Superconducting (SC) RF cavities are becoming common in accelerators for high-energy and nuclear physics, and the technologies needed to obtain high fields and high-quality factors in elliptical cavities for electron acceleration have come close to maturity, for example in the TESLA project. However, because mechanical weakness causes some difficulty in adopting elliptical cavities for lower-velocity particles, there is a demand for developing different types of SC cavities, in particular to reduce the costs of future low-energy facilities, such as spallation neutron sources, rare isotope accelerators, and accelerator-driven waste transmutation systems.
One of the promising candidates is the spoke cavity, invented in the late 1980s by Jean Delayen and Ken Shepard at the Argonne National Laboratory. With this, it is easier to extend the acceleration length of half-wave coaxial resonators by adding more spokes in one cavity. A benefit is that for the same frequency a spoke cavity is about half the size of an elliptical one; conversely, a spoke cavity would operate at half the frequency of an elliptical cavity of similar size. This increases the active length by a factor of two, and allows an operating temperature of 4.5 K, with resulting savings in the installation and operating cost of the cryoplant.
In 2002, the Los Alamos National Laboratory (LANL) began developing spoke cavities as part of its Advanced Accelerator Applications (AAA) programme to develop technology for an accelerator-driven waste transmutation system. The LANL team has designed a 350 MHz two-gap spoke cavity, with b (fraction of light velocity) = 0.175, and procured two cavities from the Italian firm Zanon SpA. The diameter of the cavity is 40 cm, the beam aperture is 5 cm and the accelerating length is 10 cm. The two cavities have reached 12.9 MV/m and 13.5 MV/m respectively at 4 K, exceeding the present AAA design goal of 7.5 MV/m by up to 80%. This will help achieve the very high reliability required for the waste transmutation application. Although there are still issues to be discussed, such as drive couplers, multipacting and higher-order modes, this result has encouraged the LANL team to strive for further development of multispoke cavities, which may also prove to be a cheaper and better option for medium velocity (b~0.6) particles.
