High-resolution cavity bunch arrival-time monitor for charges less than 1 pC

Pump-probe experiments using x-ray pulses or MeV ultrafast electrons as probes are important for studying ultrafast dynamic processes at the atomic level. Increasing demand is being focused on ultrashort x-ray pulses and electron pulses to research processes down to a few femtoseconds. Due to the limitation of the space charge force, only low charge electron bunches with critical synchronization can achieve such short pulses. A high-precision beam arrival-time monitor for the low charge is especially important. In this paper, a low $Q_l$ cavity with high sensitivity is designed to measure the arrival time of charges below 1 pC. The pickup is particularly optimized with high $R/Q$ at 4.76 GHz to provide the maximum signal, and the cold test results are in good agreement with the simulations. An IQ demodulation scheme is developed for the readout electronics. In the beam tests, the measured resolution of the beam arrival-time monitor is 33.46 fs at 0.5 pC.

Figure 1

Electromagnetic fields of the monopole mode.

Figure 2

Schematic cross section of the cavity. In the $R/Q$ optimization process, the cavity is built without the coupling structure (the blue dotted line).

Figure 3

$R/Q$ variations of the cavity at 4.76 GHz with the cavity length and radius.

Figure 4

Mechanical errors in three dimensions.

Figure 5

rf parameter variations of ${\mathrm{TM}}_{110}$ mode from the tolerance analysis.

Figure 6

BAM prototype: two parts of the pickup body and the assembly after welding.

Figure 7

Schematic diagram of the BAM electronics.

Figure 8

Amplitude linearity test results of the electronics.

Figure 9

Phase and amplitude errors of the electronics with a constant rf input.

Figure 10

Schematic diagram of the THz beamline.

Figure 11

Measured and simulated raw voltage signals from the pickup.

Figure 12

Example of the baseband signals (a). Amplitude and phase waveforms calculated from the $I$, $Q$ signals (b).

Figure 13

ADC counts of the BAM vs charge read by the SBPM.

Figure 14

Charges measured by two BAMs (a). Distribution of residual charge (b).

Figure 15

Arrival times measured by two BAMs (a). Distribution of residual arrival time (b).

Figure 16

BAM resolution dependence on the charge.