Novel laser applications such as laser-wake-field acceleration of particles require extreme parameters from ultra-short-pulse systems. We propose a concept capable to realize simultaneously multi-TW peak powers and multi-kW average powers by employing spatially and temporally separated amplification of chirped laser pulses delivered by fiber-amplifiers. As a combining element for the temporally 100 ns separated pulses (10 MHz repetition rate) we suggest a non-steady-state enhancement cavity using a fast switching-element to dump out the enhanced pulses at 15 kHz.
An Yb-based 78-MHz repetition rate fiber-amplified frequency comb is used to investigate the power scaling
limitations of a standard-design bow tie high-finesse enhancement cavity for XUV generation. With a Xenon
gas jet in the 22-μm-radius focus, the 200-fs intra-cavity circulating pulse reaches a maximum of 20 kW of time-averaged
power. A novel cavity design is presented, conceived to overcome the observed enhancement limitations
and offering the prospect of few-nm high-power high-harmonic generation. Several applications which come into
reach for the first time are discussed.
The concept of an electron gun for generating pulses with a duration <10 fs at energies suitable for electron diffraction experiments is presented. The principle is based on an rf cavity oscillating in the TM010 mode. Laser pulses for photoemission are injected at a well-defined phase of the rf oscillation such that electrons with different initial velocities and different time delays arrive at a target within a very small temporal window. Coulomb broadening is prevented by reducing the number of electrons to the level of a single electron per pulse while increasing the repetition rate to the MHz range. The fs-electron pulses generated will advance the time-resolution of electron diffraction experiments to the level of a vibrational period of molecules.
We propose to generate few-fs or as X-ray laser pulses by beating of two or more X-ray laser lines with appropriate frequency separation. X-ray lasers operating on transitions in neon- or nickel-like ions typically have gain on several lines with difference frequencies of around 1015 Hz. Moreover, it is found in specific cases that a few almost equidistant lines may exhibit gain. Beating of these lines results in a series of pulses with durations down to the attosecond range. It is shown that phase locking can be achieved by means of a Langmuir wave in the X-ray laser medium itself, which is resonant with the difference frequency.
X-ray pulses from a fs-laser plasma were focused by an X-ray capillary lens, generating a spot smaller than 100 μm. Fe Kα radiation (λ = 0.194 nm) is produced by focusing 200 mJ/130 fs pulses from the ATLAS titanium-sapphire laser at 10 Hz onto a moving iron tape. The capillary lens enhanced the intensity by a factor of about 1600. Diffraction from samples of small size is demonstrated by producing diffractograms from a Si (111) crystal in only about 10 seconds. The model of a novel ultrafast streak camera which takes advantage of the different path lengths of rays propagating through the lens is demonstrated. Preliminary experiments using a semi-lens for collimating X-rays are also reported.
A novel laser-pumped X-ray source is used to investigate generation of shock waves in a semiconductor and conformational changes in a molecular crystal. Ultrashort Cu-Kα pulses are generated by focusing 130 fs laser pulses from the ATLAS titanium-sapphire laser of our institute on a slowly moving copper tape. Irradiating Si(111) surfaces with a few 100 mJ/cm2 pulses at 800 nm we observe an increase in the integrated reflection on a relatively slow time scale of several 100 ps. This observation is explained by the increased geometrical structure factor generated by the shock wave propagating into a mosaic crystal. The work on conformational changes was performed with DMABN (dimethylaminobenzonitrile, sum formula C9H10N2). A pump-probe experiment using the third harmonic of the titanium-sapphire laser (λ = 265 nm) as the pump yields indications of an increase of the 004 reflection in a time shorter than 10 ps. Such an increase is expected owing to photo-induced rotation of the two methyl groups around the major axis of the molecule.
A summary of recent developments of x-ray spectroscopy for the application in laser produced plasma experiments is given. They are based on an advanced theoretical analysis of the radiation emission originating from autoionizing states and the realization of high resolution x-ray spectromicroscopy methods. Particular emphasize is given on non-Maxwellian particle analysis, strongly coupled plasmas and interpenetrating plasma sheaths of laser produced and compressing (pinching) plasmas.
We present measurements of electron densities of plasmas with fs resolution. The plasmas are generated by laser pulses with different intensities at different time delays. Such plasmas are of great interest as preplasmas for transient, collisionally excited X-ray lasers. The prepulse is generated by stretching part of a 130-fs laser pulse of the ATLAS titanium-sapphire laser of our institute. Focusing this radiation to a line on molybdenum and silver targets generates preplasmas highly interesting to research directed towards a 10 Hz sub-Joule soft X-ray laser. The electron density is measured as a function of distance from the target by interferometry using a Wollaston prism. The ultrashort probe pulse allows one to obtain data as close as 10 - 20 μm from the target surface. Experimental data are compared with simulations using the MULTI hydrocode. The results allow optimization of prepulse-main pulse delay times and compare ablation from a hard (Mo) and a soft (Ag) material.
The high brilliance expected from the X-ray Free-Electron Lasers (XFEL’s) now under construction suggest re-investigating the feasibility of a photopumped soft X-ray laser. We present simulations of a Lyman-α X-ray laser in hydrogenic He (λ = 30.4 nm) pumped by XFEL radiation with parameters of the TESLA Test Facility, phase II, at DESY/Hamburg. The simulations show that high gain can be achieved at a pump intensity of 1015 W/cm2. The realization of such a laser could provide a better understanding of the physics of photopumped lasers and thus help to develop table-top X-ray lasers.
Experimental investigations on the conditions to achieve transient gain in neon-like Ti and nickel-like molybdenum XUV laser pumped by a 10-Hz sub-Joule femtosecond laser are presented. The 4d-4p (J equals 0-1) (lambda) equals 18.9 nm and 4f-4d (J equals 1-1) (lambda) equals 22.6 nm lines in Ni-like Mo as well as the 3p-3s (J equals 0-1) (lambda) equals 32.6-nm line in neon-like titanium have been observed. The Ni-like laser lines show a threshold behavior with respect to the pump irradiance as they appear only above 1015 W/cm2. Simulation for the fs-laser pumped Ni-like Mo XUV laser are also presented.
Using Thomson scattering of laser light we have studied optical-field ionized Hydrogen and Deuterium plasmas with respect to electron density, electron temperature and ion temperature. The plasmas were created by focusing 120 fs, 790 nm pulses of our Hz ATLAS Ti:sapphire laser system to intensities of up to 7 X 1016 W/cm2. The measurements were performed for linear and elliptic polarization of the ionizing laser light. We find that the ion temperatures for Deuterium and Hydrogen are not identical, whereas the corresponding electron temperatures show no significant difference. Considering standard theory for optical-field ionization we would expect much higher electron temperatures for elliptic than for linear laser polarization. But for Deuterium the electron temperature for elliptic polarization. With increasing particle density ionization defocusing of the laser beam and heating of the electrons due to inverse bremsstrahlung gain in significance.
When fs laser pulses interact with solid surfaces at intensities I(lambda) 2 > 1018 W/cm2 micrometers 2, collimated relativistic electron beams are generated. These electrons can be used for producing intense X-radiation (bremsstrahlung or K(alpha )) for pumping an innershell X-ray laser. The basic concept of such a laser involves the propagation of the electron beam in a material which converts electron energy into appropriate pump photons.
Recent studies of soft-x-ray and XUV lasers in Ne- and Ni-like ions using the Asterix IV iodine laser as a pump are reviewed. Progress achieved includes the observation of new laser lines in a number of low to medium-Z Ne-like ions at wavelengths from 20 to 87 nm, the reduction of the driver energy for neon- like S to a value of 20 J and lasing in Ni-like Sn at 12 nm. We also present recent results in characterizing near- and far-field patterns and compare them with predictions of simulations, using a 2D hydrodynamic code combined with a ray tracing code.
We discuss high resolution two-dimensional near-field images of the neon-like nickel and germanium x-ray laser obtained using the Asterix laser at the Max-Planck-Institute and the Nova laser at Lawrence Livermore National Laboratory. Our imaging diagnostic consisted of a concave multilayer mirror that imaged the output end of the x-ray laser line onto a backside illuminated x-ray CCD detector. A 25 micrometer thick wire positioned at the end of the target provided a spatial fiducial. With the Asterix iodine laser, a prepulse 5.23 ns before the main pulse, was used to irradiate slab targets. A great deal of structure was observed in the near field images, particularly in the J equals 0 - 1 emission. We observed a large difference in the spatial dependence of the J equals 0 - 1 and J equals 2 - 1 lines of germanium, with the J equals 2 - 1 emission peaking farther away from the original target surface. A larger prepulse moved the peak emission farther away from the target surface. For the Nova experiments we used a series of 100 ps pulses spaced 400 ps apart to illuminate a germanium target. We obtained high resolution images of both the J equals 0 - 1 and J equals 2 - 1 lines of Ge. these measurements are compared to hydrodynamic simulations coupled with atomic kinetics and including refraction effects.
The layout and performance of the single beam Asterix IV high-power iodine laser operating either at the fundamental wavelength at 1315 nm or the second or third harmonic at 658 nm and 438 nm, respectively, is described. Every 20 minutes Asterix IV can provide output pulses with durations ranging from 0.4 ns to several ns with pulse energies of up to 2.1 kJ and pulse powers reaching 3 TW. Preliminary experiments and calculations reveal that by pumping Ti:S disks with the 3(omega) -radiation of Asterix IV much higher powers in the multi-100 TW region can be attained. Since 1988 the laser fired 6500 target shots as a reliable tool. As a selection among numerous experiments three highlights are dealt with: (1) uniform megabar shock waves in solids, (2) XUV opacities in hot dense Al, Fe, and Ho, and (3) lasing on the J equals 0 - 1 line of neon-like ions using the prepulse technique.
In this paper we present the results of the first experiments on x-ray laser using a laser- irradiated gas puff target. The gas puff targets were created by pulsed injection of gas from a high-pressure valve through a nozzle into a vacuum. An x-ray laser active medium in a form of an elongated plasma column was produced by the perpendicular irradiation of the gas puff target in a form of a long sheet of gas, created using the nozzle having a linear exit aperture, with the laser beam focused to a line. The x-ray laser experiments were carried out at Garching using the ASTERIX IV high-power iodine laser. Up to 3-cm-long hot and dense plasma columns were produced. Lasing in neon-like argon at 46.9 nm and in nickel-like xenon at 10.0 nm was demonstrated for the first time.
We report x-ray spectra emitted from plasmas generated by focusing 130 mJ/120 fs pulses of the MPQ Ti:Sapphire laser facility on solid targets. Low-Z elements (from Li, Z equals 3 to K, Z equals 19) were chosen as the target materials. By means of two cylindrical lenses a line focus was produced, and in combination with a stepped mirror the laser pulse can be brought to travel along the line focus with the velocity of light. X-ray spectra were observed in the direction of the axis of the line focus by a transmission grating spectrometer coupled to a backside illuminated x-ray CCD. Special attention was given to the emission of those lines which are candidates for lasing. We report systematic investigations with varied laser intensity and line focus geometry. Implications of the observed spectra on the development of a table- top x-ray laser are discussed and plans for similar experiments with an upscaled version of the pump laser are presented.
We have performed a systematic investigation of lasing on the neon-like 3p - 3s J equals 0 - 1 line in low-Z elements ranging from Cl to Ge using the prepulse technique. Laser pulses from the Asterix iodine laser (lambda equals 1.315 micrometers) were applied to slab targets with a low intensity prepulse 5 ns ahead of the main pulse. Prepulse energies ranged from 0.1% to 15% of the main pulse energy. In all materials lasing on the J equals 0 - 1 line was observed. In vanadium two J equals 0 - 1 lines were found to lase with approximately equal strength. The spatial position of lasing was determined for several materials and two prepulse levels. In addition, the sensitivity of lasing with respect to the mainpulse/prepulse ratio is reported.
Current successful approaches for achieving soft x-ray lasing typically require pumping laser pulses of duration approximately ns and energy approximately kJ (collisionally pumped schemes) or approximately ps pulses and powers of approximately several TW (recombination-pumped schemes). For applications, it is important to improve the efficiency of soft x-ray lasers and so reduce the required power of pumping lasers. The effect of pre- pulse on neon-like collisionally pumped lasers has been investigated using the LULI laser (Ecole Polytechnique, France). A small pre-pulse level approximately 10-3 of the main pulse energy was found to increase the J equals 0 minus 1 neon-like zinc laser output at 21 nm by an order-of-magnitude with a comparable increase in efficiency. A double pumping laser pulse on neon-like yttrium lasing output at 15 nm obtained with the VULCAN laser (Rutherford Appleton Laboratory, England) was also found to increase the x-ray lasing efficiency. With adiabatically cooled recombination lasing, it is shown that approximately 2 ps pulses are optimum for achieving the desired ionization balance for lasing output. The possibility of achieving recombination lasing at short wavelengths on lithium-like ions with longer pulse lasers has been investigated using the ASTERIX laser (Max-Planck Quantenoptik, Germany). These results are presented and interpreted to provide possible directions for improving the efficiency of x-ray lasers.
The single beam Asterix IV high power iodine laser ((lambda) equals 1.32 micrometers ) provides at present a maximum output energy of 2.1 kJ at a pulse length of 5 ns and an output power of 4 TW at 0.3 ns pulses. This laser is designed to deliver pulses with lengths ranging from 0.2 to 5 ns with a maximum power of 5 TW and an energy of up to 2 kJ. Asterix IV has been developed on the 10 years experience with the 1 TW Asterix III laser and with the support of a 1D and a 3D pulse propagation code. Special emphasis has been put on achieving a high overall system efficiency and laser beam intensity profile as homogeneous as possible. In this paper the measures for optimizing the laser performance and the results obtained will be described.
The Asterix IV high power iodine laser is designed to deliver in a single beam output pulses with lengths ranges from 0.1 to 4 ns with a maximum power of 5 TW and an energy of up to 2 kJ. This laser has now achieved its projected output energy of 2.0 kJ and an output power of 4 TW. The Asterix laser has been developed on the basis of 10 years of experience with the 1 TW Asterix III laser and with the support of a 1D and a 3D pulse propagation code. Special emphasis has been put on obtaining an optimal overall efficiency of the laser system and a laser beam intensity profile as homogeneous as possible.
Research on X-ray lasers at Max Planck Institute of Quantum Optics is reviewed. The main part of this work is performed using the Asterix IV high- power iodine laser, an installation capable of delivering a maximal output energy of 1.2 kJ for a pulse duration of 450 ps. The research areas investigated include photo-resonant pumping, charge transfer X-ray lasers and recombination lasers.
The experimental application of a scheme for travelling-wave excitation along a line focus to x-ray laser development is reported. The scheme utilizes an appropriately stepped prism or mirror inserted into the beam ahead of the focussing optics. It can be used for pulsed excitation down to about 1 ps. X- ray emission travelling with the velocity of light along a line focus is observed in an experiment using 500 fs to 1 ps KrF laser pulses.
Experimental and theoretical investigations of `self-pumped' photo-resonant X- ray lasers are reported. In these lasers the same kind of ion is used as active medium and for excitation. Simulations of the realization of this idea in a transient X-ray laser and first experiments towards demonstrating a quasi-cw self-pumped X-ray laser are reported.
The Asterix IV high power iodine laser was projected to deliver output pulse energies of up to 2 kJ at pulse lengths tp >= 1 ns in one beam. Using a mode-locked or a long-pulse oscillator the pulse duration can be adjusted in the range between 0.1 and 3 ns. In addition, in the fundamental wavelength at 1315 nm the laser can be operated at either the second or third harmonic at (lambda) equals 658 nm and (lambda) equals 438 nm, respectively. Up to now a maximum output energy of 1.3 kJ could be extracted which was -- at tp equals 0.4 ns -- limited only by the damage threshold of optical components. Emphasis has been put on improving the laser performance; e.g., the energy density of the top hat beam profile shows a modulation as small as +/- 6% and the output pulse energy can be kept stable within +/- 4% during a period of several months. Since 1988 the laser has been used for 3000 target shots and it has proved a reliable tool.