Abstract
Plasma accelerators (Esary et al 2009 Rev. Mod. Phys. 81 1229) are a potentially important source of high energy, low emittance electron beams with high peak currents generated within a relatively short distance. As such, they may have an important application in the driving of coherent light sources such as the Free Electron Laser (FEL) which operate into the x-ray region (McNeil and Thompson 2010 Nat. Photon. 4 814–21). While novel plasma photocathodes (Hidding et al 2012 Phys. Rev. Lett. 108 035001) may offer orders of magnitude improvement to the normalized emittance and brightness of electron beams compared to Radio Frequency-driven accelerators, a substantial challenge is the energy spread and chirp of beams, which can make FEL operation impossible. In this paper it is shown that such an energy-chirped, ultrahigh brightness electron beam, with dynamically evolving current profile due to ballistic bunching at moderate energies, can generate significant coherent radiation output via the process of Coherent Spontaneous Emission (CSE) (Campbell and McNeil 2012 Proc. FEL2012 (Nara, Japan)). While this CSE is seen to cause some FEL-induced electron bunching at the radiation wavelength, the dynamic evolution of the energy chirped pulse dampens out any high-gain FEL interaction. This work may offer the prospect of a future plasma driven FEL operating in the high-gain Self Amplified CSE mode.
Highlights
Introduction ce cri ptFree electron lasers (FELs) are sources of high-power coherent radiation, tunable over a wide range of the electromagnetic spectrum from the far-infrared into the hard Xray
Plasma accelerators [1] are a potentially important source of high energy, low emittance electron beams with high peak currents generated within a relatively short distance. They may have an important application in the driving of coherent light sources such as the Free Electron Laser (FEL) which operate into the X-ray region [2]
FELs operating at the shorter wavelengths are unique sources, beyond the reach of current conventional lasers, that open up new areas of scientific exploration in diverse fields of study from the creation of warm dense matter to structural and functional biology [5]
Summary
Free electron lasers (FELs) are sources of high-power coherent radiation, tunable over a wide range of the electromagnetic spectrum from the far-infrared into the hard Xray. PWFA-based plasma photocathodes combine intrinsic advantages of PWFA such as dark-current-free and phase-constant operation and long acceleration lengths with far-reaching decoupling between the plasma medium which supports the wakefield (e.g. hydrogen) and the one supporting the plasma photocathode medium (e.g. helium) In this approach, an electron driver beam sets up a hydrogen-based plasma wave and its characteristic and co-moving ’bubble’-like structure, while a spatiotemporally synchronized laser pulse is focused inside the bubble structure. The driver electron beam employed to excite the hydrogen PWFA based accelerator cri pt structure in which the central plasma photocathode mechanism can be realized may come from a linac, and may come from a compact laser wakefield accelerator, as many characteristics of LWFA electron beam output such as high current and charge density and modest energy spread and emittance are very favourable or at least not prohibitive for driving a PWFA stage [27, 29]. CSE can help mitigate the effects of a homogeneous electron energy spread in beams without an energy chirp, significantly reducing the start-up time and enhancing the generation of high intensity, short, superradiant radiation pulses from a poor-quality electron pulse [33]
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