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) 18 th 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 Center for Fusion Plasma Diagnostic and Steady State Operation, POSTECH 2
Microwave Reflectometry for Density Fluctuation • Anomalous transport has been explained based on Cutoff layer the turbulent fluctuations, so that measurements of plasma density and/or electron temperature are critical to understand the mechanism of anomalous t transport. t • 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. Center for Fusion Plasma Diagnostic and Steady State Operation, POSTECH 3
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 2-D fluctuation Center for Fusion Plasma Diagnostic and Steady State Operation, POSTECH 4
Curvature Matching • Wave front curvature of probe beam • Cut-off surface is imaged onto the matches to that of the curvature of array of detectors, eliminating y g cut-off surface for optical robustness. interference effects. Center for Fusion Plasma Diagnostic and Steady State Operation, POSTECH 5
Prototype TEXTOR MIR System E-plane mirror Reflector Source Detector Beam splitter 45deg mirror H-plane mirror • A 88 GH A 88-GHz probe beam and a specialized b b d i li d horn make a Gaussian beam. • Reflected beam is focused onto16-element Reflector having Reflector having linear array of detector corrugation Center for Fusion Plasma Diagnostic and Steady State Operation, POSTECH 6
Reflective Optics of the TEXTOR MIR System • Optical system including E-plane focusing mirror two large focusing mirrors Detector Beam splitter matches the probe beam matches the probe beam curvature to that of the reflector. Reflector Source (Rc = 315 mm) • The two focusing mirrors Th t f i i Lens set are tilted 30 degrees. Steering H-plane Gaussian beam mirror focusing mirror • The optical system for the launched to reflector reflected waves is to form Image an image of the cut-off Focusing layer onto a detector. lens Reflector or • Due to the tilting angle of cut-off layer the mirrors, the standing Reflected beam wave effect can be i imaged onto detector d t d t t minimized. Center for Fusion Plasma Diagnostic and Steady State Operation, POSTECH 7
Probe Beam Characteristics 1. Source beam 2. Probe beam after the optics before the optics Measured z=4012mm z=4512mm z=4862mm • Source beam profile measurement: (a) 90 mm Calculated 2-D profile, (b) its vertical profile, and (c) • Probe beam profile near the beam radii along the axial position reflector position. e ec o pos o • Probe beam is nearly a Gaussian beam • Probe beam is nearly a Gaussian beam • Focal position is ~ 4900 mm (waist size = 11 mm). Center for Fusion Plasma Diagnostic and Steady State Operation, POSTECH 8
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 0 mm +50 mm Reflection from flat mirror Center for Fusion Plasma Diagnostic and Steady State Operation, POSTECH 9
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. A little skewed beam shape at the reflector plane. Beam center is -5 ~ -3 mm Center for Fusion Plasma Diagnostic and Steady State Operation, POSTECH 10
Cutoff Layers for O-mode and X-mode Waves • Dispersion relations of O-wave and X-wave: ω ω ω ω − ω ω ω ω 2 2 2 2 2 k 2 2 2 c c k c c k k = − = − pe pe pe 1 1 (O-wave), (X-wave) ω ω ω − ω − ω ω ω 2 2 2 2 2 2 2 pe ce • • A probing wave is launched into the plasma along the density gradient and 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: [ ] ( ) 2 n e 1 ω = ω = ω = + ω + ω + ω 1 / 2 e 2 2 (O-wave), (X-wave) 4 ε pe R ce ce pe m 2 e e 0 0 The cutoff frequency of X-wave depends on magnetic field as well as plasma density. POSTECH 11
Cut-off Frequency on KSTAR Plasmas (a) Magnetic field and density profiles Contour of O-mode Contour of X-mode of KSTAR-like plasma cutoff frequencies cutoff frequencies cutoff frequencies cutoff frequencies (b) Characteristic frequencies Center for Fusion Plasma Diagnostic and Steady State Operation, POSTECH 12
Preliminary Optical Design of the KSTAR MIR Gaussian beam trace analysis of the probe beam for (a) curvature matching to the reflector surface Focusing lens set g Zoom lens set (shared with ECEI) ( ) Beam splitter Beam splitter Reflector (Rc = 421 mm) Source Detector Beam splitter Vacuum window (b) Ray trace analysis of the reflected beam to determine the image position the image position Center for Fusion Plasma Diagnostic and Steady State Operation, POSTECH 13
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 longer along the transmission path, that the ripple l l th t i i th th t th i l 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. Center for Fusion Plasma Diagnostic and Steady State Operation, POSTECH 14
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- off 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. Center for Fusion Plasma Diagnostic and Steady State Operation, POSTECH 15
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