Diode Mixer Array

Electron Cyclotron Emission Imaging (ECEI) diagnostics are very useful tools invented by the MMWave Plasma Diagnostic Group at UC Davis to study fusion plasma electron temperature and its fluctuations. A 2-D ECE imaging systems have been well developed and applied on Tokamak Devices such as KSTAR, TEXTOR, etc. by this group.

As illustrated in Figure 1, a typical ECEI system consists of four essential constituents: optics, a receiver box, an RF electronics, and an IF electronics. Optics creates the optical path that connects the plasma ECE radiation with the receiver array. A notch filter, a type of frequency selective surface (FSS) is implemented to protect the mixer array from stray ECRH heating power before entering the receiver box. ECE radiation from the hot plasma is collected by the optical lenses. Before it is received by the array, a planar quasi-optical notch filter is used to protect the array against strong radiation from stray ECRH heating power. Since it is necessary to perform the down conversion process in single sideband, the dichroic plate, a metal plate filled with circular holes, acting as a high pass filter is employed, to suppress the lower sideband and allow the high frequency radiation to pass through with low loss. The RF signal received by the antenna array is then mixed with a local oscillator signal and down converted to a lower IF frequency and amplified by low noise amplifiers. The IF signal is then divided into 8 discrete frequency bands by the electronic circuits, with each band corresponding to a different horizontal position in the plasma, and down converted again by the local oscillator signal provided by VCOs on the RF board. Then the signals go to the IF board which consists of IF amplifiers, low pass filters, detectors, video frequency amplifiers and high speed digitizers which are used to sample the signals.

A planar antenna is a very attractive solution for our quasi-optical imaging receiver application. The desired antenna should have compact size, length less than one free space wavelength for high spatial resolution in the vertical direction, wide RF bandwidth (above 20%) for wide plasma coverage in the horizontal direction, high directivity (3dB beam width less than 20 degrees) for increased receiver sensitivity, low side-lobe levels (less than 10dB) to reduce inter-channel crosstalk, and linear polarization for receiving signal emitted from plasma modes of interest. In order to down convert the ECE signal received by the planar antennas, we make use of the nonlinearity of Schottky beam-lead diodes.

In addition, a dual dipole antenna is chosen as the receiving antenna in current ECEI systems due to its wide frequency band, high directivity and low side lobes.

Since the RF and LO frequencies are close to each other, a single dual dipole antenna will receive both the RF and LO signals. A beam-lead Schottky diode is mounted in the middle of the dual dipole antenna mixing both RF and LO signals to down convert RF to IF (2-26.5 GHz). This antenna has a very good pattern under mini-lenses (see mini-lenses subsection) from 100-140 GHz. The measurement result is shown below.

I. Substrate Mode

For printed circuit board antennas, the radiated power is concentrated in the higher dielectric constant region and would be trapped in substrate modes, which is shown in Fig. 5. In order to take advantage of the radiated power and prevent it from being trapped in substrate modes, a dielectric lens must be used.

II. Front Side LO Coupling

A spherically, elliptically, or hyper-hemispherically shaped HDPE lens with dielectric constant of roughly 1.52 may be abutted to the substrate side of the printed lens array. It is shown in Figure 6 (a). This lens did an excellent job of collimating radiation from central elements; however it had the undesirable effect of imparting significant aberration on the edge channels which are off the center of the lenses. Because of the limited amounts of LO power available, antennas resonant with the air side LO are employed rather than the substrate side RF. The antenna geometry is then fine-tuned to provide “clean” RF antenna patterns over the range of frequencies for which the LO power could be efficiently coupled to the array. In the other words, there is a compromise which needs to be made between optimizing the antennas for operation at the substrate and free space wavelengths. We chose to optimize the air side due to the lack of LO power.

In the current ECEI systems, a mini lens dual dipole antenna has replaced the former hyper hemispherical lens dual dipole antenna. Pictures of the mini lenses and antennas are shown in Figure 6 (b). Taking advantages of the excellent optical properties ^[5], there is no need to consider the difficulty of feeding LO to the off-axis elements, since every element is positioned in the center of each mini lens in this configuration. With that freedom to feed both LO and RF from the front side, there is a much more careful optimization of the dimensions of the dual dipole antenna. The new dual dipole antenna shown in Figure 6 (b) is smaller than the previous one since we optimized it on the substrate side. As a consequence, the antenna pattern is much better.

III. Coupling of RF and LO

In order to couple RF and LO signal together, we make use of the beamsplitter. By using a 3-dB beamsplitter, RF and LO are coupled together and each half of them is received by the mini-lenses array.

A mini-lenses array is fabricated for TEXTOR ECEI system

A total of 16 miniature elliptical substrate lenses of 15 mm diameter are arranged in a vertical array measuring 205.74 mm in height (center to center). Horizontal staggering of 11.684 mm (center to center) allows for reduced channel spacing in the E-plane, but results in a small offset in the H-plane between even and odd channels at the image.

[1] Xiangyu Kong, “Millimeter-Wave Imaging Technology Advancements for Plasma Diagnostics Applications”, Doctoral Dissertation, University of California, Davis, USA, 2013.

[2] David M. Pozar, “Microwave Engineering”

[3] X. Kong, C.W. Domier, and N.C. Luhmann, Jr., “Miniature Elliptical Substrate Lenses for Millimeter-Wave Imaging” in proceedings of the 33rd International Conference on Infrared, Millimeter and Terahertz Waves, 15-19 September, 2008, Pasadena, California

[4] D.B. Rutledge and M.S. Muha, “Imaging Antenna Arrays”, IEEE Transactions on Antennas and propagation, Volume 30, Issue 4, Jul, 1982

[5] E. Hecht, “Optics”, 2nd Edition, Reading, MA: Addison-Wesley, 129-131 (1987)