Upgrade possibilities for continuous wave rf electron guns based on room-temperature very high frequency technology

The past decade was characterized by an increasing scientific demand for extending towards higher repetition rates (MHz class and beyond) the performance of already operating lower repetition rate accelerator-based instruments such as x-ray free electron lasers (FELs) and ultrafast electron diffraction (UED) and microscopy (UEM) instruments. Such a need stimulated a worldwide spread of a vibrant R&D activity targeting the development of high-brightness electron sources capable of operating at these challenging rates. Among the different technologies pursued, rf guns based on room-temperature structures resonating in the very high frequency (VHF) range (30–300 MHz) and operating in continuous wave successfully demonstrated in the past few years the targeted brightness and reliability. Nevertheless, recently proposed upgrades for x-ray FELs and the always brightness-frontier applications such as UED and UEM are now requiring a further step forward in terms of beam brightness in electron sources. In this paper, we present a few possible upgrade paths that would allow one to extend, in a relatively simple and cost-effective way, the performance of the present VHF technology to the required new goals.

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Figure 1

A CAD cross section of the existing APEX VHF gun.

Figure 2

(a) 3D CAD view of a double-cell version of APEX and of its main parts. (b) Electric field distribution inside the cells. (c) Longitudinal electric field intensity along the longitudinal axis of the gun.

Figure 3

Electric field distributions for two additional APEX-2 configuration options. (a) Single-cell with dual-reentrant nosecones. (b) Single cell with drift tube. The design field at the cathode for both configurations is $\sim 35\mathrm{MV}$/m. See the text for more details.

Figure 4

APEX-like injector layout used for the beam dynamics simulations.

Figure 5

Fronts of 100 pC solutions illustrating the trade-off between emittance (100% of particles included) and rms bunch length at the end of the injector. Top: Present APEX VHF-gun case. Bottom: The fronts for the cases of the double cell and single cell with no drift tube APEX-2 configurations described in the text.

Figure 6

High resolution simulation using astra with 250k macroparticles showing details of a particular 100 pC solution selected from the optimization front shown in Fig. 5 for the double-cell APEX-2 configuration case. The blue and orange curves indicate the results for 100% and 95% of the particles respectively.