Djamel Benredjem

Analytical and numerical ray tracing of a transient x-ray laser: Ni-like Ag laser at 13.9 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.

Modeling of the transient nickellike silver x-ray laser

Recent high-temporal-resolution nickellike x-ray laser experiments have yielded important insights into the output characteristics of picosecond-pumped x-ray lasers. However, current experimental observations do not fully explain the plasma dynamics, which is critical to gain generation within the x-ray laser medium. A numerical study of the nickellike silver x-ray laser has therefore been undertaken to complement our experimental results in an attempt to further our understanding of the processes at work in yielding the observed x-ray laser output. High gain coefficients existing with picosecond lifetimes are predicted, which is consistent with the short x-ray laser durations experimentally observed. The late onset of the continuum emission relative to the temporal peak of the x-ray laser output is explained as a sign of high electron density evolution near the target surface.

New techniques for the measurement of x-ray beam or x-ray optics quality

Metrology of XUV beams and more specifically X-ray laser (XRL) beam is of crucial importance for development of applications. We have then developed several new optical systems enabling to measure the x-ray laser optical properties. By use of a Michelson interferometer working as a Fourier-Transform spectrometer, the line shapes of different x-ray lasers have been measured with an unprecedented accuracy (δλ/λ~10-6). Achievement of the first XUV wavefront sensor has enable to measure the beam quality of laser-pumped as well as discharge pumped x-ray lasers. Capillary discharge XRL has demonstrated a very good wavefront allowing to achieve intensity as high 3*1014 Wcm-2 by focusing with a f = 5 cm mirror. The measured sensor accuracy is as good as λ/120 at 13 nm. Commercial developments are under way.

Theoretical modeling of the transient Ni-like Ag x-ray laser

Recent high temporal resolution Ni-like x-ray laser experiments have yielded important insights into the output characteristics of picosecond pumped x-ray lasers. However, current experimental observations do not fully explain the plasma dynamics which are critical to the gain generation within the x-ray laser medium. A theoretical study of the Ni-like Silver x-ray laser has therefore been undertaken to compliment our experimental results, in an attempt to further our understanding of the processes at play in yielding the observed x-ray laser output. Preliminary findings are presented within this paper.

Gain calculations of overlapping lines in Li-like recombination lasers

Line profiles taking into account ion (Stark) broadening, ion dynamic effect, and electron collisions are calculated for the Al10+ lines at 154.7 and 105.7 angstrom, and for the S13+ line at 206.5 angstrom in recombination lasers. The first two lines are formed of three fine structure components, while the third is constituted of nine components, and the resulting gain may be defined accordingly. The electron collisions yield an homogeneous broadening, while the Stark interaction with neighboring ions is responsible for an asymmetry of the whole profile. Consistently with the experimental determination of the gain, we calculate the total intensity involving the population inversions of the set of components which contribute to the lasing radiation, and deduce an effective small-signal gain coefficient. We discuss our results and compare them to experimental gains.

Theoretical and experimental investigations of spectral and temporal properties of seeded soft x-ray lasers

We present in this paper theoretical and experimental investigations of temporal and spectral properties of seeded soft x-ray lasers. Bloch-Maxwell simulations of the harmonic pulse propagation in a soft x-ray laser plasma have been performed. Results show a growing wake of coherent radiation formed after the harmonic pulse.We describe the first measurement of the spectral bandwidth of a seeded soft x-ray laser. Using a varying path difference interferometer the spectral profile of a seeded OFI x-ray laser has been experimentally determined, leading to a Fourier-transform pulse duration of 5ps. The measured bandwidth is in good agreement with simulations. Finally we present the progress toward the implementation of a seeded soft x-ray laser at 18.9 nm at the new LASERIX facility.

Temporal coherence calculations in the Ni-like Pd x-ray laser

Longitudinal coherence length in x-ray lasers depends strongly on the shape of the amplified line. We have modeled an experiment performed at the Lawrence Livermore National Laboratory. The experiment was devoted to the study of the temporal (longitudinal) coherence of the transient x-ray laser at 14.7 nm in Ni-like palladium (4d-4p transition). Only electron (collisional) and Doppler broadening play a role in the line profile of the 0-1 4d-4p transition. This allows us to use the Voigt shape in conditions where the amplifier, i.e., the plasma produced by the interaction of a high intensity laser with a slab target, is neither stationary nor homogeneous. Our calculations use a ray trace code which is constructed as a post-processor of the hydro-atomic code EHYBRID. In the saturation regime, there is need to account properly for the interaction between the x-ray laser field and the lasing ions. This is done in the framework of the Maxwell-Bloch formalism. The FWHM of the spontaneous emission profile is ~28 mÅ, while the width of the amplified x-ray line ~4 mÅ. Comparison with experiment is discussed.

Modeling of Transient Ni‐like Ag X‐ray Laser

Recent high temporal resolution Ni‐like x‐ray laser (XRL) experiments [1] have yielded important insights into the output characteristics of picosecond pumped XRL’s and the shortest XRL pulse was demonstrated. However, important issues were raised that require to enhance our understanding of plasma and population dynamics, namely (a) short pulse duration, (b) XRL pulse occurring before the peak of continuum emission and (c) the role of (over‐)ionization. A numerical study of the Ni‐like transient silver XRL has therefore been undertaken to complement our experimental results. High gain coefficients existing with picosecond lifetimes and restricted in space (∼5 μm FWHM) are predicted, which is consistent with short XRL durations experimentally observed. The simulations suggest that the gain is cut‐off by fast over‐ionization of Ni‐like ions. The late onset of the continuum emission relative to the temporal peak of the XRL output (as observed experimentally) is explained as a signature of a thermal conducti...

Analytical Ray‐Tracing of a Transient X‐ray Laser: Ni‐like Ag Laser at 13.9 nm

Numerical codes predict for transient x‐ray lasers (XRL) very high picosecond duration gains [e.g. 1] that are restricted in space to several microns at FWHM. XRL beam propagation in plasma is vital for estimation of the effective gain at the plasma output. In this paper, beam propagation in transient plasmas is analytically studied. General 2‐D formulae are developed for beams in electron density gradient media, including those with the exponential profile that describes the plasma created from a solid target. The gradient is predicted to potentially 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 XRL beams within an amplifying medium as it is defined by the full numerical simulation using the EHYBRID code.

Saturated collisionally pumped x-ray lasers: modeling and comparison to experiment

Large amplification at 25.5 nm in neon-like iron has been demonstrated in experiments where prepulses are used. We show that the interaction between the x-ray laser beam and the amplifying medium must be taken into account in a reliable modeling of the saturation regime. Two approaches for intensity calculations are presented in this contribution. The first one combines the radiative transfer equation and the population rate equations. This approach is fully consistent, in the sense that beam amplification and population kinetics are treated simultaneously. A formalism based on a paraxial Maxwell-Bloch approach is presented. The Maxwell-Bloch calculations give the variation with length of intensity, local gain...Moreover, in the small-signal regime, it is possible to define an effective gain which is comparable to the measured gain. The second approach is based on a raytrace calculation where the saturation effect has been introduced empirically. The two codes need the electron density and the electron and ion temperatures as inputs. These quantities are given by the hydrocode EHYBRID. The two approaches give similar results.