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/ NLCTA-Note #53
February 5, 1996
Subject: Special Considerations for the Klystron Mode Transducers
Authors: M. Harding, E. Wright

When assembling TE01 transmission lines where two mode transducers are used back to back , there exists the possibility of setting up resonance’s due to mode conversion of the TE01 wave into unwanted modes. If a resonance happens to fall on or near the desired operating frequency, the network can become quite lossy ( as high as 20% loss ). In order to combat this problem, measurements were made for the klystron to determine if there is a problem and what should be done to fix it. The fix in all cases is to change the distance of the transmission line between mode transducers.

Microwave measurements have been performed on networks that mimic XL2, XL3, and XL4 as they will be installed on the NLCTA. The network consists of two Mode Transducers and a Traveling Wave Window. Two varieties of mode transducers have been used on the klystrons: XL2 and XL3 have the old style Tapered Mode Transducers (TMT), while XL4 has the new Choke Mode Transducer (CMT). The window is the same for all klystrons. Mated to the output flange of the klystron will be a CMT in all cases. These measurements have shown that for all configurations, there are trapped resonance’s on or near the operating frequency. The frequencies of the nearest trapped resonance when an unmodified CMT is attached to the klystron output are:

Klystron / Closest Trapped Resonance
XL2 / f=11.420 GHz
XL3 / f=11.424 GHz
XL4 / f=11.437 GHz

The worst of these is XL3, with a total loss of 9% for the network. Modifications will need to be made in all cases to maximize the separation between the operating frequency and the resonance frequencies.

Figure 1 shows the measurement for XL2. There is an 80 MHz resonance free region just below the 11.420 GHz resonance. When a 0.200” spacer was placed between the 4-5/8” CFF, this region shifted down in frequency by approximately 40 MHz. If we shorten the CMT by 0.200”, we will presumably shift this region higher in frequency by 40 MHz, providing us with + 40 MHz of resonance free region about 11.424 GHz.

Figure 2 shows the measurement for XL3, as well as the solution to the resonance problem. We found that the addition of a 0.340” spacer between the 4-5/8” CFF gave a resonance free region of operation of approximately -35 MHz and +20 MHz about 11.424 GHz. Microwave measurements with the spacer to be used for high power test show a resonance free region of 80 MHz, symmetric about 11.424 GHz. Figure 3 shows the measurement for XL4. There is a resonance free region of operation of approximately -50 MHz and +13 MHz about 11.424 GHz. We plan to shorten the CMT here as well to center the resonance free region about 11.424 GHz.

The modifications to be performed on the CMT’s to be used on the klystron are as follows:

Klystron / CMT Modification
XL2 / Shorten CMT by ~ 0.200”
XL3 / Lengthen CMT by ~0.280”
XL4 / Shorten CMT by ~0.125”

Each will be unique for a given tube, and should be identified as such.