Quarter-Wave Resonant Coupler

Another important development that we have made is the Quarter-Wave Resonant Coupler (patented).  Particle accelerators involving a single resonator have the requirement that the amplitude and distribution of the rf fields within the resonator be appropriate for the acceleration process.  Particle accelerators involving two or more resonators, as do ours, have the additional requirement that the relative phase and amplitude of the fields in adjacent resonators be precisely controlled. 

The use of a resonant coupler greatly simplifies the rf field control problem for linacs involving two linac resonator.  The resonant coupler provides a single frequency at which the pair of resonators can be excited, even when the resonant frequencies of the individual resonators are slightly different.  The resonant coupler locks the relative amplitudes and relative phases of the field in the two resonators.  Consequently, the resonant coupler reduces the controls problem for two-resonator accelerators to that of controlling the frequency of the drive power to the single frequency offered by the resonant coupler and controlling the amplitude of either resonator to the required accuracy.  By designing the rf power system to track the resonant frequency of the combined structures, the control of the rf fields within structures is greatly simplified.

Our linac systems involve two different linac structures, namely an RFQ linac section and an RFI linac section.  We have developed a simple, compact, resonant coupler, based on a quarter-wave-resonator coupled to the rf magnetic fields of both structures.  This coupled system represents a chain of three coupled resonators, which if properly tuned, can be excited in three modes of oscillation, namely the "zero" mode. The "?/2" mode, and the "?" mode.  The mode of interest is the ?/2 mode, where the two linac structures are fully excited and the resonant coupler is nominally unexcited.  The ?/2 mode is unique in that it is the only mode in the mode spectrum where no changes are required in the amplitude or phase of the excited cells to accommodate changes in power flow through the structure.  The resonant couplers lock the relative amplitude and phase of the fields in the excited cell of the structures.  They stand by to support power flow in whichever direction is required to keep the fields in the excited structure at their design value.

Our linacs benefit a lot by being resonantly coupled into a single resonant unit.  Shown below is a 1.1 MeV RFQ linac section that is resonantly coupled to a 3.5 MeV RFI linac section by the short resonator that is near the center of the assembly.