A way around this is to choose low Q cavities, where all modes are naturally damped. Unfortunately, such cavities have small gap voltages and therefore a large number are required. This increases both capital and running costs. An innovative approach being pursued in the JHF design is to use a metallic magnetic alloy instead of ferrite. Since these materials have higher Curie temperature and saturation magnetization, higher rf flux densities and gap voltages are possible.
Resistivity can be in the realm of stainless steel.
Extremely low Q is possible in such a design, without lowering R s to the point that excessive power is required to drive the cavity. There are two advantages to this type of cavity design. Wakefields decay between bunches, so coupled-bunch modes are not dangerous, and no tuning of the cavity is required. On the other hand, the broad impedance would lead to large potential-well distortion. In other words, beam induced voltage must be compensated not only at the fundamental, but at other harmonics as well.