Standard Laboratory Vacuum System Stations

Standard Laboratory Vacuum System Stations

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.
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.

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.

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 turbo pump 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.

Common cryopump maintenance actions include:

1. Regeneration:

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. Cryopump 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 turbo'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.