Few-femtosecond time-resolved measurements of X-ray free-electron lasers

Abstract

X-ray free-electron lasers, with pulse durations ranging from a few to several hundred femtoseconds, are uniquely suited for studying atomic, molecular, chemical and biological systems. Characterizing the temporal profiles of these femtosecond X-ray pulses that vary from shot to shot is not only challenging but also important for data interpretation. Here we report the time-resolved measurements of X-ray free-electron lasers by using an X-band radiofrequency transverse deflector at the Linac Coherent Light Source. We demonstrate this method to be a simple, non-invasive technique with a large dynamic range for single-shot electron and X-ray temporal characterization. A resolution of less than 1 fs root mean square has been achieved for soft X-ray pulses. The lasing evolution along the undulator has been studied with the electron trapping being observed as the X-ray peak power approaches 100 GW.

Observation of femtosecond X-ray interactions with matter using an X-ray–X-ray pump–probe scheme

“…This condition of XFEL pulses, a pulse duration of 2 fs and a fluence of 3 × 10 6 J cm −2 (i.e., a photon density of 3 ×10 5 photons Å −2 , an intensity of ∼10 21 W cm −2 , and a radiation dose of 32 GGy to carbon atoms), would allow us to collect sufficient scattering signals from single protein molecules for determination of molecular orientation (26), which enables structural determination with atomic scale from multiple scattering images. As sub-10-fs XFEL pulses are routinely used in the current XFEL facilities (18,27), increasing the photon density by two orders higher than the value currently achieved would enable the X-ray scattering experiment of single protein molecules. Precisely shaped X-ray mirrors (28) producing an X-ray spot comparable to the size of protein molecules (although the spot size sets an upper limit of the molecule size to be analyzed) and increases in the number of emitted photons from XFELs by optimizing accelerator parameters for production of high-current and low-emittance electron beams, would provide a platform for scattering experiments of single molecule without X-ray-induced structural damage.…”

Conceptual and Technical Design Aspects of Accelerators for External Injection in LWFA

“…Many different technologies have been proposed to reach the necessary resolution: an X-band RF deflector [90] succeeded in achieving a sub-fs resolution at SLAC. Recently at UCLA the first sub-10 fs high brightness beam has been characterized using the same method [91].…”

Generating femtosecond X-ray pulses using an emittance-spoiling foil in free-electron lasers

“…To achieve even shorter pulses, a lower charge (20 pC) has been used for producing less than 10-fs X-rays. 3,4 Switching to this low-charge mode typically requires additional accelerator tuning and X-ray focusing optics adjustments. Another way to generate shorter FEL pulses is using an emittance-spoiling foil, 5 which has been in use at the LCLS since 2010.…”

“…7 For example, direct characterization of the X-ray pulses by streaking spectroscopy [8][9][10] has been demonstrated, but requires a specific experimental set-up not yet readily available as a diagnostic for user experiments. Recently, at the LCLS, another method by measuring the lasing "footprint" on the electron beam with an X-band radiofrequency (RF) transverse deflecting cavity (XTCAV) 4,11 has been developed as an online electron and X-ray temporal diagnostic. This provides a single-shot diagnostic at the full beam repetition rate.…”

“…This lasing "footprint" on the electron longitudinal phase space is measured directly with the XTCAV diagnostic. 4 The XTCAV provides a time-dependent horizontal deflection to the electron bunch, followed by a DC, vertically bending spectrometer magnet. With this setup, we make absolute measurements of the beam time and energy distribution with knowledge of the deflecting cavity strength and the bend angle.…”