Development of a long life, 2 kHz repetition rate X-ray preionizer

A simple and rugged X-ray preionizer system has been developed that is capable of sustained operation at the 2 kHz level with a demonstrated lifetime of2.5× 10⁸ shots and a potential lifetime of >10⁹ shots. At the heart of the system is an advanced form of corona plasma cathode. Cathode wear is minimised by utilising adsorbates from the dielectric surface and not cathode material to form the surface discharge from which the electron beam is extracted. The preionizer chamber background pressure (which is directly proportional to the sticking rate of adsorbates) is found to be of crucial importance in maintaining good cathode emission at high repetition rates. The device emits a rectangular X-ray beam, 5 cm×40 cm, with an X-ray risetime of 20 ns, a FWHM of 60 ns, and a peak energy of 95 keV. Ionization levels of >10⁷ cm^–3 in 1 atm of Ne could be generated after the X-rays had passed through 2 mm of aluminium and 1 mm of steel. Good beam uniformity and reproducibility at 2 kHz were shown to be maintained with the aid of a segmented electrode charge collection chamber. The system should be especially suitable for preionizing lasers containing a strongly electron attaching gas mixture.     

Control of parameters of micropinches formed in current-carrying plasma jet

The temporal evolution and spatial pattern of X-ray emission from a laser-induced vacuum discharge of moderate power has been investigated. It was found that micropinches in the initial stage of the cathode jet expansion into the vacuum ambient were formed. They generated a soft X-ray radiation and beams of accelerated electrons; therewith these phenomena occurred just when both amplitude of the discharge current and energy of the initiating laser pulse lied in the specified ranges of values. Parameters of the micropinch, namely, its position within the interelectrode gap and also, intensity of the X-ray radiation and beams of the accelerated electrons emitted from the micropinch are variable over a wide range of values through changes of energy of the laser pulse and/or amplitude of the discharge current.     

Study of charge distribution in a dust beam using a Faraday cup

After getting charged in plasma, dust grains in a dust beam are scanned by a Faraday cup and electrometer. At different operating conditions, including a dc field, and analysis of the dust current profile, dust charge distribution is studied and information on the nature of the nonuniform distribution obtained.     

Pulsed X-ray diode with a long-lifetime plasma cathode

6 shots, the X-ray dose, uniformity, and ionisation rate values guarantee an effective preionisation of excimer laser discharges. Owing to both the long lifetime and the substantial absence of maintenance, this X-ray diode seems suitable to preionise commercial gas lasers, such as excimer and TEA CO₂ lasers. Received: 28 August 1998 / Revised version: 16 November 1998 / Published online: 24 February 1999     

High-frequency discharge stability of waveguide He-Ne lasers

The paper deals with the small-signal behavior and the stability of discharges in narrow capillaries. A theory is presented, which assumes that the time constants of the particle balance and the wall sheath extension determine the small signal behavior of a positive column. The column admittance is used to analyze the stability of an electric circuit typical for a He−Ne waveguide laser discharge. Numerical evaluation shows qualitative agreement of the experimental and the theoretical stability diagrams.     

High density plasmas for recombination X-ray lasers

The scaling of recombination XUV lasers to shorter wavelengths requires laser plasmas produced at initial electron densities close to solid. With pump laser pulses longer than a few tens of picoseconds the hydrodynamic motion of the plasma during the interaction makes this difficult to achieve. In contrast, when picosecond laser pulses are used the laser energy is absorbed close to solid density since the plasma expansion is insignificant during the laser pulse. This results in hot near solid density plasmas which are needed for hydrogenic recombination X-ray lasers operating in the water window. Experimental observations have shown that a fully ionized aluminium plasma with a temperature of about 400 eV and a density well above 10²³ cm^–3 is produced when an aluminium target is irradiated with a single 3.5 ps high power KrF laser pulse.     

Application of laser-guided discharge to processing

Received: 5 May 1998/Accepted: 14 August 1998     

The influence of vaporization upon the roots of a high current arc. II

The root of an argon are was stabilized by strong cooling of the graphite cathode; both the composition and the temperature of the plasma at the arc root were determined spectroscopically. The measurements of the absolute intensities of two CI and two CII lines revealed that the plasma is composed almost entirely of atomic carbon. Immediately in front of the cathode a temperature of 12000 K was measured and the degree to which the gas was ionized was found to be 30%. The velocity of the plasma was 300 m/s. This work is based upon material presented in the final report “Basic research programme for plasma technology, high-pressure arcs in SF₆” to the Federal Department of Research and Technology, Fed. Rep. Germany.     

Laser-induced fluorescence measurements of velocity within a Hall discharge

The results of a study of laser-induced fluorescence velocimetry of neutral and singly ionized xenon in the plume and interior portions of the acceleration channel of a Hall thruster plasma discharge operating at powers ranging from 250 to 725 W are described. Axial ion and neutral velocity profiles for four discharge voltage conditions (100 V, 160 V, 200 V, 250 V) are measured as are radial ion velocity profiles in the near-field plume. Ion velocity measurements of axial velocity both inside and outside the thruster as well as radial velocity measurements outside the thruster are performed using laser-induced fluorescence with nonresonant signal detection on the xenon ion 5d[4]_7/2–6p[3]_5/2 excitation transition while monitoring the signal from the 6s[2]_3/2–6p[3]_5/2transition. Neutral axial velocity measurements are similarly performed in the interior of the Hall thruster using the 6s[3/2]⁰ ₂–6p[3/2]₂transition with resonance fluorescence collection. Optical access to the interior of the Hall thruster is provided by a 1-mm-wide axial slot in the insulator outer wall. While the majority of the ion velocity measurements used partially saturated fluorescence to improve the signal-to-noise ratio, one radial trace of the ion transition was taken in the linear fluorescence region and yields a xenon ion translational temperature between 400 and 800 K at a location 13 mm into the plume. Received: 27 September 2000 / Revised version: 2 March 2001 / Published online: 9 May 2001     

Population densities and optical gain of VUV transitions of Cu II in Cu−He hollow cathode discharges

Population densities of excited Cu II-levels between 16 and 25 eV in Cu–He hollow cathode discharges were determined by emission spectroscopy. Population inversion was detected for several 6s-4p transitions. Investigation of the enhancement of Cu II vuv lines in He compared to Ne discharges showed that the excitation of the 6s levels by charge transfer is up to 100 times higher in He than in pure Ne discharges. Using the population densities and known transition probabilities, a single pass gain of 55% m^–1 at 780.8 nm and 1.2% m^–1 at 154.17 nm at a current density of 0.4 A cm^–2 was calculated.This paper is dedicated to Prof. Dr. H. Gobrecht on the occasion of his 75th birthday     

Small volume coaxial discharge as precision testbed for 0D-models of XeCl lasers

In order to check the predictions of 0D-models experimentally, a small coaxial discharge configuration for the generation of homogeneous high pressure glow discharges (diameter 11 mm, length 20 mm) in rare gas halogen excimer laser gas mixtures under accurately controlled conditions has been developed. It uses X-ray preionization and a special pulse-forming network (PFN) delivering fast rising (8 ns) single square pulses (U ₀=25 kV; I=300 A; =100 ns). Discharge current and voltage are measured precisely by a capacitive voltage divider and a shunt integrated into the discharge chamber. All circuit data needed for the model calculations have been evaluated. Interferometric and spectroscopic diagnostics of the bulk of the discharge and of the cathode sheath have been performed. First results for Ne/Xe/HCl mixtures are compared with model calculations.     

The role of halogen donors in discharge instability of rare-gas halide excimer lasers

Experimental results on striation instability in a self-sustained discharge-pumped XeCl laser are presented. The presence of HCl proves to be critical to the evolution of this particular mode of instability. The role of halogen donors in discharge instabilities of RGH lasers is discussed. That mixtures containing HCl will be susceptible to striation instability and those with F₂ or NF₃ will not is concluded.This work is based on experiments done when the author was at UCSD     

Plasma nitriding of low alloy steels at floating and cathodic potentials

The low alloy steels were nitrided in the plasma atmosphere generated by using hollow cathode discharge. For comparison, specimens of 40 Cr steel were in two different potential states. One kind is isolated from cathode and anode between them the discharge was created. The other kind is placed on the plate which is connected to the cathode. The nitriding was carried out at different temperatures. The surface morphology, phase of compound layers and microhardness profiles were analyzed. The optical microscopy observation and X-ray diffraction showed that the compound layers were formed in all experiments. The results of the nitriding treatment are weakly dependent on the potentials applied on the specimens.     

Measurement of atom and molecule number densities in hydrogen arcs by means of VUV spectroscopy

The number density of ground state atoms in the 2mm hydrogen arc (T≈20000 K) and the number density of molecules in the 5 mm hydrogen arc (T≈12000 K) have been measured by means of vuv spectroscopy. These species' are particularly likely to deviate from the equilibrium population and may therefore falsify the hydrogen transport coefficients measured in arc experiments. In the present investigation the optically thick line wings of Ly-α and the H₂ molecular spectrum around 1600 Å have been analyzed. The results demonstrate that the former assumptions of LTE and PLTE, respectively, are completely justified on and near the arc axis. However, in the outer zones of these arcs a considerable overpopulation of the molecules must be expected as a consequence of radial diffusion.     

Space and time resolved discharge evolution of a large volume X-ray triggered XeCl laser system

The spatial distribution and the temporal development of the net gain have been measured in a ten liter active volume switchless discharge XeCl laser. The experimental results are compared with both zero-dimensional and two-dimensional kinetic code predictions. The comparison between the results of the kinetic codes and of some measurements relevant to the time-dependent discharge homogeneity allows a deeper insight into the influence of the streamer evolution on the discharge characteristics.ENEA StudentENEA Guest     

A spark channel plasma electrode for a CO2 laser gas discharge

A plasma electrode potentially suitable for dc discharge pumped lasers has been developed based on the repetitive creation of a spark channel. The high-density plasma is a source of charge carriers for the dc discharge, thereby largely eliminating the cathode and anode falls. Potential reduction improves with increasing spark repetition frequency until about 9 kHz, at which point the falls are virtually eliminated. Fortuitously, the most beneficial regime of plasma electrode operation also appears to coincide with the optimum E/N range for CO₂ laser vibrational excitation.     

Particle densities in a decaying SF6 plasma

Particle densities are determined for a decaying axially blown SF₆ are between current zero and brekdown, some 100 μs later. During the time of interest, primarily only atoms, diatomic molecules and ions can be built up by reactions. Therefore, in a first approach equilibrium densities are derived for SF₆ which has decomposed into its considemic and diatomic components. In a second approach reaction kinetics are considered. It turns out that the development of densities occurs in two steps. During the first approx. 100 μs there is a strong deviation from equilibrium. Afterwards the most abundant particles remain near their partial, diatomic equilibrium values. Near breakdown the degree of ionization is of the order of 10¹³ cm⁻³ with the most abundant ions S ₂ ⁺ and F⁻. This density is sufficient to distort the applied electric field. Future theories of breakdown in hot gases must take into account this principal difference as compared with the conditions in cold gases.     

MPD arcs as plasma sources for recombination lasers

In this study the decay of argon plasmas ejected from a MPD thruster described in a previous paper is investigated. Various parameters, e.g. the electron densityN _e , the electron temperatureT _e , and the absorption coefficient of the ArII-488 nm transition are measured by different experimental techniques. Axial and radial profiles ofN _e andT _e are determined, and used to decide on the relevant recombination mechanism. In spite of the fact that the dominant three-body recombination favours the population of the high-lying energy levels, population inversions have been observed even with the most sensitive method only in plasmas ejected from a reduced aperture of the MPD thruster. The theoretical analysis shows that the ranges ofT _e andN _e , in which recombination-lasing may be expected, are narrow. In addition, the mechanisms that limit the population inversions in discharge tubes of conventional Ar⁺-lasers restrict the dimension of the plasma perpendicular to the resonator axis. From these facts and the described measurements on population inversions we conclude that the initial diameter of the plasma has to be reduced. We therefore propose a new discharge configuration where extended regions of constant plasma parameters can be expected. With this arrangement it should be possible to reach population inversions required for laser oscillations in ArII.Polish Academy of Sciences, Gdansk, Poland     

Electrode design for a magnetically stabilized glow discharge

The operational characteristics of a magnetically stabilized glow discharge are investigated for several different experimental electrode configurations. In this parametric study, successive geometries were designed to accommodate and/or control specific aspects of the plasma glow process. In this manner, an electrode structure which optimizes discharge stability and promotes glow uniformity over an extended active volume, was ultimately achieved. The study teaches a promising new technique for significantly increasing the power density of high power gas lasers.Experimental data is in excellent correlation with predictions provided by a previous numerical study of this magnetic stabilization process.     

A simple kinetic model of a Ne-H2 Penning-plasma laser

A simple kinetic model of the Ne-H₂ Penning-Plasma Laser (PPL) (NeI 585.3 nm) is proposed. The negative glow of a hollow cathode discharge at intermediate pressures is considered as the active medium. The balance equations for the upper and lower laser levels, electrons, ions and electron energy are solved. The dependences of the laser gain on the discharge conditions (Ne and H₂ partial pressures, discharge current) are calculated and measured. The calculated values are in a good agreement with the experimental data.