Papers

AVS2001 Session VST-MoA: Dry, Cryo & Other Forms of Pumping

Monday, October 29, 2001 2:00 PM in Room 125

Monday Afternoon

Start	Invited?	Item
2:00 PM		VST-MoA-1 Towards the Single Pump Solution - A Recent Advance in High Speed Machines for Dry Vacuum Pumping A.D. Chew, P. Birch, N. Schofield, I. Stones, R.G. Livesey (BOC Edwards, UK) The goal of a single (dry) pump capable of reaching high vacuum and itself exhausting to atmospheric pressure has long existed in the vacuum field. The recent development of a single shaft high speed pump mechanism has gone some considerable way to achieving this goal. The pump exploits molecular drag and fluid dynamic mechanisms. It will be discussed in terms of its science of operation, internal flow mechanisms and technology innovations enabling the mechanism to be produced in volume quantities. Further development and application examples will be discussed to illustrate system-simplification which the pump provides.
2:40 PM		VST-MoA-3 Effect on Capture Probability for a Commercial Cryopumpin Three Different Mounting Arrangements S.B. Nesterov, Y.K. Vassiliev (Moscow Power Engineering Institute, Russia); R.C. Longsworth (IGC-APD Cryogenics, Inc.) A recent Monte Carlo analysis by the authors of the capture probability of an APD Cryogenics M8 cryopump in a test dome per the AVS recommended practice showed it to be about 11 % higher than if tested in an idealized large standard dome (LSD). The same analysis showed that a pump with a uniformly porous inlet has the same capture probability in the test dome as the LSD. The analysis suggested that the geometry of the inlet louvers in the real cryopump and the different flow patterns in the two different domes results in the capture probability in the two different domes being different. Further studies have been done to analyze the flow patterns at the inlet to the M8 cryopump in a LSD and a test dome in order to obtain a more rigorous explanation of the difference. The analysis has been extended to the case where the M8 cryopump is mounted behind a typical gate valve to see if the test dome provides a more realistic standard than the present LSD.
3:00 PM		VST-MoA-4 Water Vapor Capture Within a Vacuum Chamber - The Technology of Meissner Cryopumping Yesterday and Today D.J. Missimer (Retired) In 1955 C.R. Meissner revealed his in-chamber water vapor pumping concept, based on the early work of Dewar. Meissner employed both boiling liquid and heated gaseous nitrogen to rapidly temperature cycle a coil of tubing between cryogenic and above-ambient temperatures. Several vacuum system manufacturers incorporated advanced versions of the nitrogen cooled and heated "Meissner Coil" in their apparatus starting in the late 1960s. Although effective, it proved wasteful of nitrogen and difficult to control. Early closed loop mechanical refrigeration systems had reliability and low temperature limitations and were not widely used. Also they used a flammable refrigerant in the lower stage of a two-compressor cascade cycle. This was not always acceptable. In the mid-1980s a reliable single-compressor auto-refrigerating cascade (ARC) system with fast cycling capabilities between high and cryogenic low temperatures was developed. Its water vapor cryopumping proved more economical and operated with far lower power inputs than previous pumping methods. A mixture of Freons and an inert gas (argon or nitrogen) served as the refrigerant. The Freons were replaced in the 1990s by environmentally benign materials. Rapid temperature cycling of a cryopumping surface mounted directly within the vacuum space has proven its merits because of its high speed, reliability and energy efficiency.
3:40 PM		VST-MoA-6 Sputter-ion Pump Element Cathode to Reduce Noble Gas Instabilities C.M. Schwie (Physical Electronics) Research not yet completed. Sputter-ion pumps have long been used to create reliable ultra-high vacuum environments. However, in certain applications, sputter-ion pumps have demonstrated instability when subjected to noble gas loading. The most stable method of pumping noble gases (e.g. argon) has historically been to employ combinations of titanium with tantalum in the pumping element. This paper evaluates new pumping element design approaches to enhance the Physical Electronics noble gas element. A solution to the problem of re-release (outgassing) of gases under vacuum lies mainly in the element's anode/cathode design. Various techniques were applied to aid the vacuum stability of a sputter-ion pump. These techniques involve employing cathode materials at various volumetric ratios and configurations to customize the pump to its expected operating environment. In addition to vacuum stability, pump speed, ultimate pressure, life, and manufacturability are the chief concerns. The evaluated designs also allow for customization of the pumping element (anode/cathode assembly) to the operating environment. The variation of cathode material ratios is an attempt to tailor the vacuum pump to the gas species present in the system. This will allow the vacuum user to achieve a proper balance between pumping speed and stability. This paper evaluates vacuum pump stability, speed (in accordance with ISO 3556), ultimate pressure, and life with the new cathode configurations. For more information contact: Name: Chester Schwie Job title: Mechanical Engineer Email address: cschwie@phi.com.
4:00 PM		VST-MoA-7 Compact Trapping Device Integrated with a Throttle-Cycle Cooler Operating with Mixed Refrigerant M. Boiarski, A. Khatri (IGC-APD Cryogenics Inc.); S.B. Nesterov, A. Androsov (Moscow Power Engineering Institute, Russia) Trapping devices integrated with a throttle-cycle cooler operating with mixed refrigerant provide advantages in various applications. These devices can be efficient for water trapping at the inlet of turbo-molecular pumps and for oil trapping when used in combination with diffusion pumps. In addition, a mixed refrigerant cooler can provide predetermined temperature profiles of the trapping surface that can be used for selective trapping of desired substances. The refrigeration capacity and the power consumption of such coolers are discussed based on generalized experimental and theoretical data. Data on the refrigeration performance is presented for temperature ranges selected from 80 to 200 K for coolers operating with single-stage compressors. Experimental data is also presented for cooldown time. A rapid cooldown cooler can be designed with minor modifications of the existing low-cost coolers. Both the water pumping speed and nitrogen conductance are taken into account for the optimal design of the baffles. A comparative performance is given for the baffles of different configurations. The test particle Monte-Carlo method is used for simulation of gas flow in a high-vacuum system. A comparison of the calculated and experimental data was conducted for a water trap design consisting of parallel plates installed on the flattened cold finger. Experimental and calculated data on the nitrogen pumping speed are in good agreement. The deviation is less than 5%. This allows optimized configuration of the baffles using the software. Test data obtained for the water-trapping device integrated with turbo molecular pumps proves the performance to be reliable without a special temperature regulation system.