Abstract
Generation of attosecond x-ray pulses is attracting much attention within the x-ray free-electron laser (FEL) user community. Several schemes using extremely short laser pulses to manipulate the electron bunches have been proposed. In this paper, we extend the attosecond two-color ESASE scheme proposed by Zholents et al. to the long optical cycle regime using a second detuned laser and a tapered undulator. Both lasers can be about ten-optical-cycles long, with the second laser frequency detuned from the first to optimize the contrast between the central and side current spikes. A tapered undulator mitigates the degradation effect of the longitudinal space charge (LSC) force in the undulator and suppresses the FEL gain of all side current peaks. Simulations using the LCLS parameters show a single attosecond x-ray spike of {approx} 110 attoseconds can be produced. The second laser can also be detuned to coherently control the number of the side x-ray spikes and the length of the radiation pulse.
Highlights
Ultrashort soft and hard x-ray sources have the potential to open new regimes in atomic and electronic processes, benefiting widespread fields in physics, chemistry, and biology
We extend the attosecond two-color enhanced self-amplified spontaneous emission scheme proposed by Zholents et al to the long optical cycle regime using a second detuned laser and a tapered undulator
The two-color scheme proposed in Ref. [6] is based on the current-enhanced self-amplified spontaneous emission (ESASE) technique proposed by Zholents [10]
Summary
Ultrashort soft and hard x-ray sources have the potential to open new regimes in atomic and electronic processes, benefiting widespread fields in physics, chemistry, and biology. We extend the attosecond two-color ESASE scheme to the long optical cycle regime using a detuned second laser and a tapered undulator by taking advantage of the LSC field. Our scheme extends to FELs a technique proposed for high-order harmonic generation (HHG) to manipulate an electron wave packet for the generation of isolated attosecond soft x-ray pulses [12]. HHG experiments have shown a controlled detuning between the two colors can control the electron wave packet energy (low energy regime) in the subcycle domain using relatively long infrared laser pulses (up to 15 optical cycles). A tapered undulator mitigates the degradation effect of the LSC force in the undulator [11] and suppresses the FEL gain of all side current spikes
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