For the LCLS-II instruments we are developing laser power meters as compact intensity monitors that can operate at soft and tender X-ray photon energies. There is a need to monitor the relative X-ray intensity at various locations along an X-ray FEL beamline in order to observe a possible decrease in the reflectivity of X-ray mirrors. In addition for experiments, it is valuable to know the absolute intensity at the sample. There are two types of laser power meters based on thermopile and pyroelectric sensors. The thermopile power meters measure an average temperature and are compatible with the high repetition rates of LCLS-II. Pyroelectric power meters provide a pulse-by-pulse response. Ultra-high vacuum compatibility is being tested by residual gas analysis. An in-house development beamtime is being conducted at the LCLS SXR instrument. Measurements using both thermopile and pyroelectric power meters will be conducted at a set of photon energies in the soft X-ray range. The detectors’ response will be compared with the gas monitor detector installed at the SXR instrument.
We report experimental demonstration of capturing single-shot X-ray Free-electron Laser (FEL) beam profiles using gas fluorescence. The measurement was carried out at the Linac Coherent Light Source using 7 keV hard X-rays propagating through ambient air. The nitrogen fluorescence emitted upon the passage of the X-ray FEL beam were imaged using a highly sensitive optical setup, and there was sufficient optical yield that single-shot measurements were feasible. By taking two orthogonal and simultaneous images, the beam trajectory could be determined in a nearly non-invasive manner, and is best suited for photon energies in the soft X-ray regime, where such a diagnostic capability has been largely unavailable previously. The integrated intensity of the images could also serve as a non-invasive intensity monitor, complementary to current implementations of gas- and solidbased monitors. High repetition-rate Free-electron Lasers can greatly benefit from such a new diagnostic tool for eliminating potential thermal damages.
The generation of two X-ray pulses with tunable nanosecond scale time separations has recently been demonstrated
at the Linac Coherent Light Source using an accelerator based technique. This approach offers the opportunity
to extend X-ray Photon Correlation Spectroscopy techniques to the yet unexplored regime of nanosecond
timescales by means of X-ray Speckle Visibility Spectroscopy. As the two pulses originate from two independent
Spontaneous Amplified Stimulated Emission processes, the beam properties fluctuate from pulse pair to pulse
pair, but as well between the individual pulses within a pair. However, two-pulse XSVS experiments require the
intensity of the individual pulses to be either identical in the ideal case, or with a accurately known intensity
ratio. We present the design and performances of a non-destructive intensity diagnostic based on measurement
of scattering from a transparent target using a high-speed photo-detector. Individual pulses within a pulse pair
with time delays as short as 0.7 ns can be resolved. Moreover, using small angle coherent scattering, we characterize
the averaged spatial overlap of the focused pulse pairs. The multi-shot average-speckle contrasts from
individual pulses and pulse pairs are compared.
Intensity interferometry measurements were carried out to study the spatial coherence properties of a Free-Electron Laser (FEL) in the Self-Amplified Spontaneous Emission (SASE) mode in the hard X-ray regime. Statistical analyses based on ensemble averages of the spatial intensity correlation function were performed on a large number of pulses, overcoming challenges associated with the FEL beam being non-stationary in time and highly collimated. The second-order intensity correlation functions consistently show deviations from unity, reminiscent of the classical Hanbury-Brown and Twiss effect. They also exhibit a slow decaying spatial dependence at length-scales larger than the width of the beam, indicating a high degree of spatial coherence. These measurements are consistent with the behavior of a highly brilliant but chaotic source obeying Gaussian statistics as expected for a SASE FEL. Our study could be used to devise an in-line diagnostic capable of providing quasi real-time feedback for understanding and tuning the FEL process.
The recent success of the X-ray Free Electron Lasers has generated great interests from the user communities of a wide range of scientific disciplines including physics, chemistry, structural biology and material science, creating tremendous demand on FEL beamtime access. Due to the serial nature of FEL operation, various beam-sharing techniques have been investigated in order to potentially increase the FEL beamtime capacity. Here we report the recent development in using thin diamond single crystals for spectrally splitting the FEL beam at the Linac Coherent Light Source, thus potentially allowing the simultaneous operation of multiple instruments. Experimental findings in crystal mounting and its thermal performance, position and pointing stabilities of the reflected beam, and impact of the crystal on the FEL transmitted beam profile are presented.