X-ray Laser Studies Research Articles

Ne-like Ge soft X-ray lasers driven by fundamental- and double-frequency lasers

In this study, experiments and simulations of Ne-like Ge soft X-ray lasers driven by nanosecond fundamental-frequency (1ω) and double-frequency (2ω) lasers are performed. The results show that 2ω lasers are also able to drive and produce Ne-like Ge soft X-ray lasers, even though the requirements for the driving conditions greatly increase compared to 1ω lasers. It is expected that the laser outputs at 23.2 and 23.6 nm will be stronger at higher power densities. The results provide a good foundation for executing experiments using Ne-like Ge soft X-ray lasers at existing large laser facilities, which cannot currently sustain 1ω drive lasers.

Behavioral study of Ar x-ray lasers at 46.9 nm based on model of geometrically dependent gain coefficient

There are many experimental measurements appeared in the literature, where the researchers reported their measured amplified spontaneous emission (ASE) output energies versus medium excitation lengths. In this paper we report on the ASE analyses, made based on using intensity rate equation along with the geometrically dependent gain coefficient (GDGC) model, for different measurements carried out in different laboratories under different experimental conditions. We show that the deduced gain coefficients follow a general equation of g0ν° (cm−1) = 0.37 + 11.51/lAMP, where lAMP is the medium excitation length. For this study at least 13 experimental measurements were analyzed. The method also revealed that we can obtain the behavior of gain coefficient with respect to input current. The optimum operational current is 30 kA, and the calculated intrinsic spectral linewidth for i = 21 kA is 55.67 mÅ. With the GDGC model, consequently, it can be concluded that Ne-like Ar x-ray lasers are unified, and method simplifies greatly the analyses of complicated systems.

Relativistic forward stimulated Raman scattering of a laser in a plasma channel

A circularly polarized Gaussian laser beam propagating through a low density plasma creates a partially electron depleted channel. The laser undergoes stimulated forward Raman scattering, producing a plasma wave and two radially localized electromagnetic sideband waves. The laser and the sideband waves exert an axial ponderomotive force on electrons driving the plasma wave. The latter couples with the pump to drive the sidebands. The radial width of the electromagnetic sideband is of the order of the spot size of the pump, r0, whereas the radial width of the plasma wave is determined by the growth rate of the Raman process. The localization effect reduces the region of interaction and the growth rate. The algebraic equation of growth rate for forward Raman scattering is solved numerically for a typical laser wavelength and a plasma density The growth rate of the forward Raman process increases on increasing the normalized pump amplitude at lower values of pump amplitude, while the growth rate decreases on increasing at higher values of the pump amplitude. On increasing the laser spot size, maximum growth rate is obtained at higher values of a00. This analysis will be applicable in the study of x-ray lasers, inertial confinement fusion, and laser plasma accelerators.

Space- and time-resolved density measurements of a high-intensity laser-produced plasma for x-ray laser studies

We present a detailed study on the spatiotemporal density evolution of a plasma created by optical-field ionization of a high-pressure pulsed gas jet by a 10-TW, 60-fs Ti:sapphire laser. The plasma dynamics has been studied on a 17-ns time scale with a 60-fs time resolution and a 5-microm space resolution using a Mach-Zehnder interferometer. The density profile and the plasma radial expansion were accurately measured for conditions relevant to x-ray laser schemes in H-like nitrogen which were recently proposed [S. Hulin et al., Phys. Rev. E 61, 5693 (2000)]. The results were reproduced well by hydrocode simulations that allowed to infer the plasma temperature.

Development and characterization of a low current capillary discharge for X-ray laser studies

Capillary discharge experiments were carried out for soft X-ray laser studies. A ceramic capillary, which has an inner diameter of 3 mm and a length of 150 mm, has been used for the end-on X-ray diode observation, and a Pyrex capillary has been used for side-view observation. Spike output has been observed, when operating the device with a predischarge current of 5 to 15 A, a current of 9 to 35 kA with a rise time of 55 ns in an argon gas pressure range from 100 to 800 mtorr. It is found that without a predischarge current, spike output has been hardly observed. Observation of spike output at a low discharge current of 9 kA provides us a possibility for design of a compact soft X-ray laser device. When the predischarge is turned off, the side-view observation of the capillary discharge clearly shows the growth of instabilities during the pinch process. This suggests that the predischarge is indispensable for achieving a uniform plasma, which is required by the soft X-ray lasing in a capillary discharge.

Femtosecond laser driven XUV sources: High-harmonic and OFI X-ray laser studies

We report recent experimental results obtained at LOA on a femtosecond driven collisional x-ray laser. The experiment has consisted in studying the amplification of the 4d 9 5p-4d 9 5d line at 41.8 nm in Pd-like xenon. This lasing emission has been observed to dominate the output spectra when focusing a 20 TW 10Hz Ti: sapphire laser system in a few mm gas cell. The influence of several parameters on the x-ray laser emission is presented. A comparison with high harmonic sources is drawn.

Experiments with ASTERIX and ATLAS

We report on measurements of the equation-of-state of copper and gold in the multi-ten Megabar range, opacity measurements based on K-shell absorption in aluminium, and X-ray laser studies on a large number of neon-like ions applying the prepulse technique. For these investigations, the one-beam iodine laser facility ASTERIX emitting pulses with energies of up to 1000 J at 1315 nm and of up to 420 J at 438 nm was used. We also give a brief account of experimental and theoretical results referring to the propagation of an ultrashort pulse through underdense hydrogen or nitrogen plasmas and X-ray spectra from an optically field-ionized nitrogen plasma generated either by linearly or elliptically polarized light. For these investigations, 150-fs/200-mJ/800-nm pulses emitted from our titanium:sapphire laser ATLAS were employed.

Study of the hydrodynamics of plasmas produced by laser irradiation of thin foil targets

The one-dimensional hydrodynamic expansion of plasmas produced by laser beams focused on thin foil targets is studied by using a self-similar approach. This work is an extension of the London and Rosen model proposed in the context of x-ray laser studies. Here, cases with non-flat-top laser pulses are considered and, for some of these, an exact analytic solution is deduced. The model is useful to study long scale length plasmas, produced by uniform laser irradiation, which become fully underdense during the laser pulse. It predicts the fundamental plasma variables (scale lengths, density, temperature) as a function of the experimental parameters (laser intensity, wavelength and pulse duration, target thickness and composition).

Dielectronic recombination rate coefficients of Cu-like Ta ion

The knowledge of accurate ionization and recombination rates of heavy ions is crucial for the study of X-ray lasers and inertia confine fusion (ICF). The dielectronic recombination rates of Cu-like Ta44+ (1s22s22p63s23p63d104s1) ion through the (3d1041n’ 1’,n’ = 4, 5) configurations are given. The remarkable difference between the isoelectronic trends of the rate coefficients for dielectronic recombination through 3d104141’ and 3d104151’ is emphasized.

Analytical study of recombination X-ray lasers in an adiabatic expanding plasma

The collisional recombination process in highly ionized plasma and relevant reheating of the plasma are analyzed in the parameter regime appropriate for X-ray lasing. A method for describing the rate of collisional recombination is proposed based on a compact threelevel atomic model. Reheating of the plasma due to collisional recombination is described by a time-dependent adiabatic exponent γ. Using a self-similar description of the free expanding plasma, the gain of recombination lasing is investigated. In comparison with an open two-level atomic model, the present model gives lower gains appearing later in time. The inclusion of plasma reheating reduces the peak gain and shifts it to later times. Although the three-level model is shown to give a good qualitative description of the atomic processes, a considerable discrepancy was observed when compared with simulations for hydrogenlike carbon.

X-ray laser studies of recombining lithium plasmas created by optical field ionization

A high-intensity short-pulse KrF laser system is used to produce fast recombining plasmas by ionization of preformed lithium fluorine plasma targets. Soft-x-ray emission from these plasmas is measured, and population inversions in hydrogenic lithium are observed as the plasma recombines. For relatively low-temperature target plasma columns a nonlinear increase in the intensity of the 2–1 transition in Li III (135 Å) is observed with respect to length.

Production of centimeter-scale, high-density plasmas with a linear gas jet

A novel linear gas jet has been developed and used to produce centimeter-scale, 1019 cm−3 electron density plasmas. Long regions of high density are important to many types of experiments, including x-ray laser and laser-plasma interaction studies. This new type of gas jet has been characterized by stimulated Raman backscatter emission from the plasma.

Characterization of laser-produced plasmas by ultraviolet Thomson scattering

Space–time-resolved Thomson scattering was used to characterize the electron and ion temperatures in laser produced plasmas of various atomic numbers (Z=6, 13, 32, 73, 92). A Nd:glass laser with intensity between 1013 and 5×1014 W/cm2 was used to produce plasmas of relevance to laser fusion and x-ray laser studies. A comparison of the electron temperature and density deduced from Thomson scattering, x-ray spectroscopy, and lasnex hydrocode simulations is presented.

Radiation-dependent ionization model for laser-created plasmas

For the simulation of laser-created plasmas, hydrodynamic codes need an atomic physics package, for both the equation of state and the optical properties, which does not use the hypothesis of local thermodynamical equilibrium (LTE). However, in x-ray laser studies, as well as in indirect drive inertial confinement fusion studies, high-Z materials can be found where radiation trapping can induce a significant departure from the optically thin description. A method is presented in which an existing LTE code can be changed into a non-LTE code with radiation-dependent ionization. This method is numerically simple and its cost, in terms of computing time, is low enough to be used in two-dimensional simulations.

Modal study of the role of excess noise in x-ray lasers.

Current x-ray laser designs rely on amplifying spontaneous emission in a high-temperature plasma. A central problem in the study of x-ray lasers is determining and optimizing the degree of transverse spatial coherence for holographic applications. The inherent non-power-orthogonal character of the normal modes in a general amplifying medium necessarily results in excess noise and cross-correlation effects that can significantly affect the predicted coherence and intensity profiles of an x-ray laser. In particular, loosely bound discrete transverse eigenstates of the paraxial wave equation have previously been found to dominate the intensity and coherence in the low-amplification regime if the continuum eigenstates are omitted from the spectrum (London, Strauss, and Rosen, Phys. Rev. Lett. 65, 563 (1990)). A detailed analysis of the role of excess noise on steady-state laser intensity and coherence is presented for the simple case of a transverse square gain and density profile in a finite geometry. The inclusion of the continuum portion of the spectrum is found to substantially reduce the level of excess noise and to alter the coherence predictions for low and intermediate gain-length products.

Novel laser line focus geometry applied to X-ray lasers

Producing a long narrow plasma column for X-ray laser studies requires focussing a laser beam into a line focus. Uniformity of intensity along the line is an important consideration. An array of wedges was used in conjunction with a cylindrical lens to provide averaging over many segments of the laser beam, resulting in a more uniform intensity distribution along the line focus. X-ray and infrared images of the resulting line plasmas are shown, to demonstrate the smoothing capability of the wedge array.

DENSITY AND TEMPERATURE DIAGNOSTICS OF A Ne-LIKE GERMANIUM PLASMA CREATED BY LASER

In the frame-work of X-ray laser studies, experiments on Ne-like Ge plasma have been performed by laser irradiation at λ = 1.053 μm. Amplification in lasing transitions has been observed by varying the plasma length. Theoretical calculations of excited level population allow us to determine density and temperature from line intensity ratios.

X-ray laser studies at LLE

X-ray laser studies at LLE

Density and Temperature Diagnostics of a Ne-Like Germanium Plasma Created by Laser

AbstractIn the frame-work of X-ray laser studies, experiments on Ne-like Ge plasma have been performed by laser Irradiation at λ = 1.053 μm. Amplification in lasing transitions has been observed by varying the plasma length. Theoretical calculations of excited level population allow us to determine density and temperature from line intensity ratios.

Z-pinch implosions onto extremely low-density foam cylinders

We have imploded xenon gas-puff Z pinches onto small diameter, extremely low-density foam cylinders ( ρ=0.0045 g/cm3). The presence of the foam on the cylindrical axis had little effect on the radiation production efficiency, pulse width, or spectral details, but it significantly improved the symmetry and uniformity of the stagnation. These experiments confirm that we can stagnate and thermalize a high velocity (5×107 cm/s) plasma onto a cold, low-density target and suggest the feasibility of creating a homogeneous plasma for x-ray laser studies.