SLAC, Menlo Park, California
The Echo-enabled Harmonic Generation (EEHG) FEL uses two modulators in combination with two dispersion sections that allow one to generate in the beam a high-harmonic density modulation starting with a relatively small initial energy modulation of the beam.[1] After presenting the concept of the EEHG, we address several practically important issues, such as the effect of coherent and incoherent synchrotron radiation in the dispersion sections, the beam transverse size effect in the modulator, etc. Using a representative realistic set of beam parameters, we show how the EEHG scheme enhances the FEL performance and allows one to generate a fully (both longitudinally and transversely) coherent radiation.
[1] G. Stupakov, PRL, 102, 074801 (2009).
D. Xiang and G. Stupakov, PRSTAB, 030702 (2009).
SLAC, Menlo Park, California
We study effects of energy chirp on echo-enabled harmonic generation (EEHG). Analytical expressions are compared with numerical simulations for both harmonic and bunching factors. We also discuss the EEHG free-electron laser bandwidth increase due to an energy-modulated beam and its pulse length dependence on the electron energy chirp.
SLAC, Menlo Park, California
If the energy spread of a beam is larger then the Pierce parameter, the FEL gain length increases dramatically and the FEL output gets suppressed. We show that if the energy distribution of such a beam is modulated on a small scale, the gain length can be noticeably decreased. Such an energy modulation is generated by first modulating the beam energy with a laser via the mechanism of inverse FEL, and then sending it through a strong chicane. We show that this approach also works for the optical klystron enhancement scheme. Our analytical results are corroborated by numerical simulations.
SLAC, Menlo Park, California
Stanford University, Stanford, California
We propose and analyze a scheme to achieve a seeded hard x-ray source based on a two-stage echo-enabled harmonic generation (EEHG) FEL. In the scheme a 180 nm seed laser covering the whole bunch is first used to modulate the beam when beam energy is 2 GeV. After passing through a strong chicane complicated fine structures are introduced into the phase space. The beam is again modulated by a short 180 nm laser that only covers the rear part of the beam and then accelerated to 6 GeV. A weak chicane is then used to convert the energy modulation to density modulation. The density-modulated beam is sent through a radiator to generate intense 6 nm radiation which will be time-delayed to interact with the front fresh part of the bunch. Finally we generate in the beam density modulation at the 1199th harmonic of the seed laser. We will discuss the issues related to the realization of the seeded hard x-ray FEL.
SLAC, Menlo Park, California
We propose a scheme that combines the echo-enabled harmonic generation technique with the bunch compression and allows one to generate harmonic numbers of a few hundred in a microbunched beam through up-conversion of the frequency of an ultraviolet seed laser. A few-cycle intense laser is used to generate the required energy chirp in the beam for bunch compression and for selection of an attosecond x-ray pulse. Sending this beam through a short undulator results in an intense isolated attosecond x-ray pulse. Using a representative realistic set of parameters, we show that 1 nm x-ray pulse with peak power of a few hundred MW and duration as short as 20 attoseconds (FWHM) can be generated from a 200 nm ultraviolet seed laser. The proposed scheme may enable the study of electronic dynamics with a resolution beyond the atomic unit of time (∼24 attoseconds) and may open a new regime of ultrafast sciences.