G.J. Pert

A COMPUTATIONAL INVESTIGATION OF THE NEON-LIKE GERMANIUM COLLISIONALLY PUMPED LASER

The complex problem of a collisionally pumped Ne-like germanium laser is examined through several detailed models. The central model is EHYBRID; a 1 1/2 D fluid code which self consistently treats the plasma expansion with the atomic physics of the Ne-like ion for 124 excited levels through a collisional radiative treatment. The output of EHYBRID is used as data for ray-tracing and saturation codes which generate experimental observables. A detailed description of the models is given. The atomic physics is investigated through a three-level approximation, a steady state collisional radiative treatment and a time-dependent solution within the fluid model. The accurate calculation of the non-steady state ionization balance is identified as a key issue. Time resolved and time integrated output profiles are generated for various experimental configurations, and the effects of saturation and gain narrowing are examined. The agreement with experiment is excellent in virtually every respect.

Characterisation of soft X-ray amplification observed in Ne-like germanium

Abstract Soft X-ray amplification has been observed on five 3p → 3s transitions of Ne-like germanium at 19.6, 23,2, 23.6, 24.7 and 28.6 nm using flat, massive targets irradiated in 50 and 150 μm wide line foci up to 3.2 cm long at intensities of 0.3 → 14 × 10 13 W/cm 2 by 1.06 μm laser light in 1 ns and 0.5 ns pulses. Gain lengths up to gl ≈ 12 were measured for the J =2 → 1 transition at 23.6 nm and the temporal behaviour of four of the Ne-like germanium lasing lines has been investigated for the first time. The duration of the gain was 60 → 80% of the fwhm of the laser pulse, with the gain turning off at the trailing half-maximum of the incident laser intensity. Output powers in excess of 10 kW per line have been observed.

Output characteristics of amplified-stimulated-emission lasers

The output characteristics of an amplified-stimulated-emission laser are developed with a simple model of a rod of lasing material along which the beam propagates in both directions. Detailed expressions are derived for the saturation of gain and spontaneous emission. From these a series of analytic models is developed for calculation of the output intensity and linewidth. The exact solution is obtained for the case in which the line has a rectangular profile, and this solution is used as the basis for a complete solution for inhomogeneous broadening and to validate an approximation used for the homogeneous case. The mixed case in which the line has both homogeneous and inhomogeneous components is developed. The role of collisional relaxation in changing the nature of broadening is discussed.

Observation of high transient gain in the germanium X-ray laser at 19.6 nm

The transient-excitation pumping scheme, in which a picosecond duration pulse rapidly heats the plasma preformed by a low-intensity nanosecond pulse, was used to pump the Ne-like germanium, J=0–1 transition at 19.6 nm. A small-signal gain coefficient of 30 cm-1 was measured for targets ⩽5 mm long.

MEASUREMENT AND DETAILED ANALYSIS OF SINGLE PASS GAIN AT 81-A IN A RECOMBINING LASER-PRODUCED FLUORINE PLASMA

Abstract XUV amplication at 81 A was observed in a freely expanding adiabatically cooled plasma produced by laser irradiation of LiF coated carbon fibre targets. Measurements included time resolved on- and off-axis spectroscopy, target length variation and the variation of the absorbed laser energy. Detailed analysis of the experimental data has been compared with numerical simulations.

Optimization of the non-normal incidence, transient pumped plasma X-ray laser for laser spectroscopy and plasma diagnostics at the facility for antiproton and ion research (FAIR)

Intense and stable laser operation with Ni-like Zr and Ag was demonstrated at pump energies between 2 J and 5 J energy from the PHELIX pre-amplifier section. A novel single mirror focusing scheme for the TCE x-ray laser ~XRL! has been successfully implemented by the LIXAM0MBI0GSI collaboration under different pump geometries. This shows potential for an extension to shorter XRL wavelength. Generation of high quality XRL beams for XRL spectroscopy of highly charged ions is an important issue within the scientific program of PHELIX. Long range perspective is the study of nuclear properties of radioactive isotopes within the FAIR project.

Laser ablation of Mg, Cu, and Pb using infrared and ultraviolet low-fluence lasers

The modified two-dimensional hydrodynamic code POLLUX has been used to simulate the ablation of magnesium, copper, and lead targets in the early stage of expansion (<100 ns) by 30 ns, 0.248 μm KrF excimer and 1.064 μm Nd–YAG lasers at fluences of 3–10 J/cm2. The results of magnesium ablation using KrF laser with different fluences are compared and found to be in reasonably good agreement with the experimental results. A comparative study of laser ablation for magnesium, copper, and lead has been made by simulation using Nd–YAG lasers at fluence 10 J/cm2. The temporal evolution of surface temperature of the target, ablated vapor mass, and ionization fraction of plasma plume is studied for each material. The simulated temporal evolution and spatial distribution of electron density, electron temperature, and axial velocity of ablated plume of each material are studied and the results are compared. It is found that the plume expansion follows adiabatic behavior and is slowed down with increasing atomic mass. ...

The effects of multi-pulse irradiation on X-ray laser media

Abstract Multipulse irradiation with 100 ps pulses of stripe Germanium targets is shown to enhance by up to several orders-of-magnitude the output of Ne-like Ge lasing on the J = 0–1 line at 196 A compared to single pulse pumping. Various pre-pulse and multipulse configurations have been experimentally investigated for irradiances of ≈ 4 × 10 13 W/cm 2 with a 1.06 μm wavelength pumping laser. The ionisation balance measured by a KeV crystal spectrometer (KAP crystal) has been found to not affect the X-ray laser output. Good agreement between the experimental results and a fluid code incorporating atomic physics, gain and X-ray beam ray tracing is obtained. The code results show that the enhanced X-ray laser output is produced by multipulse irradiation reducing the electron density gradients in the gain region and simultaneously increasing the gain region spatial size. These changes reduce the effect of refraction on the X-ray laser beam propagation.

Detailed simulation of recombination XUV laser experiments

The detailed simulation of recombination XUV laser experiments is described and the calculation of gain as measured by two different experimental techniques is discussed. It is shown that a reasonable comparison between simulation and experiment can only be obtained if the self-trapping of the Lyman α line is neglected.

Soft X-ray amplification in aluminium recombining plasma produced from a thin coated fibre

Thin carbon fibres, coated with aluminium, are irradiated with approximately=120 ps, 0.53 mu m laser pulses yielding a specific absorbed energy of approximately=4 J cm-1 and time-resolved gain measurements on the n=4-3 and n=5-3 transitions of lithium-like Al XI ions in the recombining plasma are made. In particular, gain on the 4f-3d line at 15.4 nm is measured with alpha max approximately=2-3 cm-1 using both single-shot and multi-shot techniques. Gain measurements from multi-shot data on the 5f-3d line at 10.6 nm indicate alpha max approximately=1.5 cm-1. Observations are compared with theoretical calculations, discrepancies are noted and the need for reproducible plasma conditions in low gain-length product data sets is discussed.