Comparative Study between the Reflective p y Optics and Lens based - - PowerPoint PPT Presentation

Comparative Study between the Reflective p y Optics and Lens based System for Microwave Imaging System on KSTAR Imaging System on KSTAR

• W. Lee, G. S. Yun, Y. Nam, I. Hong, J. B. Kim, H. K. Park (POSTECH),
• B. Tobias, T. Liang, C. W. Domier, and N. C. Luhmann, Jr. (UC Davis)

18th Topical Conference on High-Temperature Plasma Diagnostics, Wildwood, New Jersey, USA (May 16-20, 2010)

Contents

• Microwave Imaging Reflectometry (MIR)
• Reflective optics of the prototype TEXTOR MIR system
• Preliminary optical design of the KSTAR MIR system

based on lens system based on lens system

• Future work and Plan

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Microwave Reflectometry for Density Fluctuation

• Anomalous transport has been explained based on

the turbulent fluctuations, so that measurements of

Cutoff layer

plasma density and/or electron temperature are critical to understand the mechanism of anomalous t t transport.

• Reflections from the cut-off layers contain

information of density fluctuations near the cut-off information of density fluctuations near the cut off.

• Reflectometry is very sensitive to small density

variation.

• Reflectometry is a good tool for measurement of

density fluctuations as well as density profiles.

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Microwave Imaging Reflectometry

• 1-D fluctuations: simple mirror-like interpretation
• 2-D fluctuations: the received signal is corrupted by interference from multiple ref

lected waves

• Microwave imaging reflectometry with large aperture optics can restore phase

fronts of reflecting layer. g y Cutoff layer is imaged onto array of detectors

1-D fluctuation

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2-D fluctuation

Curvature Matching

• Wave front curvature of probe beam

matches to that of the curvature of

• Cut-off surface is imaged onto the

array of detectors, eliminating

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cut-off surface for optical robustness. y g interference effects.

Prototype TEXTOR MIR System

Reflector E-plane mirror Source Detector Beam splitter

A 88 GH b b d i li d

H-plane mirror 45deg mirror

• A 88-GHz probe beam and a specialized

horn make a Gaussian beam.

• Reflected beam is focused onto16-element

Reflector having linear array of detector

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Reflector having corrugation

Reflective Optics of the TEXTOR MIR System

• Optical system including

two large focusing mirrors matches the probe beam

E-plane focusing mirror Beam splitter Detector

matches the probe beam curvature to that of the reflector. Th t f i i

Source Reflector (Rc = 315 mm)

• The two focusing mirrors

are tilted 30 degrees.

• The optical system for the

H-plane focusing mirror Steering mirror Lens set

Gaussian beam launched to reflector

Image Focusing lens

reflected waves is to form an image of the cut-off layer onto a detector.

Reflector or cut-off layer

• Due to the tilting angle of

the mirrors, the standing wave effect can be

Reflected beam i d t d t t

minimized.

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imaged onto detector

Probe Beam Characteristics

Measured

• 1. Source beam

before the optics

• 2. Probe beam after the optics

z=4012mm z=4512mm z=4862mm

• Source beam profile measurement: (a)

2-D profile, (b) its vertical profile, and (c) beam radii along the axial position

• Probe beam is nearly a Gaussian beam

90 mm

Calculated

• Probe beam profile near the

reflector position.

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• Probe beam is nearly a Gaussian beam

(waist size = 11 mm). e ec o pos o

• Focal position is ~ 4900 mm

Reflected Beams

• Reflected beams from few reflectors such as flat mirror and cylindrical mirrors

(radius of curvature = 300, 1200, 1300) were measured to verify the beam size at the detector plane. Position = 2460 ± 50 mm

• 50 mm

+50 mm 0 mm Reflection from flat mirror

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Characteristics of Reflective Optics

• One important issue in the microwave diagnostics is a standing wave effect

between surfaces of optical elements. The standing wave effect can be minimized in the reflective optics due to the relatively large tilting angle of mirrors.

• However, the tilting angle generates aberration such as coma. And the vertical

center position of the probe beam is also lower than the beam axis.

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A little skewed beam shape at the reflector plane. Beam center is -5 ~ -3 mm

Cutoff Layers for O-mode and X-mode Waves

• Dispersion relations of O-wave and X-wave:

2 2 2

ω k c

2 2 2 2 2k

c ω ω ω −

(O-wave), (X-wave)

• A probing wave is launched into the plasma along the density gradient and

2 2

1 ω ω ω

pe

k c − =

2 2 2 2 2

1

ce pe pe pe

k c ω ω ω ω ω ω ω ω − − − =

• A probing wave is launched into the plasma along the density gradient and

reflected at the cutoff layer where the electron density equals a critical value.

• Cutoff frequencies of O-wave and X-wave:

(O-wave), (X-wave)

2

ε ω ω

e e pe

m e n = =

( )

[ ]

2 / 1 2 2

4 2 1

pe ce ce R

ω ω ω ω + + + =

The cutoff frequency of X-wave depends on magnetic field as well as plasma density.

e

11 POSTECH

Cut-off Frequency on KSTAR Plasmas

(a) Magnetic field and density profiles

• f KSTAR-like plasma

Contour of O-mode cutoff frequencies Contour of X-mode cutoff frequencies

(b) Characteristic frequencies

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cutoff frequencies cutoff frequencies

Preliminary Optical Design of the KSTAR MIR

Zoom lens set (shared with ECEI) Beam splitter Focusing lens set

(a)

Gaussian beam trace analysis of the probe beam for curvature matching to the reflector surface

( ) Source Beam splitter Reflector (Rc = 421 mm) g Detector Vacuum window

(b)

Beam splitter

Ray trace analysis of the reflected beam to determine the image position the image position

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Characteristics of Lens Based Optics

• The probe beam and reflected beam always reflect

in the surfaces of the lenses, it generates interference effect by the standing wave. The off- axis reflection (by the surface curvature) is no l l th t i i th th t th i l longer along the transmission path, that the ripple ratio by the standing effect is reduced to about 1.7%.

• Relatively large curvature of the lens,

especially the third zoom lens, makes especially the third zoom lens, makes aberration such as spherical aberration.

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Future Work and Plan

• First year, two-frequency (88 and 94 GHz) microwave source will be used as the

probe beam, that will cover the radial position (r/a) from 0.4 to 0.8 for KSTAR plasmas of 3.0 ~ 3.5 T.

• 20 vertical channel will cover ~ 20 cm vertical space with the resolution of 1 cm,

and ultimately the five-frequency source enables us to measure five discrete cut-

• ff layers, simultaneously.
• For the TEXTOR MIR system so far we measured the intensity of the reflected
• For the TEXTOR MIR system, so far, we measured the intensity of the reflected

beams from the smooth surface reflectors. The phase measurement is required for the reflectors with corrugated surface. The optics of the reflected beam will be able to be verified with the phase measurement.

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