12 INCH CYCLOTRON VACUUM SYSTEM


Fig.1 The vacuum system: diffusion pump stack, automated valve, chamber roughpump (foreground), and chamber in between poles.
The vacuum system used on the 12-inch cyclotron is essentially the same as was used with the 9-inch. The high vacuum is accompolished by a diffusion pump that is backed by a direct drive mechanical pump. The Precision mechanical pump has an absolute pressure of 1E-4 Torr, and a pumping speed of 195 liters/minute. The diffusion pump is a Veeco 4 inch water cooled pump that uses Dow Corning 704 oil, with water cooled baffles, and a liquid nitrogen trap. The ultimate pressure of this four inch pump is 1E-8 Torr and has a pumping speed of 425 liters/minute. Directly after the LN2 trap the plumbing steps down from a four inch flange to a two-inch KF40 flange.

The most notable difference with this vacuum system is the reduction of the 18" long KF40 bellows to a more reasonable 6 inch long (2 inch diameter) bellows mounted for an almost straight run to the chamber. This is accompolished by a bracket that mounts the diffusion pump as close to the chamber as possible. A significant improvement in vacuum quality has been noticed, both in absolute pressure and pump down time. Pressures as low as 9E-7 have been reached, and from a cold start it takes approximately 1 hour to reach the high E-6 Torr range.


Fig.2 Vacuum gauge controllers are mounted in the magnet table and are connected to the computer.


A welcome upgrade came when the entire vacuum system was put under computer control. The cyclotron operator can monitor and control the vacuum system at the control rack. The operator can control each stage of the vacuum system or can simply start the pump down sequence and let the computer take control. Other automation features include the computer intervention in the vacuum system's safety, which includes monitoring the foreline pressure, mechanical and diffusion pump temperatures, and water flow. If any of these parameters move into a range where a pump may be damaged the appropriate valve(s) and pump(s) are shut off, a solenoid valve shuts off the hydrogen supply, and the operator is notified.

An improvement that increases time efficeincy is the addition of a manifold of valves on the chamber pumping port to allow a second mechanical pump to rough out the chamber. This is useful when wanting to make quick changes to targets or ion source filaments which requires bringing the chamber up to atmosphere. The operator simply isolates the diffusion pump from the chamber, brings the chamber up to atmosphere, performs required work, then using the second mechanical (roughing) pump, roughs out the chamber to 1E-3 torr, isolates the roughing pump, and finally reconnects the diffusion pump. This eliminates the 40 minute cool down time and 60 minute warm up time of the diffusion pump. This will prove to be useful feature once the cyclotron is put into "production" lab use.

Pressure is monitored in the foreline with a standard thermocouple gauge, pressure at the cold trap is monitored with an ion gauge, cyclotron chamber pressure is monitored with both a thermocouple gauge and an ion gauge. All pressure units have outputs that are monitored by the DAQ/Control system and are used in the operation and safety of the vacuum system.

Anyone viewing this page that has worked on reasonably high vacuum systems will sympathize with us in the task of looking for minute vacuum leak(s). One of our visiting scientists, Dan Hoffman, is skilled in vacuum leak hunting. Using an Alcatel leak detector Stuart and Dan spent many hours locating and fixing a pesky leak. It was found, the joint re-welded, and succesfully retested. The vacuum system is pretty well understood and satisfactory.


Fig.3 Dan (L) instructing Stu on leack checking.



Fig.4 Stu looking for the leak.



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