Wire-scanner design for the SNS superconducting-RF linac

The requirement for profile measurements in the SNS superconducting-RF (SRF) linac has presented diagnostic designers with unusual challenges. In addition to making accurate measurements of a high-brightness H/sup -/ beam, the device must be compatible with 10/sup -10/ Torr vacuum requirements. Also, it cannot impact reliability of the nearby superconducting structures, either by failure modes, by introducing particulate matter, by vaporization, or by producing excessive radiation levels. We have designed a device to meet these stringent requirements by using a lever-action mechanism and a formed bellows that provide a clean interface and keep all moving joints outside the vacuum. Both tungsten and niobium wires are suitable, but thermal properties and charge collection are important issues that are considered. The prototype currently being fabricated will be used to verify vacuum and reliability performance.