Molecular Drag Pump Research Articles

Molecular alignment and chirality in gaseous streams and vortices

The discovery of collisional mechanisms, leading to aligned molecules in gaseous streams, opened the way to the research of possible origin of chiral discrimination involving aligned reactants in molecular dynamics. Experimental evidence is being provided that translational-rotational motions (such as those occurring in vortices), in the liquid phase first and in the gas phase more recently, show manifestations of chiral effects, revealing interesting relations with scenarios on the emergence of chiral selectivity. In this account, we also report the results of experiments of enantiomeric enrichment induced on chiral rotamers by rotational-translational motion. Several chiral molecules were expanded in supersonic seeded molecular beams and the vortex motion was generated by two molecular drag pumps with a right- and a left-handed screw motion, respectively. Connections can be established with many processes occurring in the Universe (and thus possibly related to a prebiotic context), like the stellar accretion of low-mass stars like the Sun and the flowing in atmospheres of rotating bodies.

Optical rotamers of substituted simple alkanes induced by macroscopic translation-rotational motions

Four substituted simple alkanes – 1-bromo-2-chloroethane, 1-chloropropane, 1,1-dichloropropane, and 1,1,2-trichlorotrifluoroethane – can exist in gauche positive (G), gauche negative (G′), and trans (T) conformers. The G and G′ conformers are optical rotamers, while the T conformer is achiral. Here, the first observation of the preferential population of the optical rotamers in these alkanes induced by the molecular flow through molecular drag pumps is reported. The molecular mechanism of the observed enantiomeric excess of the studied alkanes is rationalized in the general framework of the molecular chirality generated through oriented collisions involving flowing molecules in macroscopic translation-rotational motions induced by a rotating surface.

New spiral molecular drag stage design for high compression ratio, compact turbomolecular-drag pumps

Currently, commercially available turbo-drag pumps for high vacuum systems are based on either Gaede- or Holweck-type molecular drag pumping stages used in series downstream of axial bladed stages to extend the maximum compression ratio up to the 10 mbar foreline pressure range. Modern Gaede-type molecular drag stages use a disk-shaped impeller, allowing a very compact design, and the maximum compression ratio is limited by the leakage effect to about 10 per stage. Holweck stages are able to supply a high pumping speed, thanks to the presence of many channels in parallel and a high compression ratio, but this is obtained with the use of a less compact drum-shaped impeller. In this article, a new spiral molecular drag stage design is presented with the advantages of both high compression ratio and pumping speed per stage and very compact design: a stage occupying the very small axial space of one Gaede can supply the same compression ratio and pumping speed of a Holweck stage of the same diameter and peripheral speed in a much smaller axial space. The new spiral drag stage allows the design of very compact, high compression ratio turbo-drag pumps. The comparison of a 700 l/s turbo-drag pump implementing the new spiral molecular drag pump design with the existing Gaede- and Holweck-based products of the same pumping speed is presented, showing the performance advantages of the new design.

309 表面粗さを設けた真空ポンプの排気特性(流体力学(2))

309 表面粗さを設けた真空ポンプの排気特性(流体力学(2))

Research for a clean and large throughput differential pumping system

The research is to design a differential pumping system not only to achieve the pressure transition with a large throughput, but also to achieve a clean system without oil vapour contamination. In the paper, the pressure in differential stages are calculated; the differential pumping system design and equipment choice are introduced; the tests of MBP, a new kind of molecular-drag pump with large throughout and clean vacuum are described and the system experiment result and analysis is presented.

Reduction of the Base Pressure of an UHV System by Using Turbomolecular Pumps

We present the results on the reduction of the base pressure of an ultra-high-vacuum (UHV) system by using turbomolecular pumps. The UHV range is easily obtained by using a small backing pump, such as a scroll pump, a molecular drag pump, or a turbo-molecular pump. However, the extremely high vacuum range is difficult to achieve due to the low compression ratio and thermal outgassing from the inside of the turbopump. Several tests were done to overcome these difficulties. Among them, hot oxygen injection into the foreline of the pumping system is a possible way to reduce the chamber pressure by converting hydrogen to water molecule. However, a combination pumping approach turned out to be the most effective method of achieving an extremely high vacuum.

An experimental study on the pumping performance of molecular drag pumps

The pumping performance of molecular drag pumps (MDP) has been investigated experimentally. The experimented MDPs are a disk-type drag pump (DTDP), helical-type drag pump (HTDP) and compound drag pump (CDP), respectively. In the case of the DTDP, spiral channels of a rotor are cut on both upper surface and lower surface of a rotating disk, and the corresponding stator is a planar disk. In the case of the HTDP, the rotor has six rectangular grooves. The CDP consists with the DTDP, at lower part, and with the HTDP, at upper part. The experiments are performed in the outlet pressure range of 0.2–533 Pa. The inlet pressure and compression ratio are measured under the various conditions of outlet pressure and throughputs, and nitrogen is used for the test gas. At the outlet pressure of 0.2 Pa, the ultimate pressure has been reached to 1.0 × 10−2 Pa for the HTDP, 1.3 × 10−4 Pa for the DTDP, and 3.6 × 10−5 Pa for the CDP. The maximum compression ratio of the CDP is much higher than those of the DTDP or HTDP. Consequently, the ultimate pressure of the CDP is the lowest one.

Computational fluid dynamic model of a tapered Holweck vacuum pump operating in the viscous and transition regimes. I. Vacuum performance

Holweck molecular drag pumps are used as high-pressure stages in hybrid turbomolecular vacuum pumps, where they can operate in the transition and the viscous regime. In this article we develop a Navier-Stokes model of a Holweck pump with tapered pumping channels, applying slip-flow boundary conditions, to predict vacuum performances with and without gas flow. The commercial computational fluid dynamic code FLUENT is used to solve the model equations and to predict the pressure profile along the grooves. A specifically designed experiment is presented, whose arrangement provides the boundary conditions as input to the model and whose results are used to validate it.

Measurements of the self-sustained enhancement of field emission by carbon fiber microemitters

Two types of self-sustained enhancement in field emission by carbon fibers are described. In the first, the field is increased until the emission current switches from zero to between 1 and 10 μA. Next the field is reduced, but not so far that the current would drop. Then the current remains for several hours to several days, with transient increases from the 10 μA to between 14 and 22 μA. It is believed that the transients are caused by the activation of new microtips on the fiber surface. These effects were noted when the carbon fiber tip was mounted in a closed glass vacuum bulb pumped by barium getters, and also in a vacuum system using the combination of a molecular drag pump and ion pumps. The second type of enhancement occurs under ultrahigh vacuum conditions, during in situ thermal treatment of the carbon fiber tip while the emission current is about 2.5 μA. A specially built cathode assembly enables heating the tip to ∼725°C. After continuous heating at 570°C for 20 to 35 h, the current suddenly increases to between 13 and 25 μA. This enhancement is reversible if the emitted current is kept at the newly increased value for at least 30 min. The current–voltage characteristics at several temperatures were recorded and analyzed. Similar field-forming phenomena were previously observed with Molybdenum and ZnO–W tips.

AVS Hosts Meetings in San Francisco

AVS Hosts Meetings in San Francisco

Flow Characteristics of the Molecular Pump of Holweck Type in the Slip Regime

A computational method is used to analyze the viscous flow in the spiral grooves of the molecular drag pump of Holweck type. The flow is assumed in the slip flow regime and, thus, the slip boundary condition is imposed at walls. Tests are conducted to examine the effects of clearance gap, spiral angle, channel height, number of channels, and rotating speed. The appearance of clearance brings about lower pressure gradient between side walls of the channel and, thus, reduces the pressure rise in the channel. Testing on spiral angle and channel height indicates that these parameters need to be optimized to achieve better performance. Results also reveal that increase of rotating speed is an effective way to promote compression ratio. In calculations, pressure rise is enhanced when the number of channel is decreased. However, it should be understood that by reducing channel number the cross-sectional area of the channel is decreased, which has the effects of reducing the pressure rise.

Design characteristics of molecular drag pumps

This paper represents a model of a drag pump, to be employed as a part in modern turbo-molecular pumps. The important function of these pumps is their efficient working in the range of medium and high vacuum. The design characteristics of a molecular drag pump, such as, ultimate vacuum, pumping speed, compression ratios, and the efficiency have been experimentally determined. A close proximity has been found between the theoretically computed values and the experimental evaluation. At the end, some applications of these pumps in various industrial and technological areas have been highlighted.

Measurement of axial pressure distribution on a rotor of a helical grooved molecular drag pump

The theory in regard to the pumping performance of a helical grooved molecular drag pump proposed by the authors has been employed successfully as a design and evaluation tool. However, some researchers presented results different from the authors’ using the direct simulation Monte Carlo method. Their method included the inlet effect and the secondary flow in grooves, both of which were neglected in the authors’ theory. This study was planned to examine the validity of the authors’ theory precisely. A helical grooved cylindrical rotor was located concentrically in a smooth sleeve. In order to obtain the pressure distribution on the rotor, several taps were installed on the sleeve along the axial direction. The taps led to one side of a differential pressure gauge via valves and the inlet side of the rotor was connected to the other side of the gauge. The pressure was measured by switching the valves in turn. The measurements show that (a) the pressure in the free molecule flow regime increases exponentially from the inlet to outlet, (b) the pressure increases almost linearly from the inlet to outlet in the slip and viscous flow regime, and (c) the inlet effect becomes remarkable in the turbulent flow regime. The validity of the theory has been proved from the free molecule to viscous flow regimes, but the theory needs modification so as to include the inlet effect in the turbulent flow regime.

Pumping mechanism of helical grooved molecular drag pumps

The flow on a rotor of a molecular drag pump varies from viscous to slip to free molecule flow according to the decrease in pressure. As the first step, flow through a groove that is facing a wall which is moving along the groove is analyzed. The Navier–Stokes equations are simplified in the viscous and slip flow regimes and can be solved numerically with relative ease. In the free molecule flow regime, drag is caused by the momentum carried to a wall piece by gas molecules colliding with the wall piece, and the drag must be equated to the force exerted by the pressure on the two cross sections sandwiching the wall piece. A weighted linear combination of the two equations for slip and free molecule flows can illustrate the flow through the three flow regimes. The flow in ridges which leads to a leak is treated in a similar way to the flow in grooves. Then, the flows in grooves and ridges are hitherto treated separately and connected by the continuity condition of the mass flow rate normal for the groove-ridge interface. The pressure gradient which is discontinuous at the groove-ridge interface is smoothed by Boon and Tal’s “narrow groove theory” [E. F. Boon and S. E. Tal, Chemie-Ingenieur Technik 31, 202 (1959)]. The theory proposed here satisfactorily predicts the measured pumping performance throughout the three flow regimes including the transition regime from free molecule to slip flows.

Developments in helium leak detection at JET

Helium leak detection on fusion devices such as JET is hampered by the presence of a high deuterium partial pressure emanating mainly from the loaded tiles of the first wall. At JET selective pumping methods have been developed to enable the detection of very small leaks. A novel reference leak system has also been designed to provide for the calibration of the leak detection system. Leak detection is performed by backing a 3500 l/s turbomolecular pump with several pumping systems placed in series: • LN 2 cooled trap – to pump all condensable gases. • Molecular drag pump – to enhance gas flow through the getter pump. • JET designed non-evaporable getter pump – to pump hydrogen and its isotopes. • Helium mass spectrometer leak detector. A key to the success of the system is the unique design of the JET getter pump which, with small volume, low conductance and operated at elevated pressure (∼10 mbar) ensures maximum absorption of the unwanted hydrogen species, in particular Deuterium. The design of the novel leak detection components is outlined. Details of results and experience are given, which show that leak detection sensitivity in the 10 -9 mbar l s -1 range can now be achieved in the presence of high deuterium partial pressure.

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.

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.

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.

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.