Papers

AVS2001 Session VST-MoM: Sealed and Insulating Vacuum Systems

Monday, October 29, 2001 9:40 AM in Room 125

Monday Morning

Start	Invited?	Item
9:40 AM		VST-MoM-1 RHIC Insulating Vacuum System: Status and Performance^* R. Davis, H.C. Hseuh, D. Pate, L. Smart, D. Weiss (Brookhaven National Laboratory) Superconducting magnets are used to bend and focus the heavy ions in Relativistic Heavy Ion Collider (RHIC), which has been operational for the past two years. The RHIC insulating vacuum systems consist of 40 large volumes and 112 smaller transfer lines with a cumulative length and volume over 10km and 2000 m³, respectively. Numerous pumps, valves, gauges and residual gas analyzers are deployed to maintain and monitor the integrity of the insulating vacuum, and thus minimize the heat transfer from the ambient cryostat wall to the magnet cold mass. The overall design, the construction, commissioning and operation experience of the RHIC insulating vacuum systems will be presented. The measurements and methods for locating internal helium leaks at various helium line temperatures and pressures will also be presented. ^*Work performed under the auspices of the U.S. Department of Energy.
10:00 AM		VST-MoM-2 Getter Requirements for a CRT with a Diamond Coated FE Electron Source V. Nemanic, M. Zumer, B. Zajec (ITPO, Institute of Surface Engineering and Optoelectronics, Slovenia); T. Tyler (North Carolina State University) A single diamond coated field emitter (FE) tip could be used as a replacement for the thermionic cathode in small electron beam devices offering better opto-electronic performance. The most important parameter limiting the use of field emitting sources is their short lifetime, which is closely related to the pressure level within the pinched off device. After the initial evacuation of the tube and bake-out, getter selection and the activation procedure would seem to play the crucial role. In the present study, the pressure of miniature CRTs was measured by a spinning rotor gauge (SRG) after the initial evacuation and bake-out procedures, again after activation of the getters, and finally during operation under typical operating conditions. Two commercial getter types were studied: Ba evaporable (St15/AM/O/9.5) and non-evaporable (NEG, St 172), made by SAES and built into the CRTs with either an oxide coated thermionic cathode or a diamond coated FE tip. The later were fabricated by electrochemically etching 125µm molybdenum wire in KOH, resulting in tips with radii of ≤ 50nm. The bare tips were then coated with nanodiamond via electrophoresis using a 2g /L suspension of diamond in ethanol. After the evaporation of barium getters by the prescribed procedures, the pressure did not drop, but increased slowly from 1x10^{- 5} mbar up to 1x10^-3 mbar, later shown to be methane. In CRTs with oxide cathode, getter pumping action could be triggered by switching on the cathode heater, which efficiently cracked the methane generated. In CRTs with a FE tip, there was no way to decrease the residual methane pressure to the desired level. The FE tips, previously approved and characterised in UHV, were thus irreversibly damaged ab initio. In CRTs with NEGs, no pressure increase was registered by the SRG during several days. This means that they can maintain the residual atmosphere at the level that may provide a long-term operation of devices with diamond coated FE electron sources.
10:40 AM		VST-MoM-4 Applicability of Spinning Rotor Gauges to Pressure Measurements in Sealed System J. Setina, B. Erjavec, L. Belic (Institute of Metals and Technology, Slovenia); V. Nemanic (ITPO, Slovenia) Spinning rotor gauge (SRG) is truly inert vacuum gauge and has remarkable resolution and long term stability. Measuring element is a small steel ball, which is kept inside a measuring thimble, while all other necessary elements for the gauge operation are kept outside vacuum. The measuring head easily decouples from the ball/thimble assembly and the volume of the thimble is only few ml. All this makes the gauge especially usable for pressure measurement in sealed systems. However the accuracy of pressure measurements in a sealed systems is affected by the gauge residual drag. The residual drag changes whenever the suspension head is removed but remains stable during the continuos suspension if the ball. This makes gauge more suitable for studying pressure changes in a sealed system than for absolute pressure measurements. Pressure changes in a sealed-off device can be caused by leaks and permeation, outgassing or gas pumping effects. Through the years we successfully used SRGs for studies of gas pressure in different sealed-of devices. We will describe our experience of using SRGs for measurements in image intensifiers, miniature cathode ray tubes and recently in channel electron photomultipliers, flat vacuum insulation panels, ultrasensitive leak detection and studies of hydrogen outgassing from stainless steel. We will discuss how the unknown gas composition in the sealed system and the ball-thimble geometry influences accuracy of the gauge in the transition from molecular to viscous regime.
11:00 AM		VST-MoM-5 Sealed Vacuum Systems including Vacuum Insulation I.P. Della Porta (SAES Getters SPA, Italy); L. Rosai (SAES Getters’ Group Strategic Marketing Director) Vacuum systems can be continuously pumped discharging to air the gasses generated by the process or released by the internal surfaces. However an enormous number of vacuum devices after an initial processing and evacuation are insulated, and sealed-off. The maintenance of a good vacuum for long times requires both a sufficient pumping and efficient baking processes. In most cases an internal chemical pump, a getter, is used to take care of the gasses developed during these processes and the operations of the systems. Getter materials, getter pumps, ion-pumps, cryo-pumps are the most economical and efficient solutions. We will examine the requirements and the solutions of vacuum systems operating in the vacuum better then 1x10^-10 torr, typically particle accelerators; of applications where the required vacuum has to be better than 1x10^-8 torr, typically certain processes for the semiconductor industry; of devices in which the vacuum or the contaminants partial pressures are in the range of 1x10^-2- 1x10^-8 torr, typically electron tubes, displays (CRTs,FEDs,etc. ) and lamps. We will examine also sealed off stainless steel devices like dewars or vacuum thermally insulated pipes with vacuum in the range of 1x10^-6 - 1x10^-4 torr. Finally, we will also consider a new generation of vacuum insulated panels for thermal insulation based on plastic materials where the vacuum is in the range of 1x10^-3 to 1 torr. We will illustrate the materials used, the treatments of the surfaces, the various types of tailor made getter materials for each group of vacuum systems. We will also mention the key factors and the mathematical models used to predict the vacuum evolution and the effects of the deterioration of vacuum on the working characteristics of the sealed- off devices.