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Quasi-Optical Notch Filter for ECEI systems

Introduction

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

In an ECEI system, 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. [1] 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 Fig.1) 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[1] 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.[3]

Description: Description: 1

Fig. 1 Photograph of a section of a test FSS notch filter with square loop structure.

Design of Quasi-Optical Notch Filter for the ECEI system

140 GHz Quasi-Optical Notch Filter Simulation Result

Ansoft Designer® [4] is used for the 140 GHz notch filter simulation. It employs a FSS with a square loop structure as shown in Fig. 2.

Fig. 2 FSS schematic in Ansoft Designer®

When the incident angle is zero, the simulation results are plotted in Fig. 3 and Fig. 4. This filter exhibits 34.4 dB rejection at normal incidence.

Fig. 3 Normal incident insertion loss S21 in wide frequency range

 

Fig. 4 Normal incident insertion loss S21 in narrow frequency range

From Fig.3, we can see that the insertion loss in the pass band is larger than -1dB, far larger than -3dB, which makes it possible to cascade two or three such notch filters to provide a larger rejection in the stop band.

The incident plane wave direction and the observed direction are specified in terms of an azimuthal angle and a scan angle, as shown in Fig.5.

Fig.5 V Polarization and H Polarization

As mentioned above, the FSS notch filter should be relatively insensitive to the angle of incidence. When the incident angle is not zero, but rather 8 degrees, the insertion loss is shown in Fig.6 and Fig.7.

Fig. 6 Insertion loss S21 when the horizontal incident angle is 8 degrees

 

Fig. 7 Insertion loss S21 when the vertical incident angle is 8 degrees

 

From Fig.6 and Fig.7, we see that when the vertical incident angle is 8 degrees, the resonant frequency is shifted to a lower frequency, where the shift is about 0.4 GHz, while when the horizontal incident angle is 8 degrees, the resonant frequency is shifted to higher frequency by a slight amount, which is only 0.1GHz. Table 1 is a summary of the 140 GHz notch filter simulation result.

 

Incident angle

-1dB frequency

-1.5dB frequency

Resonant frequency

Minimum rejection

Rejection @140GHz

normal

132.6 GHz

133.3 GHz

140 GHz

-34.42 dB

-34.42 dB

Theta=8°(H)

132.66 GHz

133.30 GHz

140.1 GHz

-34.46 dB

-33.44 dB

Theta=8°(V)

132.47 GHz

133.13 GHz

139.6 GHz

-33.68 dB

-28 dB

Table 1 Summary of 140 GHz notch filter simulation result

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.

140 GHz Quasi-Optical Notch Filter Fabrication and Measurement

The 140 GHz notch filter has been fabricated commercially and characterized.

Fig.8 140GHz notch filter

Fig. 9 Insertion loss S21 comparison of measurement and simulation for normal incidence

Fig. 10 Insertion loss S21 comparison of measurement and simulation

when the horizontal incident angle is 8 degrees

Fig. 11 Insertion loss S21 comparison of measurement and simulation

when the vertical incident angle is 8 degrees

From Fig.9 to Fig.11, we can see that the measurements show a close match to simulations.

Fig. 12 Summary of notch filter performance over a wide frequency range

Fig. 13 Summary of notch filter performance over a narrow frequency range

From Fig.12 and Fig.13, 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 Fig. 14.

 

Fig. 14 Three cascaded 140 GHz FSS notch filters

have been installed on the TEXTOR ECEI system[5]

 

170 GHz Quasi-Optical Notch Filter Simulation Result

Ansoft designer is also used for the 170 GHz notch filter simulation. It employs an FSS with a square loop structure as shown in Fig. 15.

Fig. 15 FSS schematic in Ansoft Designer

 

When the incident angle is zero, the simulation results are plotted in Fig. 16. This filter exhibits 31.15 dB rejection at normal incidence.

Fig. 16 Normal incident insertion loss S21

From Fig.16, we can see that the insertion loss in the pass band is larger than -1 dB, far larger than -3 dB, which also makes it possible to cascade two or three such notch filters to provide a larger rejection in the stop band.

When the incident angle is not zero, but 8 degrees, the insertion loss is shown as Fig.17 and Fig.18.

Fig. 17 Insertion loss S21 when the horizontal incident angle is 8 degrees

Fig. 18 Insertion loss S21 when the vertical incident angle is 8 degrees

From Fig.17 and Fig.18, we can see that when the vertical incident angle is 8 degrees, the resonant frequency is shifted to lower frequency by about 0.2 GHz, while when the horizontal incident angle is 8 degrees, the resonant frequency is exactly 170  GHz.

The 170 GHz notch filter employs the periodic square loop structures on a Rogers RT/duroid 5870 substrate with a relative permittivity of 2.33. The substrate thickness and metal patch dimensions are chosen to obtain the desired resonant frequency.

170 GHz Quasi-Optical Notch Filter Fabrication 

The 170 GHz testing notch filter has been fabricated a Rogers RT/duroid 5870 substrate with a relative permittivity of 2.33 and 5 mils thickness. The small square dimension is chosen to be 3 inches * 3 inches for the experiment window. Because of the lacking of G band BWO, the 170 GHz full size notch filter is not fabricated.

 

Fig. 27 Test board of 170 GHz notch filter

Summary

In this work, 140 GHz and 170 GHz notch filters for the ECEI system have been designed. Experimental results correspond well with simulation results, which verify the feasibility of the design. The 140 GHz notch filter has already been used in the TEXTOR ECEI system and works well.

 

 

 

Reference

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

[2]         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

[3]         Z. Shen, N. Ito, E. Sakata, C.W. Domier, Y. Liang, N.C. Luhmann, Jr. and A. Mase, 2006 IEEE Ant. Prop. Society Int. Symp., 4191 (2006).

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

[5]         http://www2.fz-juelich.de/ief/ief-4//textor_en/

 

 

 

    
 
 
 
 
 
 
 
 
 
 
 
 
              
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