Note: Study of extreme ultraviolet and soft x-ray emission of metal targets produced by laser-plasma-interaction
Review of Scientific Instruments
Different metal targets were investigated as possible source material for tailored laser-produced plasma-sources. In the wavelength range from 1 to 20 nm, x-ray spectra were collected with a calibrated spectrometer with a resolution of λ/ λ = 150 at 1 nm up to λ/ λ = 1100 at 15 nm. Intense line emission features of highly ionized species as well as continuum-like spectra from unresolved transitions are presented. With this knowledge, the optimal target material can be identified for the
... ed application of the source in x-ray spectrometry on the high energy side of the spectra at about 1 keV. This energy is aimed for because 1 keV-radiation is ideally suited for L-shell x-ray spectroscopy with nm-depth resolution. While increased effort is placed on research for the improvement of large-scale facilities such as synchrotron radiation facilities leading even to novel concepts such as the free-electron-laser, soft x-ray laboratory sources are still not available off-the-shelf. In the laboratory scale mainly plasma sources, either discharged or laser produced, can be used for applications in soft x-ray spectroscopy, 1, 2 nano-scale imaging, 3 or extreme ultraviolet (EUV) metrology. 4, 5 Discharged based EUV and soft x-ray sources have experienced a great progress in the last ten years but show two issues concerning possible routine usability. Firstly, the source size is relatively large (typically a few 100 μm up to 1 mm) reducing the possible spectral brightness and secondly these sources are not easily scalable to photon energies as high as 1 keV. Laser-produced plasma-sources (LPP) are well established in laboratory x-ray microscopy 3, 6 and x-ray transmission spectroscopy 7 in the water window as well as in EUV metrology. 4, 5 They exhibit a small source size (typically a few 10 μm) and dependent on the driver laser a sufficient high average power. The latter one is in principle scalable through the repetition rate of the laser. These factors and the possibility to use a broad emission range render LPP sources into interesting x-ray sources, especially in the soft x-ray regime. For an efficient operation in a wide energetic range, the choice of the optimal target material is crucial. While for the emission of low-Z material intense line emission from highly ionized species is a characteristic property, high-Z material produces a broad continuum-like spectrum. Thus, depending on the requirements posed by the experiment at hand, different target concepts must be chosen. The use of solid-state metal rods has proven to be a flexible and easy-to-operate concept 8 for a broad variety of target materials. When designing a LPP source another important aspect is debris mitigation for the protection of following optics. Numerous mechanisms using magnetic fields, 9 a) Electronic mail: email@example.com. gas atmosphere, 10 or foil traps 11 have already been published which may be combined for optimal operation. In this short note, we present a study of the x-ray emission of four low-Z metal targets as opposed to the emission of two high-Z targets. The investigated spectral range lies between 1 and 20 nm, respectively 100 to 1200 eV. In Fig. 1 the setup used in this work is depicted. The laser beam is focussed by an adapted telescope and focussing system onto a rotating and translating metal cylinder. The x-ray emission is detected with a variable line-space (VLS) -grating spectrograph. The used laser system consists of a regenerative amplifier based on Thin Disk Technology by TRUMPF Inc. The seed pulse is provided by a new type of pulsed Distributed Bragg Reflector (DBR) diode laser, developed by Ferdinand-Braun Institute, Berlin. It delivers output pulses at 1030 nm, with a variable pulse length between 200 ps and a few ns and an output pulse energy of >250 pJ. The seed pulse is amplified in the regenerative amplifier to pulse energies up to 250 mJ. For the measurements presented below, the laser has been operated at a pulse duration of 1.1 ns, energies up to 250 mJ and a repetition rate of 100 Hz. The variation of laser intensity is smaller than 2% over a time period of 1 h. The focal spot of the laser delivered by the adapted telescope and focussing system is highly symmetric resulting for a pulse energy of 230 mJ in an intensity of about 2 10 14 W/cm 2 on the target. Thus, using a simple blackbody radiation calculation an electron temperature of about 200 eV is estimated. For intense line emission in the soft x-ray region, we have chosen a cylinder target system with easily exchangeable cylinders made of Cu, Ti, Fe, and Al. These materials were chosen based on tabulated x-ray emission lines derived from the NIST database but also due to their availability, costs, and ease of machining as further criteria. Additionally, Mo and Ag were investigated as targets for broadband emission. In our test setup, the cylinder length of 50 mm and the diameter of 20.5 mm result in a possible operation time of about 3 h. The cylinder was continuously rotated and translated after a few revolutions of the target.