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Page last updated
February 28, 2005

User Advisory

Overview

All endstation and beamline equipment must be operated so as to avoid contamination of beamline components, and must include proper safeguards to protect the storage ring vacuum from an accidental break in the beamline or endstation vacuum system. This advisory gives an overview of the vacuum policy for endstations and provides guidelines for compliance with ALS requirements.

Policy

This advisory defines vacuum policy for experiment endstations, which is slightly different from the policy for beamlines.* This endstation policy must be followed in the initial design, operation, and modification of endstations.

* Beamlines usually contain optics, and because of their close proximity to the storage ring, have more stringent vacuum requirements than endstations. Vacuum policy for ALS beamlines is found in Advanced Light Source Vacuum Policy and Vacuum Guidelines for Beamlines and Experiment Endstations, LSBL-280, and in ALS Beamline Design Guide, Rev 2, PUB-3114.

Requirements for Operation and Equipment

Vacuum chambers

A vacuum-only chamber is one that will only be exposed to vacuum or purge pressures less than 3 psi (20 kPa) above atmospheric pressure. Commercially designed and fabricated vacuum vessels are generally acceptable; any noncommercial vacuum vessels must be designed to withstand atmospheric forces safely. Contact William Thur (ext. 5689) for additional information.

Materials

To avoid contamination of beamline optics, special care is required when heated samples, gases, vapors, or hydrocarbons (plastics, elastomers) are being used. Zinc, cadmium, and phosphate-bearing materials such as brass and most solder alloys are not allowed due to their significant vapor pressures at bake-out temperatures. Phosphor bronze is acceptable, as is lead-free solder such as 95% tin/5% silver. Electronics components are not allowed unless it can be demonstrated that there is a large safety factor on component burnout. Contact the Vacuum Support Group (ext. 4552) for further information.

Machining Restrictions

Sulfur or silicone bearing oils, lubricants, or coolants must not be used during component machining processes, to avoid embedded contaminants. Resin-bonded or rubber-bonded abrasives must not be used. Ceramic-bonded, tungsten carbide, and diamond abrasives are acceptable.

Cleaning

Hydrocarbon based solvents are not to be used for cleaning parts exposed to vacuum. Acetone and alcohol are recommended, along with various aqueous cleaners. The ALS has cleaning facilities and supplies available for users. Detailed specifications for cleaning UHV hardware can be found in LSME Notes 479 and 500A.

Non-UHV endstations

If endstations do not operate under UHV conditions (above 10^-9 torr, 10^-7 Pa) they must be isolated from the beamline by differential pumping or a thin window to avoid contamination of beamline components or storage-ring vacuum. Elastomer-sealed joints perform adequately for non-UHV systems, and a light coating of vacuum grease may be used to aid in O-ring sealing. Apiezon Type L low-vapor-pressure vacuum grease can be cleaned from surfaces and is the only vacuum grease acceptable for use at the ALS. Silicone vacuum grease (such as Dow-Corning) is prohibited on equipment brought to the ALS, since it is nearly impossible to remove from contaminated surfaces in the event of a vacuum mishap. In addition, Krytox LVP vacuum grease has been shown to vaporize when hit by UV light, and therefore must not be used at the ALS.

Vacuum pumps

Backstreaming of pump oil into the ALS beamlines must be avoided. This is greatly simplified when a modern "dry" backing pump is used with an oil-less ceramic or magnetic-bearing turbomolecular pump. A complete ban of oil-sealed or oil-lubricated vacuum pumps is under consideration at the ALS. In the meantime:

• If an oil-sealed backing pump or an oil- or grease-lubricated turbomolecular pump is used, an rpm-sensing interlock system is required. Acceptable systems are specified in ALS User Advisory 14, Interlock Requirements for Turbopump Systems on User Endstations.
• Diffusion pumps are not permitted because of the high risk of oil contamination of the beamline and/or optics.

Gauges

ALS users should normally provide their own vacuum ion gauges and controllers, which may also serve as sensors for various required interlock systems. Glass-tube ion gauges are not allowed at the ALS because of the significant risk of contaminating the beamline or possibly even the storage ring in the event of a broken gauge. Protecting glass ion gauges with mechanical guards and cable strain reliefs is not adequate. Only unbreakable "nude" ion gauge heads are acceptable. Since these typically protrude beyond their Conflat mounting flanges, a short spool piece may be needed to create a direct replacement for a glass-tube ion gauge. A few nude gauges are available for loan if not otherwise in use. Contact a Beamline Coordinator (ext. 7464) or the Beamline Coordination Section Leader (ext. 5527).

Water Cooling

Vacuum-to-water joints are not permitted in ALS beamline vacuum systems. Vacuum-to-water joints must be avoided as much as possible in vacuum systems supplied by users and are not allowed without explicit permission from ALS management. For additional information, see ALS User Advisory 19, Water-to-Vacuum Joints in User Endstations at the ALS or contact William Thur at ext. 5689.

Venting

Large ALS liquid-nitrogen dewars are available to supply clean boil-off nitrogen for letting users' vacuum chambers up to atmospheric pressure. These dewars are equipped with pressure-reducing regulators and small 0.5 psi (3.5 kPa) pressure relief valves in the line that is connected to the user's chamber. The 0.5 psi (3.5 kPa) relief valve serves to protect against blowout of glass vacuum viewports and it also gives a positive indication of completion of the venting process.

Purging

In some cases a significant flow of clean nitrogen is needed to flush a vacuum chamber. A specially equipped ALS nitrogen dewar is available for this purpose. It will supply a substantial flow of clean nitrogen limited to 3 psi (20 kPa) pressure by a large relief valve. An overpressure of 3 psi (20 kPa) was chosen to drive a significant flow of nitrogen while still providing protection against blowout of glass viewports.

Relief valves

The ALS does not require pressure relief devices on vacuum-only chambers. However, any system that is used to introduce pressurized gas into a vacuum chamber must include a suitable relief valve set for no more than 3 psi (20 kPa). No relief valve is needed if calculations demonstrate that the quantity of pressurized gas available is incapable of producing a positive pressure in the vacuum chamber.

Viewports

The implosion or explosion of glass or crystalline windows is a significant personnel hazard. The use of cataloged commercial viewports with apertures of 15 cm (6 inches) or less is encouraged. Any viewports larger than 15 cm must be pressure tested to 2 atm (30 psi or 200 kPa) differential pressure applied from the atmosphere side of the window. All glass or crystalline viewports should be protected by the heavy glass (bakeable) safety covers provided by the ALS. Anyone using an uncovered glass or crystalline viewport must wear safety glasses.

Any noncommercial glass or crystalline windows must be simply supported on both sides by soft members to minimize bearing stress concentrations. Glass windows must be designed with a safety factor of at least 10 on ultimate stress. Detailed desgin advice is given LSME Note 936, Design of Custom Vacuum Viewports. A written Safety Note to be reviewed by William Thur in ALS Engineering is required for noncommercial windows.

Vacuum interlocks

Endstation vacuum must be monitored with an appropriate vacuum gauge. An output from the gauge controller will be used as an input for the beamline Equipment Protection System (EPS) to protect the beamline in the event of a vacuum accident at the endstation, and also to protect the endstation from the effects of vacuum mishaps by other users on nearby branch lines. The configuration of beamline and endstation valves and gauges should be examined carefully to ensure protection against conceivable vacuum accidents. William Thur (ext. 5689) and Ken Woolfe (ext. 7739) make such an inspection before approving the Experiment Safety Sheet for a new experiment; they may be contacted with questions about vacuum interlocks.

Gas cells

Experiments at the ALS often involve a gas chamber (a small vacuum vessel) containing a gas that is at low pressure and is to be illuminated by synchrotron radiation. A membrane or thin window is commonly used to segregate the gas from the vacuum of the upstream beamline. Thin windows are inherently delicate, and they will blow out if they are subjected to any significant gas pressure.

A failed thin window could contaminate a beamline at the ALS (depending on the gas used), and conceivably the storage ring itself if fast valves and sensors fail to operate as intended. For this reason, gases to be used in gas cells must be reviewed by the Vacuum Support Group (ext. 4552). Gases that pose a contamination hazard may require an interlock system interfaced to the EPS.

Differentially pumped gas cells must have interlocked vacuum pumps and may be tested by an upstream RGA scan for each gas to be used at the maximum pressure.

Beryllium windows

Beryllium is often chosen as the material for windows that transmit the photon beam into a chamber at atmospheric pressure. Beryllium has minimal absorption characteristics, has good thermal conductivity, and is relatively strong. However, there are special concerns about thermal stress and about corrosion and subsequent failure of beryllium windows in beamline-to-atmosphere applications. The photon beam emerging from the surface of the window interacts with the atmosphere to produce ozone and nitric acid, which react with the beryllium surface. The resulting corrosion will cause an eventual failure of the window. Further information and advice are given in LSME Note 659, Beryllium Windows at the ALS. If your experiment includes a beamline-to-atmosphere transmission window, contact ALS Engineering (William Thur, ext. 5689).

RGA Policy

One of the major safeguards against contamination of the storage ring and beamlines is the ALS Policy on Residual Gas Analysis (RGA). Optics at the downstream end of the beamline are particularly vulnerable to contamination from the endstation. Contamination can result from photon/residual gas interaction at or near the optic surface. It can also be caused by deposition of molecules from non-UHV-compatible materials in the endstation. Because of these concerns, an RGA is required for any user apparatus that could possibly contribute to the residual gas in the beamline. The results of the analysis must meet certain criteria before an endstation can run. It is required that the ion gage reading (N₂ equivalent) be less than or equal to 2 x 10^-9 torr (2 x 10^-7 Pa) and that the sum of the partial pressures of constituents above mass 46 be less than 1 x 10^-11 torr (1 x 10^-9 Pa). ALS Vacuum Technicians will do the RGA, but the user must arrange to have it done before exposing the endstation to the beamline. This can be arranged by contacting Steve Klingler (ext. 5177), Vladimir Moroz (ext. 5177), or Frank Zucca (ext. 4552). It is helpful if one of them is notified a day or two in advance. For more information about RGA requirements at the ALS, please contact, Frank Zucca (ext. 4552), or Steve Klingler (ext. 5177).

More Information

Technical questions about beamline or endstation vacuum design requirements should be addressed to William Thur (Tel: 510-486-5689). Contact the Beamline Coordinator on duty (ext. 7464) for questions regarding vacuum safety or ALS procedures concerning beamline operation. Additional information on the ALS Vacuum Policy for beamlines can be found in Advanced Light Source Vacuum Policy and Vacuum Guidelines for Beamlines and Experiment Endstations, LSBL-280, and in ALS Beamline Design Guide, Rev 2., PUB-3114.