna receives all frequencies. In ECEI, a vertically aligned antenna/ mixer array is employed as the receiver.

• Advantages: high spatialand temporal resolution,2-D correlation.

• Real time 2-D imaging using wideband IF electronics and single sideband detection (16×8=128 channel system installed on TEXTOR; Two 24×48=1152 channel systems toroidally separated envisaged for KSTAR).

• Real time fluctuations can be studied down to ~1% level.

2-D ECE Imaging (ECEI)

• Measures the electron temperature in 2D• 2nd harmonic X-mode ECE radiation intensity• First 2D ECE Imaging diagnostic on TEXTOR

very successful– 8 x 16 observation volumes– High spatiotemporal resolution– Tuneable

• Detailed measurements of the 2D structure of instabilities

ECE ImagingECE Imaging

xo

ECEI

TS

1D ECE

.

ECRH

xo

ECEI

TS

1D ECE

.

ECRH

Experimental TEXTOR setup for magnetic island heat transport study (left), and measured Te profile in this study as determined by Thomson scattering (right).

ECEI ApplicationsECEI ApplicationsIslands and NTMsIslands and NTMs

ELMsELMs

Sawtooth crashSawtooth crash

H.K. Park et al.

G.W. Spakman et al.

Te fluctuationsTe fluctuationsSpatial resolution 1.5 cm in all directions; fluctuations must be larger

Fluctuation amplitude generally much smaller than thermal noise level:200 kHz sampling: 1.7%1 MHz sampling: 3.7%2 MHz sampling: 5.3%

So: Correlation needed: only time averaged behavior survivesThe smaller the amplitude, the longer the needed integration time.1s integration at 1MHz: noise level 0.1%

Cross correlation between different ECEI channels in poloidal and radial directions.

IF

V

R

R

B

BN

tT

tdT4/1

2

)(

)(

Double Downconversion Approach (1)

A characteristic frequency plot forthe TEXTOR tokamak (BT=2.1 T) is shown left, showing 2nd and 3rd harmonic X-mode ECE spanning94 GHz to >160 GHz

Mixers

Detectors

Antennas

Mixers

LP FiltersLO

LOnNotch Filter

ADCs

PlasmaOptics

Dichroic Plate

LO1

90

100

110

120

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150

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140 160 180 200 220

Fre

qu

en

cy (

GH

z)

Major Radius (cm)

3fC

2fC

fECRH

Double Downconversion Approach (2)

Quasi-optical notch filter prevents transmission of a narrow band of frequencies to protect against stray 140 GHz ECRH

Mixers

Detectors

Antennas

Mixers

LP FiltersLO

LOnNotch Filter

ADCs

PlasmaOptics

Dichroic Plate

LO1

90

100

110

120

130

140

150

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140 160 180 200 220

Fre

qu

en

cy (

GH

z)

Major Radius (cm)

3fC

2fC

fECRH

Double Downconversion Approach (3)

Dichroic plate ensures single sideband operation: effect offcutoff = 110 GHz plate shown left

Mixers

Detectors

Antennas

Mixers

LP FiltersLO

LOnNotch Filter

ADCs

PlasmaOptics

Dichroic Plate

LO1

90

100

110

120

130

140

150

160

140 160 180 200 220

Fre

qu

en

cy (

GH

z)

Major Radius (cm)

3fC

2fC

fECRH

Double Downconversion Approach (4)

Antennas receive broadband ECE, downconvert by fLO (at or near fcutoff), and amplified by low noise amplifiers: example shows case of fLO=110 GHz combined with 2-20 GHz amplifiers

Mixers

Detectors

Antennas

Mixers

LP FiltersLO

LOnNotch Filter

ADCs

PlasmaOptics

Dichroic Plate

LO1

90

100

110

120

130

140

150

160

140 160 180 200 220

Fre

qu

en

cy (

GH

z)

Major Radius (cm)

3fC

2fC

fECRH

Double Downconversion Approach (5)

Downconverted 2-20 GHz signals are split into n bands and downconverted a second time by frequencies fLO1 through fLOn in the 2-8.4 GHz range: shown left are two such channels

Mixers

Detectors

Antennas

Mixers

LP FiltersLO

LOnNotch Filter

ADCs

PlasmaOptics

Dichroic Plate

LO1

90

100

110

120

130

140

150

160

140 160 180 200 220

Fre

qu

en

cy (

GH

z)

Major Radius (cm)

3fC

2fC

fECRH

f (GHz)2.4 8.0

Double Downconversion Approach (6)

Final step is to lowpass filter the n band signals, reducing the radial spot size and providing sharp band edges suitable for cross correlation studies

Mixers

Detectors

Antennas

Mixers

LP FiltersLO

LOnNotch Filter

ADCs

PlasmaOptics

Dichroic Plate

LO1

90

100

110

120

130

140

150

160

140 160 180 200 220

Fre

qu

en

cy (

GH

z)

Major Radius (cm)

3fC

2fC

fECRH

f (GHz)2.4 8.0

TEXTOR Study of “Sawtooth Oscillation”

• ECEI allows direct comparison between simulation and experimental data

• Time evolution of the island and m=1 mode based on the “full reconnection model” (Kadomtsev) agrees well with the measurement except the crash time

H.K. Park et al., Physical Review Letters 96, 195003 (2006). H.K. Park et al., Physical Review Letters 96, 195004 (2006).H.K. Park et al., Physics of Plasmas 13, 055907 (2006).

Everything.wmv

TEXTOR Study of “Sawtooth Oscillation”

ECEI demonstrated “random 3-D reconnection zone,” in which the reconnection zone has been observed to occur everywhere (including high field side, see video left)

Magnetic Island Evolution under ECRH

• High power ECRH employed to suppress m/n = 2/1 tearing modes

–Tearing modes induced by 1 kHz dynamic ergodic divertor (DED)

–Modes suppressed by depositing 400 kW, 140 GHz ECRH on the same minor radius as tearing mode

• Tearing mode evolution observed by ECEI

–Known DED frequency enables reconstruction

–Time history mapped poloidally

–Represents Low Field Side geometry

One frame every rotation period (2 ms)Total movie length = 200 ms

Classen, et al. “Effect of Heating in the Suppression of Tearing Modes in Tokamaks,” Physical Review Letters, 98, 2007.

Ongoing Advances: Next Generation ECEI

• Miniaturized substrate lens or mini-lens

• Front-side LO illumination

• New antennas with increased sensitivity and bandwidth

• New notch filters with enhanced ECRH rejection

• Vertical zoom capability

• Horizontal zoom capability

FiltersFilters

Upper side bandLower side band

LO

0 2.4 8-2.4-8F [GHz]

Different filter for each LO frequency: Remote controlled filter changer

Dichroic plates for lower side band rejection

Notch filter for 140 GHz ECRH protection

-30

-25

-20

-15

-10

-5

0

110 115 120 125 130 135 140 145

Tra

nsm

issi

on

(d

B)

Frequency (GHz)

New Antennas and Array (air side) Measured LO Field Intensity

Compared to the Mini-Lens Array

ECEI Array

Beamsplitter

Notch Filters (3)

New Tandem

Notch Filters

Next Generation ECEI

ECEI Array

LO Source

Zoom Control Lenses

Focal Plane Translation Lens

Beamsplitter

New Mini-Lens ECEI System OpticsNotch Filters (3)

Independent Vertical ZoomExperimental Verification

Narrow Zoom Configuration is obtained providing minimum spot sizes and approximately 20 cm of total plasma coverage in the focal plane. In the Wide

Zoom Configuration, coverage is increased to 33 cm. Spot sizes (2*w0) in either transverse plane scale with plasma coverage, or image height (I). Depth of field (zR) scales with the square of plasma coverage. Choice of focal plane position is

independent of the zoom configuration!

Ver

tical

pos

ition

rel

ativ

e to

Tok

amak

Mid

-Pla

ne

(mm

)

Tokamak Minor Radius (mm)

200 , wzwI R

50 cm Translation of Focal Position Demonstrated at 120 GHz in Wide Vertical

Zoom Configuration

Tokamak Minor Radius (mm)

Ver

tical

Pos

ition

rel

ativ

e T

okam

ak M

id-P

lane

(m

m)

New horizontal zoom capability via upgraded RF boards

– RF spacing control, selectable between 500 and 900 MHz on a module-by-module basis

– 500 MHz spacing:3.6–8.0 GHz coverage

– 900 MHz spacing:2.0–9.2 GHz coverage

New Horizontal Zoom Capability

7.85GHz

8.8 GHz

2.4 GHz

3.3GHz

4.2 GHz

5.1 GHz

6.0 GHz

6.9 GHz

12 dB

12 dB

12 dB

6.7 GHz LPF

0-31.5 dB

DigitalAttenuator

3.8 GHz HPF

Power divider

7.05GHz

7.6 GHz

4.0 GHz

4.5GHz

5.0 GHz

5.5 GHz

6.0 GHz

6.5 GHz

Mixer VCO

TEXTOR: High and Low Field Composites

Demonstration of New “Zoom” Capability

The Future of ECEI is Bright!

• ECEI systems are installed and operating on TEXTOR, HT7, and LHD

• ECEI systems are envisaged for ASDEX-UG, DIII-D, KSTAR, EAST and HL-2M, with design and fabrication underway for many of these devices

• Capabilities of ECEI continue to grow in terms of both resolution and plasma coverage

ASDEX-UG

• TEXTOR system to transfer to ASDEX-UG in Jan. 2009

• Initially employ TEXTOR array and electronics, to be replaced later with new horizontal zoom electronics

DIII-D• System design and

development have commenced

• Employ both horizontal and vertical zoom control with full remote capability

• Two array system

– High field side, 100-130 GHz, 16×8 expandable to 16×24

– Low field side, 82-104 GHz, 16×8 expandable to 16×24

• First results on DIII-D anticipated in Fall 2009

ECEI ECEI

NOVA calculated Te perturbation for n=3 RSAE (left), and n=3 TAE (right) modes for DIII-D discharge 122117 [from M.A. Van Zeeland et al., PRL 97, 135001 (2006)]. Envisioned ECEI coverage is shown in yellow. Te perturbation amplitude (in eV) shown to right of each figure.

Interference with 3rd harmonicECE may limit viewing here

DIII-D Coverage

Steady State Devices: KSTAR• KSTAR system design under US-KSTAR

collaboration program

• Initial 2 T operation: Two toroidally separated ECEI systems. Both with dual array configuration for simultaneous low and high field measurement capability– 4x 1152 channels

• (3.0-3.5 T) operation: In-vacuum mirrors to minimize window area and heat load, and maximize coverage– Simultaneous ECEI and MIR

• ECEI (beams shown in red) employs 4in-vacuum mirrors, with additional optics positioned outside the window

ECEI on KSTAR: Plasma Coverage

Narrow zoom coverage

Wide zoom coverage

KS

TA

R C

as

sett

e

Window

ECEI on KSTAR: Plasma Coverage

Narrow zoom coverage

Wide zoom coverage

KSTAR CassetteWindow

Zoom control optics

Translation stages

KSTAR ECEI: Top View

Plasma

Focal lenses

Vacuum window

Zoom lenses

Beam splitter

H plane lenses

CassetteLow field array

High field arrayMirror

ECEI on ITER ??

• Suggestion by Alan Costley at EC-15 to examine the possibility of employing the KSTAR mirror approachon ITER

• ECRH steering mirrors envisaged for ITER have sizes of 230 × 196 mm

• ECEI using similar sized mirrors may be possible

Need for Microwave Imaging Reflectometry (MIR)

1-D fluctuations: straightforward interpretation

2-D fluctuations: Interference when observing beyond the diffraction distance

Imaging can restore phase front!

What is Microwave Imaging Reflectometry?Microwave reflectometry is a radar technique similar to ionospheric sounding, employed here for density fluctuation detection

c dr

02 2

θ

2

1 σ Δ

kD

k

when

kr k0(k0 Ln)1/3

Effect of Fluctuations on 1-D Reflectometry

• Reflectometer signals (here TFTR) corrupted by interference from reflected wave components

• Power spectrum and amplitude distribution verify randomized interference pattern

TFTR microwave signal phase plots

E. Mazzucato, et al., Phys. Rev. Lett. 77, 15 (1996)

Weak turbulence

(clean signal)

Strong turbulence burst

(distorted signal)

Spectral information lost for strong turbulence case

Microwave Imaging Reflectometry (MIR)

Probing beam illuminates extended region of cutoff layer

Curvature of the illuminating beam matched to that of the cutoff surface (toroidal and poloidal) for optical robustness

Cutoff layer imaged onto detector array (3 example points shown), eliminating the interference effects of multiple reflections

Detection system shares the same plasma-facing optics

Millimeter Wave Imaging Combined ECEI/MIR

MIR System ConfigurationCombined ECEI/MIR System The frequencies

for ECEI and MIR systems are close but separable. ECEI and MIR share same optics and window They are separated by a dichroic plate

TEXTOR Combined ECEI/MIR SystemECEI and MIR share two front-end optics and window

Mesh beamsplitter separates the ECEI and MIR signals

Dual dipole antenna arrays are used for both ECEI and MIR

ECEI/MIR optics are designed to minimize image spot size

Video Amps

IF Amp I-Q Mixer

Antenna

Mixer

Filters

LO

DACs

PlasmaOptics

Beam Splitter

Toroidal Mirror

Window

MIR ArrayLOSource

Illumination Source

Plasma

Poloidal Mirror

MIR Electronics

MIR - System Overview

Characterization of MIR system • Known corrugated

reflectors used to characterize complete MIR system response to range of k and n/n, and to compare performance of 1-D and imaging techniques

• Surface corrugation precisely measured with Leica “Laser Tracker” visible interferometer, used as reference for measurements

Test results of MIR system (laboratory)

Blue curve is measured reference

k = 1.25 cm-1 2 ( ñ/n 0.3%)

•1-D System

d=10 cm

•1-D System

d=30 cm

•Imaging System

d at image focus

• 1-D system correlation near unity for d<20 cm, decorrelated as d~30 cm

• MIR system near unity in focal range, falls off beyond ‘depth of field’

• Amplitude modulation suppressed near focus in both systems

Analytic model (1-D system) precisely duplicates data

into focus back out of focus

TEXTOR Quadrature (I-Q) Signals

unfocused spectrum focused spectrum

Complex field amplitude from the prototype TEXTOR MIR system as the cutoff layer is swept through the focal plane of the imaging optics.

out of focus

Initial TEXTOR MIR Results

Multichannel TEXTOR MIR Data

Further lab tests of MIR system for robust operation

• MIR system has been applied to the plasma measurement– Curvature matching condition from plasma cut-off layer is not as

sharp as expected from infinite conductivity assumption of modeling

– Correlation length based on phase information is not consistent with that based on amplitude of reflected waves (inherent conventional reflectometry problem)

• MIR system sent to POSTECH to understand the issues that we learned from plasma application – Fundamental difference between plasma cut-off and perfect

reflector: dielectric multi-layer reflector versus metal surface.– Doppler reflectometry shares the same fundamental problem of

the conventional reflectometry.• Extensive laboratory tests will be conducted with

simulation study– 1.5D and/or 3D EM simulation (PPPL) will be compared with

laboratory test to clarify the outstanding issues.

Multi-frequency Illumination for 2-D turbulence

• A simultaneous “comb” of illumination frequencies can probe multiple cutoff layers, as each distinct frequency reflects from a distinct cutoff layer

• Measurement of multi-layer turbulence flow such as “zonal flow” in the core of tokamak plasma

Schematic illustration of the principles governing Doppler reflectometry

2-D simulations of microwaves reflected from a circular plasma, with an illumination beam curvature-matched to the plasma applicable to MIR and synthetic imaging

Zonal flow 2-D reflectometer (57-61 GHz) simulations of a circular plasma with imposed poloidal flow velocity marked by solid lines.

Thank You for Your Attention