This article reviews the progress in the development of practical table-top sources of soft x-ray laser radiation. The field is rapidly approaching the stage at which soft x-ray lasers sufficiently compact to fit onto a normal optical table will be routinely utilized in science and technology. This is the result of recent advances in the amplification of soft x-ray radiation in both compact laser-pumped and discharge-pumped devices. The use of excitation mechanisms that take full advantage of new ultrafast high power optical laser drivers and multiple pulse excitation schemes has resulted in the demonstration of saturated soft x-ray amplification at wavelengths as short as 14 nm using several Joule of laser-pump energy. Moreover, several schemes have demonstrated significant gain with only a fraction of a Joule of laser-pump energy. In addition, the demonstration of saturated table-top soft x-ray lasers pumped by very compact capillary discharges has shattered the notion that discharge-created plasmas are insufficiently uniform to allow for soft x-ray amplification, opening a route for the development of efficient, high average power soft x-ray lasers. Recently, a table-top capillary discharge laser operating at 46.9 nm has produced millijoule-level laser pulses at a repetition rate of several Hz, with a corresponding spatially coherent average power per unit bandwidth comparable to that of a beam line at a third generation synchrotron facility. This review summarizes fundamental and technical aspects of table-top soft x-ray lasers based on the generation of population inversions in plasmas, and discusses the present status of development of specific laser systems.

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Table-top soft x-ray lasers

The interference pattern of two laser beams induces periodical variations of the optical properties of matter (refractive index and absorption coefficient). Induced grating structures have been observed in absorbing liquids and solids, plasmas, laser materials, dye solutions and semiconductors. These gratings have been applied to thermal diffusion measurements, excitation of high frequency normal and second sound waves, determination of electronic lifetimes, distributed feedback lasers and real-time holography. The literature on transient grating materials and applications is surveyed.

Laser-induced Grating Phenomena

Charge transfer, or charge exchange, describes a process in which an ion takes one or more electrons from another atom. Investigations of this fundamental process have accompanied atomic physics from its very beginning, and have been extended to astrophysical scenarios already many decades ago. Yet one important aspect of this process, i.e. its high efficiency in generating X-rays, was only revealed in 1996, when comets were discovered as a new class of X-ray sources. This finding has opened up an entirely new field of X-ray studies, with great impact due to the richness of the underlying atomic physics, as the X-rays are not generated by hot electrons, but by ions picking up electrons from cold gas. While comets still represent the best astrophysical laboratory for investigating the physics of charge transfer, various studies have already spotted a variety of other astrophysical locations, within and beyond our solar system, where X-rays may be generated by this process. They range from planetary atmospheres, the heliosphere, the interstellar medium and stars to galaxies and clusters of galaxies, where charge transfer may even be observationally linked to dark matter. This review attempts to put the various aspects of the study of charge transfer reactions into a broader historical context, with special emphasis on X-ray astrophysics, where the discovery of cometary X-ray emission may have stimulated a novel look at our universe.

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Charge Transfer Reactions

One of the great barriers to improving our understanding of the molecular basis of life is that we do not yet have any way of examining individual macromolecules in living tissue. The progress made so far in unravelling the structures of biological macromolecules is based on painstaking chemical and x‐ray crystallographic analyses of pure, crystalline samples of a given macromolecule.1 This situation could, however, be completely revolutionized if a source of coherent x rays were available. Although, as we shall show below, the realizaton of this goal is beset with difficulties, experiments aimed towards its accomplishments are under way, as shown in figure 1.

X-Ray Lasers.

A simple two‐state molecular theory for calculating the cross section for the photon‐assisted charge‐exchange collision process A++B+hν→A*+B+ is described. The semiclassical impact‐parameter method is applied to such collisions and for simplicity the nuclear trajectory is assumed to be rectilinear. Ignoring the Stark broadening and trajectory distortion of the molecular levels the exact solution of the resulting coupled charge‐transfer equations is obtained. A simple model of the required molecular transition moment is discussed and applied to obtain numerical results. The cross section, averaged over the molecular orientation, as a function of the applied field intensity and relative nuclear translational energy is determined. Using parameters characteristic of an atomic system the cross sections for photon‐assisted charge transfer are presented for intensities of 109–1011 W cm−2 and relative nuclear energies varying from 10 to 1000 eV. Our results demonstrate that considerable enhancement of the charge‐exchange cross section may be possible with the moderate field intensities presently available. 148

Photon‐assisted nonresonant charge exchange: A simple molecular model

A soft X-ray laser utilizing charge exchange☆

For potential applications in scanning and measurement, we consider two interacting laser beams in an active nonlinear medium. The nonlinear medium generates two new beams at the same frequency and in directions different from the incident beams. The Kerr effect is used as a means of phase matching in the isotropic medium.

Interacting laser beams in a resonant medium.

A scheme for a soft x-ray laser utilizing electron pickup in an excited state of a multiply ionized species is presented. Using a current source of 10 to 20 mA, we estimate a yield of ≈ 106 x-ray quanta per pulse. The present scheme seems to be well within the realm of modern technology.

William H. Louisell

Papers

A soft X-ray laser utilizing charge exchange☆

Interacting laser beams in a resonant medium.

A soft X-ray laser utilizing charge exchange