Table of contents

Solid state lasers have undergone a renaissance since the development of reliable and cheap laser diodes which can be used as pump sources. The result is compact, efficient laser sources which are attractive for a wide range of applications. This review summarizes the properties of laser diodes which are advantageous for pumping solid state lasers and describes the diverse laser materials and configurations of solid state lasers reported in the literature.

In this work an analytical formula is derived for the calculation of resonance integrals weighted by the adjoint flux. The expression obtained is a function of the temperature and lattice cell parameters and can be written in terms of the well known functions J( theta , beta ). The derived formula is general and suitable for all types of resonances (narrow, wide or intermediate) and is applied to a nuclear reactor lattice cell with different moderator to fuel volume ratios at different temperatures. The results show that in this case the resonance integrals weighted by the adjoint flux have negative Doppler coefficients while those weighted by the direct flux have positive Doppler coefficients.

In this paper the authors show that the same equivalent reflectivity distributed in different ways leads to widely different behaviour in the case of pulsed lasers. In a typical comparison, where the transmission loss is either from both ends or from one end as in a conventional laser, they show that the laser power and pulse duration are substantially different. The results are supported by experimental verification.

The authors have observed laser oscillation on the B'² Delta -X² Pi (3-10) and (3-11) of an F₂ laser pumped nitric oxide laser. The maximum total output pulse energy was 280 mu J. They present here the results of an investigation to determine the maximum intra-cavity loss which still allows laser oscillation to occur in this system. This information has been used to find the effective gain coefficient of the NO laser, operating on the B'² Delta -X² Pi (3-10) transition at 218 nm by applying the condition for cw laser oscillation. The effective cw gain coefficient was found to be 0.25 cm^-1 for pump laser intensities of 20 MW cm^-2.

A two-wavelength (10.6 mu m/632.8 nm) Mach-Zehnder interferometer is employed to provide radially and temporally resolved measurements of the electron density in a longitudinally discharge-excited strontium vapour laser over a wide range of operating conditions. From these results it is possible to gain an estimate of both the electron temperature and the Sr²⁺ ion density.

Magnetic storage systems, like disk-head and tap-head interfaces, machine tool guideways, and many other friction pairs, are vulnerable to so-called 'stiction'. Stiction is a contact phenomenon whereby static friction increases relative to kinetic friction. As a rule, this effect increases dramatically with time of stationary contact. Thus, the time dependence of static friction is an important subject of tribological studies. The motivation of this work is to understand the static friction mechanism at both extremes of the rest time or dwell range: short-term dwell is in the order of seconds or minutes, and long-term dwell is in the order of days or months. The stiction models reviewed cover a wide spectrum of static friction processes related to material deformation, meniscus formation and viscous lubricant flow.

The axial distributions of the optical emission intensity and metastable atom density are shown experimentally for DC and RF helium discharges. Significant differences in the cathode glow adjacent to the cathode sheath are perceived between the DC and RF discharges. These differences seem to lead to differences in the consistencies of their cathode sheaths. It is found that the RF glow is caused by a direct excitation of non-thermalized high-energy electrons; on the other hand, the DC glow is caused by a dissociative recombination of old molecular ions and thermalized low-temperature electrons in addition to the former process. A theoretical model of metastable atoms in the cathode glow region is proposed, and the theoretical curves are compared with the experimental results of metastable density distribution. The validity of this model is then assured, at least qualitatively.

The ion energy distribution of ions bombarding a negative wall depends on the sheath potential and on the ion kinetics in the boundary layer. In most discharges the electron Debye length lambda _D is small compared to the ion mean free path lambda _C. For weakly negative walls the sheath is, therefore, nearly collision free. In the case of highly negative walls, however, the sheath thickness increases and collisions in the sheath may become important. The effect of collisions on the ion distribution function and sheath potential variation is investigated theoretically and experimentally. The theory starts from the charge exchange model of ion kinetics, presents a convenient self-consistent description of the unipolar ion sheath and extends a former asymptotic analysis lambda _D/ lambda _C to 0 to the Debye sheath. In the experimental part a retarding field analyser is used to determine the ion energy distribution. Measurements were made in the negative glow of a DC glow discharge at pressures of several Pa in argon, nitrogen and oxygen.

The range of temperatures over which Rayleigh scattering is a useful diagnostic of temperature in a noble gas thermal plasma at atmospheric pressure is examined. The various scattering processes that can occur a light passes through such a plasma are outlined. It is shown that while Rayleigh scattering by ground-state atoms is the dominant process at low temperatures, Thomson scattering becomes important at temperatures at which significant ionization occurs. Resonance scattering is also a significant source of scattered radiation when the laser wavelength is approximately equal to the wavelength of a transition of a species present in the plasma. Radial profiles of the polarized and depolarized radiation scattered from argon and helium arcs have been measured and analysed to confirm the relative importance of the respective scattering mechanisms. The occurrence of Thomson scattering is found to set a fundamental upper-temperature limit to the applicability of Rayleigh-scattering techniques to temperature measurement. For a laser wavelength of 514.532 nm and a detection system bandwidth of less than 0.1 nm, Thomson scattering becomes significant at approximately 9000 K in argon and 11000 K in helium. Above these values, the derivation of plasma properties from scattering measurements requires that all scattering mechanisms be taken into account. The temperature of the argon arc is calculated to exemplify this.

The effect of Coulomb collisions on the electron energy distribution function (EEDF) in surface-wave produced argon plasmas in the RF and microwave regime is studied via solution of the Boltzmann equation, including a Fokker-Planck term, in the well known Lorentz approximation. The maintenance field strength is determined self-consistently by simultaneously solving a particle balance equation subjected to a boundary condition, which accounts for the sheath formation at the wall. The modifications to the EEDF and some derived parameters introduced by the Coulomb collisions will be discussed. The theoretical predictions are compared to experimental results, which have been obtained by a Langmuir probe technique on a surface-wave produced argon discharge at 60 MHz.

Resonance cavity diagnostics are applied to the magnetized plasma of a reflex discharge as well as an inductive RF discharge. Special attention is focused on the evaluation of the collisional frequency in the cavity Q and the underlying physical mechanism. When the conventional transmission method is used, a strong broadening of the frequency response curve is observed: the time-averaged transmission profile turns out to be a Voigt profile that arises from the Lorentzian profile of the instantaneous transmission curve and the Gaussian probability distribution of the density fluctuations. Obviously, the resonance broadening is not caused by collisional damping, but mainly by large scale density fluctuations. To measure the real absorption of the microwave power, the temporal decay of the electromagnetic wave energy in the cavity is detected. Two characteristic features are observed: (i) At magnetic field strengths well below the electron cyclotron resonance field, the measured (effective) collisional frequency nu _eff shows a threshold behaviour as a function of the electron density. Below 7*10¹⁵m^-3 the measured collisional frequency fits the electron-neutral collisional frequency, whereas above 9*10¹⁵m^-3, nu _eff tends to a value twice as high. (ii) Near the cyclotron resonance a strongly enhanced absorption is observed at low densities, though the TM₀₁₀ cavity mode used has no electric field components perpendicular to the constant magnetic field. Neither effect can be attributed to the observed density fluctuations in the discharges. Various absorption processes are discussed to explain the findings.

The sequences primary streamer-secondary streamer-arc transition are investigated in a positive point-to-plane electrode configuration for air pressure P in the range 1 to 10 bar. The evolution of the current and the emitted light during the predisruptive sequences are similar, whatever the pressure value, if the usual scaling factor Pd (but also P rho , rho being the point radius) and the applied positive potential remain constant. It was observed that the primary streamer velocity and product TP, T being the time between two discharges, remain constant. The anomaly of the decrease in the current was understood in terms of hydrodynamic phenomena which are more influential at high pressure than at atmospheric pressure.

The spectrum of an ablation-stabilized arc has been recorded using ice as the test material. The intensity of neutral oxygen lines was found to reach the blackbody radiation limit and subsequently the temperature of the vapour layer was determined by the Planck function. The arc diameter, pressure at stagnation point, arc column temperature and vapour temperature were measured by spectroscopic methods. The results are in agreement with those of Kovitya and Lowke (1984) for a two-zone, isothermal arc model.

A numerical simulation, including transport phenomena, is presented in this paper for the calculation of ozone generation in an oxygen-fed wire-to-cylinder ozonizer. The ozone density dependence on the first excited states of atomic and molecular oxygen and ozone is analysed, introducing three kinetic models. The influence of excited states on ozone production is shown as well as the correlation between the first excited state of atomic oxygen O(¹D) and the second excited state of molecular oxygen O₂(¹ Sigma ⁺_g). Furthermore, these results confirm the validity of the plug-flow reactor hypothesis in the case of an oxygen-fed ozonizer.

This paper reviews the properties of the cathode ion flux generated in the vacuum arc, concentrating on the characteristics of the ion energy distributions of the cathode ions. The cathode ion flux is quite energetic, with average ion potentials much larger than the arc voltage, and generally contains a considerable fraction of multiply-charged ions. The authors' calculations are based mainly upon the energy distribution data of the different ion fluxes, the data available for various cathode materials are summarized. They assume that the integrated energy distribution (IED) of the ion flux for a particular material may be treated as the sum of all the fractional ion energy distributions and present plots of such IEDS. They further assume that a shifted Maxwellian type velocity distribution gives the best fit to the IED, and calculate the characteristic parameters (shifted velocity v₀, thermal velocity beta and normalization constant C₁) for each material. IEDS calculated using these parameters agree reasonable well with the experimental data. The ratios v_p/v₀ (v_p is the most probable velocity) and v₀/ beta are evaluated and used in discussing theoretical ion acceleration mechanisms. Their analysis suggests that the mechanism making the greatest contribution to the energy of the ion flux is the electro-ion collision mechanism, but that the contributions of the electric field force and flow interaction (pressure gradient) mechanisms are also significant. Their results should be applicable to any vacuum arc where the cathode spots may be treated as individual emitting sites, i.e. where collective interactions between cathode spots may be neglected.

The authors have studied the phenomenon of changes in glow peak sensitivity in LiF:Mg, Ti(TLD-100, Harshaw) by monitoring changes in the intensity of peaks 4 and 5 following 100 degrees C for 30 min post-irradiation and pre-irradiation annealing. In an attempt to aid the dosimetric application of this material they have investigated these changes in sensitivity in both 400 degrees C annealed and unannealed materials and as a function of glow curve heating rate from 100 degrees min^-1 to 1000 degrees min^-1. Computerized glow curve deconvolution (CGCD) is used to resolve peaks 4 and 5 and thus monitor the behaviour of peaks 4 and 5 separately. In a previous publication they found that it was only peak 4 which seemed to show a propensity to possible clustering and consequent increases in sensitivity following room-temperature storage. In the work reported here they also find that peak 4 exhibits significantly anomalous behaviour (changes in sensitivity following 100 degrees C for 30 min annealing, differences in these changes in sensitivity between 400 degrees C annealed and unannealed material and a tendency to shift in temperature following annealing). At high heating rates above 500 degrees min^-1 peaks 4 and 5 merge and become increasingly difficult to resolve, so that changes in sensitivity of peak 4 can be easily misinterpreted and assigned to peak 5.

This paper presents experimental results of space-charge distribution in cross-linked polyethylene (XLPE) by a pulsed electroacoustic method. The influence of polarity reversal on space-charge distribution in degassed XLPE and as-received XLPE is studied to understand the transport and injection of charges under DC voltage. Homocharge distribution is dominant in degassed XLPE whereas heterocharge distribution is dominant in as-received XLPE. After the applied voltage is reversed, for degassed XLPE, previously injected homocharges are neutralized by newly injected ones with opposite polarity; while for as-received XLPE, previously accumulated heterocharges are replaced by newly accumulated ones with opposite polarity. A new data display method of a three-dimensional colour plot is proposed to display all measurement data on one plot by which the time dependence of space-charge distribution can be understood easily and conveniently.

The effect of infrared radiation on the resistivity of polymer-based thick film resistors was studied. It was found that the resistivity decreases with both the density and duration of the infrared and with the number of exposures. The change in resistivity was attributed to the viscous flow of the polymer at the temperature attained by the film when exposed to infrared radiation. A model has been proposed through which the percentage change in resistance can be estimated in terms of the number of exposures, duration of exposures etc. Based on these studies it was possible to suggest a downward trimming method for polymer film resistors in which the polymer film is exposed to intermittent infrared radiation pulses of the duration of the order of a few seconds. This method of trimming appears to be a clean process, with the resistors being free from hot spots.

Measurements are reported on the amplitude and phase of the photoacoustic signal for a range of light chopping frequencies between approximately 0.2-800 Hz. At low chopping frequencies existing theories deviate markedly from observation. A simple phenomenological model has been devised to remedy this, with encouraging results.

Polycrystalline SrTiO₃ thin films have been prepared on Si(100) substrates by RF magnetron sputtering. The films were deposited at 400 degrees C and were annealed at 600 degrees C. The films had a dense microstructure with fine grains. The electrical properties of the films were dramatically controlled with annealing. From the result of the capacitance-voltage (C-V) characteristics the dielectric constant and the flat band voltage of the annealed films were measured to be 75 and -0.31 V, respectively. These oxide thin films with high dielectric constant are potentially useful in ULSI applications.

Since the superconducting transition temperature of a NbN thin film is a function of a hysteresis cycle it is important to know the conditions under which such a hysteresis cycle exists. Agarwal and Shenoy (1981) have determined conditions in the vicinity of the spinodal curve for the existence of a hysteresis window. Using the work of Agarwal and Shenoy and available experimental data, estimates are made for the size of the hysteresis window related to the NbN thin film fabrication process.