Theory, Design and Tests on a Prototype Module of a Compact ...

2. RF MEASUREMENT ON A SINGLE CAVITY 69
design parameters are reachable. This step is repeated for all the tanks
in the same way.
On the other h<strong>and</strong>, the bridge coupler pieces are stacked to make
coupled cells measurement <strong>and</strong> to control that the design values are in
the movable tuner range.
Once these steps are finished, the whole module brazing is made,
<strong>and</strong> after, one deals with a structure where it is still possible to move
the rod tuners in the cavities <strong>and</strong> the tuners in the three bridge couplers.
Last, the hard point has to be fixed: after regulations on all those
tuners, one control the π/2 mode, by means of a frequency measurement,
<strong>and</strong> the axial electric field by means of a bead pulling measurement.
The procedure go ahead with the help of the empirical criteria
found with the equivalent model <strong>and</strong> with lots of patience <strong>and</strong> skill
operators. At the same time, one has to control the coupling between
the waveguide <strong>and</strong> the whole module <strong>and</strong> has to choose the length of
the short-circuited waveguide (see figure 3.11).
Finally, after the parameters are fixed within the design ranges, the
rod tuners are also brazed <strong>and</strong> the module is ready for the power tests.
2. RF Measurement on a single cavity
As it is shown in the previous chapters, a resonant mode of an electromagnetic
cavity can be fully represented by only three parameters:
resonant frequency, quality factor <strong>and</strong> shunt impedance.
By using a network analyzer <strong>and</strong> some mechanical <strong>and</strong> electrical
tools, it is possible to measure all these three parameters. In principle,
these measurements are very easy, but a big effort has to be made in
order to get correct results <strong>and</strong> to avoid systematic errors when one
measures the cavities of a structure such a LIBO.
2.1. Measurement of the resonant frequency. The measurement
of resonant frequency on a closed cavity (excepted for the beam
holes) is performed using probes which penetrate until the inner surface
of the cavity. One probe allows the measurement of reflection
coefficient, whereas two probes allow measurement of transmission coefficient.
It is apparent that the cavity should has one or two holes for
the probes, unless the beam holes are used. The probes are realized in
the following two ways:
• electric pick-up which is a rigid coaxial cable, whose internal
conductor juts out of about 4 mm over the external one.
This type of antenna should be used where the electric field is
strong 1 . In figure 5.2 an example of realization is shown.
• magnetic loop which is a rigid coaxial cable, whose internal
conductor juts out over the external one <strong>and</strong> realizes a loop
1 It could be put along the axis of the cavity through the beam hole where the
longitudinal electric field should be strong.