Beamline description

BEAMLINE LAYOUT AND OPTICAL COMPONENTS

General layout of FEL2 and the seed laser for users (SLU) photon delivery systems at MagneDyn. The intensity of the FEL pulses is first monitored by means of the undulator and safety hutch ionization chambers. The FEL intensity can be adjusted by using either a gas (nitrogen, neon, or helium) cell or solid-state thin film free-standing metallic filters. Then, the first deflecting mirror deviates the FEL2 pulses along the beamline. Pulse-to-pulse spectral monitoring is performed by means of a high-resolution spectrometer (TARDI). The pulses are then refocused by means of the Kirkpatrick–Baez active optics (KAOS) mirror bending system. A dedicated beamline delivers the SLU to MagneDyn, where a breadboard is assigned for the control and manipulation of the optical laser. The SLU consists of a portion of the seed laser that is delivered to the experimental stations via a low-vacuum beamline terminated by a quartz window. The main experimental chamber consists of an electromagnet coupled to an XUV polarimeter, a cryogenic sample environment, and pump–probe characterization tools and monitors. The second experimental station is equipped with an x-ray emission spectrometer for time/resolved RXES/RIXS experiments. The upstream chambers are equipped with a SLU–FEL coupling system made of in-vacuum mirrors, intensity monitors, solid-state filter wheels, and beam position monitors.
C. Svetina J. Synchrotron Rad.  (2016). 23, 98-105

beamline transmission

In addition to the mirror’s reflectivity, another important cause of loss of beamline transmission are the geometrical losses. These are due to the finite size of the mirrors as well as the presence of apertures as a function of the incoming wavelength. In fact the photon beam divergence is linearly correlated to the wavelength meaning that the longer the wavelength of the radiation, the more the transverse spot will enlarge along the beamline. Their effect will be negligible at wavelength below 20 nm where the beam is confined within the length of the mirrors. At longer wavelength, radiation emitted in the first stage, the geometrical cuts become relevant with a transmission going from 87% at 30 nm down to 45% at 60 nm.The overall beamline transmission is shown in figure having taken into account both reflectivity and geometrical losses in the whole wavelength range. Considering a photon flux at the source of about 100 µJ, we expect to obtain an intensity at the sample of the order of 58 µJ at 30 nm and 20 µJ at 6 nm. These intensities will correspond to a fluence at the sample of the order of 2.5e¹² W cm^-2 and 8.2e¹¹ W cm^-2 respectively. Of course the photon beam can be attenuated by means of a set of dedicated solid state filters (such as Aluminum, Zirconium, Palladium, etc.) and/or by using the PADReS gas attenuator already operative. Moreover the beam size can be easily enlarged by acting on the KAOS mirrors shape in order to drop even more the fluence if needed.

Seed Laser for Users (SLU)

The pulses dedicated to user experiments (referred to as SLU) are transported to the experimental stations via low-vacuum laser transport beamlines terminated by a quartz window and used for pump–probe experiments. A dedicated beam pointing feedback system secures the stability of the pointing of the laser beam to a few-μrad level. The laser pulse energy is up to 3 mJ and it can be decreased to a sub-μJ level by a two-stage variable polarization-based attenuator. The fundamental pulse duration is about 55 fs in FWHM at the maximum compression and can be lengthened by adding either positive or negative chirp in a grating-based compressor/stretcher.  The SLU system is on a dedicated optical table attached to the beamline chambers where pulse diagnostics, manipulations, and controls are performed. These include polarization state adjustment, harmonic conversion, pulse compression, pulse duration measurement, beam steering, pointing stabilization, and focusing.

SLU layout

A setup for non-linear frequency conversion can be used to generate the second [SHG (second harmonic generation) = 400 nm] and third [THG (third harmonic generation) = 266 nm] harmonics of the fundamental wavelength, allowing us to optically pump a large variety of metals, semiconductors, and rare-earth/transition metals oxides. This system uses a common-path type scheme and β-BaB2O4 (BBO) crystals for SHG and THG generation. Using different combinations of crystal thicknesses allows for optimizing the conversion efficiency or pulse duration depending on the needs of the measurements. The time delays between the fundamental and the SHG/THG pulses are compensated. The polarization of all harmonics can be varied ad-hoc by wave plates. Depending on the required wavelength, dielectric coated mirror sets are positioned after the SHG/THG stage for beam steering and removal of undesired lower harmonics. A compact fourth harmonic generation setup, delivering 200 nm light, is also available. If a tunable optical wavelength is required, the fundamental. SLU pulse can be, instead, used to pump a parametric amplifier TOPAS-C (Light Conversion). For this system, up-and-downconversion units are available, allowing to provide 60–150 fs range pulse spanning from UV to mid-IR, with energy per pulse from 10 to about 200 μJ. In this case, the optical setup for transporting and focusing the desired wavelength needs to be decided well in advance before the beamtime and may be limited by the availability of optimized optical components. The laser-FEL pulse-to-pulse timing at the sample can be set in the 1 fs to 1 ns range. The focused laser beam enters the high vacuum chambers of the MagneDyn end-station via a viewport flange equipped with a broadband anti-reflection coated optical quality window. A hollow-fiber-based pulse compression setup (HFC) has been recently added as an option to deliver sub-10 fs range IR pulses. The timing structure of the optical laser with 10 or 50 pulses per second reproduces the timing pattern of the FEL.

SLU intensities

Typical SLU pulse parameters at the sample. A lower repetition rate of the SLU can be customized with respect to the standard frequency (10 and 50 Hz). The abbreviation NM stands for “not measured.”

SLU temporal overlap and time zero

(left) relative change in the transient reflectivity (ΔR/R) of a single crystal of silicon measured at different probing wavelengths as a function of the delay time between the FEL pump and the SLU probe pulses. (right) Experimental SLU and FEL parameters for fine time zero measurements. SLU is used as a probe with almost comparable pulse energy (0.5 μJ) for all wavelengths.

 

 

Last Updated on Sunday, 30 April 2023 13:50