Optics Design for Microwave Imaging Reflectometry in LHD

Abstract

An optics system for microwave imaging reflectometry (MIR) in the Large Helical Device (LHD) was newly developed to optimize the performance of the two-dimensional microwave receiver array. Reflected microwaves from the plasma and the first local oscillator (LO) wave are transmitted to the receiver array via the optics from the front. Finite-difference time-domain (FDTD) calculation was used to design the ellipsoidal or hyperboloidal shapes of the quasi-optical mirrors. It is confirmed that the LO beam in the constructed system covers the receiver antenna aperture area as intended. The S/N ratios of the signals are improved with this optimized optics system from those in the previous system. Understanding confinement and transport in magnetically confined plasmas is one of the important issues for realizing fusion reactor. Microscale instabilities such as turbulence are considered to activate anomalous transport. Microwave imaging reflectometry (MIR) would be a sensitive fluctuation diagnostics in turbulent plasmas. MIR is a multi-receiver reflectometry that can potentially visualize the 3-D structures of electron density fluctuations by projecting images of the cutoff surfaces onto the image focal plane [1].In the Large Helical Device (LHD), a prototype MIR system with three commercial horn antennas, has successfully received scattered microwaves from plasmas [2,3]. The received microwaves were transmitted from the receiver antenna to mixer components remote from the diagnostic port by oversized waveguides. In a full-scale imaging system, however, it is difficult to use waveguides to transmit received signals from dozens of receiver channels. This signal transmission problem was solved by a newly developed 2-D horn-antenna mixer array (HMA), which consists of arrayed quasi-optical antenna-mixers covered with pyramidal horn apertures [4,5]. This HMA was developed to down-convert received microwaves into intermediate frequency (IF) signals by Schottky diodes, which are placed inside each of the HMA apertures. After downconversion, the signal handling becomes very convenient, since printed circuit boards or ordinary coaxial cables can be used for amplification, filtering, or transmission. This paper presents the methodology for mixing the received microwave signal with the first local oscillator (LO) signal for down-conversion. Our technique is to cast the LO microwave (55.8 GHz) from the front of the HMA aperture together with the received microwave signals. The status of the optics system for LO wave projection is also described.The newly designed optics system for MIR in LHD is illustrated in Fig. 1. It consists of aluminum alloy mirrors

Development of 3D microwave imaging reflectometry in LHD (invited)

“…This paper presents the 3D MIR system in LHD. This system consists of 2D microwave imaging device, 10,11 four frequency microwave source, 12 microwave imaging optics with adjustable mirror, 13 and electronics for intermediate frequency (IF) amplification, frequency separation and quadrature detection. By using the 3D MIR, 3D structure of the edge harmonic oscillation (EHO) is observed.…”

Simultaneous projection and detection system of four different frequencies for microwave imaging reflectometry in Large Helical Device

“…10 Figure 3 compares the MIR signals under a high ion-temperature discharge with the signals from other diagnostics. The signals with the illuminating frequencies ͑f IL ͒ at 60.410, 61.808, and 64.610 GHz from the central HMA channel of the MIR, whose field of view is on the optical axis, are plotted.…”

“…However, the imaging optics system becomes rather complicated since the reflected signal waves and the first LO must be simultaneously focused and projected onto the front of the horn aperture. 10 Using a downconversion method for the first IF signals at the receiver antennas significantly simplifies the succeeding signal handling processes, such as transmission, amplification, filtering, and further downconversion. All of these processes use the low cost, mass-produced telecommunications elements that are crucial for a MIR system with many receiver channels.…”

Development and application of radar reflectometer using micro to infrared waves

“…The focusing point or the wavefront at the cut-off layer is determined from the plasma shape. The optics design is characterized using a ray tracing code, such as, Code V and Zemax and FDTD (finite difference time domain) simulation code [36][37][38][39]. Figure 4 shows the propagation behavior of incident, reflected, and local oscillator waves using an FDTD code for the LHD (large helical device) plasma [38,39].…”

“…The optics design is characterized using a ray tracing code, such as, Code V and Zemax and FDTD (finite difference time domain) simulation code [36][37][38][39]. Figure 4 shows the propagation behavior of incident, reflected, and local oscillator waves using an FDTD code for the LHD (large helical device) plasma [38,39]. The ellipsoidal or hyperboloidal surfaces of the curved aluminum alloy mirrors were installed in a vacuum chamber of LHD for simultaneous focusing of the components (the illumination wave, the reflected wave, and the LO wave).…”