Ceramic-supported traveling-wave structures for SNS fast beam chopper

The current structure for the fast 2.5-MeV beam chopper for the Spallation Neutron Source (SNS) project was originally developed [1] to provide rise and fall times around 1 ns. The structure is based on the meander-folded notched strip line with low-dielectric-constant supports and metal separators. Since then the requirements of the chopper rise-time has been significantly relaxed, up to 10 ns, as a result of beam dynamics simulations and to simplify the voltage pulse generators. In addition, initial runs with the beam showed that this structure was prone to damage when accidental beam spills occurred. We suggest alternative meander structures for the SNS chopper that employ high-dielectric-constant substrate (e.g., alumina). Time-domain simulations show their electromagnetic performance to be well within the requirements, while their resistance to beam spills and thermal properties are expected to be much better and fabrication significantly simpler.