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.
Stanford University, Stanford, California
SLAC, Menlo Park, California
We propose a single-stage scheme to produce coherent soft X-ray radiation directly from a UV seed laser. Seeding an electron bunch prior to compression simultaneously shortens the laser wavelength and increases the modulation amplitude. The final X-ray wavelength is tunable by controlling the compression factor with the RF phase. Photocathode beams with large energy spreads require corresponding large modulation amplitudes, leading to strong over-bunching during compression in the first chicane. We introduce a second chicane to unwind and restore the bunching. We also show that transportation of fine compressed modulation structure is feasible due to recompression in the second chicane.
SLAC, Menlo Park, California
LBNL, Berkeley, California
We present experimental studies of the gain length and saturation levels from 1.5 nm to 1.5 Å for a variety of conditions at the Linac Coherent Light Source (LCLS). By disrupting the FEL process with an orbit kick, we are able to measure the X-ray intensity as a function of the undulator length. This kick method is cross-checked with the method of removing undulator sections. We measure the FEL gain length as a function of X-ray wavelength, laser-heater induced energy spread, beta function and peak electron current. We also study the X-ray intensity level and FEL-induced electron energy loss after saturation as a function of undulator K value to determine the optimal taper. The experimental results are compared to analytical formulae and simulations.
SLAC, Menlo Park, California
The very bright electron beam required for an x-ray free-electron laser (FEL), such as the LCLS, is susceptible to a microbunching instability in the magnetic bunch compressors, prior to the FEL undulator. Using a 'laser heater', the uncorrelated electron energy spread in the LCLS can be increased by an order of magnitude to provide strong Landau damping against the instability without degrading the FEL performance. In this paper, we report the commissioning experience with the LCLS laser heater. We present detailed measurements of laser heater-induced energy spread, including the unexpected self-heating phenomenon when the laser energy is very low. We discuss the suppression of microbunching instability with the laser heater and its impact on the LCLS x-ray FEL performance. The experimental results are compared with theory and simulations where possible.
SLAC, Menlo Park, California
The effect of the drive laser transverse shape upon the electron beam emittance and FEL performance at 1.5 angstroms was studied at 250 pC for the Linac Coherent Light Source X-Ray FEL. Rectangular grids and cylindrically symmetric shapes were imaged onto the cathode and the emittance and FEL output were measured. Each pattern was truncated by a 1.2 mm diameter iris. The projected and time-sliced emittances as well as the electron bunch shape were measured at 135 MeV using a one micron thick optical transition radiation foil and a transverse RF deflecting cavity. The beam was then compressed and accelerated to 13.7 GeV and transported through the undulator. In our initial measurements, the 1.5 angstrom FEL pulse energy was determined from the energy loss of the electron beam. Future experiments will use an x-ray calorimeter. The gain length was obtained by measuring the FEL output along the undulator by deflecting the electron beam off the optical axis. These emittances and the FEL performance are compared with the nominal uniform transverse shape.
SLAC, Menlo Park, California
LCLS capabilities can be significantly extended with a second undulator aiming at the soft x-ray spectrum (0.5- 5 nm). To allow for simultaneous hard and soft x-ray operations, 14 GeV beams at the end of the LCLS accelerator can be intermittently switched into the SLAC A-line (the beam transport line to End Station A) where the second undulator may be located. Recently, a new optics has been designed to transport the LCLS beam through the A-Line while preserving the beam brightness. In this paper, we discuss the A-Line Soft X-ray FEL design ─ parameter selections and performance expectations with an energy-chirped LCLS beam as required by the A-Line optics. Start-to-end simulations using realistic LCLS beams show that it is possible to generate ~100 GW FEL power with the pulse duration as short as 1-fs.
SLAC, Menlo Park, California
Recent experiments on the LCLS accelerator have demonstrated low emittances for 20-pC bunches, with evidence for few-femtosecond electron bunch lengths, although the existing beam diagnostics do not allow a direct measurement of the bunch length. Simulations confirm that the LCLS accelerator can be operated at low charge (20 pC) while maintaining the nominal 3 kA peak current and with transverse emittances below 0.4 microns. An x-ray pulse duration of 2 femtoseconds with 3× 1011 photons is predicted, and nearly a single longitudinal spike may be obtained for soft x-ray wavelengths. We report on the operation of the accelerator and undulator with short electron bunches and present supporting simulation results.
SLAC, Menlo Park, California
LLNL, Livermore, California
Stanford University, Stanford, California
European X-ray Free Electron Laser Project Team, c/o DESY, Hamburg
Precision in-situ measurements of relative undulator segment K parameters were made at the LCLS and are reported here. We describe the methods used, systematics errors, and signal levels. A method for determining the central ray from each undulator segment was developed to control the effect of angle-energy correlation of the spontaneous radiation on the photon energy spectrum. A variety of photon-energy sensitive detectors were employed, including: Ni foil, the yttrium component in a YAG screen, and a narrow band monochromator followed by either a photodiode or a YAG screen. Different harmonics of the spontaneous radiation were also used.