Critical Gas Pressure Research Articles

Temperature control of a liquid helium cooled Eigler-style scanning tunneling microscope

A procedure for operating an Eigler-style, low temperature scanning tunneling microscope (STM) at variable temperatures has been developed. A critical exchange gas pressure regime was found to allow for controlled variation of the STM temperature while it is encapsulated in a liquid helium Dewar. The sensitivity of various parameters to the ability to generate stable variable temperatures above 4 K is discussed.

Steady state detached solidification of water at zero gravity

Steady-state detached solidification of water was calculated using the Moving Meniscus Model. Similar to the experimental observation of many materials in microgravity, detached solidification of water is predicted to occur in a sealed ampoule at zero gravity under proper conditions. For steady detachment, the freezing rate must exceed a critical value, Henry's constant of the dissolved gas must be below a critical value, the temperature of the top of the water must be below a critical value, the contact angle of water on the ampoule wall must exceed a critical value, and the diffusion coefficient must exceed a critical value. Each critical value depends on the physical properties and the other operating conditions. Thus different results are obtained for InSb and water. The critical gas pressure above the melt for water is much smaller than for InSb, the critical freezing rate is larger for water, and the critical contact angle of the melt on the ampoule wall is larger for water. For the gases examined here, the solubilities of Ar, N 2 and Ne in water are sufficient for detachment to occur, while the solubility of He is not.

Dense carrier gas effect in vapor phase nucleation

By simple Monte Carlo simulation it is demonstrated that the capture of vapor molecules by a drop is affected by a dense background gas far from the Knudsen regime, and deep within the free molecule regime. The corresponding effect on steady-state nucleation is considered. For several representative examples the slopes of critical supersaturation vs carrier gas pressure are obtained and compared with experimental data.

Analyses of gas storage capacity in unlined rock caverns

Groundwater control is a means of ensuring that no gas leakage occurs from unlined rock storage caverns. To achieve a better understanding of the principles of gas containment under groundwater and how to obtain maximum gas storage capacity, analyses of groundwater conditions surrounding storage caverns and the effects of different factors on gas storage capacity have been performed and are reported in this paper. The analysis results indicate that the critical gas pressure of a storage facility is considerably less than the natural hydrostatic pressure around the caverns and somewhat less than the water curtain pressure. High gas storage capacity can be obtained by optimizing the design of cavern configuration, water curtain layout, cavern depth, etc. The principles of such optimization are outlined in this paper.

Dead zone characteristics of SQS and GM modes in Kr-, Ar-, and Ne-mixtures

Dead zones of a gas counter have been studied systematically for various gas mixtures at gas pressures equal to and lower than 1 atm. Evaluated values of the dead zone for the self-quenching streamer (SQS) mode were 190-390 mu s-cm at 1 atm. Characteristics of the dead zone were explained by considering the effect of SQS ions. The critical gas pressure at which the operation mode changes from the SQS mode to the GM mode was determined for each gas mixture on the basis of the dead zone measurement. Operation mode diagrams of the counter were drawn according to those results, and they are given here. >

Transition from moving to stationary double layers in a single-ended Q machine

Large-amplitude (≲100%) relaxation oscillations in the plasma potential are known to be generated when the cold endplate of a single-ended Q machine is biased positively. These oscillations are associated with double layers that form near the hot plate (plasma source) and travel toward the endplate at about the ion-acoustic velocity. At the endplate they dissolve and then form again near the hot plate, the entire process repeating itself in a regular manner. By admitting a sufficient amount of neutral gas into the system, the moving double layers were slowed down and eventually stopped. The production of stationary double layers requires an ion source on the high-potential side of the double layers. These ions are provided by ionization of the neutral gas by electrons that are accelerated through the double layer. The dependence of the critical neutral gas pressure required for stationary double-layer formation on endplate voltage, magnetic field strength, and neutral atom mass has been examined. These results are discussed in terms of a simple model of ion production and loss, including ion losses across the magnetic field.

Correlation of light fluctuations in the positive column of an argon discharge

The correlation coefficient of light fluctuations has been measured in an argon plasma in the positive column of a glow discharge at gas pressures between 0.02 and 0.08 torr and at discharge currents from 10 to 100 mA. At such conditions there are no stationary striations. The mean frequency and mean wavelength of the fluctuations are determined from correlation functions measured at zero time delay distance between sensors respectively. It has been found that the maximum of the correlation coefficient envelope moves towards the anode. The velocity of this movement has been determined. As some critical gas pressure at constant discharge current is approached all these quantities change sharply and well observed ionization waves appear.

Formation of Rotating Plasma in Crossed Field

The present paper describes the formation process of a rotating plasma in a homopolar-like short coaxial-electrode system carried out in a H 2 , N 2 or A discharge. A Kerr cell shutter with a 50 ns exposure time was used for optical records. It is found that the process is divided into two stages: first a negative streamer appears and develops to a spoke, and later the spoke spreads to a disk. The streamer process is hardly affected by the magnetic field. A plausible mechanism by which the spoke spreads to the disk is ionization of neutrals by E × B drift motion of ions. There is a critical gas pressure above which the spoke never spreads to the disk. The reason is considered to be that ions cannot drift in E × B direction above the critical pressure due to a large frictional force between ions and neutrals.

Oscillations and Noise Generation in a Beam-Generated Plasma

An electron beam at a few kilovolts is used to generate a plasma in helium or argon at 10−4 to 10−2 Torr in a glass tube aligned with an axial uniform magnetic field of 0.3 to 2 kG. At and above a critical beam current and gas pressure, the discharge changes from a glow region the size of the beam to a glow discharge filling or nearly filling the glass tube (total glow). Measurements of the plasma density, the velocity distribution of the beam, and the spectrum of the rf noise generated in the plasma as functions of beam current, voltage, and gas pressure have been made. These data are consistent with an explanation of the effect involving a convective forward-wave beam-plasma oscillation which has a starting current equal to the measured critical current for the transition to total glow and which is determined by the net gain of the system including reflections at the ends. The oscillation, once started, is not a coherent oscillation, but can have a very wide noise bandwidth extending essentially from zero frequency to near the electron plasma frequency.

Influence of Space Charge on the Potential Distribution in Mass Spectrometer Ion Sources

The role of space charge as a factor influencing the potentials and potential gradients in a mass spectrometer ion source of the electron bombardment type is calculated. Planar equipotential surfaces are assumed, and the analysis then becomes that of a plane parallel positive ion diode. The ``cathode'' of the ion diode may be either emission-limited or space-charged-limited. The analysis considers the charge of the electrons in the ionizing sheet and the charge of the positive ions in the diode. At a critical gas pressure the influences of the positive and negative charges on the potential of the ionizing region are equal and opposite, for small ionizing current. This concept leads to a pressure normalization in terms of the critical pressure. For a given source geometry and electron bombarding energy one can construct universal curves which give the potentials and the potential gradients as a function of the normalized gas pressure and the ratio of the ionizing electron current density to the repeller voltage. Experimental data are in agreement with the predictions of the theory.