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 $2/3\text{ }\text{ }\mathrm{ns}$ 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.
Highlights
The Compact Linear Collider (CLIC) is a proposed electron-positron collider with a maximum center of mass energy of 3 TeV
A cavity beam position monitors (BPMs) pickup consists of at least one cavity with the beam pipe passing through the middle
An output signal excited by a single bunch in a cavity BPM pickup is the decaying sinusoid of Eq (4)
Summary
The Compact Linear Collider (CLIC) is a proposed electron-positron collider with a maximum center of mass energy of 3 TeV. In order to achieve the target luminosity, each main beam must be transported through the main linac and beam delivery system without compromising the beam size at the interaction point. For this reason, the accelerator elements in these sections must be aligned more precisely than is possible with surveying methods alone. As well as using multiple beam pulses for the dispersion free steering, the possibility of using a single bunch train with an energy chirp along it must be accommodated. Each BPM must be able to make multiple independent position measurements within a single 156 ns long bunch train [3]. In order to achieve these resolutions, the use of cavity beam position monitors is proposed since they have been shown to routinely reach resolutions in the order of 10 nm [4]
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