Standard Laboratory Vacuum System Stations

Standard Laboratory Vacuum System Stations

Introduction

In order to achieve high vacuum (nominally ~ 1 x 10E-6 Torr) it is necessary to combine various vacuum technology components into a system, sometimes colloquially referred to as a "stack." Throughout our lab, there are vacuum stations that are used to achieve high vacuum in different sections of the accelerator. Typically, anywhere the beamlines or accelerator can be separated by isolation valves, a station exists between those valves. It is undesirable to have a "dead section" of beamline - a section that is isolated by valves but without a vacuum station. This page explains the basic stacks or systems in used at each station throughout our lab.

Turbo-Pump Based Stations

The Turbo-Pump Based Vacuum Station is a system used to produce high vacuum ( 10^-6 < X < 10^-9 Torr ) in beamlines and experimental chambers through out the lab. Turbo pump, or more exactingly turbomolecular pump, systems are throughput pumping systems, which means that the gases that are removed from the evacuated space are continuously discharged into the atmosphere. Turbo pumps cannot discharge to atmospheric pressure and require a roughing pump to interface between the turbo's discharge port and atmosphere. In our lab, this roughing pump is typically an oil-sealed, rotary-vane vacuum pump but may sometimes be an oil-free, dry scroll pump.

The standard turbo pump station consists of the following components:

• A single turbomolecular vacuum pump.
• A single roughing pump.
• An electro-pneumatically operated, high-throughput, gate valve.
• A hand operated (usually) or an electro-pneumatically operated (rarely) foreline valve.
• A molecular-sieve foreline trap.
• A foreline vacuum gauge.
• Dual chamber/beamline vacuum gauges.
• A vacuum pump gate valve interlock circuit.
• A panel-mounted indicator & control panel.
• Various interconnecting vacuum hoses and fittings.

Operation

Normally, the turbo and rotary pump (RP) run 24/7 and maintain chamber high vacuum. If lab operations dictate that the beamline or chamber be vented, then the turbo inlet gate valve is manually shut to protect the pumping system. The gate valve interlock should be left in the PROTECTED mode while the work is performed as it will prevent the inadvertent opening of the inlet gate valve. Once the work is completed, the beamline or chamber should be evacuated w/ a portable system, preferably a dry-scroll pump-out cart until the chamber pressure is less than 50 mTorr. The larger the volume, the lower the pressure one should try to obtain before opening to the pump inlet gate valve.

When opening the chamber to the turbo pump station, the following steps should be performed:

1. Verify that the chamber pressure is < 50 mTorr or more preferably, as close to the ultimate pressure of the portable pumping cart being used.
2. Verify that the turbo is in NORMAL OPERATION (running at full speed) and that the foreline pressure is good ( < 50 mTorr ). Some of our turbos have a LOW SPEED setting that can be employed when not in use to prolong bearing life. The turbo should be brought to full speed before placing online.
3. Isolate the portable pump-out cart by shutting the valve on the chamber or beamline.
4. Place the Gate Valve Interlock system in BYPASS mode. This allows operation of the gate valve w/o protection. We do this to prevent gate valve oscillation, a full explanation of which occurs later.
5. Place the Gate Valve switch in the open position.
6. Monitor the foreline pressure. It should rise rapidly, slow, and then begin to drop in a relatively brief time. Ideally, the foreline pressure should stay below a few hundred millitor and quickly recover. Danger to the turbo can occur if the foreline remains well above 50 mTorr for a prolonged period (10's of minutes). Prolonged exposures to high foreline pressures will cause bearing overheating and pump shut-down. Never walk off and leave a turbo running w/ high foreline pressures. If the foreline pressure fails to start dropping within a few minutes, shut the gate valve, place the interlock in the PROTECTED mode, and contact staff for assistance.
7. Restore the Interlock. Once the foreline pressure has dropped to 50 mTorr or less, you should be able to place the interlock back in the PROTECTED mode. The foreline pressure gauge will indicate whether the interlock threshold setting is satisfied or not. If the gate valve shuts at this point, this threshold is not satisfied. Simply place the interlock back in BYPASS mode and the valve switch in the OPEN thus re-opening the gate valve and wait for the foreline pressure to drop further.

CAUTIONS

• NEVER leave a turbo pump based vacuum system in the UNPROTECTED mode unsupervised without staff approval.
• Be mindful of turbo pump temperature and foreline pressure until both are back to nominal values.
• Gate Valve Oscillation occurs with throughput systems when the gate valve is left in PROTECTED mode while pumping down the chamber. It is caused when the turbo's discharge pressure exceeds the foreline interlock threshold pressure as it begins to take in gas from the chamber. When this happens, the interlock will trip and shut the gate valve. With the gate valve shut, the turbo is no longer taking in gas from the chamber and thus its discharge pressure drops below the interlock's threshold pressure. Now the interlock is once again satisfied and opens the gate valve, starting the whole process once again. Typically, this will continue until the the system restores pumping and will be fine, but it causes excess wear on our gate valves and should be avoided when possible.
• MagLev Crash: A rapid gas inrush in a magnetically levitated turbo can overwhelm the magnetic field bearings and cause the turbo to "sit down" on it's emergency bearings. When this occurs the turbo will "scream" loudly and can easily startle nearby personnel. MagLev crashes should be avoided if at all possible. Most maglev turbos have a limited number of times that this may occur before the emergency bearings must be replaced at significant cost.

Maintenance

Students are not expected to perform maintenance on these systems, however an understanding will help students know when to alert staff to maintenance issues.

All of the turbo pumps in operation in our lab use one of two bearing types: (1) static or dynamic magnetic bearings, or (2) permanently lubricated ceramic ball bearings. In both of these cases, the pumps are run until failure. Some of our turbos have a hybrid bearing technology where one bearing is magnetic, but the other is ceramic ball. The Re-Buncher turbo does have a battery that must be replaced periodically. Cooling fan operation must be maintained when in operation unless approved by staff. The oil-lubricated, rotary-vane roughing pumps require oil changes, shaft seal changes, and occasional rebuilds. Dry-scroll roughing pumps require tip-seal replacements periodically.

Cryopump Based Stations

The Cryopump Based Vacuum Station is a system used to produce high vacuum ( 10^-6 < X < 10^-9 Torr ) in beamlines and experimental chambers through out the lab. Cryopumps are capture pumps, which means that the gases that are removed from the evacuated space are stored within the pump until the pump's capacity is reached. These gases must then be expelled through a process known as regeneration. Modern cryopumps can do this automatically. Sadly, our cryopumps are not modern cryopumps and regeneration must be done manually.

The standard cryopump station consists of the following components:

• A single cryopump.
• A single cryopump compressor.
• An electro-pneumatically operated, high-throughput, gate valve.
• A pump-mounted vacuum gauge.
• Dual chamber/beamline vacuum gauges.
• A vacuum pump gate valve interlock circuit.
• A panel-mounted indicator & control panel.
• Various interconnecting vacuum hoses and fittings.

Operation

A cryopump is a capture pump and will run continuously until one of the following occurs:

1. Operation is interrupted long enough to allow the pump to warm and break its vacuum insulation
2. Pump capacity is reached.
3. Pump Failure occurs.

In a cryopump based vacuum station, the cryopump runs 24/7 and captures gas from the chamber by cryo-condensation and cryo-adsorption. If lab operations dictate that the beamline or chamber be vented, then the cryopump inlet gate valve is manually shut to protect the pumping system. The gate valve interlock should be left in the PROTECTED mode while the work is performed as it will prevent the inadvertent opening of the inlet gate valve. Once the work is completed, the beamline or chamber should be evacuated w/ a portable system, preferably a dry-scroll pump-out cart until the chamber pressure is less than 30 mTorr. The larger the volume, the lower the pressure one should try to obtain before opening to the pump inlet gate valve.

When opening the chamber to the cryopump station, the following steps should be performed:

1. Verify that the chamber pressure is < 30 mTorr or more preferably, as close to the ultimate pressure of the portable pumping cart being used.
2. Verify that the cryopump is cold (< ~ 25K) and that the pump pressure is low (< ~ 10 mTorr).
3. Isolate the portable pump-out cart by shutting the valve on the chamber or beamline.
4. Place the Gate Valve switch in the open position.
5. Monitor the chamber/beamline pressure and ensure that it begins to fall. This should occur rapidly, as our standard cryopump provides a very high pumping speed for water vapor and nitrogen - the predominant gases in atmosphere. Check periodically to ensure that the system achieves a nominal pressure and the pump temperature remains nominal.

CAUTIONS

• Opening the cryopump when the chamber pressure is too high can cause the cryopump to "crash" or warm up beyond recovery.
• When warming, cryopumps can overpressure. All cryopumps have a pressure relief device of some kind, typically a spring-loaded, elastomer-sealed valve. Cryopumps designed for ultra-high vacuum systems may also have a single-use burst disk which ruptures violently when actuated. (none of our cryopumps have burst disks)
• When warming, the external cryopanel housing can become so cold that it condenses water vapor and can create a drip hazard for equipment located below it.

Maintenance

Students are not expected to perform maintenance on these systems, however an understanding will help students know when to alert staff to maintenance issues.

Maintenance actions involving cryopumps almost always involve thermal cycling and therefore can involve prolonged periods of time. It is not uncommon to lose the better part of a day of data collection due to a cryopump issue.

Common cryopump maintenance actions include:

1. Regeneration. This is the process of ridding the pump of the captured gas. Regeneration is needed when the pump reaches capacity or develops a cold short--an ice bridge between the interior cryopanels and exterior housing which prevents the pump from achieving it's operating temperature. Regeneration requires thermal cycling. Typically regeneration is performed by staff during normal business hours. All other maintenance actions will require regeneration as part of the maintenance action. The following procedure for regeneration is provided in case those unfamiliar with the process are required to perform it to restore operations.
   1. Isolate the cryopump from the vacuum chamber or beamline by shutting the gate valve. The gate valve interlock should be left in the PROTECTED mode while the work is performed as it will prevent the inadvertent opening of the inlet gate valve.
   2. Turn off the cryopump compressor.
   3. Vent the cryopump. Locate a source of dry nitrogen, if possible, and use this to vent the cryopump. Allow the cryopump to come to atmospheric pressure. Use caution when connecting the dry nitrogen source and ensure that the pump is never over-pressured.
   4. Open the relief valve. Carefully pull the relief valve open and place something relatively soft in the elastomer seal to prevent closure. A wooden cotton swab handle or a nylon tie-wrap works well.
   5. Start Dry Nitrogen Bleed. Establish a slow-flow from the dry nitrogen line and connect it to the pump-out valve so that a small amount of dry nitrogen flows through the pump and out of the relief valve.
   6. Start the Heater. If one exist, initiate heating on the housing. This is typically done w/ a silicone heat-tape wrapped around the pump housing. CAUTION: Be sure to follow any instructions on the heater control. Over heating the pump can melt the indium components!
   7. Monitor the pump temperature. Allow the pump to come to room temperature, or slightly above. Some of our pumps have silicon diode thermometry and will read the temperature fairly well throughout the range to ambient. Other pumps have hydrogen vapor pressure thermometry and do not accurately read the temperature above the low Kelvin range. Allow these pumps to warm/flow for at least two hours, and up to four if possible.
   8. Stop Dry Nitrogen Flow. Once the pump has reached ambient temperature or slightly above, stop the flow of nitrogen. Remove the nitrogen source connection. Remove the device that was used to hold the relief valve open and visually confirm that it closes properly.
   9. Turn Off Heating. If a heater was used to warm the pump during flow, turn it off now.
   10. Pump out the Cryopanels. Connect an oil-free pumping cart to the cryopump and initate pumping. It can take a while to pump out a cryopump, but if the pump was fully warmed and flowed, it should be done in less than two hours.
   11. Test the Pump Out. Verify that the pump is fully evacuated by ensuring the following criteria: (1) pump pressure is 30 mTorr or less, and (2) the pressure in the pump has a rate of rise < 15 mTorr/min when the roughing cart is isolated from the pump. If these criteria are met, re-open the valve to the scroll cart and proceed to the next step.
   12. Restart the Cryopump. Using the switch on the compressor, restart the cryopump and allow it to run.
   13. Stop the Pump Out. Within about 10 mins of starting the compressor, shut the valve to isolate the scroll cart.
   14. Monitor the Pump. On pumps with silicon diode thermometry, one need only monitor the temperature display and verify that the pump is cooling down. On pumps with hydrogen vapor bulb thermometry, there will be no visible sign of temperature change until the pump is in the tens of Kelvin range. On these pumps, the pump pressure should be monitored for indications that the pump pressure is dropping and that this pressure drop occurs within a few tens of minutes. Once temperature and/or pressure drop are confirmed, one need simply wait for the pump to achieve nominal operating temperature. This should usually occur in 2-3 hours.
   15. Place Pump in Service. Once the pump has achieved nominal operating temperature, verify that the chamber pressure is below 30 mTorr (the lower the better) and then open the gate valve. Verify that the chamber pressure drops rapidly. Monitor the pump pressure for the next few tens of minutes to ensure the cryopump doesn't crash (become overwhelmed and warm up).
   16. Paperwork. If there exists a tag on the cryopump, mark the date and the fact that regeneration was performed for future reference.
2. Cold Head Purge. This involves removing contaminant gases from the cold head. During normal operation, gases other than helium will tend to freeze withing the cold head's (pump's) helium circuit and impair operations. If allowed to build too much, this ice can mechanically damage components in the cold head. Similar to regeneration, the pump is stopped and the cold head is disconnected quickly from the compressor. Once warm, the thawed contaminant gases are purged from the cold head's helium circuit. Cold head purge require implicit regeneration of the cryopanels.
3. Compressor Purge. When contaminant gases are significant, it can be necessary to purge the compressor as well. Compressor Purges require implicit Cold Head Purge and Cryopanel Regeneration.
4. Adsorber Change. This is a scheduled, preventive maintenance action, typically performed annually. The adorber is basically an oil filter that prevents compressor oil from entering the cold head. Failure to perform this maintenance on schedule will result in contamination and failure of the cold head and necessitate a costly rebuild. Adsorber changes require implicit Cryopanel Regeneration, and usually include Cold Head Purges as well.

Diffusion Pump Based Stations (rare)

The Diffusion Pump Based Vacuum Station is a system used to produce high vacuum (10^-6 < X < 10^-9 Torr ) in some specialized systems within the laboratory, notably the SNICS source and one of the target lab evaporators. Diffusion pumps, in general, are being replaced by more modern alternatives and are typically no longer included in new designs. Feel free to skip this section if you're short on time. Diffusion pump systems are throughput systems, which means that the gases that are removed from the evacuated space are continuously discharged into the atmosphere. Diffusion pumps cannot discharge to atmospheric pressure and require a roughing pump to interface between the diffusion pump's discharge port and atmosphere. In our lab, this roughing pump is an oil-sealed, rotary-vane vacuum pump.

The standard diffusion pump station consists of the following components:

• A single diffusion pump.
• A single roughing pump.
• An electro-pneumatically or manually operated, high-throughput, gate valve.
• An electro-pneumatically or manually operated foreline valve.
• A foreline vacuum pump gauge
• Dual chamber/beamline vacuum gauges.
• A vacuum pump gate valve interlock circuit. (on some systems)
• A panel-mounted indicator & control panel. (on some systems)
• A source of water cooling.
• Various interconnecting vacuum hoses and fittings.

Contact

Powell Barber mailto:pbarber@fsu.edu