Quasi-Optical Notch Filter

Electron Cyclotron Emission Imaging (ECEI) is a passive millimeter wave imaging and visualization technique for fusion plasma diagnostics and it has been successfully applied on Tokomaks such as the Rijnhuizen Tokamak Project (RTP) and Tokamak Experiment for Technology Oriented Research (TEXTOR) devices.

In an ECEI system, as illustrated in Figure 1, a band-stop filter (notch filter), is required to reject spurious gyrotron heating power and thus protect the receiving mixer arrays from damage or saturation. Because the filter must be mounted between the optical lenses in the imaging system, a frequency selective surface is suitable as a thin planar filter and is easy to implement.

A frequency selective surface (FSS) often consists of an array of periodic metallic patches or a conducting sheet periodically perforated with apertures ^[2]. FSSs have been intensively studied since the mid 1960s. Early FSS filters were mostly band pass filters, such as the Cassegainian subreflectors in parabolic dish antennas. An FSS was first used as a band-stop filter in a microwave plasma diagnostic system in the Rijnhuizen Tokamak project in 1999^[2].

As a planar, light weight, and low cost structure, the frequency selective surface (FSS) (shown in Figure 2) is suitable for incorporation into imaging optics to protect the mixer arrays from spurious ECRH power. However, there exist several challenges for the design of the FSS notch filter. Unlike waveguide filters, FSS notch filters must provide low pass band insertion loss with high notch rejection over a wide range of incident angles. This strongly affects both the filter steepness as well as the depth of the notch that can be obtained. The optical beams that require filtering are quite large, and filters of size 25 cm × 15 cm or more are required. The latter [3] requirement makes it difficult to employ photolithographic techniques, and our focus is therefore on standard PCB board fabrication methods. However, the resolution limitations of this fabrication method bring challenges to the design process, especially at high frequencies.

There are more restrictive requirements on the band-stop filter. First, the filter must cover an 8” diameter lens. Second, the FSS notch filter should be relatively insensitive to the angle of incidence of the millimeter waves because the filter is mounted on a lens where the input waves impinge at different angles. What is more, the filter is required to exhibit low loss in the pass band, in addition to large rejection in the stop band, resulting in a requirement for high Q ^[4].

The 140 GHz notch filter employs the periodic square loop structures on a Rogers RT/duroid 5870 substrate with a relative permittivity of 2.33 and 10 mils thickness. The substrate thickness and metal patch dimensions are chosen to obtain the desired resonant frequency. The 140 GHz notch filter has been fabricated commercially and characterized as Fig. 3.

From Figures above, we can see that the measurements show a close match to simulations.

From Figure 6 and Figure 7, we can see that the notch filter performs as well as the simulation predictions. Under the V and H polarization, the frequency shifts are both relatively small, especially for the case of H polarization. The largest frequency shift is 0.6 GHz, that is to say the requirement of angle insensitivity is met. Under all three situations, the notch frequency values are all smaller than -30 dB, and S21 value in the pass band are all larger than -1 dB. Three cascaded 140 GHz FSS notch filters have already been installed on the TEXTOR ECEI system, as shown in Figure 8.

Fig. 8. Three cascaded 140 GHz FSS notch filters have been installed on the TEXTOR ECEI system ^[6]

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

[2] Ben A.Munk, “Frequency Selective Surfaces Theory and Design”, Wiley, 2000

[3] H.J.van der Meiden, “Application of band-stop filters for the 30-200 GHz range in oversized microwave systems”, Review of Scientific Instruments, VOLUME 70, NUMBER 6, 2861-2863, June 1999

[4] Z. Shen, N. Ito, E. Sakata, C. W. Domier, Y. Liang, N. C. Luhmann Jr., and A. Mase, “Frequency selective surface notch filter for use in a millimeter wave imaging system,” in Proc. IEEE Antennas and Propagation Society Int. Symp., Albuquerque, NM, Jul. 9–14, 2006, pp. 4191–4194

[5] http://www.ansoft.com/products/hf/ansoft_designer/

[6] Textor, Institute of Energy and Climate Research, Institute for Energy Research/Plasma Physics, EU, URL: <http://www2.fz-juelich.de/ief/ief-4//textor_en/>