High-power 20-channel neodymium glass laser and combined laser and beam heating of a plasma

A high-power 20-channel neodymium-glass laser is described (energy i kJ, pulse duration 2 nsec). It is part of the Flora facility intended for combined simultaneous heating of a plasma by powerful laser radiation (PLR) and by a relativistic electron beam (REB) produced in a facility of the plasma focus type. The paper deals theoretically with the processes of combined interaction of PLR and REB with a current plasma and a solid target.Translated from Trudy Ordena Lenina Fizicheskogo Instituta im. P. N. Lebedeva, Vol. 103, pp. 202–230, 1978.     

Relativistic filamentation of laser beams in plasma

We describe the results of a numerical code which models the relativistic selffocussing of high-intensity laser beams in plasmas by the nonlinear relativistic dependence of the optical constants on laser intensity. The plasma dynamics of 10¹³ W Nd glass lasers of 30 m initial beam diameter in nearly cut-off density plasmas is followed for a few picoseconds interaction time and 25 m depth into the plasma. Rapid relativistic selffocussing down to a beam diameter of one micron in a distance of the order of the original beam diameter is observed, as well as the production of GeV ions moving against the laser light.     

Feasibility of a laboratory X-ray laser pumped by ultrashort UV laser pulses

In order to allow widespread application of soft X-ray lasers there is a strong effort worldwide to use as small as possible pump lasers for plasma production. Short pulse lasers ( 1 ps), particularly in the UV, have attracted much interest, since extremely high intensities (up to 10¹⁸ W/cm²) can be achieved with a relatively high repetition rate. In this article we discuss their merit for soft X-ray laser pumping and possible solutions to the specific problems, for instance pulse front distortion, nonlinear absorption in window materials, plasma formation by short laser pulses and the relatively low total pump energy.     

An argon ion ring laser as a gyroscope

As a prototype gyroscope for a precision measurement of the Earth's rotation _E, we set up an argon ion ring laser with an area of 1.4 m². Different cavity geometries were tested in order to achieve a cancellation of the effect of the plasma flow. In a set-up with two laser tubes of the same type facing towards opposite directions, a stability of the beat frequency of 2–3 _E was measured. In a configuration with a double transition of the laser beam through the tube, the lock-in threshold was too high for the Earth's rotation to be measured, however, no effect of the plasma flow was observed.     

The effect of prepulse on X-ray laser development using a powerful subpicosecond KrF laser

A high power UV laser has been developed as a pump source for short wavelength (down to 1 nm) X-ray lasers. Various schemes are considered and theoretical analysis is discussed. Spectroscopic studies of laser-target interaction have been performed and, in particular, the effect of a prepulse on plasma generation has been investigated. Analysis of the observed spectra indicates that reduction of the prepulse energy results in a higher temperature plasma. Investigation of the interaction using thin layered targets is also presented. These data provide evidence for initially hot plasma conditions generated from target layers 150 Å. Discussions of proposed laser schemes at 1–5 nm are presented.Also at the Mechanical and Aerospace Engineering Department, Princeton University, Princeton, NJ     

X-ray emission from short-pulse laser plasmas

Highly intense picosecond and subpicosecond laser pulses interacting with solids can create hot and dense plasmas which emit x-ray pulses in a broad spectral range from 100 eV up to MeV. The duration of these x-ray pulses depends on the transient behaviour of the relaxation and recombination mechanisms, as well as on the lifetime of energetic electrons produced via nonlinear processes in the plasma. This paper reports experiments using a 1.5-ps laser pulse with high constrast ratio (up to 10¹⁰) and intensities up to 10¹⁸ W cm^-2 irradiating solid targets. Both the line spectrum characteristics of a magnesium plasma, recorded using crystal spectrometers with high spectral resolution, and kinetic calculations have allowed the deduction of plasma parameters in the process of plasma evolution. In addition, hard x-ray pulses from a tantalum plasma were measured and their scaling was explained as bremsstrahlung emission from energetic electrons. Absolute dose values of x-ray pulses are given.     

Short pulse scattering from a laser produced plasma

Experiments are reported on 1.06m scattering from a laser produced plasma. Generally in such experiments scattering is observed on the shock wave front at 90° from the incident beam. In our set-up the plasma is expanding in a Fabry-Perot resonator and we note a strong emission at 1.06m.     

A combined fluid dynamic and chemical model to investigate the laser induced plasma expansion

In this paper a theoretical model to study the time and space evolution of the plasma generated during the interaction between a KrF laser and a Ti surface has been developed. An optimisation of the initial parameters in the continuous evaporation approximation, such as temperature (T₀), rate production of matter (z_d), velocity (v₀) and ionization degree (), has been carried out to reproduce the experimental profiles obtained by time and space resolved laser induced plasma spectroscopy. The best initial conditions have been found by Downhill simplex method and their effects on the plasma evolution have been discussed. PACS 47.70.Fw; 52.25.Dg; 52.38.Mf; 82.20.Wt     

A free-electron laser pumped by the soliton laser

We propose a scheme for building a free-electron laser in the soft X-ray region pumped by the soliton laser. Making use of soliton laser wave evolution shape and single-pass small signal analysis, we find that this laser has two special advantages over the previous electromagnetic wave undulator free-electron lasers. One is a very small mass-shift effect because of the special characteristics of soliton laser; the other is that it has an additional frequency tuning effect based on the conventional free-electron laser's tunability. We also obtain the small signal gain and present some discussion.     

Recent results on development of a table-top soft X-ray laser

Gain measurements have been performed in a recombining carbon plasma generated by a line focussed Nd-glass laser beam on a multi-fin target with the aim of developing a table-top soft X-ray laser. With a laser beam of only 4 J (2ns) and a 6 mm long target gain up to 7.1 cm^–1 was measured for the C VI 18.2 nm line (gain-length 4.3). During these measurements high gain (6.5 cm^–1) was also measured for the first time on the C VI 13.5 nm line (4–2 transition). These measurements were done in the process of preparation for two-target experiments. High gain was measured independently for two separate, identical 6 mm targets. Changes in the influx into the plasma of iron ions from cooling blades significantly affected the location of the gain region and can be used as a mechanism to control the gain region in two-target experiments.     

Preionization of TEA CO2 laser by sliding discharge

TEA CO₂ laser preionization by plasma sheet formed by discharge sliding over a dielectric surface is described. The preionization electron number density in order of 10⁹cm^–3 was measured in the CO₂ N₂ He=113 gas mixture. The plasma sheet was also tested as a main discharge electrode in TEA CO₂ laser.The authors would like to acknowledge with thanks the current interest and the help of P. Gavrilov and V. Krajíek in experiments.     

Investigation of laser energy conversion into plasma x-rays in the system Iskra-4

We present results of experimental and computational investigation of laser plasma emission in the soft x-ray spectrum. Plasma has been produced by irradiating a gold target with subnanosecond pulses at the first and second harmonics of an iodine laser. It has been shown that approximately half of the absorbed laser energy is converted into soft plasma x-rays.Presented at bilateral international seminar of High Temperature Laser Plasma and High Gain Iodine Lasers held on 4 July 1991 in the Inst. of Physics, Czechosl. Acad. Sci., in Prague (organized by Division of Optics of the Inst. of Physics, Czechosl. Acad. Sci., and Physical Section — Plasma Division of the Union of Czech Mathematicians and Physicists).     

Simulations of grazing-incidence pumped Ni-like Sn X-ray laser at 11.9 nm

Grazing-incidence pumped Ni-like Sn X-ray laser media at 11.9 nm (4d-4p, J = 0-1) is modelled using code EHYBRID and a post-processor code. The required atomic data are obtained using the Cowan code. In this study the pre-formed plasma is pumped on longitudinal direction with a grazing angle. Detailed simulations were performed to optimize the driving laser configurations. Relatively high gain is produced for the Ni-like Sn X-ray laser at 11.9 nm with long pre-pulse and short main pulse drive energy of only 100 mJ on 4 mm slab targets. Using low intensity pre-pulse prior to long pulse decreases the electron density gradient. X-ray resonance lines between 13 and 25 Å emitted from tin plasma have been simulated using post-processor coupled with EHYBRID. The ratio of these resonance lines can be used to measure electron temperature of the laser produced Sn plasma.     

Effect of process parameters on the mechanical properties of carbon nitride thin films synthesized by plasma assisted pulsed laser deposition

We present an investigation of the effect of the process parameters, namely deposition pressure and laser intensity, on the growth and mechanical properties of carbon nitride (CN_x) thin films synthesized by plasma assisted pulsed laser deposition. Deposition at high remote plasma pressure (200 mTorr) enhances both growth rate and nitrogen incorporation (up to 40 at.%), but nano-indentation measurements indicate that these films are very soft and have poor mechanical properties. At low remote plasma pressure (0.5 mTorr), the nitrogen content varies from 24 to 16 at.% with increasing laser intensity as the films become much harder and more elastic, with hardness and Youngs modulus values reaching 24 GPa and 230 GPa, respectively. These effects are explained in terms of a thermalization of the laser plasma at 200 mTorr and indicate that plasma activation of nitrogen does not provide any particular benefit to the film properties when deposition is performed at high pressure. However, at low pressure, the benefit of plasma activation is evidenced through enhanced nitrogen incorporation in the films while preserving the highly energetic species in the ablation plume. Such conditions lead to the synthesis, at room temperature, of hard and elastic films having properties close to those of fullerene-like CN_x. PACS 81.15.Fg; 81.05.Gc; 68.60.Bs     

Optical properties of low-temperature magnesium laser plasma

Results are reported of an experimental study of ehe optical properties of the plasma produced when magnesium vapor is irradiated by resonant laser emission ( = 285.2 nm). The spectral and temporal characteristics of the vapor luminescence were measured. An additional laser source was used to study the kinetics of the plasma formation and decay. The conditions for obtaining a plasma with a large ion content were determined. At 1 kW resonance radiation and at a magnesium-atom concentration 4·10¹⁵ cm^-3 the ion concentration was 2·10¹⁴ cm^-3. Lasing on the 4³S₁–3³P₂ transition of the magnesium atom ( = 518 nm) was obtained for the first time and was the result of rapid transfer of the excitation from the singlet system of levels to the unperturbed triplet system via secondary processes accompanying the particle collisions.Translated from Lazernye Sistemy, pp. 35–45, 1982.     

Iodine photodissociation laser system Perun II

A construction and exploitation of a medium size iodine photodissociation laser system Perun II is reported. This laser produced pulses of infrared light (=1.315m) up to 50 J in energy and 300 ps in duration. The diameter of the beam is 82 mm. The beam divergence is about 4×10^–4 rad. The laser beam can be focused in a focal spot of a power density exceeding 10¹⁴ W/cm². The resuls of measurements of basic plasma parameters on an Al foil target are also presented. A recent improvement of the system includes a conversion to the second harmonic by a DKDP crystal.Presented at bilateral international seminar of High Temperature Laser Plasma and High Gain Iodine Lasers held on 4 July 1991 in the Inst. of Physics, Czechosl. Acad. Sci., in Prague (organized by Division of Optics of the Inst. of Physics, Czechosl. Acad. Sci., and Physical Section (Plasma Division) of the Union of Czech Mathematicians and Physicists.     

Submillimeter laser interferometer for high density plasma diagnostic

There are presented the results of investigation of the one-channel homodyne laser interferometer =119 µm made on the basis of the hollow dielectric beamguide and quasioptical functional devices. The interferometer is designed for determination of the plasma electron density of the TOKAMAK-7. The density response threshold is 0.7% from the expected plasma density and the phase difference measurement total error is 5°     

Plasma laser with convective gas flow

A system for convective gas flow through the active zone of a plasma laser is considered. Expressions are obtained for the energy rating of the active zone at which convective flow ensures the required thermal regime. The kinetics of a helium-strontium plasma produced by high-energy particles is analyzed. The threshold values of the energy rating at which gain is ~chieved on the transition Sr II(s(1/2)Sr II (5P_1/2) ( = 4305 Å) are obtained. The calculations have shown that convective gas flow can ensure the required heat removal even at energy ratings substantially exceeding the threshold values.Translated from Trudy Fizicheskogo Instituta im. P. N. Lebedeva, Vol. 120, pp. 75–84, 1980.     

Influence of strong laser fields on the inverse bremsstrahlung collision frequency

A new model is described for the absorption of laser light by a plasma. Two issues important for high laser intensity are considered: changes in the electron velocity distribution and the absorption non-linearity. The effects of anisotropy and non-Maxwellian distribution function are analysed with a hard sphere model. It is shown that anisotropy alters the electron-ion collision frequency by an amount which depends on the ratio of the temperatures in direction of and perpendicular to the laser field.The effect of high laser intensity is considered assuming Coulomb collisions, and a new expression obtained for the collision frequency as a function of quiver velocity. It will be shown that the result is in agreement with the Spitzer result in the case of small fields and with the result of Catto and Speziale in the case of very strong fields, but shows an unexpected maximum of the collision frequency in an intermediate regime.     

Intense laser generation from an atomic-fluorine laser

An intensive generation of radiation from a discharge-pumped atomic-fluorine gas laser is reported. A peak power exceeding 330 kW and a total energy of more than 2 mJ is obtained for a number of lines in the red, using a NF₃He (1100) gas mixture at total pressure of 500 Torr. The circuitry optimization is described and the conditions for effective operation of the atomic-fluorine laser are discussed. The temporal and the spectral characteristics of the laser emission are also presented.