Molecular Drag Research Articles

Creation of extreme high vacuum with a turbomolecular pumping system: A baking approach

Pressures in the low 10−12 Torr range are produced by a 200 °C bakeout with a turbomolecular pumping system. A small molecular drag pump in conjunction with the membrane pump turns out to be an effective means of achieving low backing pressures. The stainless-steel test chamber of 6800 cm2 surface area has been prebaked to 450 °C. An ultimate pressure of 1×10−12 Torr is obtained on the extractor gauge and to date this is the lowest pressure attained with turbomolecular pumps only. This system offers simplicity of arrangement, ease of operation, rather low cost, and fast pumpdown to ultrahigh vacuum. The pressure reaches 2×10−9 and 4×10−11 Torr at 2 and 83 h, respectively, after startup without bakeout when vented to atmospheric pressure using extremely dry nitrogen.

New technique for the measurement of the intrinsic drag torques of high temperature superconductor bearings

Abstract A new technique providing a non-intrusive means for the isolation and accurate measurement of the intrinsic drag torques of high temperature superconductor bearings is reported. It involves measuring the rotational acceleration and deceleration of a rotating symmetric object suspended in vacuo on a bearing formed by the combination of a magnet and an HTS pellet. Accelerations are produced by exploiting molecular drag torque in medium vacuum while deceleration from low starting speeds in ultra high vacuum, in which gaseous drag and eddy current effects are minimized, enables intrinsic drag torques to be accurately determined. Experiments have been carried out on YBa 2 Cu 3 O 7 pellets and improvement in pellet grain structure is shown to decrease the size of the intrinsic drag torque. Theoretical estimates are in reasonable agreement with measured values. The effect of the magnetic field intensity of the suspended magnet and the suspension height is also reported. A threshold type starting phenomenon was observed in which the torque required to accelerate the suspension was two to three orders of magnitude higher than the intrinsic drag torque obtained from deceleration measurements. Observations of spontaneous reacceleration and the potential for using the device to measure vacuum pressures is also discussed.

Use of the Clausing's equation to evaluate the pumping action of molecular Gaede pumps

The pumping action of molecular drag pumps is theoretically analyzed using the Clausing's integral equation. This equation allows one to calculate the transmission probability of a duct of arbitrary shape in molecular flow and it has been solved by different authors in the case of tubes with circular cross-section and stationary walls. This approach is extended to the case of a tube with moving walls, such as a molecular pump of Gaede type. Numerical solutions of Clausing's equation are provided for both the cases of stationary and moving walls. With this method, it is possible to evaluate compression ratio and pumping speed of an ideal pumping channel without leakages, as a function of the geometrical parameter length to radius ratio and the velocity of the moving surface. The results are also compared with those of previous theories and the possibility of applying the method to the design of molecular stages is discussed. In contrast to the design of a turbine, the Gaede pump does not, in principle, require a stator.

Electrostatic orrery for celestial mechanics

A device is described in which negatively charged submillimeter particles are trapped in orbits about a rod with a positive potential. Hollow glass microspheres about 50 μm in diameter are dropped past an emitting filament and accumulate a charge of approximately 5×105 electrons. The microspheres fall into a Kingdon trap (concentric cylinders with end caps) and are trapped by a rising potential of order 10 kV. The orbital decay time is determined by molecular drag and is about 40 min or 105 revolutions at 2×10−6 Torr. Orbiting particles are visible to the unaided eye and may be recorded photographically with a CCD camera. Possible applications from celestial mechanics include experiments on orbital resonances, Kirkwood gaps, and planetary ring phenomena.

PC-based spinning rotor gage controller

The spinning rotor gage (SRG) is a molecular drag vacuum gage. Commercial versions are available that operate between about 10−4 and 1 Pa. In this paper we describe the design, and present performance data for a new SRG controller that uses a plug-in peripheral board and a personal computer (PC) to perform all control, signal processing, and data analysis functions. This controller offers several advantages over those presently available, including the simultaneous operation of multiple SRGs (at least four SRGs with a single PC), complete software control of all operating parameters, and significantly improved low-pressure performance.

Contamination particle traps due to a cone, cube and disk

This paper studies asymmetric particle clouds confined within electrostatic particle traps in the presence of a molecular drag force. The drag force, due to flow of neutral Ar gas around a particle, is proportional to the gas flow, which can be controlled by a mass flow controller. Our system has a component of the drag force parallel to the wafer electrode surface. The particle clouds associated with a cube and a disc are seen to be highly asymmetric in the direction of the drag force, as would be expected. The asymmetry increases as gas flow increases. In order to visualize the clouds quantitatively, we have introduced a technique called spot scanning in which we move the laser beam slowly with a serpentine path through the particle cloud. The forward scattered light is recorded by a charge-coupled device camera onto videotape. The image of the scattered light on a frame of the videotape is a nearly circular red spot. Using commercially available computer software, the red spots can be used to determine the boundary of the cloud as well as to find contours of constant power density scattered from the cloud.

Comparison between several new molecular drag pumps and turbomolecular pumps

Three prototypes of full molecular drag pump (MDP) have been discussed. The pumping speed is nearly equal to and even could be higher than that of a turbomolecular pump (TMP). Performances at higher pressure are much better than that of TMPs. Configurations of these new pumps are simple. A figure of merit G, defined as product of pumping speed and the logarithm of compression ratio, is introduced to compare the performances between the new MDPs and TMPs. It is possible that these new pumps would be developed in the near future to be viable alternatives to the present TMPs, diffusion pumps, and roots pumps.

The effects of electrostatic, molecular drag and gravitational forces on the behavior of particle clouds in an RF discharge

A Gaseous Electronics Conference (GEC) Reference Cell was used to investigate the behavior of graphite particles sputtered in an argon plasma. By configuring the reactor in a normal (wafer facing upward) and in an inverted (wafer facing downward) configuration, and by varying the direction and flow of the argon, the relative importance of gravity, molecular drag and electrostatic trapping could be observed. It was observed that particle clouds in specific locations adjacent to the wafer (referred to as particle traps) were identical in both configurations. Drag forces from the showerhead grounded electrode were sufficient to prevent particles from accumulating in the traps over the wafer. Drag forces from flow parallel to the wafer (/spl ges/25 cm/s) easily emptied the traps. Gas flow from a ring source mounted above a downward facing driven electrode were observed to affect the manner in which particles were removed from the ring-shaped trap surrounding the cathode. In that case, particles were observed to flow down eight fluid streamlines toward the eight openings through which the gas flowed from the reaction chamber to the pump. Comparison of results for the normal and inverted configurations also permitted estimates of the number of electrons attached to each particle.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Influence of clearance on the pumping performance of a molecular drag pump*

The pumping performance of a molecular drag pump of the disk type has been studied using a three-dimensional dynamic model based on leakage flow through the clearance between the rotating and the stationary disks of the pump and the Monte Carlo method. The relationship between the theoretical values of the pumping performance and the structural parameters of the pump has been studied. The results showed that clearance has no big effect on the pumping speed, but dramatically affects the compression ratio of the pump, especially in cases of high rotation speed and heavy element gases. The amount of leakage through the clearance is determined not only by the amount of clearance, but also by its ratio to the depth of pumping channels.

Design of disk molecular pumps for hybrid molecular pumps*

In recent years, the various types of hybrid molecular pumps combining a molecular drag pump (MDP) with a turbomolecular pump (TMP) have been developed in order to raise their pumping throughput and operating pressure. Some of them have been put on the market. In this article the pumping performances of the disk MDP, whose pumping channels are cut on a rotating disk, were calculated using a new model based on the Monte Carlo method. The relationship between the pump performance to the structural parameters was studied. The above pumping performance is also compared with that of the pumps having channels on stationary disks. When a pump is constructed in a series of two stages, the pumping speed and the compression ratio of the present pump for nitrogen gas increases by 35% and 64%, respectively, as compared with the previous type. It is believed that the pump having channels on the rotating disk has both kinds of pumping actions. Hence, it possesses the advantages of both MDP and TMP.

Combination of turbomolecular pumping stages and molecular drag stages

Conventional turbomolecular pumps consist of a succession of rotating and stationary bladed disks. In order to increase compression ratios, the open angle of the disks is decreased from the upper to the lower stages of the pump: a compression ratio of ≳ 107 can be obtained for nitrogen in nine stages. However, the maximum compression ratios are obtained only when the pump is working in free molecular flow conditions, corresponding to a foreline pressure in the range 10−3–10−2 mbar; at higher pressures the compression ratio decreases dramatically thus limiting the total efficiency of the pump. Molecular drag pumps, on the contrary, can work with high compression ratios also in the transition flow region, in the range from 10−2 to 10 mbar. Combination of turbomolecular pumping stages and drag stages have been tested. Molecular drag stages of a new design have been incorporated in a conventional turbomolecular pump without adding to its dimensions. From these tests it appears possible to have high compression ratios with the foreline pressures up to 10 mbar: the forevacuum pump could be a conventional, double-stage rotary pump or a less conventional diaphragm oil-free pump. The behavior for different gases has been analyzed and advantages of this arrangement are examined.

Rotating disc gauge for absolute total pressure measurement in high vacuum

An absolute vacuum gauge exploiting molecular drag between coaxial discs, a configuration suggested by Langmuir, is being further investigated. The device operates in a range similar to that of the spinning rotor gauge. It consists of a silicon disc 92 mm in diameter suspended on a calibrated torsion fiber a few mm above a metal disc of the same size which rotates at speeds up to 50 000 rpm. Molecules leaving this rotor communicate a torque to the suspended disc by molecular drag. The angular deflection thus produced is sensed optically and is about 2° at 10−6 mbar. The critical factors relevant to absolute measurement, such as tangential momentum accommodation, geometry, and edge effects have been investigated and experimental comparisons with a spinning rotor gauge demonstrate that absolute pressure measurements with uncertainty better than ±5% are possible at pressures ∼ 10−6 mbar. Measurements of similar uncertainty will be possible at 10−7 mbar.

Testing of turbomolecular pumps

Turbomolecular pumps (TMPs) are used to provide clean pumping to attain pressures down into the 10−10 Torr range. Some of our data indicated that the pumping speeds and compression ratios claimed by some manufacturers of TMPs were greatly overstated. To measure these parameters, an apparatus was constructed following AVS recommended practices. Hydrogen, helium, nitrogen, and argon were used as test gases, with their flow rates controlled to provide inlet pressures over the range 10−6–10−2 Torr for pumping speed measurements and exhaust pressures over the range 0–1 Torr for compression ratio measurements. Ten pumps from eight manufacturers were tested, all with 6 in. o.d. Conflat flanges at their inlets. Wide variations were found between the pumps, with the specifications of the manufacturers met by only a few pumps. Pumping speeds varied from 0.3 to 1.1 times those specified by the manufacturers. Compression ratios were frequently exaggerated by a factor of 10 or more. As expected, four of the pumps, equipped with coaxial molecular drag pumping sections, had much larger compression ratios than conventional pumps, particularly for the light gases.

Vacuum performance of molecular drag stages

Considerable interest has been raised in recent years in molecular drag pumps. The relevant characteristic of these pumps is the possibility to work at higher pressures compared with turbomolecular pumps. They are often used in combination with conventional turmomolecular pumps: the combined pumps allow the use of totally oil-free foreline pumps. A novel arrangement of drag stages, that presents an alternative to what has been offered before, has been developed and tested. The vacuum performance of the stages is presented. A three-stage pump has been developed. Compression ratios and pumping speeds for different gases have been measured: the compression ratio for air was about 400 at discharge pressure of 1 mbar. The results of experiments are presented and possible applications are discussed.

Re-entry aerodynamics derived from space debris trajectory analysis

This paper considers the technique of orbital analysis as a means of determining the ill-defined gas-surface interaction between spacecraft and atmospheric molecules in low Earth orbit. The interaction is a major uncertainty in trajectory predictions for a body moving within an atmosphere. The rate of change of the orbital period of a debris object, the uncontrolled Salyut 7/ Kosmos 1686 space station, is analysed in order to determine the free molecular drag coefficient. The results are compared with theoretical values for the drag coefficient calculated using a complex representation of the vehicle configuration and motion and applying the Monte Carlo Test Particle method. Results suggest a nature of re-emission very close to the classical diffuse, totally accommodated case was occurring at the surface of the debris object as it approached re-entry. However the determined drag coefficient and therefore the derived interaction are found to be very sensitive to the neutral density and therefore the atmospheric model used in the analysis.

Preliminary observations of the behaviour of an absolute gauge of the rotating disc type in high vacuum

A proposal by Langmuir for an absolute vacuum gauge exploiting the torque communicated by molecular drag between a high speed rotor and a sensor in close proximity has been further investigated experimentally. The rotor consists of a metallic disc which replaces the normal blade structure of a turbomolecular pump, and which rotates at speeds up to 50,000 rpm. The sensor is a silicon disc of matching size suspended on a torsion fibre and placed coaxially and parallel to it. Early observations in the pressure range 10 −5−10 −6 mbar are reported, together with the measurement of edge effects and other phenomena.

The Molecular Theory for the Viscosity of Nematic and Smectic C Liquid Crystals

Abstract The rotational diffusion of a rod-like molecule in nematic and smectic C liquid crystals, and also in two-component nematic mixtures, is considered in the molecular-field approximation. The microscopic friction constant. which determines the molecular rotation drag, possesses an exponential temperature dependence with the activation energy determined by the isotropic part of the intermolecular interaction energy. The nematic Leslie coefficients are obtained in terms of molecular parameters. Their values, signs and the temperature variation qualitatively correspond to experimental data. An additional activation energy, proportional to the nematic order parameter, appears in the expressions for the rotational viscosity coefficients y1 and y2 both in nematics and C smectics. All Leslie coefficients possess complicated behavior with varying mixture concentration. In particular, the rotational viscosity cocfficient y1 is proportional to a third-order polynomial in concentrations pA and pB with coeffic...

Precision Ion Polishing System -A New Instrument for TEM Specimen Preparation of Materials

AbstractThe article describes the design, construction and performance of a new bench top instrument for high speed ion beam thinning and polishing of materials. In this system, the combination of very low angle ion milling and powerful ion guns has led to the rapid production of high quality TEM specimens. The main subassemblies are (1) a work chamber (2) gas control system (3) vacuum system and (4) electrical system. The work chamber consists of a pair of newly designed Penning type ion guns and Faraday cups to measure ion currents. The Whisperlok™ mechanism provides specimen rotation, pneumatically driven airlock for very fast specimen exchange and transmission/reflection illumination for specimen viewing. The ion guns are mounted to deliver a nominal, 4° milling angle on the specimen surface with precision alignment of ±2° about horizontal and vertical axes. The actual thinning is undertaken from one side using a single, post-type specimen holder which minimizes the specimen heating and contamination. The ion beam current of each gun can be individually optimized by varying the flow rate of the ionizing gas. The main chamber is evacuated by diaphragm and molecular drag pumps to produce a clean, dry vacuum in the 10−6Torr range. The discharge and accelerating voltages required for the operation of each gun are provided by a dual high voltage power supply capable of delivering ion energies in the range; 1 keV to 6keV. TEM micrographs of typical ion polished specimens of semiconductors, metals, ceramics and composites are included to illustrate the performance of the instrument.

Rotational viscosity coefficients of nematic and smecticC liquid crystals: Statistical theory

The rotational diffusion of a rodlike molecule in a nematic and smecticC liquid crystal is considered in the molecular-field approximation. The microscopic friction constant, which determines the molecular rotation drag, possesses an exponential temperature dependence with the activation energy determined by the isotropic part of the intermolecular interaction energy. The rotational viscous coefficients,γ 1 andγ ⊥ are obtained by averaging of the corresponding microscopic stress tensor with the nonequilibrium distribution function. The additional activation energy, proportional to the corresponding order parameter, appears in the expressions for the rotational viscosity coefficients both in nematics andC smectics.

An ultra high vacuum low temperature gyroscope clock

We propose to perform a null-gravitational redshift experiment by comparing a mechanical gyroscope clock with atomic clocks. The Gravity-Probe-B Relativity Gyroscope Experiment provides the opportunity for this co-experiment. The goal is to measure the effect to an accuracy of 0.01% of the gravitational redshift due to the eccentricity of the orbit of the earth about the sun. This corresponds to an integrated frequency measurement over one year of Av/v=3∗10 −14. A major disturbance torque on the gyroscope is due to fluctuations in the molecular drag of the residual gas caused by temperature variations. We propose to use a low temperature bake-out technique in order to achieve the required vacuum of 10 −17torr.