Fermilab (FNAL) is contributing to the SLAC Linac Coherent Light Source II (LCLS-II) project with superconducting radio-frequency (SRF) accelerator technology and cryogenics. The LCLS-II will produce a high-power free-electron laser for research from nano/bio-technology to materials engineering. The SRF technology will increase the pulse rate from 120Hz in the current LCLS to 1MHz in the LCLS-II.
SRF cavities can store energy with extremely low losses. In order to keep their resonant frequencies exactly in tune, a combination of mechanical coarse tuners (slow) and fine piezomechanical tuners (fast) are implemented. Piezo actuators provide motion based on solid state effects. Reasons for choosing piezo actuators are low-energy requirements, high force output, sub-millisecond responsiveness, and virtually unlimited positional resolution. Piezo actuators are not subject to wear such as traditional mechanical actuators and if designed right, will provide billions of cycles of precision motion.
The reliability requirements for SRF cavity tuners are very high. They need to work over a period of 20 years in a harsh environment: cryogenic temperatures, vacuum, and radiation.
The latest designs for the LCLS II project were run through a number of accelerated lifetime tests. The piezoelectric fine-tuners provided by PI are based on the experience gathered with many previous applications and similar projects. Key to avoiding detrimental forces on the piezo ceramics is a mechanical housing design that keeps tensile and shear forces away from the ceramics.