DESY, Hamburg
CERN, Geneva
RIKEN/SPring-8, Hyogo
PSI, Villigen
XFELs require high precision orbit control in the long undulator sections. Due to the pulsed operation of these systems the high precision has to be reached by single bunch measurements. So far cavity BPMs achieve the required performance and will be used at the European XFEL between each of the 116 undulators. Coupling between the orthogonal planes limits the precision of beam position measurements. A first prototype build at DESY shows a coupling between orthogonal planes of about -20 dB, but the requirement is lower than -40 dB (1%). The next generation Cavity BPM was build with tighter tolerances and mechanical changes, the orthogonal coupling is measured to be lower than -43 dB. This report discusses the various observations, measurements and improvements which were done.
DESY, Hamburg
Future accelerators like the International Linear Collider and Free-Electron Lasers require beam position measurements with resolutions between few nanometres and 1 μm. Cavity Beam Position Monitors (BPM) are able to achieve the resolution. This paper shows the basic principles of this type of monitor, followed by a brief history of the developments. Since different institutes are designing Cavity BPM system for different projects, an overview is given on their recent developments including results and limitations compared with their requirements.
PSI, Villigen
A new current monitor has been built and installed during the last maintenance period in prevision of the high intensity beam operation (3mA, 1.8MW) which is planned in the near future. It is a re-entrant cavity tuned at the 2nd RF harmonic (101 MHz). Compared to the current monitors already in operation, the design had to be modified to improve its cooling. Indeed, this monitor is placed 8 m behind a graphite target and is exposed to scattered particles. The resulting heat load would raise the monitor temperature well above 200 deg C without cooling. The modifications include a slightly different structure to improve the heat conduction, a blackening of the external surface to increase the thermal radiation and an active water cooling. Thermocouples placed on the cavity will monitor the temperature of the system. The new design was supported by simulations for heat load resulting from the scattered particles and by calculations concerning the cooling efficiency. Results obtained during laboratory tests and at the beginning of operation will be presented. Comparison between expected heat load and temperature with the actual measured values will be also discussed.