H. Stiel, H. Legall

Laboratory Table Top Transmission X-ray Microscope (LTXM)

Funding: This project is funded by the BMBF (#13N8913).

Collaborations: Bruker ASC , FhG- ILT Aachen, KTH Stockholm, HZB-BESSY, FhG-IOF Jena, IfG GmbH

X-ray microscopy is today mostly performed at synchrotron facilities. Laboratory x-ray microscopy requires compact x-ray sources with high brigthness. The so called water window (2.3 - 4.4 nm) is of special interest for x-ray microscopy because in this spectral range the absorption of the surrounding water is smaller than that of the (organic) sample. As shorter the wavelength as higher the penetration depth of the radiation. If the operating wavelength of the microscope is near to the oxygen K-edge a sample thickness in the order of some mikrometers can be achieved.

Therefore the main goal of this project is the development of a laser plasma source with an emission wavelength as close as possible to the short wavelength edge of the water window. The Ly-alpha emission of highly ionized nitrogen is located at 2.48 nm and meets this requirement. In order to get a stable long-term operation of the source we chose a liquid nitrogen cryo-jet as target system. The design of the jet is based on developments at the KTH Stockholm.

Fig. 1: Laser plasma source for the water window.

In order to get the a high average photon flux of the source a high repetition rate pump laser system delivering 100 mJ pulses at < 1 ns duration is required. The pump laser system based on slab technology has been developed at FhG-ILT, Aachen. A prototype of this system delivers pulses of 100 mJ with 1.3 kHz repetition rate and a (adjustable) pulse duration as low as 400 ps.

Fig. 2: The pump laser for the water window laser plasma source developed at FhG-ILT

Parallel to the installation and optimization of the laser plasma source multilayer and zone plate condensor optics for the operating wavelength of the microscope are under development at FhG-IOF Jena and HZB-BESSY. A scheme of the setup for the laboratory table top transmission x-ray microscope is shown in Fig.1.

Fig. 3: Scheme of the Laboratory Transmission X-ray Microscope.

The laser plasma source including the 1.3 kHz laser system is now installed and first tests are already performed. A photograpg of the jet and the plasma is shown in Fig. 4.

Fig. 4: Laser induced plasma from the the liquid nitrogen jet.

First measurements of the spectral emission using 1.3 kHz laser system with pulse duration of 0.4 ns were already performed. The resulting emission spectrum is shown in Fig. 5. From this spectrum a photon flux per sterad and shot in the Lya-line emission of the liqiud nitrogen jet was estimated. The results obtained in these experiments show that the expected photon flux of 5*10e14 ph/s/sr can be ensured.

Fig. 5: Emission spectrum of the laser plasma source.

In a next step the laser plasma source will be equiped with a spherical multilayer condensor mirror developed at FhG-IOF Jena. The further system integration includes the installation of a sample stage, which will be provided by Bruker ASC. After comissioning of the completed LTXM first user experiments are planned together with HZB-BESSY paving the way for an final installation of the prototype at BLiX..

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