SLAC, Menlo Park, California, USA
Cavity BPMs commonly use the fundamental TM010 mode (excited either in the x/y cavity itself or in a separate "reference" cavity) which is insensitive to beam position as a reference signal, not only for amplitude normalization but also as a phase/time reference to facilitate synchronous detection of the signal derived from the position-sensitive TM110 mode. When taking these signals into the digital domain the reference and position signals need to be acquired by a synchronous clock. However, unless this clock is also locked to the accelerating RF, absolute timing information is lost which affects the relative phase between reference and position signals (assuming they are not carefully tuned to the same frequency). This contribution presents a method for estimating the necessary time of arrival information based on the sampled reference signal which is used to make the signal detection insensitive to the phase of the digitizer clock. Running an unlocked digitizer clock allows for considerable simplification of infrastructure (cabling, PLLs) and thus decreases cost and eases maintenance.
SLAC, Menlo Park, California, USA
The LCLS2 is a CW superconducting LINAC driven X-ray free electron laser under construction at SLAC. The high beam rate of up to 1MHz, and ability to deliver electrons to multiple undulators and beam dumps, results in a beam diagnostics and control system that requires real time data processing in programmable logic. The SLAC Technical Innovation Directorate has developed a common hardware and firmware platform for beam instrumentation based on the ATCA crate format. The FPGAs are located on ATCA carrier cards, front ends and A-D / D-A are on AMC cards that are connected to the carriers by high speed serial JESD links. External communication is through the ATCA backplane, with interlocks and low frequency components on the ATCA RTM. This platform is used for a variety of high speed diagnostics including stripline and cavity BPMs.