Vacuum System
Why do we need such low pressure?
- In air, intense electric and magnetic fields of the microwaves cause lightning-like sparking, thereby destroying the microwaves. This could also damage the waveguide or accelerator structure. Extreme low pressure avoids this breakdown.
- If particles in the accelerator beam collide with any air molecules, they are knocked out of the beam bunch and lost. Extreme low pressure minimizes the chance of such collisions.
How do we achieve such low pressure?
At such low air pressures, ordinary suction pumps are not effective -- there is essentially nothing left for them to pump out. At ordinary pressures, molecules of a gas collide frequently with each other. These collisions set up a flow in the gas towards a region of lower pressure. Thus one can use a rotary pump (which works like a fan) to induce a flow of gas out of any container by creating a low-pressure region at the pump.
Below about a millionth of atmospheric pressure (10-6), a gas is so dilute that molecules hardly influence each other at all. In fact, they collide more frequently with the walls of the vacuum vessel than they do with each other. A fan has no effect when there is so little gas around. There is no fluid to flow, just isolated molecules. In this regime, the only way to "pump"' is simply to trap any molecules or ion that happen to run into a collection area, or "ion pump."
Contamination from surfaces
It is extremely important to keep the vacuum vessel itself very clean, so no molecules are unnecessarily introduced. Any molecules loosely stuck on any surface will eventually be knocked off by collisions of other molecules with the wall, or by synchrotron radiation from the beam. These molecules must then be removed by the pumps. Components for any high-vacuum system must be fabricated in clean-room conditions and then baked at high temperature to ensure that not so much as a fingerprint is left to introduce impurities into the vacuum.
Pump down time
Any leak or scheduled maintenance that requires a shut-down of the vacuum system costs considerable time. It takes more than a day to get the entire accelerator system pumped out to the desired level.
Vacuum seals and joints
Wherever two sections of the accelerator or beam transport lines join there must be a close-to-perfect seal, so air cannot leak in. The accelerator structure is copper because this provides the necessary electrical conductivity to make electromagnetically resonating cavities. In the copper sections of the accelerator, joints are made by brazing together the separate components, so it actually becomes a single copper structure.
Beam transport sections are typically aluminum or stainless steel because these metals are cheaper than copper. Joints between two stainless steel sections of beam transport pipe are made with flanges.
A circular V-shaped groove is milled in the surface
of one flange and a matching ridge in the surface of the other.
A stainless steel O-ring is placed between the two surfaces
which are then bolted very tightly together, so that the O-ring
is deformed into the groove by the knife-edge of the opposing
ridge. Very effective seals can be made in this way.
Most difficult of all to achieve is a flexible joint between two sections of pipe that are both to be held at high vacuum. Such joints are needed because it would be extremely difficult to build and maintain such a large system as a single rigid structure.
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The trick used here is to form an accordion-like structure (a bellows) made of thin aluminum foil. The accordion provides flexibility to fit together two parts which are not perfectly aligned. The depth of its pleats provides the strength needed to support the pressure of the outside air without buckling or tearing |
