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

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 $2/3\mathrm{ns}$ 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.

Figure 1

Signal rise in amplitude and phase against the number of bunches as predicted by Eqs. (7) and (8).

Figure 2

Convergence limits in amplitude and phase of the multiple bunch signal as the number of bunches tends to infinity.

Figure 3

Image of the cavity BPM pickup assembly (left) with the position cavity at the bottom and the reentrant reference cavity at the top and the vacuum geometry (right).

Figure 4

Diagram of the CALIFES beam line showing the location of all major diagnostics, the prototype cavity BPM installation and the two dipole corrector magnets that were used to measure the position sensitivity of the pickup.

Figure 5

Design of a single digital down-converter channel used for the beam measurements.

Figure 6

Example digitized signals from the horizontal position channel of the cavity BPM excited by a short beam pulse and a long beam pulse.

Figure 7

Peak signals from the horizontal and vertical position cavity channels normalized by the reference cavity signal for the different settings of the digitally controlled variable attenuator. The attenuation in the reference channel was kept constant. Data from the bench measurement of the variable attenuator is included with an artificial offset.

Figure 8

Phase of the signals from the horizontal and vertical position cavity channels relative to the phase of the reference cavity signal for different settings of the variable attenuator. The attenuation in the reference channel was kept constant. Data from the bench measurement of the variable attenuator is included and all three data sets are mean subtracted.

Figure 9

Maximum reference signal amplitude for different beam pulse lengths.

Figure 10

Signal frequency measured every five samples along the length of 40 ns long pulses.

Figure 11

Total signal energy from the reference cavity for different bunch charges and pulse lengths of 30 and 60 ns.

Figure 12

Total signal energy from the horizontal position cavity channel normalized by bunch charge squared during a sweep in the horizontal beam position.

Figure 13

Signal amplitudes from the different channels during a vertical calibration with the phase of the signal from the vertical position cavity channel also shown.

Figure 14

Fit of the quadrature-phase component of the vertical position cavity signal against the in-phase component during a vertical position scan to measure the IQ rotation angle.

Figure 15

Fit of vertical position signal amplitude against the predicted beam position to measure the position scale factor. The tilt signal from the bunch tilt and beam trajectory angle is also shown.