This review is focused on free-electron lasers (FELs) in the hard to soft x-ray regime. The aim is to provide newcomers to the area with insights into: the basic physics of FELs, the qualities of the radiation they produce, the challenges of transmitting that radiation to end users and the diversity of current scientific applications. Initial consideration is given to FEL theory in order to provide the foundation for discussion of FEL output properties and the technical challenges of short-wavelength FELs. This is followed by an overview of existing x-ray FEL facilities, future facilities and FEL frontiers. To provide a context for information in the above sections, a detailed comparison of the photon pulse characteristics of FEL sources with those of other sources of high brightness x-rays is made. A brief summary of FEL beamline design and photon diagnostics then precedes an overview of FEL scientific applications. Recent highlights are covered in sections on structural biology, atomic and molecular physics, photochemistry, non-linear spectroscopy, shock physics, solid density plasmas. A short industrial perspective is also included to emphasise potential in this area.

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Short-wavelength free-electron laser sources and science: a review

A review is made of the literature covering the three types of electric arcs, i.e., (1) gas-stabilized, (2) wall-stabilized and (3) free-burning, which have contributed most to the field of quantitative spectroscopy in the role of light sources. Most of the paper is devoted to a description of the atmospheric pressure, argon arc burning freely between cooled metal electrodes. Methods for computing the composition of the argon plasma in local thermal equilibrium have been discussed in detail, including corrections required to account for coulomb interactions. Methods for measuring the absolute intensities of all species of plasma radiation, including corrections for self-absorption and for converting them to temperatures, are discussed. Temperatures determined from atom and ion line intensities and from the electron continuum agree within ± 1 per cent. Absolute transition probabilities for selected lines of the ArI and ArII spectra have been measured with an accuracy of ∼ 10 per cent. Some experimental work on spectral line shapes has been briefly discussed. The existence of local thermal equilibrium has been experimentally demonstrated further establishing the usefulness of this arc as a light source for quantitative spectroscopy.

The electric arc as a light source for quantitative spectroscopy

The calculation of electron density and temperature in Ar spectroscopic plasmas from continuum and line spectra

https://aip.scitation.org/doi/pdf/10.1016/j.mre.2016.10.002

Review of quantum collision dynamics in Debye plasmas

The early 21st century is witnessing a breakthrough in the study of the thermal radiation of neutron stars. Observations with modern space telescopes have provided a wealth of valuable information, which, when properly interpreted, can elucidate the physics of superdense matter in the interior of these stars. This interpretation is underlain by the theory of formation of the neutron star thermal spectra, which is in turn based on plasma physics and on the understanding of radiative processes in stellar photospheres. In this paper, the current status of the theory is reviewed, with particular emphasis on neutron stars with strong magnetic fields. In addition to the conventional deep (semi-infinite) atmospheres, radiative condensed surfaces of neutron stars and 'thin' (finite) atmospheres are considered.

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Atmospheres and radiating surfaces of neutron stars

In a plasma the ionization energy is decreased due to the presence of the microfield. In the past several attempts have been made to calculate this effect. These calculations, which use statistical and thermodynamical procedures, give different results. They produce either a ``polarization term'' or a ``lattice term'' or both of them. Moreover there are quantitative discrepancies. The lowering of the ionization energy is here derived by a statistical method, which is physically conceivable. The results cover a wide density range below and above the so‐called critical density. The new results are compared with the results of all earlier calculations and reveal the cause of their discrepancies.

G. Ecker

Papers

Lowering of the Ionization Energy for a Plasma in Thermodynamic Equilibrium