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

4. BRIDGE COUPLERS DESIGN 35
4. Bridge Couplers design
The bridge coupler has a fundamental role in a Side Coupled Linac.
It allows to have tanks which are separated by magnetic quadrupoles
<strong>and</strong> fed by the same RF waveguide. In the figure 3.10 a sketch of this
functionality is shown.
Figure 3.10. Schematic view of the Bridge Coupler
functionality. The Bridge Coupler cavity couple two
tanks <strong>and</strong> leaves the space for a Permanent Quadrupole
Magnet by which the particles beam is focused.
Of course, the Bridge Couplers are particular cells, the design of
which should be conceived paying attention to different claims with
respect to the accelerating <strong>and</strong> coupling cavities.
Not so many Bridge Couplers have been realized in the accelerators
community. The few examples are the ones for the 800 MHz Side
Coupled Linac in the Los Alamos National Laboratory [19], <strong>and</strong> the
800 MHz Side Coupled Linac in the FermiLab in Chicago. For these
linacs, the bridge couplers was connected to coupling cells, therefore,
from the electromagnetic point of view they were analogous to accelerating
cells.
In this case, one could think to a single cavity bridge coupler which
directly connects two accelerating cells but mode mixing could arise 7 .
The resonant mode we choose has to be far away from the others to
avoid mode mixing.
In a LIBO module there are three Bridge Couplers. One is connected
with the feeding waveguide through an iris (see figure 3.11),
<strong>and</strong> the other two have pumping ports for the vacuum. In order to
reduce the overall dimensions, it has been chosen a bridge coupler including
also two coupling cells, which, of course, are specially designed.
The principal quality of such a structure is the transit time of energy
which is equal to the length of the bridge coupler divided by the group
velocity of the cavity mode. As the bridge coupler length is determined
by other considerations, the principal figure of merit for a particular
bridge coupler c<strong>and</strong>idate is its group velocity, the higher the better
[29].
7 Due to a finite quality factor of the resonant cavities, more than one mode
could be excited from a frequency tone <strong>and</strong> this phenomenon is called Mode mixing.