Novel hybrid reflection zone plates as dispersive elements will allow time-resolved Near Edge X-ray Absorption Fine Structure (tr-NEXAFS) studies in a wide photon energy range from 100 – 1500 eV. We describe two tr-NEXAFS setups using a laser produced plasma (LPP) as well as a high harmonics generation (HHG) source.
Laboratory based laser driven short pulse X-ray sources like laser produced plasmas (LPP) and high harmonic generation (HHG) exhibit a great potential for spectroscopy in the soft X-ray range. These sources are complementary to large scale facilities like synchrotrons or free electron lasers. For applications of LPP or HHG sources for time-resolved X-ray absorption spectroscopy in the water window or beyond a high photon flux is crucial. The available photon flux strongly depends on energy, pulse duration and repetition rate of the pump laser. Depending on the experimental needs in timeresolved experiments pulse durations of the X-ray pulse ranging from nanoseconds to sub-femtoseconds are required. In our contribution we will present a highly brilliant LPP source emitting soft X-rays in the photon energy range between 50 and 1500 eV based on CPA and thin disk laser technology as well as the high average power thin disk laser based OPCPA system for high photon flux HHG. In addition we present a new generation of reflection zone plates on spherical substrates, that promises a remarkable high resolution over a wide spectral range making it an ideal and highly efficient diffractive optic for time-resolved NEXAFS experiments in the lab.
In this contribution, we report about nanoscale imaging using a laser produced plasma source based laboratory transmission X-ray microscope (LTXM) in the water window. The highly brilliant soft X-ray radiation of the LTXM is provided by a laser-produced nitrogen plasma source focused by a multilayer condenser mirror to the sample. An objective zone plate maps the magnified image of the sample on the super resolution camera. This camera employs a deep cooled soft-X-ray CCD imaging sensor sandwiched with a xy piezo stage to allow subpixel displacements of the detector. The camera is read out using a very low noise electronics platform, also directing low µm shifts of the sensor between subsequent image acquisitions. Finally an algorithm computes a high resolution image from the individual shifted low-resolution image frames.
In this article we present experimental results from a counter-propagating two laser pulse experiment at high intensity
and using ultrathin gold and plastic foil targets. We applied one laser pulse as a pre-pulse with an intensity of up to
1x1018 W/cm2. By this method we manipulated the pre-plasma of the foil target with which the stronger laser pulse with
an intensity of 6x1019W/cm2 interacts. This alters significantly subsequent processes from the laser plasma interaction
which we show the ion acceleration and high harmonic generation. On the one hand, the maximum kinetic ion energy
and the maximum charge state for gold ions decline due to the pre-heating of the target in the time range of few ps, on
the other hand the number of accelerated ions is increased. For the same parameter range we detected a significant raise
of the high harmonic emission. Moreover, we present first experimental observations, that when the second laser pulse
is applied as a counter-propagating post-pulse the energy distribution of accelerated carbon ions is charge selective
altered. Our findings indicate that using this method a parametric optimization can be achieved, which promises new
insights about the concurrent processes of the laser plasma dynamics.
A method, how electrons can be directly accelerated in intense laser fields, is investigated experimentally and discussed with numerical and analytical simulation. When ultrathin foil targets are exposed with peak laser intensities of ~ 1x1020 W/cm2 , slow electrons ( ~ keV kinetic energy), that are emitted from the ultrathin foil target along laser propagation direction, are post-accelerated in the transmitted laser field. They received significant higher kinetic energies (MeV), when this interaction was limited in duration and an enhanced number of fast electrons were detected. The decoupling of the light field from the electron interaction we realized with a second separator foil, blocking the transmitted laser light at a particular distance and allowing the fast electrons to pass. Variation of the propagation distance in the laser field results in different energy gains for the electrons. This finding is explained with electron acceleration in the electromagnetic field of a light pulse and confirms a concept being discussed for some time. In the experiments the effect manifests in an electron number amplification of about 3 times around a peak at 1 MeV electron energy. Measurements confirmed that the overall number in the whole bunch is enhanced to about 109 electrons covering kinetic energies between 0.5 to 5 MeV. The method holds promise for ultrashort electron bunch generation at MeV energies for direct application, e.g. ultra-fast electron diffraction, or for injection into post accelerator stages for different purposes.
E. Schleifer, E. Nahum, S. Eisenmann, M. Botton, A. Baspaly, I. Pomerantz, F. Abricht, J. Branzel, G. Priebe, S. Steinke, A. Andreev, M. Schnuerer, W. Sandner, D. Gordon, P. Sprangle, K.W. Ledingham, A. Zigler
Compact size sources of high energy protons (50-200MeV) are expected to be key technology in a wide range of scientific applications 1-8. One promising approach is the Target Normal Sheath Acceleration (TNSA) scheme 9,10, holding record level of 67MeV protons generated by a peta-Watt laser 11. In general, laser intensity exceeding 1018 W/cm2 is required to produce MeV level protons. Another approach is the Break-Out Afterburner (BOA) scheme which is a more efficient acceleration scheme but requires an extremely clean pulse with contrast ratio of above 10-10. Increasing the energy of the accelerated protons using modest energy laser sources is a very attractive task nowadays. Recently, nano-scale targets were used to accelerate ions 12,13 but no significant enhancement of the accelerated proton energy was measured. Here we report on the generation of up to 20MeV by a modest (5TW) laser system interacting with a microstructured snow target deposited on a Sapphire substrate. This scheme relax also the requirement of high contrast ratio between the pulse and the pre-pulse, where the latter produces the highly structured plasma essential for the interaction process. The plasma near the tip of the snow target is subject to locally enhanced laser intensity with high spatial gradients, and enhanced charge separation is obtained. Electrostatic fields of extremely high intensities are produced, and protons are accelerated to MeV-level energies. PIC simulations of this targets reproduce the experimentally measured energy scaling and predict the generation of 150 MeV protons from laser power of 100TW laser system18.
The effective action of strong electrical fields on a beam of protons passing through a laser irradiated thin foil has been investigated. The energy distribution function of protons propagating along the surface normal changes in a pronounced way, exhibiting a gap in the spectrum accompanied by up to two local maxima. The temporal behavior is set into context with expectations derived from the evolution of strong electrical fields at the plasma-vacuum interface, usually being considered responsible for fast ion acceleration during the initial stage of laser driven plasma expansion. Our investigation reveals complex field effects in thin foils when irradiated with intense and ultra-short pulses with a very high temporal contrast. The experiments were performed with a laser accelerated proton beam, the probe, traversing a “plasma slab” created by ultra-short ( 80fs), high-intensity (~ 1 × 1019 W/cm2) laser irradiation of a 30 nm to 800 nm thick foil. Laser pulses with different temporal contrast and pulse duration have been used, both for the probe and for the plasma slab creation (the pump). An analytical model is discussed to approach an understanding of the observation.
Beams of energetic negative ions and neutral atoms are obtained from water and ethanol spray targets irradiated by high
intensity (5×1019 W/cm2) and ultrashort (50 fs) laser pulses. The resulting spectra were measured with the Thomson parabola spectrometer, which enabled absolute measurements of both: positive and negative ions. The generation of a beam of energetic neutral hydrogen atoms was confirmed with CR-39 track detectors and their spectral characteristics have been measured using time of flight technique. Generation is ascribed to electron-capture and -loss processes in the collisions of laser-accelerated high-energy protons with spray of droplets. The same method can be applied to generate energetic negative ions and neutral atoms of different species.
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