High bandwidth pickup design for bunch arrival-time monitors for free-electron laser

In this paper, we present the design and realization of high bandwidth pickup electrodes with a cutoff frequency above 40 GHz. The proposed cone-shaped pickups are part of a bunch arrival-time monitor designed for high ($>500\mathrm{pC}$) and low (20 pC) bunch charge operation mode providing for a time resolution of less than 10 fs for both operation modes. The proposed design has a fast voltage response, low ringing, and a resonance-free spectrum. For assessing the influence of manufacturing tolerances on the performance of the pickups, an extensive tolerance study has been performed via numerical simulations. A nonhermetic model of the pickups was built for measurement and validation purposes. The measurement and simulation results are in good agreement and demonstrate the capability of the proposed pickup system to meet the given specifications.

Figure 1

Cross section of the cone-shaped pickup with the dimensions.

Figure 2

Plot of the slope vs the number of mesh cells and two settings of the local mesh refinement. Two symmetry planes are used for the simulation such that only one quarter of the model has to be considered. The slope converges to approximately $417\mathrm{mV}/\mathrm{ps}$.

Figure 3

Simulation results of one pickup in time domain (top) and the respective frequency spectrum normalized by the spectrum of the particle beam (bottom).

Figure 4

Influence of geometric variations on the slope (top) and on the ringing (bottom).

Figure 5

Realization of a nonhermetic pickup prototype. Top: Pickup system with four integrated pickups. Bottom: Disassembled pickup element.

Figure 6

Reflection coefficients measured at the connectors.

Figure 7

Measured transmission coefficient between the ports. Top: Between neighboring ports. Bottom: Between opposite ports.

Figure 8

Comparison between simulation and measurements. Top: Between neighboring ports. Bottom: Between opposite ports.

Figure 9

Proposed rf cabling diagram for low and high charge operation mode of the bunch arrival-time monitor (BAM).