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

2. RF MEASUREMENT ON A SINGLE CAVITY 71
In figure 5.3 the necessary mechanical tools <strong>and</strong> a particular solution
of electric mirror are shown: the half cavity rests on a plane <strong>and</strong> is
closed on a circular surface whose diameter is bigger, of a few millimeters,
than the cavity diameter. The pressure on the contact reference
surface can be controlled by using a dynamometric key <strong>and</strong> should be
kept below the mechanical limit of plastic deformations for the material
used for the cavities.
Figure 5.3. Mechanical tools for RF measurement on
the LIBO tiles. On the top right the use of these tools is
shown.
2.2. The statistical approach to the measurement of the
frequencies of LIBO cells. Frequency measurement of half cavities
on electric mirror can be source of systematic errors. In fact, it is very
difficult to realize a good contact between the cavity <strong>and</strong> the conductor
plane, even if a reference contact surface is realized. The measurement
is not reliable for each cavity, but a statistical approach can be used.
As it is explained in section 4, in LIBO project the single tiles which
form a tank contain two half cavities with a ring. The height of this ring
can be machined in order to change the volume of magnetic field, <strong>and</strong>
then the resonant frequency. This operation implies only a decrease
of resonant frequency. For this reason, the half cavities are designed
with a resonant frequency which is higher than the desired value <strong>and</strong>,
in this sense, there are no cells which do not need interventions, since
all the cells have to be corrected to the required resonant frequencies
using the machinable ring.
In figure 5.4 is shown the dispersion of frequencies value for the half
cells in the tiles around the mean value. The frequencies are measured
with an unknown systematic error which, due to the mechanical tools,