Long bunch trains measured using a prototype cavity beam position monitor for the Compact Linear Collider

Abstract

The Compact Linear Collider (CLIC) requires beam position monitors (BPMs) with 50 nm spatial resolution for alignment of the beam line elements in the main linac and beam delivery system. Furthermore, the BPMs must be able to make multiple independent measurements within a single 156 ns long bunch train. A prototype cavity BPM for CLIC has been manufactured and tested on the probe beam line at the 3rd CLIC Test Facility (CTF3) at CERN. The transverse beam position is determined from the electromagnetic resonant modes excited by the beam in the two cavities of the pickup, the position cavity and the reference cavity. The mode that is measured in each cavity resonates at 15 GHz and has a loaded quality factor that is below 200. Analytical expressions for the amplitude, phase and total energy of signals from long trains of bunches have been derived and the main conclusions are discussed. The results of the beam tests are presented. The variable gain of the receiver electronics has been characterized using beam excited signals and the form of the signals for different beam pulse lengths with the 2=3 ns bunch spacing has been observed. The sensitivity of the reference cavity signal to charge and the horizontal position signal to beam offset have been measured and are compared with theoretical predictions based on laboratory measurements of the BPM pickup and the form of the resonant cavity modes as determined by numerical simulation. Finally, the BPM was calibrated so that the beam position jitter at the BPM location could be measured. It is expected that the beam jitter scales linearly with the beam size and so the results are compared to predicted values for the latter.

The CLIC project

“…A low-Q cavity BPM for the CLIC main beam was constructed and tested at CTF3. Its ability to measure the beam position for a 200 ns long train of bunches with a time accuracy better than 20 ns was demonstrated [53].…”

Development of a real-time cavity-based injection efficiency monitor system at SSRF

“…The signal amplitude of the first monopole-mode TM 010 excited in a pillbox cavity is independent of the beam position under paraxial approximation but proportional to the bunch charge [14]. In addition, the mode signal of the cavity monitor has a high signal-to-noise ratio (SNR) which allows high-resolution bunch charge measurement [15,16].…”