Thin Gas Layer Research Articles

Energy straggling of 60 MeV 58Ni q+ ions in thin carbon foils and gases

Energy straggling of 60 MeV 58Ni q+ ions in thin carbon foils and gas layers has been measured with a Q3D magnetic spectrograph in dependence of the incident and analysed charge state of the ions. The straggling increases with the number of electrons per gas molecule, thus showing strong correlation effects. In solid carbon, however, energy straggling is close to the data of deuterium gas. The small straggling in carbon is interpreted as an effect of the long range interaction of the heavy ions in dense solids. Furthermore, it is observed that energy straggling depends on the charge state of the projectiles unless the equilibrium charge state is reached.

IMPLEMENTATION AND EXPERIMENTAL VALIDATION OF A NEW VISCOTHERMAL ACOUSTIC FINITE ELEMENT FOR ACOUSTO-ELASTIC PROBLEMS

The propagation of sound waves in thin gas or fluid layers can be significantly affected by viscothermal phenomena. In this paper, a new viscothermal acoustic finite element is presented. The element includes the effects on inertia, viscosity, compressibility and thermal conductivity. The acoustic element can be coupled to flexible structural elements. This enables fully coupled acousto-elastic calculations to be made for complex geometries. The model is experimentally validated with measurements on a rigid airtight box with a flexible coverplate. Eigenfrequencies, damping and mode shapes of the plate were measured and compared with numerical results for a number of gas layer thicknesses. The agreement between measurements and calculations is good. The results indicate that there can be large shifts in eigenfrequency as a function of the layer thickness. In addition, for small layer thicknesses a high damping value is found, which can be attributed to viscous effects in the layer.

Theoretical model to describe the coupling between a vibrating membrane and thin gas layers

A theoretical model to describe the behavior of a membrane loaded by gas films has recently been developed. This analytical model is required by the current level of miniaturization of acoustical transducers and by new devices and applications, especially in the high-frequency range (up to 100 kHz and more). It is based on the fundamental acoustical and mechanical equations (the Navier–Stokes equation, the conservation of mass equation, the Fourier equation for heat conduction, the equation of motion of the membrane) and includes realistic boundary conditions. These equations are simplified by making assumptions based on the fact that the thicknesses of fluid films are very small. Then, conventional models, based on equivalent electrical networks, generally used to describe the behavior of electrostatic or piezoelectric microphones or loudspeakers, are improved. For example, the influence of the viscosity, the heat conduction, and the deformation of the membrane in the high-frequency range can be accurately taken into account.

Simulation of shock-generated instabilities

Direct 2-D numerical simulation of the fluid instability of a shock-accelerated thin gas layer shows flow patterns in agreement with experimental images. The Eulerian-based hydrodynamics code features Adaptive Mesh Refinement that allows the code to follow the vorticity generation and the complex flow resulting from the measured initial perturbations. These experiments and simulations are the first to address in quantitative detail the evolution of the Richtmyer–Meshkov instability in a thin fluid layer, and to show how interfluid mixing and vorticity production depend sensitively on initial perturbations in the layer.

Thermometry of gas layers in thermal plasma CVD by pulsed-laser Rayleigh scattering

The application of Pulsed-Laser Rayleigh Scattering Thermometry (PLRST) to Chemical Vapor Deposition (CVD) processing of diamond films in a thermal induction plasma is demonstrated. Temperature profiles were measured in the reactive thermal boundary layer, characteristic features of which are a steep temperature gradient and strong chemical non-equilibrium. The temperature in the layer varies between 5000 and 1000°C. The stray light rejection problem typical for Rayleigh scattering measurements was solved successfully. In the plasma core, where atomic emission spectroscopy can be applied to temperature measurement, agreement has been obtained between plasma temperatures measured by emission spectroscopy and PLRST. In the boundary layer both point-by-point measurements and laser sheet measurements were made and adequate agreement was obtained. Technical reasons for observed variations were identified. It is concluded that PLRS is a viable non-invasive thermometer for thin reactive gas layers typical or various CVD deposition processes and technical improvements are suggested.

Maximum-payload trajectories for a laser-propelled launch vehicle

The feasible trajectories of a laser-propelled launch vehicle are restricted by the laser power available, the power lost to atmospheric absorption and beam spreading, and the thruster design. These constraints result in optimal trajectories that are significantly different from those for conventional launch vehicles. The problem formulation and constraints for a launch vehicle using a pulsed-laser thruster with two laser power transmission models are presented. A maximum payload problem for launch to various circular orbits is solved for both transmission models using sequential quadratic programming. An unusual backs wing trajectory feature is obtained for most of the optimal trajectories presented. The optimal trajectories for various circular orbit altitudes are discussed, and the effects of altering the power transmission efficiency are shown. ASER-PROPELLED launch vehicles were first proposed in the early 1970s.1 Advances in laser and optics technology since then have increased the feasibility of very high power, ground-based, laser launch systems.2'3 Such systems offer the potential of thousands of launches to low Earth orbit (LEO) per year at greatly reduced costs over current launch vehicles.4 Kantrowitz5 has proposed an extremely simple (and hence cheap) design for a laser-propelled launch vehicle. The vehicle is essentially a right circular cone of some solid propellant with a payload in the apex as shown in Fig. 1. The conical shape allows the vehicle to turn relative to the laser beam without exposing the payload to the laser. When struck by a double-pulsed laser beam, the entire base of the vehicle acts as a double-pulse planar thruster (DPPT) as described by Kare. 4 A low-power pulse evaporates a desired amount of propellant from the thruster surface to form a thin layer of gas. The gaseous propellant is then heated by a laser-supported detonation (LSD) wave initiated and sustained by a second, highpower laser pulse. The hot gas then expands one dimensionally producing thrust.

Stabilization of Biological Specimens by the Use of Plasma Polymerized Hydrocarbon Film for Imaging with an Atomic Force Microscope

An atomic force microscope makes imaging of biological molecules possible at high resolution. To this end all samples have to be fixed securely to a flat solid support so that they cannot be displaced by the scanning tip. We herein describe a new method to fix the biological samples to glass surfaces by depositing a thin layer of plasma polymerized methane gas in a high vacuum chamber. Such samples can thus be imaged repeatedly by an atomic force microscope without any loss of image quality.

Nonlinear growth of the shock-accelerated instability of a thin fluid layer

Richtmyer–Meshkov instability causes spatially periodic perturbations initially imposed on a shock-accelerated, thin gas layer to develop into one of three distinct flow patterns. Planar laser-induced fluorescence imaging of the evolving layer, produced by a perturbed SF6 planar jet in air, shows an apparent flow bifurcation that is observed as mushroom-shaped or sinuous-shaped interfacial patterns. Analysis of this nonlinear instability growth, accomplished by modelling the flow field as a row of line vortices, predicts that the layer thickness grows logarithmically at later times and compares well with our measurements. Because the row of vortices is unstable, the model also provides an explanation for the appearance of the three observed interfacial patterns.

Ellipticity and polarization effects in harmonic generation in ionizing neon.

We report observations of the ellipticity dependence of harmonic generation in a thin layer of neon gas at an intensity of ${10}^{15}$ W/${\mathrm{cm}}^{2}$ and a wavelength of 7540 \AA{}. We observe a markedly different ellipticity dependence and polarization for harmonic energies below and above the ionization potential of neon (21.56 eV). The ellipticity dependence of the higher harmonics can be understood in terms of free-bound transitions of ionizing electrons within the first cycle of the fundamental subsequent to tunnel ionization, while other mechanisms must be invoked to explain the characteristics of the lower harmonics.

Theory of X-ray photoacoustic spectroscopy

PhotoAcoustic Spectroscopy (PAS) in the X-ray region is becoming a new field in PAS research and poses some new problems, such as heat production mechanisms and non-exponential heat distribution due to multi-excitation processes, additional signals caused by escaped fluorescence and electrons, special backing and fronting setup for comparison with absorption experiment, etc. In this paper we treat these problems and include them in an extended theoretical model which can be reduced to the RG model in a special case. The results indicate that the non-exponential heat distribution has no effect on the PA phase. The contribution by gas heating of the escaped electrons can be neglected. The thermally thin gas layers between the sample and the Be windows are sensitive factors to both amplitude and phase. The PA signal generated in the backing gas layer by the backing surface of the sample should be considered to understand the frequency dependence of the PA signal. The expansion of the backing gas layer is the reason for the phase change in PA-EXAFS.

圧力変動を考慮したＧＭ冷凍機シャトル熱損失の解析

The so-called shuttle loss in a displacer-type cryocooler is caused by temperature distribution in the radial direction of the thin gas layer between the cylinder and the displacer. The temperature oscillation is due to the following two reasons: one is reciprocating motion of the displacer and the other is pressure oscillation of the gas. We refer to the heat loss induced only by the former as the shuttle heat transfer. To evaluate the shuttle loss in a Gifford-McMahon (GM) refrigerator including the influence of the pressure oscillation, we have employed a numerical approach based on the linearized thermoacoustic theory. The theory can be a very useful tool to treat the problem if the oscillating amplitude is fractional. The results show that the pressure oscillation of the gas functions as a prime mover in usual operating conditions. Compared with the shuttle heattransfer characteristics, several characteristics of the shuttle loss were drawn from the calculation results. Further, it is confirmed that coating on the surface of the solid walls with a low thermal-conductivity material is effective in reducing the shuttle loss.

Possibility of inverse Mach reflection in association with a laser spark explosion over a plane surface

A numerical experiment has established that inverse Mach reflection may be realized when spherical shock waves interact with a plane surface in the presence of a thin layer of hot gas having certain dimensions and a certain temperature.

Plane shock wave cumulation in a tube with a thin wall low-density gas layer

A new effect of focusing of a shock wave disturbed by a thin low-density gas layer localized near the side wall of the cylindrical tube, followed by the wave reflected from the flat tube end, is analyzed numerically.

Instability growth patterns of a shock-accelerated thin fluid layer.

Laser-induced fluorescence imaging of a shock-accelerated thin gas layer, produced by a planar ${\mathrm{SF}}_{6}$ jet in air, shows multiple flow evolutions. Richtmyer-Meshkov instability causes spatially periodic perturbations initially imposed on the jet to develop into one of three distinct flow patterns, indicating nonlinear instability growth. Slight differences in the vorticity distribution deposited on the air-${\mathrm{SF}}_{6}$ interfaces by the shock interaction produce a bifurcated flow, observed as mushroom-shaped or sinuous-shaped interfacial patterns.

Extreme-infrared brightness profile of the solar chromosphere obtained during the total eclipse of 1991

THE solar chromosphere is a thin layer of gas that is several thousand degrees hotter than the underlying photosphere, and responsible for most of the Sun's ultraviolet emission. The mechanism by which it is heated to temperatures exceeding 10,000 K is not understood. Millimetre and submillimetre radiometry can be used to obtain the chromospheric temperature profile, but the diffraction-limited resolution for the largest telescopes is at best 17 arcsec, or ∼12,500 km at the Sun's distance. This is greater than the thickness of the quiet chromosphere itself. The total eclipse of July 1991, which passed over the Mauna Kea Observatory in Hawaii, provided a rare opportunity to make limb occultation observations with a large submillimetrewavelength telescope, the 15-m James Clerk Maxwell Telescope, and in this way we obtained a temperature profile in 1.3-mm radiation with ∼300 km resolution at the Sun. Our observations indicate that spicules (magnetically entrained funnels of gas) reach a temperature of 8,000 K at 3,000–4,000 km above the photosphere, a temperature lower than those of many spicule models.

Calculation of primary-specific ionization produced by relativistic charged particles in some molecular gases

Primary-specific ionization produced by relativistic charged particles in NH 3, CO 2, CH 4, C 2H 2, C 4H 10, O 2, and N 2 are calculated taking into account the polarization of the medium. The values of the primary-specific ionization in minimum of ionization are calculated to be respectively 24.7, 38.0, 27.7, 30.8, 99.3, 29.7 and 23.7 ion pairs per cm at normal temperatures and pressures. Those values (except O 2) differ from the experimental values by no more than 10%. The plateau exceeds the minimum by 36, 36, 30, 29, 29, 41 and 40% correspondingly. All calculations were performed using the model of a medium with a discrete harmonic oscillators set. The oscillator sets obtained from spectrometric data can be used in calculations of truncated energy losses and simulations of energy losses in thin gas layers.

Ethanol removal from fermentation broth by gas membrane extraction

A new type of membrane extraction for in situ removal of ethanol from fermentation broth is presented. Aqueous solutions of propylene glycol are used as extractants. The extractatant and the broth are separated by a microporous hydrophobic membrane which is not penetrated by the broth or by the extractant. As a consequence a thin gas layer, essentially air, is immobilised within the membrane pores and separates the two liquid phases (i.e. a gas membrane). Vapour-liquid equilibria are established at both membrane sides; because glycols reduce the ethanol content of the equilibrium vapour phase with respect to the binary system, ethanol vapours preferentially diffuse through the stagnant gas layer.

W values and other transport data on low energy electrons in tissue equivalent gas

The degradation of low energy electrons in a methane based, tissue equivalent gas was studied experimentally by ionization chamber experiments and theoretically by Monte Carlo electron transport simulation in the energy range between 25 eV and 5 keV. From measured ionization yields and calculated ionization and energy dissipation profiles, the mean energy W required to produce an ion pair, the differential omega value, backscatter coefficients, different types of mass ranges, and stopping powers were determined. A comparison of the authors' experimental results with those of the calculations shows a very satisfactory agreement in the whole energy range. In addition to these data some quantities of interest in the fields of radiation biology, and microdosimetry, such as the statistical fluctuation of ionization yields, the energy transfer to and the ionization yields in thin material layers of the tissue equivalent gas, together with the distance distribution of energy transfer and the dose average of the specific energy, have been studied. A comparison of these results with those for electrons in water showed the great similarity of both stopping media for many purposes in the fields of radiation research.

Fast particle identification based on the relativistic increase of the threshold efficiency in multilayer proportional detectors

The possibility of fast charged particle identification by the measurement of the relativistic increase of the threshold efficiency in a thin gas layer

Optical plasma resonance in semiconductors: Novel concepts for modulating far-infrared light

A novel concept for modulating far-infrared light is presented based on the ‘‘photon-plasmon, plasmon-photon’’ interaction with a thin layer of a free-electron gas in a n-doped semiconductor. For oblique incidence of p-polarized light at the plasma frequency of the electrons, high-reflection properties arise due to plasma resonance. The influence of all the constituent parameters (angle of incidence, electron mobility and density, thickness of the layer, etc.) on the reflectivity is discussed. The plasma frequency, and hence the reflectivity, can be modulated by changing the effective mass or the density of the electrons. This leads to a small, integrable, and fast modulable mirror, which can be used as a switching device (electrically or optically driven) or even as a way of frequency modulation.