Abstract
We present a design study of an innovative scheme to generate high rep rate (MHz-class) GeV electron beams by adopting a two-pass two-way acceleration in a superconducting (SC) linac operated in continuous wave (CW) mode. The electron beam is accelerated twice by being reinjected in opposite direction of propagation into the linac after the first passage. Acceleration in opposite directions is accomplished thanks to standing waves supported in rf cavities. The task of recirculating the electron beam when it leaves the linac after first pass is performed by a bubble-shaped arc compressor composed by a sequence of double bend achromat. In this paper we address the main issues inherent to the two-pass acceleration process and the preservation of the electron beam quality parameters (emittance, energy spread, peak current) required to operate x-ray free electron lasers (FEL) with low jitters in the amplitude, spectral and temporal domain, as achieved by operating in seeding and/or oscillator mode a CW FEL up to 1 MHz rep rate. Detailed start-to-end simulations are shown to assess the capability of this new scheme to double the electron beam energy as well as to compress the electron bunch length from picoseconds down to tens of femtoseconds. The advantage of such a scheme is to halve the requested linac length for the same final electron beam energy, which is typically in the few GeV range, as needed to drive an x-ray FEL. The AC power to supply the cryogenic plant is also significantly reduced with respect to a conventional single-pass SC linac for the same final energy. We are reporting also x-ray FEL simulations for typical values of wavelengths of interest (in the 200 eV--8 keV photon energy range) to better illustrate the potentiality of this new scheme.
Highlights
Current x-ray free electron lasers (FELs) in operation are delivering c-ray high brilliance photon beams carrying a large number of photons per pulse, up to 1013, at quite modest rep rate up to 120 Hz
We present a design study of an innovative scheme to generate high rep rate (MHz-class) GeV electron beams by adopting a two-pass two-way acceleration in a superconducting (SC) linac operated in continuous wave (CW) mode
A CW operation at 1 MHz regular rep rate is ideal for spectroscopy with x-ray FEL pulses, in conjunction with high pulse-to-pulse spectral and amplitude stability, as typically achieved in full seeding operation mode of FELs, opposed to SASE operation, which is instead characterized by large amplitude and spectra jitters
Summary
Current x-ray free electron lasers (FELs) in operation are delivering c-ray high brilliance photon beams carrying a large number of photons per pulse, up to 1013, at quite modest rep rate up to 120 Hz. This paper reports the study of this newly conceived two-pass two way linear accelerator capable to significantly reduce footprint and AC power requirements, adding considerably to the compatibility of such kind of installation with typical university campus layouts We believe that this concept to implement two-pass acceleration in a linac greatly improve the sustainability of large-scale facilities where multi-GeV high beam power linacs are envisaged. The paper is organized as follows: in Sec. II we discuss the accelerator design criteria, issues concerning beam dynamics of high rep rate, the electron gun choice and high brightness electron beams in the specific context of two-way two-pass acceleration for driving x-ray FELs. Particular care is taken in examining the problems arising from the two-pass operation and compression in the arc; solutions are proposed for brightness preservation and matching. V, underlying that the two-way SC CW linac new paradigm promises a scheme suitable to reduce construction and operational costs, besides its footprint, allowing to conceive compact research infrastructures compatible with the typical size of a University Campus
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