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

Abstract

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.

Knife-edge based measurement of the 4D transverse phase space of electron beams with picometer-scale emittance

“…Ultrafast electron beams with emittance values below the nanometer and beam size in the sub-micrometer regime are finding wider applicability, from ultrafast electron diffraction and microscopy, to Dielectric Laser Acceleration [14] and external electron injection in laserplasma accelerators. High brightness electron source R&D projects are being pursued in order to get higher brightness from the emission surface [31] [26] and increase the accelerating field, output energy together with the average electron flux [32]. We believe that techniques like the one presented here will become crucial as the R&D on the sources progresses.…”

High duty cycle EUV radiation source based on inverse Compton scattering

“…Recently, a two-cell version of VHF gun called APEX-2 was proposed with the fields and beam energy comparable to those targeted by SRF guns but with a lower cost and complexity. The gun can be CW operated at 162.5 MHz (the 1/8 sub-harmonics of 1.3 GHz) with an enhanced launching field (34 MV/m) at the cathode [Sannibale et al, 2017]. Initial simulation indicates that electron beam with 0.13 µm emittance at 100 pC bunch charge and 2 MeV energy could be pursued.…”

Two-pass two-way acceleration in a superconducting continuous wave linac to drive low jitter x-ray free electron lasers

“…High gradient, room temperature guns, allowing for emittance compensation scheme [6][7][8], are ruled out by their low rep rate operation (100 Hz class) [9], stemming from heat dissipation issues. On the other hand, DC and CW rf guns feature lower electric peak fields at the photocathode, ∼5 MV=m and ∼20 MV=m [9] respectively, resulting in a relatively low output beam energy of ∼400 keV and 800 keV, but at much higher rep rate or even in CW. Such a low beam energy strongly affects the beam dynamics (BD) along the machine, since efficient emittance compensation schemes fail in presence of a high bunch peak current.…”