Advanced photon source plan hurdles vacuum obstacles.
So maybe this is a good time to look at the kind of technologies needed to make these machines work from a vacuum point of view--before, that is, the hype about relative beam intensities drowns out discussion of anything else. Let's take Argonne's APS as an example.
Its main storage ring will be about two-thirds of a mile (1,100) in circumference, and will serve as a running track for positrons. It will maintain the positively charged antimatter electrons at an energy of 7 GeV. Magnets will steer and focus the particles, producing intense x-ray beams at 68 experimental beam lines (34 will be instrumented initially).
Ideally there should be no gas molecules at all in the positrons' path as they rush along at nearly the speed of light, because every collision will remove an x-ray producer from the 300-mA current. Practically speaking, therefore, the vacuum level must be extremely good.
The main storage ring is designed to achieve a base pressure in the mid-|10.sup.-11~ torr range, with a buildup to only about |10.sup.-9~ torr while the current is flowing. To achieve that, it is to be constructed as a series of 40 nearly identical sectors, each consisting of five or six extruded aluminum-alloy vacuum chamber sections.
The chambers are now being tested individually. Each must pass a leak check and achieve a vacuum of less than |10.sup.-10~ torr.
"There are approximately 2,400 ultrahigh vacuum-quality weld joints in the storage ring, and not one can leak," says George Goeppner, who is the APS vacuum chamber production manager. To make sure they don't, the welds are made by a state-of-the-art, computer-controlled robotic welding system.
The vacuum chambers are made of an extruded aluminum alloy because this has a high thermal conductivity. Thermal conductivity is an important consideration because the storage ring is very sensitive to even minute displacements.
"For example, beam position monitors that are attached to the storage ring vacuum chambers must be aligned to within 200 |micrometer~ relative to the magnets. Heating the chamber non-uniformly could easily cause distortions of that size," says John Noonan, vacuum group leader in the APS Accelerator Systems Div.
Thermal gradients are kept to a minimum by water-cooling channels along the inside and outside edges of each arrow-shaped sector. Positrons flow through the tip portion and x rays through the shaft, while the tail houses a special "getter" strip that functions as an auxiliary vacuum pump.
When the APS storage ring is ready for use, each sector will be isolated by gate valves, then leak checked and pumped at opposite ends with turbomolecular pumps on carts. A 150-C bakeout accomplished by running heated deionized water in the water channels will be part of the process. After a vacuum in the high-|10.sup.-10~ torr range is achieved, the turbo pumps will be disconnected and wheeled to the next sector.
At this point, the activated iron-zirconium-vanadium non-evaporable getter (NEG) strips are to take over the pumping chore. They will reduce the storage ring pressure to about |10.sup.-11~ torr.
"When you heat the getter to 450 C, the gases that are adsorbed on its surface diffuse into its bulk, opening up more adsorptions sites at the surface. That's how it pumps," explains Goeppner. "You need to pass a 45-amp current through the material for about an hour to get it to 450 C," he adds.
If the APS were merely a positron racetrack, our story would end right here. Surprisingly, the x rays coming off the APS complicate vacuum pumping enormously. That's because of an effect called stimulated desorption.
You see, when metals are exposed to the atmosphere, they tend to adsorb a large quantity of gas on their surface. This amount increases greatly when a metal oxide forms. Then when the metal is exposed to x rays, some of this gas is driven off--that's stimulated desorption.
The main storage ring is designed so that the aluminum chamber is exposed to only a small fraction of the x-ray beam. Most of the beam is intercepted by beam stops, or absorbers, that are placed at strategic locations in the chamber. The absorbers are meant to intercept most of the x rays, in order to protect vacuum flanges and other critical components, and to define the size of beam that is transmitted to the experimental stations.
Only 5% of the synchrotron's radiation is transmitted to the beam lines. The beam stops themselves absorb 13kW of power, which results in releases of an enormous amount of gas on a continual basis.
To handle the heavy gas loads, every section of every sector gets an ion pump. The 13-kW beam stops also get a getter pump, which consists of a high concentration of NEG strips folded accordion-style. The pump's initial pumping speed is about 10,000 1/sec, but this figure drops logarithmically with use.
Even with all this pumping, the positron beam is expected initially to last only about 20 hrs. That's because, for a while, stimulated desorption will be a bit too much for the pumping system to handle. Gradually, however, the beam stops will become depleted of adsorbed gas, and the beam lifetime will rise significantly.
There is much more to this story than space to tell it. After all, you don't get 7-GeV positrons by just flipping a switch. The APS begins with a linear accelerator that directs electrons down a vacuum chamber toward a target that converts the electrons into positrons. The conversion rate is so inefficient, though, that the positrons are sent to another evacuated chamber, a 30-m positron accumulator ring.
They come out of this with an energy of 450 MeV. A 370-m ring then boosts them to an energy of 7 GeV and injects them into the main storage ring.
Because the booster ring stores positrons for only about 2 sec, it can operate at a relatively high pressure. The pressure difference between this and the main storage ring is maintained through differential pumping.
When in full operation, the Advanced Photon Source will accommodate more than 300 scientists at any one time, performing more than 1,000 experiments annually. The APS's high brilliance x rays will be used for experiments in materials science, chemistry, physics, geophysics, biology, and medicine. Potential medical uses include new drug development.
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|Title Annotation:||R & D Vacuum Technology.; Argonne National Laboratory's Advanced Photon Source in Illinois|
|Publication:||R & D|
|Date:||May 1, 1993|
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