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Project ID: HPRI-CT-1999-50004
Finanziato nell'ambito di: FP5-HUMAN POTENTIAL
Paese: France

Ultrafast x-ray beamline (synchrotron)

One new and one upgraded beam lines have been constructed at Synchrotrons (LLC/MAXLAB & ESRF) to perform few 10ps and sub-picosecond x-ray experiments.

Beam time is in very high demand by European and National users owing to the increased interest in novel applications with ultrafast x-rays. The new beam line at LLC/MAXLAB will provide increased beam time for ultrafast x-ray experiments. It will be open to the European pool of users through different channels (Access with Research Infrastructures like “LaserLab” and collaboration between teams within Networks like “FLASH”). The ESRF beam line is already well known in the field as a pioneered x-ray installation for picosecond experiments. The upgrades achieved within the project will provide significant added value to the users as it will be described below. The beamlines are commissioned and opened to the European pool of users.

LLC/MAXLAB (D611) Beam line. It works with a bending magnet associated with the MAX II pulsed source operating at 100MHz. The duration of the pulses has been measured to be approximately 150ps using a streak camera. A laser providing pulses with durations of 20-30fs has been synchronized to the ring, and the fast x-ray streak camera which yields sub-ps time resolution has been installed. The laser operates at a maximum of 10kHz, which sets the data accumulation repetition rate. When using the single-photon counting streak camera developed in WP4, the repetition rate is reduced to 1kHz.

The characteristics are the following:
- Source: Bending magnet: 4mrad
- Focusing optics: Toroïd.
- Monochromator: Double Crystal monochromator with Si (111) or InSb (111) crystals.
- Energy range: 2.5 - 9 keV.
- Energy resolution: E/DE = 2 10{4}.
- Photon flux on sample: ~ 10{6}ph/s. (on laser repetition rate)
- Spot size on sample: 0.2(v) x 0.2(h)mm².
- Experimental station: The system consists of a short-pulse laser synchronised to the storage ring. A vacuum chamber providing pressures in the 10{-6} mbar range is available for combined laser/synchrotron radiation experiments. A streak camera detector can be used for sub-ps time-resolution or alternatively an avalanche photo diode for the ns range can be used.

ESRF Beam line. Its performance has been boosted in terms of: laser excitation to provide a complete photolysis of samples; optimisation of the sample environment; and time resolution of the x-ray pulse from 100ps to few 10ps.

X-ray Intensity on sample: A new so-called in-vacuum undulator, based on a 17mm magnetic period, has been designed and now gives a 10-20 increase in flux at an x-ray wavelength of 15keV. A second in-vacuum undulator, the U20, which has its first harmonic at 9.0keV has also been constructed. The third harmonic at 27keV will be 6 times more intense than the second harmonic of the U17. The first optical element in the beam line is the monochromator and its cooling system from water cooling has been upgraded to liquid nitrogen cooling. The improvement in flux density on the sample is 4-6 due to the improved flatness of the atomic net-planes. Finally, the toroïdal mirror, placed down stream the monochromator, has been replaced with a single-crystal silicon mirror of unprecedented optical finish. The new mirror produces a round focal spot of 0.100-mm diameter, with an increase of the flux density by 4 times. To summarize the overall gain is 40 on a day to day basis, which will open up new opportunities such as the use of streak cameras and fine-scanning the x-ray pulse.

Improvement of the time resolution: The time-resolution of our pump and probe experiments is given by the convolution of the x-ray pulse length (100ps) and the laser pulse length (125fs). In a single-shot experiment, the x-ray the pulse length is thus determining (and limiting) the time resolution. In more realistic experiments, the x-ray signal is accumulated on a CCD detector, and the stability of the time-stability of the delay between the laser and x-ray sources, has to be included. We have now measured the timing stability to be 3-5ps (rms) and that opens up the possibility to fine-slice the x-ray pulse with the laser pulse. Specifically one collects scan the time-delay in steps of 10-20ps around time zero and try to extract the differential signal.

Laser for sample excitation: The chirped-pulsed amplifier in our femtosecond laser used to excite the sample was replaced by a new diode-pumped system, the Hurricane from Spectra Physics. The energy per pulse has been raised from 0.45mJ to 1mJ. This will provide 2-3 times more laser photons on the sample, which will increase the signal to noise similarly in many chromophore systems. The set-up for time resolved X-ray experiments has then be improved with a new laser, the so-called TOPAS wavelength shifter (OPA), which increased the wavelength range and the available pulse energy.

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ESRF - European Synchrotron Radiation Facility
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