Transient X-ray Laser Research Articles

Study of rapid ionisation for simulation of soft X-ray lasers with the 2D hydro-radiative code ARWEN

We present our fast ionisation routine used to study transient soft X-ray lasers with ARWEN, a two-dimensional hydrodynamic code incorporating adaptative mesh refinement (AMR) and radiative transport. We compute global rates between ion stages assuming an effective temperature between singly-excited levels of each ion. A two-step method is used to obtain in a straightforward manner the variation of ion populations over long hydrodynamic time steps. We compare our model with existing theoretical results both stationary and transient, finding that the discrepancies are moderate except for large densities. We simulate an existing Molybdenum Ni-like transient soft X-ray laser with ARWEN. Use of the fast ionisation routine leads to a larger increase in temperature and a larger gain zone than when LTE datatables are used.

Direct measurement of the temporal profile of the amplification gain of the transient collisional excitation neonlike manganese x-ray laser medium

The temporal profile of the amplification gain of the neonlike manganese x-ray laser in the transient collisional excitation scheme was measured by the use of the 29th-order harmonic light at a wavelength of 26.9 nm as a probe beam. The raise-up time of the gain was delayed by 20 ps from the peak of the pumping laser pulse, and the duration of the gain was 50 ps. This method is a promising powerful tool to realize the highly coherent x-ray laser beam by using a combination of an x-ray seeder and an x-ray amplifier.

Dynamical Description of Transient X-Ray Lasers Seeded with High-Order Harmonic Radiation through Maxwell-Bloch Numerical Simulations

The Maxwell-Bloch code COLAX has been upgraded to use detailed hydrodynamical and collisional-radiative simulations of a soft x-ray laser plasma with traveling-wave pumping. The seeding of short pulses of high-order harmonics of the pump laser into the x-ray laser medium has been simulated. The amplification is shown to be a dynamic, two-stage process: the atomic dipoles of the lasing ions are first coherently excited by the short pulse, and subsequently generate a radiation wake which is amplified along its path through the plasma, with consequences on the experimentally recorded spectra.

Pulse duration measurements of grazing-incidence-pumped high repetition rate Ni-like Ag and Cd transient soft x-ray lasers

We have measured the pulse duration of gain-saturated 13.9 nm Ni-like Ag and 13.2 nm Ni-like Cd transient collisional lasers excited by grazing-incidence-pumping for several pumping conditions. High-resolution streak-camera measurements yielded FWHM pulse durations close to 5 ps for both lasers under optimum pumping conditions. The very high brightness and short pulse duration of these new high repetition tabletop soft x-ray lasers make them an attractive source for dynamic applications.

Longitudinal coherence and spectral profile of a nickel-like silver transient soft X-ray laser

The spectral width of a Ni-like silver transient soft X-ray laser (4d-4p λ= 13.9 nm) was experimentally inferred from the measurement of its temporal coherence, performed with a novel wavefront division interferometer. The measured coherence time of 3 ps corresponds to a spectral linewidth of 0.7 mA which is narrow. We compare this experimental result to numerical simulations of the amplified line profile, performed along two different approaches. Both calculations predict a spectral linewidth that is about 3 times larger than the experimental value. We discuss several effects that might explain this discrepancy. We show that, due to the short duration of the X-ray laser pulse, the assumption of a stationary electromagnetic field used in both the experimental analysis and in the calculations need to be reconsidered.

Homogeneous focusing with a transient soft X-ray laser for irradiation experiments

We report the work done on a transient soft X-ray laser (SXRL) beam to deliver a proper extreme UV irradiation source for applications. The same optical tool was first demonstrated on a quasi stationnary state (QSS) soft X-Ray laser at the PALS Institute in Prague. The problem set by the transient soft X-Ray laser developed by the LIXAM at the LULI installation in Palaiseau is more crucial, first because the beam spatial profile is more irregular secondly because high repetition rate soft X-ray laser facilities in the future are based on this SXRL type. The spots obtained show a 20 micron average diameter and a rather homogeneous and smooth profile that make them a realistic irradiation source to interact with targets requiring relatively high fluence (near 1 J/cm 2) or intensity (near 10 11 W/cm 2) in the extreme UV domain.

Spatial coherence measurements of a 13.2 nm transient nickel-like cadmium soft x-ray laser pumped at grazing incidence

The spatial coherence of a 13.2 nm transient collisional Ni-like Cd soft X-ray laser pumped at 23 degrees grazing incidence was measured in a series of Young's double-slit experiments. We observed pronounced fringe visibility variations associated with microstructures in the beam's intensity profile. The transverse coherence length was measured to be about 1/20 of the beam diameter and did not significantly improve with longer plasma columns. The equivalent incoherent source size is determined to be 10 mum and the laser's peak spectral brightness ~ 3 x 10(23) photons/sec/mm(2)/mrad(2) within less than 0.01% spectral bandwidth.

Hydrodynamic model of transient Ni-like X-ray lasers driven by Gaussian laser pulse

The self-similarity model of hydrodynamics of transient collisional excitation X-ray lasers is modified for drive laser pulse with a Gaussian temporal profile. The results show that the modified model can more accurately describe the hydrodynamic process of tramsient collision emission X-ray laser than the self-similarity model. Using the modified model, the experimental design of the Ni-like Mo X-ray lasers is optimized.

Experimental study of a transient X-ray laser pumped by a picosecond pulsed laser

This paper presented the results of experimental studies on transient soft X-ray laser using a picosecond pulsed laser facility.A rather intensive Ni-like Ag X-ray laser at 13.9nm with output energy of about 5—10nJ was obtained by irradiating the solid flat targets with a several hundred picosecond long laser pulse in combination with an 1ps ultra-short laser pulse.

Modeling of the saturated Ni-like silver x-ray laser

This paper reports on the modeling of the Ni-like silver transient x-ray laser at the wavelength of 13.9 nm. Time-dependent populations and gain are calculated consistently with the output intensity. Two-dimensional refraction, i.e., in the direction of the driving laser and parallel to the slab target surface, is modeled by a ray trace code which is a postprocessor of a hydrodynamic code. Temperatures and electron-density variations are given by the hydrocode. Our calculations show that interaction of the x-ray laser field with the amplifying medium, and refraction, affect the output intensity and reduce the gain values by a large factor: from many hundreds per cm, as predicted by collisional-radiative models ignoring the above interaction, to one hundred per cm, at most.

Experimental study of the temporal coherence and spectral profile of the 13.9 nm transient X-ray laser

A new Fresnel interferometer is used to investigate the temporal coherence of the 13.9 nm Ni-like Ag X-ray laser. The results obtained show a coherence time of ≈ 3 ps , corresponding to a spectral linewidth Δ λ of 0.7 mÅ, which is narrow. The pulse duration, also measured in this experiment, is shown to be close to the Fourier-transform limit. Finally, we discuss the implications of the high temporal coherence of the X-ray laser on the structure of the far-field pattern of the beam.

Single-Pulse Low-Energy-Driven Transient Inversion X-Ray Lasers

The paper deals with some aspects of a new pump method applied to collisional transient X-ray lasers working with ions of the Ni-like isoelectronic sequence. Specifically, Ni-like silver is the element of main interest. It is shown that a single profiled picosecond pulse with energy of about 1 J is able to excite efficiently the active medium. A small amount of the produced plasma facilitates its heating and following excitation. Application of other available pump laser systems and the resulting medium kinetics are analyzed. Pump pulse profiling and control over the pulse shape in a laser driver are highlighted on the background of the recent development in laser driver technology. Prospects and possible development trends are discussed as well.

Diagnostics of laser-induced plasma with soft X-ray (13.9�nm) bi-mirror interference microscopy

X-ray interference microscopy diagnostics of a nanosecond-pulse pumped Al (Z=13) plasma are presented. A transient picosecond Ni-like Ag X-ray laser (13.9 nm) is used as the probe beam and a wavefront division bi-mirror interferometer is adopted as the diagnosis tool. From the fringe shifts, the two-dimensional plasma density profile at 1 ns after the peak of the heating pulse is deduced. Opposite fringe shifts during the plasma development are observed and possible explanations for this behavior are discussed. This study will contribute to the validation of 1D and 2D hydrodynamic codes as well as to the better understanding of the physics during the development of laser produced plasmas.

Analytical and numerical ray tracing of a transient x-ray laser: Ni-like Ag laser at 139 nm

Transient soft-x-ray lasers are generated from a solid target irradiated by two intense pump laser pulses. Amplification is achieved in the plasma column thus produced. Knowledge of the beam propagation is vital for the intensity and quality of the x-ray laser output. In this paper, x-ray laser beam propagation in transient plasmas is studied both analytically and numerically. General one-dimensional formulas are developed for beams in electron density gradient media, including the exponential profile that describes the plasma created from a solid target. The gradient is predicted to limit the amplification length within the maximum gain to <2.6 mm in standard experiments. The result given by the analytical model is confirmed by numerical ray tracing of x-ray laser beams within an amplifying medium as it is defined by the full numerical simulation results of the ehybrid code.

Effects of delay time on transient Ni-like x-ray lasers.

In transient collisional excitation scheme, a long (nanosecond) prepulse is used to perform and ionize plasmas. After a delay time, a short (sub- or picosecond) intense laser pulse is used to rapidly heat the plasma. This results in transient x-ray lasers with high gain. Effects of delay time on transient collisional excitation nickel-like x-ray lasers are investigated analytically using a simple model. The calculations show that the longer delay time can greatly relax the density gradient. This is very critical for the propagation of x-ray lasers. However, a too long delay will reduce the electron temperature of the plasma before the arrival of the short pulse. Increasing the intensity of the long pulse or extending the pulse duration can keep the temperature required to maintain a high percentage of Ni-like ions while the delay time is longer. Similarly, increasing the intensity of the short pulse or extending the duration can also raise the electron temperature, resulting in higher gain coefficient. Our results indicate that extending the pulse duration is more efficient than that of increasing the intensity.

Transient characteristics of a neon-like x-ray laser at 19.6 nm

A transient Ne-like x-ray laser at 19.6 nm (J=0→1, 3p→3s) is numerically investigated using a hydrodynamic code for a 100 μm thick Ge planar target irradiated by a nanosecond prepulse followed by a subpicosecond or a picosecond main optical laser pulse. Simulations are performed for different main pulse lengths of 50 fs, 500 fs, 5 ps, and 50 ps but with fixed pulse energy. For a drive pulse even as short as 500 fs, the x-ray laser gain still shows quasisteady state behaviors. A real transient nature can only be seen when the x-ray laser is driven by a laser pulse with a duration as short as tens of femtoseconds.

Demonstration of a 2-ps transient x-ray laser

A time-resolved measurement of the output from the Ni-like Ag transient-collisional-excitation x-ray laser is described. An ultrafast x-ray streak camera was used to diagnose the output of the $J=\stackrel{\ensuremath{\rightarrow}}{0}1$ $4d\ensuremath{-}4p$ lasing line at 13.9 nm. The full width at half maximum duration of the x-ray pulse is measured to be of $1.9\ifmmode\pm\else\textpm\fi{}0.7 \mathrm{ps}$ at optimum conditions of pump laser irradiation. This is the shortest x-ray laser duration directly demonstrated to date and illustrates the great potential of transient x-ray lasers as a high brightness, picosecond x-ray source for applications.

High Frequency Behavior of La&lt;SUB&gt;0.7&lt;/SUB&gt;Sr&lt;SUB&gt;0.3&lt;/SUB&gt;MnO&lt;SUB&gt;3&lt;/SUB&gt; with Giant Magnetoimpedance Effect

The generation of coherent soft X rays is studied using a terawatt picosecond Nd:glass laser system. Two different methods are investigated as candidates for efficient generation of such radiation, namely, longitudinally pumped transient collisional excitation nickel-like molybdenum X-ray laser, and high-harmonic generation from solid-vacuum interfaces. In the course of experiments on longitudinally pumped X-ray lasers, unexpected jetlike structures are observed in the visible emission of the molybdenum plasma, extending over a length of several millimeters. An interesting characteristic of this phenomena is that clear jets are observed only for longitudinal pump intensities between 5 x 10(14) and 7 x 10(14) W/cm(2). The effects of a finite scale length density gradient on surface harmonics is also investigated. The efficiency of harmonic generation from near-solid density plasma is found to increase by a factor of 2 to 3 when using prepulses. The scale length of the preplasma is simulated using a one-dimensional hydrodynamic code, and the increase in efficiency is verified to be in accordance with particle-in-cell simulation results.

Coherent short wavelength radiation via picosecond Nd:glass lasers

The generation of coherent soft X rays is studied using a terawatt picosecond Nd:glass laser system. Two different methods are investigated as candidates for efficient generation of such radiation, namely, longitudinally pumped transient collisional excitation nickel-like molybdenum X-ray laser, and high-harmonic generation from solid–vacuum interfaces. In the course of experiments on longitudinally pumped X-ray lasers, unexpected jetlike structures are observed in the visible emission of the molybdenum plasma, extending over a length of several millimeters. An interesting characteristic of this phenomena is that clear jets are observed only for longitudinal pump intensities between 5 × 1014 and 7 × 1014 W/cm2. The effects of a finite scale length density gradient on surface harmonics is also investigated. The efficiency of harmonic generation from near-solid density plasma is found to increase by a factor of 2 to 3 when using prepulses. The scale length of the preplasma is simulated using a one-dimensional hydrodynamic code, and the increase in efficiency is verified to be in accordance with particle-in-cell simulation results.

Temporal resolution of a transient pumping X-ray laser

We report on a time-resolved study of a Ni-like transient collisionnal X-ray laser with a resolution as high as 1.9 ps. The FWHM duration of the Ni-like x-ray laser pulse at 13.99 nm Ag J = 0→1 4d-4p line is measured to be as short as ∼2 ps at optimum conditions of pump laser irradiation. This is about four times shorter than was estimated in previous experiments. The x-ray laser signal appears in the rising edge of the continuum emission. The x-ray laser duration rises significantly when the short (heating) pulse duration is increased and when doubling the peak-to-peak delay of the two irradiation pulses. It does not change when the short pulse energy is increased. The results presented are the first direct measurements of the temporal profile of the x-ray laser output at a high resolution.