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 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.
Highlights
Electron beam applications such as free electron lasers (FELs), light sources based on energy recovery linacs, inverse Compton scattering sources, and ultrafast electron diffraction (UED) and microscopy (UEM), all require electron sources capable of generating beams with high transverse brightness and a properly controlled longitudinal phase space
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
In a gun scheme based on VHF technology, the core of the system is represented by a room temperature normal conducting (NC) copper cavity resonating in the very high frequency (VHF) range (30–300 MHz)
Summary
Electron beam applications such as free electron lasers (FELs), light sources based on energy recovery linacs, inverse Compton scattering sources, and ultrafast electron diffraction (UED) and microscopy (UEM), all require electron sources capable of generating beams with high transverse brightness and a properly controlled longitudinal phase space. For the case of relatively low repetition rate (100 Hzclass) applications, a number of radio frequency (rf) gun schemes, based on high frequency (≳1 GHz) room temperature resonant structures (cavities) and on photocathodes, have already demonstrated the required performance level to operate an x-ray FEL (notably transverse normalized emittances ranging from about 0.2 to 0.6 μm for charges per bunch ranging from about 10 to 300 pC—a more complete list of parameter requirements can be found elsewhere [1]). Higher fields at the cathode in combination with higher beam energies at the gun exit, allow for “quickly” accelerating the beam to relativistic energy, minimizing emittance dilution and longitudinal phasespace distortions due to space charge forces Such distortions, if not properly controlled, can severely affect the bunch compression capability required by FEL schemes for achieving the kA-class peak currents necessary for an efficient lasing process. We present and discuss possible upgrade options that would allow one to extend the VHF-gun performance towards these new goals
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