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Section 5.5. In the following two sections, measurements of the HOM frequency, QL
and polarization, which are used to characterize the HOMs, are discussed in detail.
4.6.1 RF Measurements of the Zone 3 Cryomodule
In February 2004, the zone 3 cryomodule was moved into the FEL vault for
installation and commissioning. Initially, the cryomodule sat parallel to its final
destination on the beamline so it could be commissioned in parallel with standard
FEL operations. When the cryomodule was cooled to 2 K, and before the final
waveguides were installed, measurements of the HOM parameters were performed.
For each of the eight cavities, the frequencies and loaded quality factors of the TM010
fundamental passband and TE111 and TM110 dipole mode passbands were measured.
The details of the setup for measuring the HOM parameters are shown pictorially
in Fig 4.5.
A network analyzer (NWA) was used to manually measure the S21 scattering
transmission parameter (see Appendix B). This involves using port 1 of the NWA
to excite the cavity through the fundamental power coupler. The FPC is connected
to a WR-650 waveguide-to-coaxial adapter (frequently called a top hat). The top
hat is used to provide the proper impedance match from the waveguide to a 50 Ω
coaxial cable. Port 2 of the NWA is connected to the cavity’s HOM1 port while the
field probe and HOM2 ports are terminated in 50 Ω loads. This completes the S21
measurement.
A manual search and measurement of each HOM is necessary. The frequency of
individual HOMs are measured with an accuracy of up to 1 kHz. The loaded quality
factor of each mode is found from the center frequency divided by the bandwidth
between the −3 dB points. A typical HOM resonance curve with markers at the
−3 dB points is shown in Fig. 4.6. In some instances, the interference of neighboring
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