The rate of laser ablation at irradiances of ~2x1014 Wcm-2 of solid iron and aluminum has been measured using the
transmission of a neon-like zinc X-ray laser at 21.2 nm through thin iron and aluminum targets. It is shown that the
opacity of ablated material falls rapidly with increasing temperatures and decreasing density from the solid value. As
ablated plasma becomes transparent to the X-ray laser flux, the thickness of solid, unablated material and hence the rate
of ablation can be measured from time resolved X-ray laser transmission. A self-regulating model of laser ablation and
fluid code simulations with absorption to thermal plasma of 5-10% show agreement with our measured ablation rates.
The possibility of generating short wavelength laser action in plasma is considered by two different methods. In
the first we examine the conditions under which laser action can be generated using laser heating of thin foils
of materials with a high atomic number. It is shown that there is a way forward using reasonable pump laser
energies tailored to the specific x-ray wavelength and material. Recombination lasers, in principle, have a high
quantum efficiency, particularly if transitions to the ground state can be used. ATI offers a possible means to
achieve this objective. We examine the conditions under which such inversion can be generated. Some simple
scalings for the density and temperature are derived.
We examine the development of soft X-ray lasers using collisionally pumped Ni-like ions to identify the possibilites for further improvements in the pumping efficiency as we move to shorter wavelengths. The underlying requirements of the pumping system are reviewed. Silver and samarium are examined by detailed simulation in the light of this analysis. It is found that there is significant potential for further reductions in the pump demands for silver using long pre-pulse/mainpulse delays and grazing incidence pumping. Samarium however offers less possibilty of improvement as recombination in the expanding plasma plume inhibits the use of long delays. We find that grazing incidence pumping at an appropriate angle can lead to significant reduction in pump energy.
Experimental measurements of the opacity of plasmas at densities close to solid state and temperatures ~ 60 - 300 eV using a probing X-ray laser are presented. Utilizing thin targets, opacities of iron have been measured using x-ray lasers of photon energy 89 eV created by pumping with the VULCAN RAL laser. The thin targets are separately heated by spot focus laser pulses. We have demonstrated that X-ray laser brightness is sufficient to overcome the self-emission of hot plasma so that useful opacity measurements can be made. Due to their high brightness, x-ray lasers can fulfill a useful niche in measuring opacity and other phenomena associated with laser-plasma interactions. Quantities such as opacity measured in laser-plasmas are useful elsewhere. For example, plasma opacity is important in understanding radiative transfer in the sun.
Output characteristics of a transient Ni-like Ag X-ray laser pumped by a single picosecond laser pulse are demonstrated. The small-signal gain coefficient, beam divergence, output pulse profile and beam dimensions in the near-field were measured. The influence of the pump pulse structure on the lasing process and the output signal is analysed theoretically.
Below saturation, X-ray laser output shows a reduction in pulse duration and frequency bandwidth as the gain-length product increases. Above saturation, both quantities can be expected to rebroaden. The duration of gain can be close to an order-of-magnitude longer than the output pulse duration. With gain-length products just below saturation, X-ray lasing at 13.9 nm in nickel-like silver has been measured with a pulse duration Δt of 3 - 4 ps and an estimated frequency bandwidth Δv of 5×1011 Hz. Such values imply that the pulses are close to transform limited with Δt Δv ≈ 1.5. Measurements of x-ray laser pulse-lengths and gain duration will described in this paper.
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.
We present a review of new progress performed in several laboratories (Laboratoire pour l'Utilisation des lasers Intenses, Rutherford Appleton Laboratory, Prague Asterix Laser System, Institute of Laser Engineering, Laboratoire d'Optique Appliquee). Concerning the realization of x-ray lasers sources, using different laser pumping techniques (600 ps, 100 ps, ns/ps, OFI) and the optimization of their optical properties, using curved and plane half-cavity mirrors. In parallel of these developments, we present the main results obtained with x-ray laser in interferometry applications. These studies concern on the one hand the Michelson interferometry with an x-ray laser emitting at 13.9 nm (recently realized at LULI), and on the other hand the Fresnel bi-mirror with an x-ray laser emitting at 21.2 nm (recently realized at PALS).
Saturated operation of an X-ray laser is desirable as a high output irradiance is obtained with reduced shot-to-short variation. The potential of saturated X-ray laser output in probing plasma samples is first investigated. The laser pumping requirements to scale Ni-like saturated X-ray laser output to shorter wavelengths is then analyzed using published atomic physics data and a simple 4-level laser model for gain. A model of amplified spontaneous emission has been modified to accurately predict experimentally observed saturation behavior obtained in different experiments at the Rutherford Appleton Laboratory. In particular, the effects of traveling wave pumping with short duration (approximately 1 ps) laser pulses are investigated. Simulations of Ne-like Ge resonance line emission are compared to experimentally measured spectra.
Ne-like and Ni-like ions have been pumped in the transient gain regime using intense picosecond pump pulses from Vulcan in its CPA mode. High gain coefficients of at least approximately 30/cm are observed for the Ne-like ions Ti XIII, Ge XXIII and Sn XXIII at wavelengths 31.2, 19.6 and 12.0 nm respectively and approximately 20/cm for the Ni-like ion Sm XXXV at 7.3 nm. Saturated output is found in all cases for target lengths shorter than 10 mm and the effect of traveling wave pumping has been studied and unequivocably demonstrated. An experimental campaign to observe four wave mixing using a soft x-ray laser and an optical laser in a sum-difference frequency mixing scheme has been initiated. Preliminary results are described and future directions discussed.
We report the first demonstration of saturation in nickel-like x-ray lasers, specifically nickel-like Ag, In, Sn, and Sm x- ray lasers at wavelengths of 14, 12.6, 12.0 and 7.3 nm respectively. These x-ray lasers were found to be very monochromatic x-ray sources with the laser lines completely dominating the output spectra. Using high-resolution spatial imaging and angularly resolved streaking techniques, the output source sizes as well as the time histories, divergences, energies and spatial profiles of these x-ray lasers have been fully characterized. The output intensities of these x-ray lasers were measured to be in the range of 0.7 - 2 X 1011 W (DOT) cm-2 in approximately 40 ps. The high monochromaticity, narrow divergence, short pulse duration, high efficiency and high brightness of these x-ray lasers make them ideal candidates for many applications.
The use of multi-pulse irradiation of neon-like ions has been shown to produce orders-of-magnitude enhancement of x-ray laser output. Recent results obtained at the Rutherford Appleton Laboratory are reviewed with an emphasis on understanding the reasons for the enhancement. Simulations with the fluid and atomic physics code EHYBRID are used to show that enhancement occurs because of a spatial enlargement of the gain region and consequent better propagation of the x- ray laser beam along the gain region.
Neon-like ions are routinely pumped by electron collisional excitation from the ground 2p6 state to levels in the n equals 3 manifold, generating a number of different transitions. The experiments are performed in two geometries: foils and slabs. In this paper we examine slab systems using germanium as the prototype medium. The aim is to identify ways in which design modifications can be used to improve the output performance of the x- ray lasers. We examine the role of pump laser pulse modification and target curvature (bending) to reduce refraction effects in the radial direction, and the potential of density wells to produce a guiding effect in the transverse direction.
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.
Michael Key, W. Blyth, Gerald Cairns, A. Damerell, A. Dangor, Colin Danson, J. Evans, Graeme Hirst, M. Holden, Chris Hooker, J. Houliston, J. Krishnan, Ciaran Lewis, J. Lister, Andrew MacPhee, Z. Najmudin, David Neely, Peter Norreys, Allen Offenberger, Karoly Osvay, Geoffrey Pert, S. Preston, Stuart Ramsden, Ian Ross, Wilson Sibbett, Gregory Tallents, C. Smith, Justin Wark, Jie Zhang
An injector-amplifier architecture for XUV lasers has been developed and demonstrated using the Ge XXIII collisional laser. Results are described for injection into single and double plasma amplifiers. Prismatic lens-like and higher order aberrations in the amplifier are considered. Limitations on ultimate brightness are discussed and also scaling to operation at shorter wavelengths. A preliminary study has been made of UV multiphoton ionization using 300 fs pulses at high intensity.
A theoretical basis of ASE action in collisional excitation slab lasers is described. Good agreement with experiment for the germanium laser is obtained, and reasonable agreement with limited data for yttrium. It is found that the gain on the 0-1 transition is only accounted for if the collision rate to its upper state is reduced by a factor 1/2 over the predicted value.
Systematic study of the collisionally pumped Ne-like germanium 3p-3s laser system has led to the observation of saturated output on the J = 2-1 lines at 23.2 and 23.6 nm when a double slab target is double passed using a normal incidence concave mirror. The about-1-MW output power on the same lines from a simple double plasma has been used to record preliminary images with a Schwarzchild condenser and zone plate microscope arrangement. Initial analysis of measurements on the 4d-4p J = 1-0 line at 7.3 nm from nickel-like samarium indicate a gain coefficient of between 0.4/cm and 0.8/cm for an incident irradiance of about 2 x 10 exp 13 W/sq cm using a l.06-micron heating laser on slab targets of SmF3.
The problem of the anomalously rapid decay rate of the principal radiative transition in interpreting expansion cooled recombination lasers is examined. The radiative transfer of the line is analyzed, and it is shown that, in typical situations, the net radiative bracket may be sufficiently larger than the commonly used planar escape factor to account for this discrepancy. The role of doubly excited states in the recombination cascade is also examined.
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