SLAC, Menlo Park, California, USA
SLAC is developing a new X-band Cavity BPM receiver for use in the LCLS-II for use in the LCLSII. The Linac Coherent Light Source II (LCLS-II) will be a free electron laser (FEL) at SLAC producing coherent 0.5-77 Angstroms hard and soft x-rays. To achieve this level of performance precise, stable alignment of the electron beam in the undulator is required. The LCLS-II cavity BPM system will provide single shot resolution better than 50 nm resolution at 200 pC *. The Cavity BPM heterodyne receiver is located in the tunnel close to the cavity BPM. The receiver will processes the TM010 monopole reference cavity signal and a TM110 dipole cavity signal at approximately 11 GHz using a heterodyne technique. The heterodyne receiver will be capable of detecting a multibunch beam with a 50ns fill pattern. A new LAN communication daughter board will allow the receiver to talk to an input-output-controller (IOC) over 100 meters to set gains, control the programmable dielectric resonator oscillator, enable self-test, and monitor the status of the receiver. We will describe the design methodology including noise analysis, distortion analysis.
* Commissioning and Performance of LCLCS Cavity BPMs, Stephen Smith, et. al, Proceedings of PAC 2009, Vancouver, BC, Canada.
SLAC, Menlo Park, California, USA
Transverse beam size measurement for emittance tuning of the LCLS electron beam relies on wire scanners strategically placed on the beam line. It was originally intended to use Optical Transition Radiation (OTR) screens to obtain single shot measurements of the transverse beam size but it was soon discovered that the Coherent OTR that is a feature of ultra-short bunch length operation in linac FELs swamped the signal and rendered this diagnostic unusable. The original SLAC wire scanners that have been in operation for about 20 years are fairly slow and not optimized for the rapid measurements needed to monitor and fine tune the very small LCLS beam emittances, taking over a minute to complete a measurement in both planes. Two new wire scanner designs are being tested at LCLS that make use of state-of-the-art mechanical slides and position read-back systems to make fast, vibration-free measurements of the beam size. One uses an in-vacuum piezo-controlled linear slide and the other uses a dc linear servo motor coupled through a pair of vacuum bellows. The merits of the two designs are compared to the original SLAC design that relies on a ball screw driven by a stepping motor.
SLAC, Menlo Park, California, USA
DESY, Hamburg, Germany
LBNL, Berkeley, California, USA
The technique of streaking an electron bunch with a RF deflecting cavity to measure its bunch length is being applied in a new way at the Linac Coherent Light Source with the goal of measuring the femtosecond temporal profile of the FEL photon beam. A powerful X-band deflecting cavity is being installed downstream of the FEL undulator and the streaked electron beam will be observed at an energy spectrometer screen at the beam dump. The single-shot measurements will reveal which time slices of the streaked beam have contributed to the FEL process by virtue of their greater energy loss and energy spread relative to the non-lasing portions of the electron bunch. Since the diagnostic is located downstream of the undulator it can be operated continuously without interrupting the beam to the users. The resolution of the new X-band system will be compared to the existing S-band RF deflecting diagnostic systems at SLAC and consideration is given to the required RF phase stability tolerances required for acceptable beam jitter on the monitor. Simulation studies show that about 1 fs (rms) time resolution is achievable in the LCLS over a wide range of FEL wavelengths and pulse lengths.