NLC BD VACUUM Thermal Outgassing Rate for Various Beam Line Materials
The main parameters in designing a vacuum system are: Conductance 12.3 (D3/L) [l/s] Outgassing QTOT=(D Q)/L[Torr l-1 cm-2] Pumping speed S=Q/P [l s-1] Desired pressure [Torr] Other decision criteria for choosing an UV or UHV material for particle accelerators: Electrical conductivity MPS considerations Cost Ease of machining Introduction -1
The physics driven parameters are: Conductance, (beam line I.D) Desired pressure Electrical conductivity MPS considerations Engineering driven parameters are: Cost Ease of machining Outgassing Pumping speed The driving engineering parameter for vacuum design is the outgassing rate. It will determine choice of material. Introduction - 2
Through trial and error a handful of vacuum materials have evolved as being suitable for UV and UHV applications. Stainless steel Copper Aluminum Glass Ceramics They all have one thing in common, a low thermal outgassing rate. The origin of the thermal outgassing rate is: Hydrogen trapped in the molecular structure of the material and from decomposition of water vapor. Other sources of thermal outgassing are: Surface contamination from lack of proper cleaning Gases trapped on the material surface such as CO and CO2 Virtual leaks Design Thermal outgassing rate - 1
The “other” sources of outgassing for the various UV and UHV materials are dealt with through: Surface contamination - Refined Cleaning Methods Water Vapor - Outgassing CO and CO2 - Glow Discharge, Electron Bombardment Virtual Leaks - Engineering Design - Engineering Typical thermal outgassing rates for various materials: Stainless Steel: For stainless steel the outgassing is small. An accepted value for baked and cleaned, 304 or 316 stainless steel is 1.0E-12 torr liter s-1 cm-2. (Source: AVS) Copper: Comparable to stainless steel as far as the outgassing rate goes. Thermal outgassing rate - 2
Non-metallic materials Hydrogen outgassing rate is not an issue. Aluminum: The accepted outgassing rate for aluminum at SLAC is 5.0E-11 Torr liter s-1 cm-2. This value is based recent PEP-2 experiences. (Source: Private conversations with Dan Wright and Julia Weinberg) How does the value hold up to other measured values of the thermal outgassing rate for aluminum? 1. Hajime Ishimaru, KEK, measured a thermal outgassing rate of E-13 Torr l s-1 cm-2 on an, “EX” processed, UHV cleaned and baked system. E-10 Torr l s-1 cm-2 before bakeout. “EX” process: AL part dipped in sodium hydroxide, rinsed in de-mineralized water, thermally processed in an oxygen/argon atmosphere for 24 hr’s, the water forms an hydroxide layer which is transformed in to a thin oxide layer during the thermal treatment. [Source; AVS Series 8, ISBN 0-88318-756-6 p. 139] Thermal outgassing rate - 3
Two identical vacuum systems, one of stainless steel, one of aluminum, will not achieve the same base pressure. The aluminum system will always be approximately two orders of magnitude higher in pressure than a stainless system. If the thermal outgassing rate on aluminum could be lowered, it would be close to an ideal beam line material. What are the main ways to lower the thermal outgassing rate? Cleaning Bake out Coating of inner vacuum chamber surface Cooling the vacuum chamber Effects of thermal outgassing rate - 2
The beam line aperture must be maximized. Provide for in situ bake out Consider a design with an ante chamber to maximize pumping speed. Re-evaluate the current pressure specifications for Collimation and Big Bend. What to do next? Determine the exact vacuum specification for NLC BD. Discuss maximizing beam line aperture. Quantify MPS specification Quantify “Electrical” properties. Do a cost evaluation of Aluminum alternative. Investigate the consequences of a cooled beam line. If we were to use Aluminum...